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INTEGRATED SILICON THERMOPILE INFRARED DETECTORS

INTEGRATED SILICON THERMOPILE INFRARED DETECTORS

Infrarooddetectoren op basis van geintegreerd silicium thermozuilen

Proefschrift

ter verkrijging van de graad van doctor in de technische wetenschappen

aan de Technische Universiteit Delft op gezag van de Rector Magnificus, prof.dr. J.M. Dirken, in het openbaar te verdedigen ten overstaan van een commissie,

door het College van Dekanen daartoe aangewezen, op donderdag 1 oktober 1987, te 16.00 uur

door

Pasqualina Maria Sarro geboren te Piedimonte Matese, Italië

dottore in Fisica

TR diss 1571

Dit proefschrift is goedgekeurd door de promotor Prof.dr.ir. S. Middeihoek

ai miei genitori aan René en Marco ed alia mia nonna

TABLE OF CONTENTS Page

1. INTRODUCTION 1 1.1 Aim of the work 1 1.2 Organization of the thesis 2

2. OVERVIEW OF INFRARED DETECTORS 3

2.1 Introduction 3 2.2 Detection of infrared radiation 3

2.2.1 Infrared radiation 3 2.2.2 The photon detection process 6 2.2.3 The thermal detection process 10

2.3 Thermal detectors 10 2.3.1 Thermopile detectors 11 2.3.2 Bolometer detectors 13 2.3.3 Pyroelectric detectors 15 2.2.4 Others 17

2.4 Optical detectors versus thermal detectors 18

3. THE SILICON THERMOPILE INFRARED DETECTOR 21

3.1 Introduction 21. 3.2 Thermoelectric effects 22

3.2.1 The Seebeck effect 22 3.2.2 The Peltier effect 25 3.2.3 The Thomson effect 27 3.2.4 The Seebeck coefficient 28 3.2.5 Figure of merit 33

3.3 Integrated silicon thermopiles 35 3.3.1 Thermopile performance 35 3.3.2 Use of thermopiles in thermal sensors 38

3.4 The silicon thermopile infrared detector 39 3.4.1 The working principle 39 3.4.2 Design criteria 40

vn

4. FABRICATION PROCESS 45

4.1 Introduction 45 4.2 The cantilever beam structure 46

4.2.1 The etch process 46 4.2.2 Influence of the oxide thickness 55 4.2.3 Influence of the aluminum

interconnection pattern 57 4.2.4 Influence of other parameters 58

4.3 IR detector fabrication process 61

5. EXPERIMENTAL RESULTS 67

5.1 Introduction 67 5.2 The single detector 67

5.2.1 The detector layout 67 5.2.2 Responsivity to blackbody radiation 73 5.2.3 Relative detectivity, NEP and

time constant 81 5.2.4 Spectral response 83 5.2.5 Spatial homogeneity 84

5.3 The infrared sensing array 86 5.3.1 The array layout 86 5.3.2 Responsivity to blackbody radiation 88 5.3.3 Relative detectivity, NEP and

time constant 91 5.3.4 The array as part of a monochromatic

radiation sensor 92

6. DISCUSSION AND CONCLUSIONS 99

REFERENCES 103 LIST OF SYMBOLS 108 SUMMARY 110 SAMENVATTING 112 ACKNOWLEDGMENTS 114 ABOUT THE AUTHOR 116

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1. INTRODUCTION

1.1 Aim of the work

Electronic measurement and control systems in general consist of an input transducer, a signal processor and an output transducer [1.1]. In the input transducer, often called the sensor, a measurand such as temperature, pressure, radiation, chemical composition or magnetic field direction, is converted into an electrical signal. In the signal processor, the electronic signal is modified (amplified, filtered, etc.). In the output transducer, the electronic signal is converted into a signal which can be perceived by one of our senses (display) or which can cause some action (actuator). While an abundance of very sophisticated low-cost microelectronic components is available today, sensors with performance/price ratios comparable to that of microelectronic circuits are much in demand. One group of sensors of current interest are silicon sensors.

Silicon is a very promising material for sensors not only because it shows many large physical effects which may be used for sensing purposes [1.2], but also because a dependable, diverse and sophisticated silicon planar technology is available nowadays. The application of silicon planar technology to sensors offers several advantages [1.3,1.4]:

- The dimensions of the sensor can be very small, so that the measurand will not be significantly influenced by the sensor, the power consumption can be very small and the frequency response can be good.

- The batch-fabrication technique allows large quantities of sensors to be produced, thereby reducing their price.

- The sensor and the signal processing electronics (or a part of it) may be integrated on the same chip, to obtain a so-called smart sensor.

Silicon also has very good mechanical properties and micromachining of three-dimensional structures is feasible. Further, it exhibits no hysteresis if subjected to repeated stress and in terms of their chemistry silicon and its oxide are inert in many hostile environments. Of course, the use of silicon also has some drawbacks such as a limited temperature range of operation (most sensors only work properly between - 50 and + 150°C). In addition, packaging often presents some difficulties (the sensor may have to operate in a hostile environment in which the usual integrated circuit (IC) encapsulation is inadequate). However, the advantages of integrated sensors greatly outweigh the disadvantages.

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One of the physical effects that can be exploited for thermal sensing is the Seebeck effect. This self-generating effect, in which a temperature difference is converted into an electric voltage, is rather large in silicon. Thermocouples or thermopiles (a pile of thermocouples connected in series) based on this effect have been used to measure temperature differences or to convert thermal energy into electrical energy. Several thermal sensors, based on the Seebeck effect and able to measure mechanical, radiant and chemical signals, have been realized in silicon and some of them are fabricated by integrated circuit technology [1.5].

The aim of this work was to investigate the possibility of realizing one of these thermal sensors, namely a thermal infrared detector based on an integrated silicon thermopile. The use of infrared (IR) detectors, both thermal and photon, is not confined to research and development laboratories, but has many applications in industry, medicine, meteorology, astronomy and defence. In fact, without touching an object, IR technology can determine its existence, its shape, its temperature and its composition [1.6-1.7]. Thermal detectors, although generally slower and less sensitive than photon detectors, are still widely used, because they respond equally well to a broad range of infrared radiation, operate at room temperature and are inexpensive. These unique properties make them suitable for various tasks that cannot be fulfilled by photon type detectors, and as such are sufficient reasons to continue to develop them, particularly for applications where inexpensive, but reliable detectors are required [1.8]. The device presented in this thesis is a thermal type detector of infrared radiation, based on an integrated silicon thermopile. By using silicon not only as a supporting structure, but also as one of the two thermocouple materials, such a device benefits from both of the above-mentioned advantages offered by silicon IC technology and from the large value of the Seebeck coefficient in silicon.

1.2 Organization of the thesis

In Chapter 2 the infrared radiation detection process will be briefly described and an overview of the thermal type infrared detectors will be presented. The Seebeck effect in silicon and the integrated silicon thermopile, the device exploiting this effect, will be investigated in Chapter 3. In that chapter the infrared detector based on the thermopile will be analyzed theoretically. The fabrication process used to fabricate both single detector and linear arrays will be described in Chapter 4, while the experimental results will be extensively presented in Chapter 5. Finally, a discussion of these results together with some conclusions will be the subject of Chapter 6.

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2. OVERVIEW OF INFRARED DETECTORS

2.1 Introduction

Although the infrared part of the spectrum was discovered in 1800 by Herschel, detection of infrared radiation goes back a long way. Man has always been aware of the heating effects of first the sun and then of fire. The only heat sensors available then were the skin sensors distributed over the body, with those in the hands and the face being the most convenient for use. The first experiment to sense heat emitted by a terrestrial object appears to be that made by della Porta at the end of the 16th century [2.1]. He noted (using his face) the concentration by a concave metal mirror of the heat of a distant candle and the cold from a block of ice. This experiment was repeated in Florence by the Accademia del Cimento in 1660 when, for the first time, a detector replaced the hand or face. The detector used was a thermometer, which was a prototype of the modern liquid-in-glass thermometer. Since then many detectors for infrared radiation have been discovered and are usually classified into two general classes: thermal detectors and photon detectors [2.2]. In photon detectors, the incident radiation excites electronic transitions which change the electronic state of the detector. In thermal detectors, the energy of its absorbed radiation raises the temperature of the detecting element. This increase in temperature will cause changes in the temperature dependent properties of the detector. Monitoring one of these changes enables the radiation to be detected.

In the following section, after a short review of the infrared radiation characteristics, we will describe briefly these two types of detection processes. In the last section of this chapter we will review the most important types of thermal detectors, pointing out their characteristics and their limits. Finally, the advantages and disadvantages of both optical and thermal detectors will be briefly discussed.

2.2 Detection of infrared radiation

2.2.1 Infrared radiation

The radiation emitted by a body as a result of its temperature is called thermal radiation. All bodies emit such radiation to their surroundings and

3

absorb such radiation from them. The emitted radiation has a continuous spectrum, whose details depend strongly on the body temperature and somewhat on the composition of the body. However, experiments show that there is one class of hot bodies that emits thermal spectra of a universal character. These are called blackbodies, i.e. bodies which absorb all the thermal radiation incident upon them. Independently of the details of their composition all blackbodies emit thermal radiation with the same spectrum. The spectral distribution of blackbody radiation is specified by the spectral radiancy RT iy), defined so that RT (i/) Av is equal to the energy emitted per unit time in radiation of frequency in the interval v to v + di/ from a unit area of the surface at absolute temperature T. The integral of the spectral radiancy over all frequencies is the total energy emitted per unit area from a blackbody at temperature T, i.e.,

oo /?T= f RT(v)év (2.1)

o

RT is called the radiancy and it increases rapidly with increasing temperature (see Fig. 2.1). This result was first stated in 1879 in the form of an empirical equation, called Stefan's law

RT = oT4 (2.2)

where a is the Stefan-Boltzmann constant. Experiments show also that the spectrum shifts towards higher frequencies - - o r lower wavelengths - - as T increases. This result is known as Wien's displacement law

^max T = const (2.3)

where Amax is the wavelength at which the spectral radiancy has its maximum value for a particular T. By considering the energy emitted by a blackbody as a discrete variable, Planck obtained the following formula for the energy density in the blackbody spectrum:

Zithc dA ,- ., M A ) d A — hc/XkT

( 2" 4 )

A C — 1

known as Planck's radiation law. This formula is in complete agreement with the experimental results at all temperatures. Stefan's law (2.2) and Wien's law (2.3) can be derived from this formula. Stefan's law is obtained by integrating Planck's law over the entire wavelength range, while Wien's law is obtained by setting dp(A)/dA = 0 [2.4].

4

STELLINGEN

appendix to the Ph. D. thesis

"Integrated silicon thermopile infrared detectors"

by

Pasqualina M. Sarro

1. Any type of signal which can generate an on-chip temperature difference (or alter an existing known temperature difference) across an integrated silicon thermopile can be detected by the thermopile.

This thesis, chapter 3; A. W. van Herwaarden and P. M. Sarro, Thermal sensors based on the Seebeck effect, Sensors and Actuators, 10 (1986) 321-346.

2. For an integrated thermopile infrared detector, a cantilever-beam structure is preferable because its sensitivity is higher than that of a membrane structure of comparable dimensions.

This thesis, chapter 3.

3. Micromachining of silicon by means of an etching process which is compatible with silicon planar technology, is a necessary tool for persuing new types of integrated silicon sensors.

This thesis, chapter 4.

4. Very thin dielectric membranes (400A) resting on a silicon substrate, fabricated by anisotropic etching of silicon, are excellent substrates for TEM analysis of sputtered or evaporated metals.

5. In heterojunction photovoltaic devices, large lattice mismatch and high solar conversion efficiency are not mutually exclusive.

R. R. Arya, P. M. Sarro and J. J. Loferski, Efficient CdS on silicon solar cells, Appl.Phys.Lett., 41 (1982) 355-357.

6. Growth of good quality I-III-VI2 and II-IV-V2 ternary chalcopyrite semiconductors and their alloys is a worthwhile research topic. Alloy systems of these materials offer the possibility of attaining independent control of the bandgap and of the lattice constant of the material. This has great potential for new classes of LEDs, laser diodes and solar energy convenors.

7. Team work can give excellent results only if all the team members fully understand the meaning of these words.

8. The fact that, in The Netherlands, technical studies are concentrated in separate universities (Universities of Technology) has dramatic consequencies on the social development of the students and employees.

9. To give a good talk is almost as difficult as achieving the results which are to be presented, and at least as important.

10. The decisional power of managers in large organizations should at least match their responsibilities.

WAVELENGTH A (/jm)

Figure 2.1: Spectral radiancy R(\,T) of a blackbody at the temperature in kelvins shown on each curve. The diagonal line intersecting each curve at its maximum shows the Wien's displacement law [2.3].

The relation between the body temperature and the frequency spectrum of the emitted radiation can be used to estimate the temperature of the body. There is a continuous spectrum of the thermal radiation emitted, the eye seeing chiefly the color corresponding to the most intense emission in the visible region. However, objects at temperatures below 1000 K emit a spectrum which has a maximum at a wavelength larger than 3 micron. This means that a large part of the energy is emitted in the infrared region of the spectrum. Consequently, infrared detectors are necessary to detect the radiation emitted by these objects.

Let us now examine how this radiation can be detected. Electromagnetic radiation can interact with materials in many ways. However, the choice of materials and the experimental arrangement usually cause one of the effects to predominate. The two most important categories are photon effects (photons interact directly with the electrons in a material) and thermal effects (certain properties of a material change due to a change in temperature arising from absorption of radiation) [2.5]. Infrared detectors are generally classified into photon or thermal detectors according to the mechanism of radiation detection involved. Let's first examine the photon detection process.

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2.2.2 The photon detection process

Several forms of photon effects are possible when incident photons interact with electrons within the material - - generally a semiconductor - -whether bond-to-lattice or free atoms. These effects can be subdivided into internal and external ones (see Table 2.1). The internal effects are those in which the photoexcited carriers (electrons or holes) remain within the sample, while the external effects are those in which an incident photon causes the emission of an electron from the surface of the absorbing material. Among the many different photon effects the photoconductive, the photovoltaic and the photoemissive have been widely exploited for infrared detection.

Table 2.1: Classification of photon effects [2.5].

1. Internal 1.1 Excitation of additional carriers

Photoconductivity Electrically biased

Intrinsic Extrinsic

Microwave biased Photovoltaic effect

p-n junction Avalanche p-i-n Schottky barrier Heterojunction Bulk

Photoelectromagnetic effect Dember effect Phototransistor

1.2 Free carrier interactions Photon drag Hot electron bolometer Putley detector

1.3 Localized interactions Infrared quantum counter Phosphor Photographic film

2. External (photoemissive) 2.1 Photocathodes

Conventional Negative electron affinity

2.2 Gain mechanisms Gas avalanche Dynode Multiplication Channel electronmultiplication

Photoconductivity: The radiation changes the electrical conductivity of the material upon which it is incident. This effect, which can be observed in virtually all semiconductors, can be intrinsic or extrinsic. Intrinsic photoconductivity requires the excitation of a free hole-electron pair by a photon having an energy equal to or greater than the energy gap Eg (see Fig. 2.2a). The long wavelength limit A0 of an intrinsic photoconductor is therefore:

^ 0 = he

(2.5)

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Many of the semiconductors employed as photodetectors, such as Si, Ge, III-V and II-VI compounds, have energy gaps ranging between 0.4 eV and 2.4 eV at room temperature, which means that they have values of A0 ranging from 0.52 (for CdS) and 5 /zm (for PbSe), and consequently they can be used only in the visible and near IR region. By operating the detector at a low temperature (77 K), a larger value of A0 can be obtained in the case of some compounds like Pb0 2 Sn0 8 Te and Hg0 8 Cd0 2 Te, which have a long wavelength limit of 12 /xm [2.5].

APPLIED ELECTRIC FIELD

ELECTRON CONDUCTION / / / / / / / V t f ? / / / / / / / / BAND

PHOTOEXCITATION-*-T

Eg « hc /X 0

+ ////-ujy////////// VALENCE

HOLE B A N 0

APPLIE0 ELECTRIC FIELD

ELECTRON CONDUCTION

/A/////AH-H//////// BAND E; - hC/X0

T ■PHOTOEXCITATION

PHOTOEXCITATION «̂ 1 E i « h c / X 0

777777/

DONOR LEVEL ACCEPTOR LEVEL

/////■//////+. HOLE

7 7 VALENCE BAND

Figure 2.2: Photoconductive processes: a) Intrinsic; b) Extrinsic [2.5].

Extrinsic photoconductivity occurs when an incident photon, lacking sufficient energy to produce a free hole-electron pair, can produce excitation at an impurity center in the form of either a free electron-bound hole or a free hole-bound electron. In this case the long wavelength limit is given by

he K = — (2.6)

where Ei is the impurity ionization energy (see Fig. 2.2b). Since Si and Ge can be doped by many impurities a large range of values for Ei is made available by using doped crystal of these two semiconductors. The long wavelength limits are generally larger than 8 ̂ m [2.5].

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Photovoltaic effect: The incident radiation generates electron-hole pairs which are separated by a built-in electrical field due to an internal potential barrier, such as in the case of p-n junctions (Fig. 2.3), p - i -n junctions, Schottky barriers, heterojunctions and avalanche photodiodes. While photoconductivity is a majority carrier phenomenon (it is the increase in the number of majority carriers accompanying irradiation which makes up for the photocurrent), the photovoltaic effect depends largely upon the minority carrier lifetime. This is because the presence of both the photoexcited electron and hole are required for the intrinsic effect to be observed.

N-REGION P-REGION

ELECTRON

S T CONDUCTION BAND

PHOTOEXCITATION

Eg -- h c ' \ >

1 VALENCE BAND

At HOLE

Figure 2.3: Photoexcitation at a p-n junction [2.5].

Photoemissive effect: The incident radiation causes the emission of an electron from the surface upon which it is incident (photocathode) into the surrounding space, where it is to be collected by an anode. Vacuum phototubes, gas filled phototubes and the widely used photomultiplier are typical applications of the photoemissive effect. The spectral properties of detectors based on this effect are controlled by the photocathode, which can be a metal or a semiconductor. If it is a metal, the incident photon to cause photoemission must have an energy at least equal to the work function of the metal, which lies in the order of several electron volts (see Fig. 2.4a). This implies that metallic photocathodes can be used only to detect visible or ultraviolet radiation. For semiconductor materials with a positive electron affinity, whose energy band diagram is illustrated in Fig. 2.4b, the minimum energy required for a photon to cause photoemission is that which will raise the electron to an energy level higher than the potential barrier at the surface. Although less energy than for metallic photocathodes is required for photoexcitation, these photocathodes extend at best only into the very near infrared. The near

8

infrared can be reached by some photocathodes which use semiconductors having a negative electron affinity. In this case the incident photon energy must only equal or exceed the energy gap of the semiconductor for photoemission to occur (see Fig. 2.4c). It appears that photocathodes are not efficient detectors of infrared radiation at wavelengths exceeding 1 /mi.

METAL-I—VACUUM

PHOTOEXCITATION

FERMI LEVEL

T \

n 1

/

SEMICONDUCTOR — I — VACUUM

PHOTOEXCITATION

CONDUCTION BAND _

FERMI LEVEL —

VALENCE ^^y//////A/7?>

IT

SEMICONDUCTOR— —VACUUM

CONDUCTION BAND

PHOTOEXCITATION

FERMI LEVEL 4-VALENCE BAND 7 ' *— V,

q +

Figure 2.4: Photoemissive processes: a) Metal photocathode; b) Semiconductor photocathode with positive electron affinity; c) Semiconductor photocathode with negative electron affinity [2.5].

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2.2.3 The thermal detection process

The thermal detection process consists of two stages. In the first one the radiation is absorbed by the material, generating phonons and causing the lattice to heat up. In the second one, this increase in the temperature of the absorbing material causes variations in the material properties. By monitoring these changes, the radiation can be detected. Several thermal effects can be used to detect the incident radiation, but those which have found greater utility in infrared systems are the thermoelectric, bolometric and pyroelectric effects.

2.3 Thermal detectors

Thermal detectors are generally classified according to the thermal effect used for the detection. However, the primary effect in which the absorbed fraction of the incident radiation causes a temperature increase in the absorbing material is common to all types of detectors. The basic model of a thermal detector is shown in Fig. 2.5.

DETECTOR-^ V SIGNAL WIRES

Figure 2.5: Basic model of thermal detector [2.6].

The detector is represented by a thermal mass H, which is connected by a link of thermal conductance G to a heat sink at temperature T. Without incident radiation, the detector is also at temperature T, while in the presence of incident radiation its temperature rises to TB = T + 6. The temperature rise 6 is found by solving the equation [2.6]:

r)P = H(d6/dt) + Gd (2.7)

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where P is the incident power, of which the fraction r? is actually absorbed. The solution of this equation

6 = r) P (G2 + w Z / 2 ) 4 (2.8)

shows that it is advantageous to reduce the heat conductance G from the detector to the heat sink (the heat losses from the detector have to be minimized). Further, for modulated radiation, it is desirable that the heat capacity of the detector itself be minimized. The time constant r of the detector is, as in electrical circuits, the product of a heat resistance and a heat capacitance, i.e.

r = RthH (2.9)

where Rth is the detector thermal resistance.

Before we review several types of thermal detectors, classified according to the thermal effect used in the second stage of the detection process, let us define two other quantities, which together with the time constant, are used to characterize the detector performance: the responsivity R and the relative detectivity D*. The responsivity R of an infrared detector is the ratio of the detector output and the input power. Generally, the term blackbody responsivity is used if the source is a blackbody, while the term spectral responsivity is used if the source is monochromatic radiation. For most of the thermal detectors the output signal is a voltage, so the responsivity is generally expressed in V/W. The relative detectivity is an area independent figure of merit defined as

D* = (2.10) NEP

where AD is the absorbing area (or active area) of the detector, B is the bandwidth of electronic equipment and NEP is the noise equivalent power. NEP represents the minimum detectable power of the detector, and is given by the ratio of the noise signal and the responsivity.

2.3.1 Thermopile detectors

Thermopiles (several thermocouples connected in series) are the oldest radiation detectors, after thermometers. They are based on the Seebeck effect, in which a temperature difference between the junctions of the two different conductors forming the thermocouple is converted into an electric voltage. This electric voltage is related to the temperature

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difference through the Seebeck coefficient of the conductors, i.e.

AF = (a 8 ] 1 - a 8 ] 2 )Ar (2.11)

where a8 x and aB 2 are the Seebeck coefficients of the two conductors. (For a detailed description of the Seebeck effect and for typical values of the Seebeck coefficient of metals and semiconductors, see the next chapter).

The structure of the thermopile infrared detectors is generally as follows: the hot junctions are supported by a thin insulating membrane, in order to reduce the thermal conductivity of the device, and are located in proximity to a radiation absorber. A thin layer of an absorbing material is necessary to absorb the incident radiation efficiently over a broad spectral range. The cold junctions are on a thick frame, which acts as a heat sink. The two different thermocouple materials are generally deposited on the membrane by vacuum deposition and patterned through a photolithographic process. Materials which have high Seebeck coefficient values and are easy to handle are usually chosen.

Lahiji and Wise [2.7] have used two types of thermocouples for their detector: bismuth antimony and polysilicon gold. In both detector types, the thermal properties of the device are dominated by the silicon membrane. As a result, the polysilicon thermopile exhibits the same speed as the Bi-Sb device with a responsivity which is increased by a factor of about 2, reflecting the larger thermoelectric power of the polysilicon-gold couple. The minimum detectable power for polysilicon devices is somewhat larger than for the Bi-Sb detector due to the higher Johnson noise associated with the high polysilicon lead resistance. For applications involving the detection of very low incident energy, Bi-Sb couples are advantageous because of their minimum detectable power. For applications involving a higher incident level, polysilicon-gold couples are preferred, since they offer higher responsivity without compromising speed. Because of its large Seebeck coefficient (=* 400 /xV/K), tellurium has been chosen by Kimura [2.8] in combination with Ag for a thermopile made on a floating Si02 film (microbridge) on a silicon substrate and by Shibata et al. [2.9] in combination with InSb.

Table 2.2 summarizes typical performance characteristics (N is the number of couples, AD is the active area of the detector, Rtp is the electrical resistance of the thermopile and the other parameters are as defined above) of some thin-film thermopile detectors [2.7-2.12]. The best result has been achieved by Elbel et al. [2.10], with a detector consisting of Bi-Sb thermocouples evaporated onto a 1 /mi thick Si02/Si3N4 membrane anisotropically etched in a silicon wafer.

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Table 2.2: Typical performance characteristics of some thermopile infrared detectors.

Materials N A^ R T Rtp D* Ref. (mm 2) (V/W) (msec) (kfi) (K^crnHzfyvv)

Bi-Sb p-polySi/Au n-polySi/Au Te-Ag Te-InSb Bi-Sb

n-polySi/Au p-polySi/Au

60 60 60 11 54 50 6

15 12 32 32

0.36 0.36 0.36

1.0 1.0 1.0 0.78 0.78

6 7

9.6 1.1 95

30-50 15 23 9.5

20 -25 52-56

15 15 15

300 30

15-30 80 32 40 25 25

36

70 500 20 8 3

3.25 80

250

0.6 0.3.5 0.48

1.1 3.0 1.3 3.0 1.1 0.55 0.7

[2.7] [2.7] [2.7] [2.8] [2.9]

[2.10] [2.11] [2.11] [2.11] [2.12] [2.12]

Thermopile detectors are very good detectors for noncontact temperature measurements, since temperature radiation is a dc or low frequency signal and the thermopile can respond directly without requiring the use of a chopping system. Thermopiles are also used for NDIR (Non Dispersive IR) gas analyzers and for passive IR intrusion alarms.

2.3.2 Bolometer detectors

The thermal effect on which bolometer detectors are based is that of change in resistivity of a material in response to the heating effect of the incident radiation. This can be expressed by means of the temperature coefficient a defined as

1 dR

where/? is the resistance and T the temperature of the sample. Bolometers consist of a resistive element constructed from a material with a large value of a. The detector is generally called metal bolometer if the material is a metal, and a thermistor (thermally sensitive resistor) if it is a semiconductor. Metals have a positive temperature coefficient of resistance. Typical values of a for metals commonly used in bolometers (platinum, gold, nickel, etc.)

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range between 0.3%/°C and 0.6%/°C at room temperature. Thermistors use the considerably larger but negative resistance temperature coefficient of certain semiconductor materials, usually oxidic mixtures of manganese, nickel and cobalt. The temperature coefficient for these materials is in the order of - 4%/°C, that is, about one order of magnitude larger than that of metals. Another type of bolometer detector is the cryogenic bolometer developed especially as a detector for far infrared radiation. These detectors employ single crystal of semiconductors (carbon, Ga or In doped Ge, InSb and even Si) which have a large negative temperature coefficient of resistance at cryogenic temperatures (< 4 K). Although they have a high sensitivity and low noise, their cooling system makes them expensive and unpractical for many applications.

To operate a bolometer a constant current / is driven through the bolometer from a regulated current supply. The incident radiation produces a change AR of the resistance. The power supply needed to keep the current / constant will adjust the voltage by a small amount AV, which is given by

AV = rRaAT (2.13)

where the temperature AT is the solution of Eq. (2.7). The responsivity AV/P can be improved by minimizing the heat conductance and the heat capacity or by increasing the current. However, there is a critical limit to how high a bias voltage may be applied, because of the self-heating effect, which introduces error into the measurement. Bolometers are preferably used with chopped radiation, so that with ac amplification the large dc dark signal is suppressed. Most commercial detectors consist of two identical bolometers connected in a bridge circuit, one being irradiated, the other being shielded, and serving as dark current and ambient temperature-drift compensation.

Several metal and thermistor bolometers operating at room temperature are reported in literature. The essential components of a bolometer are the sensing element and associated electrical contacts, the detector substrate and a suitable package fitted with an IR-transmitting window. The sensing element, generally very thin to minimize heat capacity and maximize the temperature change resulting from the absorption of radiation, is usually mounted on a heat sink to provide high-speed response and to dissipate bias current power. Metals such as copper form an excellent heat sink, but electrical insulators like quartz, sapphire and beryllium oxide are preferred, since the sensing element can be cemented directly to them. The sensitive area is usually blackened to improve absorptivity to radiation.

14

Thermistors made by sintering powdered mixtures of ferrous oxides are reviewed by Tebo [2.13]. The thermistor elements are usually 10 //m thick, with the sensitive areas between electrodes ranging from 0.05 x 0.05 mm to 5 x 5 mm. Thermal time constants range from 1 to 10 msec and relative detectivities from 1 to 3 x 108 cmHzfyw. A very thin (4 nm) Pt film deposited on an amorphous dielectric pellicle has been used in the bolometer realized by Liddiard [2.14]. Since the pellicle materials must have a good thermal match and good adhesion to the substrate (a silicon wafer), they were prepared from thin films of alumina or silicon nitride deposited on the front side of the wafer and then etched from the back side to form the membrane. Detector elements with different sizes and resistivities were realized. For a typical detector size of 100x75 jim a maximum responsivity in vacuum of 50 V/W , falling to 4.5 V/W in air, was measured. The maximum detectivity of this detector is 1.6 x 108 cmHzVW in vacuum and 3 x 107 cmHzVW in a Xe atmosphere, with a time constant lower than 1 msec. Very good responsivity values are obtained with a Te or Bi bolometer made on a floating Si02 film (microbridge) on a Si substrate [2.8]. These devices, fabricated by silicon IC fabrication techniques and metal evaporation, showed responsivity up to 130 V/W for the tellurium type and somewhat smaller values for the bismuth type. Both devices have time constants in the order of 200 msec, since they are mainly determined by the heat capacity of the microbridge. A similar structure was also used in the air-bridge microbolometer [2.15]. It consists of a bismuth microbolometer, typically four micrometers square and one-tenth of a micrometer thick, suspended in the air above the substrate. In this way the major pathway out of the detector is removed and a maximum responsivity of 99 V/W is obtained.

Metal bolometers are still used for temperature measurements, while in infrared measurements thermistor bolometers are preferred because of their higher responsivity. Cryogenic bolometers are widely used in infrared astronomy where they have, over most of the infrared spectrum, a uniform performance comparable in sensitivity to the best photon detectors.

2.3.3 Pyroelectric detectors

The pyroelectric effect is exhibited in temperature sensitive pyroelectric crystals. Such crystals have an internal electric dipole moment. Although the external field produced by this dipole will normally be neutralized by an extrinsic charge distribution near the surface of the metal, in good pyroelectric materials (which are good insulators) this extrinsic charge distribution is relatively stable so that even quite slow changes in the sample's temperature, which produces changes in the internal dipole

15

moment, produce a measurable change in surface charge. Consequently, if a small capacitor is fabricated by applying a pair of electrodes to the sample the change in temperature and hence the incident thermal radiation can be detected by measuring the charge on the condenser. An output signal appears only if the temperature is changing. Therefore, this type of detector can be used only with chopped radiation. Another restriction is the temperature range. The materials involved lose their electric polarization if they are heated above a certain temperature, called the Curie temperature.

If the detector is irradiated with modulated radiation, an alternating temperature change AT will rise. Accompanying the temperature change is an alternating charge AQ on the external electrodes given by

AQ = pAAT (2.14)

where p is the pyroelectric coefficient of the material (in C cm"2 K"1) and A is the area over which the incident radiation is absorbed. The pyroelectric coefficient depends on the material and may also be a function of the temperature. It describes the charge C in Coulomb, which is generated per square centimeter by a temperature change of one degree Kelvin. Typical values of p vary from 0.4 to 4 x 108 C/cm2 K [2.16].

The most commonly used materials in pyroelectric detectors are: triglycine sulfate (TGS), strontium barium niobate (SBN), lead zirconate titanate ceramics, lithium tantalate, polyvinyl fluoride film (PVF) and polyvinylidene fluoride film (PVF2). TGS provides the most sensitive detectors, but disadvantages such as the low Curie temperature (49 °C) make the other materials preferable as long as the lower responsivity is acceptable. A ceramic wafer of PZT modified with Pb(Sn0 5Sb0 5 ) 0 3 has been used as the pyroelectric material by Murata and Ito [2.17], while PbTi03 was used by.Kaneko et al. [2.18] to realize detectors with a D* of 5.1 x 107 cmHzVW and a time constant of 10 msec. Very good results have also been achieved with plastic polymers such as PVF2 [2.19]. Thin membranes (0.5 - 0.8 y.m) of this material were used for detectors packaged in vacuum containers, giving a maximum D* of 109cmHzV\Vat 10 Hz.

Apart from their use for modulated radiation, pyroelectric detectors, being capacitive, may also be used for pulse measurements and even as storage elements of the charges generated in arrays.

16

2.3.4 Others

Several other thermal effects, such as the temperature variation of the dielectric constant [2.20], the pyromagnetic effect (the magnetic equivalent of the pyroelectric effect) [2.21], and the Nernst effect in suitable semiconductors [2.22, 2.23] have received a certain amount of attention, although they are not widespread in use. Another thermal detector that needs to be mentioned for its high responsivity is the Golay cell, first developed by Golay in 1947. Radiation absorbed by a receiver inside a gas-filled chamber (usually xenon for its low thermal conductivity) heats the gas, causing its pressure to rise which distorts a flexible membrane on which a mirror is mounted (see Fig. 2.6).

CELL LINE GRID

LED

ABSORBER FLEXIBLE MIRROR

I I PV DETECTOR

Figure 2.6: The Golay cell [2.2].

The movement of the mirror is used to deflect a beam of light shining on a vacuum photocell and so produce a change in the photocell current as the output. In modern Golay cells, the tungsten filament used to provide the beam of light is replaced by a light emitting diode and the vacuum photocell by a solid-state photodiode. These detectors are of interest because of their extremely high responsivity (~106V/W). However, they are complex and difficult to handle. They must be protected from shock and vibration and kept at a stable ambient temperature, and they are limited to radiation levels below 3 MW. Consequently, they are mainly suitable for laboratory applications.

17

2.4 Photon detectors versus thermal detectors

Many types of photon and thermal detectors for infrared radiation have been developed, each of them presenting advantages and disadvantages. Thus, there is no detector which is absolutely the best. What is more likely is that for each particular application or measurement condition one or the other will be the most suitable. However, by comparing the two categories of detectors, we can say that, in general, photon detectors are more sensitive (generally two orders of magnitude) and they are faster since they are based on a single-step transduction process rather than the two-step process associated with thermal detectors. One big disadvantage of the photon detectors is that they are characterized by a sharp long-wavelength cut-off (photons with energy lower than the bandgap produce no signal), which means they can be used only in a specific - - and usually quite narrow - - wavelength range. On the contrary, thermal detectors have a continuous response over a broad spectral range. This non-selectivity is a rather important requirement in many applications, such as radiometry - - where almost exclusively thermal detectors are used - - and in standardizing laboratories for basic radiometric calibration. On the other hand, there are a few applications, such as photometry and colorimetry, where selective detectors are needed. Another important advantage of thermal detectors is that they operate well at room temperature, unlike semiconductor detectors which must be cooled, because thermal generation-recombination noise limits their sensitivity. Although higher detectivity - - or lower noise - - is obtained by cooling, there are many cases in which a cooling system, generally complex and costly, is unpractical or undesirable.

Among the thermal detectors the most used are bolometers, pyroelectrics and thermopiles. Pyroelectric detectors have a higher responsivity and are generally faster, but they need modulated radiation. Bolometers, on the other hand, need an external bias. This introduces 1/f noise, making them less sensitive at low frequencies than thermopile detectors, for which Johnson noise is the limiting factor. Thermopile detectors, based on a self-generating effect, do not need any external bias as bolometers do, and the incident radiation does not need to be chopped as in pyroelectric detectors. It is in fact due to their ease of operation, and for reasons concerning performance, cost and reliability, that thermopile detectors are generally preferred to pyroelectric or bolometer detectors for applications such as noncontact thermometers, passive IR intrusion alarms, NDIR gas analyzers and the like [2.11].

Thermopile infrared detectors have been generally realized using vacuum evaporation and shadow masking of the thermocouple materials on thin plastic or alumina substrates [2.24, 2.25]. This approach resulted in

18

relatively large structures, which lack the batch fabrication and the process flexibility typical of devices employing the highly developed silicon integrated circuit technology. In order to profit from this technology, thermopile detectors have been realized which did make use of silicon, but only as a supporting structure [2.7, 2.9, 2.10].

The thermopile infrared detector which we developed and which is described in the following chapter utilizes silicon not only as a supporting structure, but also as one of the two thermocouple materials, thus benefitting from both of the above-mentioned advantages offered by silicon IC technology and from the large values of the Seebeck coefficient of silicon.

19

3. THE SILICON THERMOPILE

INFRARED DETECTOR

3.1 Introduction

Like all thermal type detectors, the thermopile infrared detector presented in this thesis uses a two-step transduction process. Firstly, the radiation is absorbed in the interaction area of the detector and transformed into heat, which will flow to the heat sink and thus generate a temperature difference. In the second step this on-chip temperature difference is converted into an electrical voltage by the integrated silicon thermopile.

In order to describe the working principle of the detector, we first need to discuss some important properties of the integrated thermopile, since the thermopile is the basic element of the detector. We will start by reviewing the Seebeck effect, the physical effect exploited by the thermopile. This effect is a self-generating effect, that is the power for the output signal is supplied by the input signal itself, instead of by an auxiliary power supply. This means that the thermopile is offsetless (no output signal is present without an input signal), which is a very attractive feature for a sensor to have. (Note that for the sake of completeness, the two related thermoelectric effects, the Peltier and Thomson effects, will be briefly discussed as well). Next, the Seebeck coefficient of metals and semiconductors will be reviewed and the criteria motivating the choice of the proper thermoelectric material will be discussed. In the following section the optimum performance of integrated silicon thermopiles will be investigated. We will see that it is not only the silicon doping concentration and type which play a role, but that the geometry of the thermopile and the device structure affect the device performance as well.

The investigations carried out in order to characterize the integrated thermopile and its performance are necessary to explain the working principle of the cantilever-beam shaped infrared detector based on such an integrated silicon thermopile. Once this has been accomplished, we will illustrate the design criteria of the device.

21

3.2 Thermoelectric effects

3.2.1 The Seebeck effect

If two (semi) conductors a and b are joined together at a hot point and a temperature difference AT is maintained between this point and a cold point (see Fig. 3.1), then an open circuit voltage AV is developed between the leads at the cold point. This effect, called the Seebeck effect after T.J.Seebeck (1770-1831) who discovered it in 1821, can be mathematically expressed by:

AV = asAT (3.1)

with as as the Seebeck coefficient expressed in V/K (or more commonly in /iV/K). It was found that only a combination of two different materials, a so-called thermocouple, exhibits the Seebeck effect. For two leads of the same material no Seebeck effect is shown for reasons of symmetry. It is present, however, because the Seebeck effect is a bulk property which depends neither on a specific arrangement of the leads or of the material, nor on a specific way of joining them. This bulk property can be expressed as:

VEF/q = aBVT (3.2)

with EF as the Fermi energy, with q as the elementary charge, and with the Seebeck coefficient as depending, among other things, upon the chemical composition of the material and upon the temperature.

lead a

lead b hot point ^ / / ' ' ' ' ' ' = Z ^ co[d po jn t

Figure 3.1: The Seebeck effect: the appearance of a AV due to the presence of a AT.

22

The Seebeck coefficient of, for example, silicon, can be obtained by setting as equal to the derivative of EF to the absolute temperature (see Fig. 3.2):

a„ = 1 dS, q èT

(3.3)

>-

TEMPERATURE!K)

7"+A7"

^"REF

Figure 3.2: The Seebeck effect: the variation of EF due to VT.

For non-degenerate n-type silicon the Seebeck coefficient may be approximated by using simple Maxwell-Boltzmann statistics. Three main effects are present.

Firstly, with increasing temperature the silicon becomes more intrinsic:

1 d£ q dT (Ec £F) = --r(ln(Arc/«)+f] (3.4)

with Ec as the conduction-band-edge energy, JVC as the conduction-band density of states, n the electron density (fixed by the doping concentration) and k the Boltzmann constant.

Secondly, with increasing temperature the charge carriers have a higher average velocity. Therefore, the charge carriers in the warm regions move faster towards the cold regions of the silicon than the charge carriers in the cold regions move towards the warm regions. This leads to charge build-up on the cold side of the silicon. Moreover, the scattering of charge carriers is usually energy (and thus temperature) dependent, likewise leading to charge build-up on the cold or hot side of the silicon, depending on whether the hot carriers can move more freely than the cold carriers or are "trapped" by increased scattering:

23

1 d£ F

~g~df ( r ) - > + * - ) ( 3 - 5 )

in which r is the relaxation time (mean free time between collisions) and sn is the exponent describing the relation between r and the charge-carrier energy.

Finally, the temperature difference in the silicon causes a net flow of phonons from hot to cold. In a certain temperature region (10-500K) and for non-degenerate silicon, a transfer of momentum from acoustic phonons to the charge carriers can occur. As there is a net phonon momentum directed from hot to cold, this will drag the charge carriers towards the cold side of the silicon. This effect may be represented by:

j dEFI ^

7 d F l<*»>" " 7 ' B (3-6)

in which <j>n denotes the phonon drag effect. In sum, the total Seebeck coefficient in non-degenerate n-type silicon becomes:

* (ln(iVe/«) + 4 + *n + ^n ) ""type (3.7) S q v 2

A similar reasoning can be applied to non-degenerate p-type silicon, which results in:

a . - + y ( l n ( t f v / p ) + - | + s p + t f p ) p _ t y p e ( 3 . 8 )

The terms \n(Nz/n) and ln(Nv/p) are typically of the order of 0 to 3 for the doping concentrations used in thermopiles, and 5 is of the order of -1 to 2. The phonon-drag contribution 0 ranges from 0 for highly doped silicon to approximately 5 for low-doped silicon at 300 K, while at low temperatures (100 K) </> is of the order of 0 for highly doped silicon to 100 for low-doped silicon [3.1]. In practice, for the range of interest for use in sensors (0.3 - 1 mV/K) and at room temperature (300 K), the Seebeck coefficient may be approximated (see Fig. 3.3) as a function of the electrical resistivity:

cta=^ln(p/p0) (3.9)

with p0 ~ 5 x 10" Qm (5 x 10" flcm) and m =; 2.6 as constants [3.2].

24

^ > FE

1—

-z. LU L J LU LU O l_J

^; L_I LU

m LU LU L/l

Z

1

0

_

/ /.

/ *

/ /

/ / a

/ x &

/ A

i i

A

A<

I

A N -type a N-type o N-type x P-type ■ P -type

10 RESISTIVITY(Qm)

10

Figure 3.3: The Seebeck coefficient of silicon as a function of the electrical resistivity at room temperature (300 K), where the symbols represent the experimental results and the dotted line the approximation of Eq.(3.9) [3.8-3.11].

3.2.2 The Peltier effect

In 1834 J.Peltier (1785-1845) discovered that when an electrical current flows through the junction of two different materials, heat is absorbed from or released to the ambient. This is caused by the thermal-energy current which is generally associated with the electrical current. The ratio of these currents usually differs for different materials. Therefore, when an electrical current crosses the junction, the difference in the thermal energy current will be released or absorbed at the junction (see Fig. 3.4).

\ZL lead a

/ / / / ■

lead b ZZIZL

J-

Figure 3.4: The Peltier effect: the absorption of heat at the junction of two different leads due to an electric current.

25

Mathematically this can be represented as follows:

<2 = - n a b 7 a b (3.10)

with Q as the heat absorbed from the ambient, II the Peltier coefficient for a junction of materials a and b, and J the current flowing through the junction from material a to material b. The Peltier coefficient II can be expressed in terms of the Seebeck coefficient a8 by using the following linear set of equations:

J=-aVET/q + aaaVT (3.11)

J;=^jrVEF/q - { ^ ) VT (3.12)

These equations interrelate the electrical and thermal quantities in materials when only electrical and thermal forces are present. Onsager [3.3] showed that - - if the correct terms are chosen - - the coefficients describing the cross effects are equal to each other in a linear theory of irreversible thermodynamics. In this way we arrive at the first Kelvin relation:

n = a8r (3.13)

named after Lord Kelvin (1824-1907), who first derived this equality. From Eqs. (3.11) and (3.12) we find, in isothermal conditions (Vr = 0), that:

Ö = - n / (3.14)

which means that an electrical current / is accompanied by an entropy flux (Q/T). The absolute Peltier coefficient is therefore nothing but the ratio of the electrical current and the associated thermal current mentioned above (see Fig. 3.5).

In thermal devices using heating resistors the Peltier effect may give rise to considerable asymmetries. For instance, the Peltier coefficient of 200 n/D resistors made of the shallow-p-type transistor base diffusion is 300 mV at room temperature. When a heating voltage of 3 V is applied across such resistors, a Peltier heat flow of 10% of the generated (irreversible) Joule heat will flow from one contact to the other, leading to

26

significant asymmetries. In critical situations the lay-out of heating resistors should therefore be planned carefully.

• • . • • • • •

^c »

•

•

* F n-type silicon(Si) aluminum(Al)

Figure 3.5: The Peltier effect: schematic representation of electrons releasing thermal energy when crossing the junction of silicon to aluminum.

3.2.3 The Thomson effect

While considering energy conservation Lord Kelvin (born with the name Thomson) argued that if an electric current flows in a material where a temperature gradient is present, heat is absorbed from or released to the ambient. The following relation applies:

ÖTh = 7 T h ^ V r (3.15)

with 7 T h as the Thomson coefficient. Kelvin showed that the Thomson coefficient is closely related to the Seebeck coefficient and formulated the second Kelvin relation:

7Th = ^ s (3.16)

The Thomson effect is useful in the determination of the absolute Seebeck coefficient of lead (Pb). Lead serves as a reference for all other materials at temperatures up to room temperature, because its Seebeck coefficient is low and can be accurately measured as a function of temperature.

27

3.2.4 The Seebeck coefficient

The absolute Seebeck coefficients of certain metals at two different temperatures are shown in Table 3.1. These values [3.4-3.6] are based upon the absolute thermoelectric power of lead. Much higher values of the Seebeck coefficient have been measured for semi-metals (such as bismuth) and for semiconductors, making them more suitable for practical applications. Therefore, measurements of the Seebeck coefficient for several semiconductors have been carried out [3.1,3.2,3.7-3.24]. The results for silicon, monocrystalline silicon as well as polycrystalline and amorphous silicon, are discussed below.

Table 3.1: The absolute Seebeck coefficient of some metals and standard thermocouples (in juV/K).

Material

Metal

Nickel Palladium Platinum Aluminum Lead Vanadium Tungsten Rhodium Silver Copper Gold Molybdenum Chrome

Seebeck coeff. at 273 K (MV/K)

-18.0 -9.00 -4.45

-0.995 0.13 0.13 0.48 1.38 1.70 1.79 4.71

18.8

Thermocouple: type and composition

J: Fe/CuNi K: NiCr/NiAl R: Pt/Ptl3%Rh S: Pt/PtlO%Rh T: Cu/NiCu

50 39

5 5

39'

Seebeck coeff. at 300 K 0iV/K)

-9.99 -5.28 -1.7 -1.047

1.0 1.07 0.4 1.51 1.83 1.94 5.57

17.3

51 41

6 7

41

Ref.

[3.4] [3.4-3.5] [3.4-3.5]

[3.4] [3.6]

[3.4-3.5] [3.5]

[3.4-3.5] [3.5] [3.5] [3.5] [3.5]

[3.4-3.5]

[3.6] [3.6] [3.6] [3.6] [3.6]

28

Single-crystal silicon The Seebeck coefficient of single-crystal silicon samples with varying impurity concentration has been measured by Geballe and Hull [3.8], and is shown in the temperature range from 20 to 320 K - - f o r three values of impurity concentration - - i n Fig. 3.6.

10

> E

- 5 u_ LU O

CO LU LU I/)

/ /

—

•

/ \ 16/ 3 ' \2 .4x10 /ciTi

\ \ \ \

\ \

\ 1.0x1018/cm3 V ^

1.5x10/cm3

i ~~~T I i i 100 200 TEMPERATURE (K)

300

Figure 3.6: The Seebeck coefficient of p-type single-crystal silicon in the temperature range of 20-320 K as a function of doping concentration.

Around room temperature the highest value of a8 was obtained for the samples with the smallest concentration difference between the donor and acceptor atoms, that is, those with the smallest density of mobile charge carriers. A marked rise in a8 at low temperatures and, for large carrier concentrations, a low-temperature reversal of the sign of the Seebeck voltage were observed.

From Fig. 3.3, where the Seebeck coefficient of silicon at room temperature is shown as a function of the electrical resistivity, it is evident that the Seebeck coefficient depends strongly on the impurity concentration. Planar IC technology makes it possible to have different types of doped silicon so that a broad range of Seebeck coefficient values is available. Integrated devices have been fabricated [3.2,3.9-3.12] in order to investigate the possibility of exploiting the Seebeck effect for thermal sensors (see Figs. 3.7 and 3.8).

29

Figure 3.7: A schematic cross-section of a p-type-Si/Al thermopile.

Figure 3.8: One of the ICs (ETMA 353) used to measure the Seebeck coefficient of monocrystalline silicon.

30

Figure 3.7 shows a schematic cross-section of a silicon chip with p-type strips created in an n-type epilayer by diffusion or ion implantation. The hot end of one strip is connected to the cold end of its neighboring strip by aluminum interconnection (the thermoelectric effect of aluminum is negligible). In this way a thermopile is created in which the Seebeck voltages of many strips have been added. Such thermopiles have been used to measure the Seebeck coefficient of the many different types of doped silicon.

Figure 3.8 illustrates one of the ICs used to measure the Seebeck coefficient of different types of doped single-crystal silicon. It contains 6 thermopiles, each generally consisting of 10 series-connected couples 2 mm in length and of different widths, and fabricated by various diffusion and ion implantation processes. The chip also includes two heating resistors to dissipate the power needed to create a temperature difference between the hot and cold junctions of the thermopiles. The resistor at the top of the photograph is a diffused shallow n-type resistor, while the one at the bottom is a Ni-Cr metal resistor. They have about the same resistance (~ 150 ft), but a different temperature coefficient. The temperature difference created by the heating resistor is measured by means of diodes positioned at the opposite junctions of the thermopiles in three different points of the chip. The values obtained with this and other ICs are in good agreement with the values measured in bulk silicon samples with the same carrier concentration.

1250

1000

>. r 750 —

o

*: LU CD LU LU

500

250

—

-

—

~ ~~^^-ü

I

JEPI

—X~~— .^.^JDPJHlm

DP 30Llm P

DN

SN

300 350 400

TEMPERATURE(K)

450

Figure 3.9: The Seebeck coefficient of silicon strips fabricated with various processing steps in the temperature range of 300-450 K.

31

In Fig. 3.9 the values of ae are plotted vs. temperature in the range from 300 to 450 K for thermopiles in which the silicon strips are fabricated by the processes listed in Table 3.2. For samples having a lower sheet resistance, a8 depends less strongly on the temperature than for those having a higher sheet resistance.

As the Seebeck coefficient depends strongly on the impurity concentration, inhomogeneities in the doping profile - - which are present in silicon strips made by diffusion or ion implantation techniques - - will affect the value of the Seebeck coefficient [3.2, 3.10]. These inhomogeneity effects are particularly visible in thermopiles with a rather different strip width/strip depth ratio, as in the case of the two 10/zm-deep p-type diffusion thermopiles shown in Fig. 3.6. These have a strip width/strip depth ratio of respectively 0.8 (DP 8 /xm wide) and 3 (DP 30 /xm wide).

Table 3.2: The processes used to fabricate the integrated silicon thermopiles.

Process

Diffusion Shallow P-type Buried N-type Half-deep P-type Deep P-type Deep N-type Shallow N-type

Epilayer N-type

Boron implantation 1017/m2, 150 keV 1018/m2, 150keV 1019/m2, 150 keV

Letter code

SP BN TP DP DN SN

EPI

H K P

Sheet resistance (fl/D)

200 20 35 10

5 7

400

2500 528

91

a8 at 300 K (mV/K)

0.95 0.9 0.7 0.7 0.3 0.2

1.2

1.1 0.8 0.5

Polysilicon The possibility of using polysilicon as a thermocouple material has been investigated by Lahiji and Wise [3.13]. Polysilicon-gold thermopiles have been used to measure the thermoelectric power of polysilicon films

32

deposited from silane in a Chemical Vapor Deposition (CVD) process at 670°C. An as of 117 /xV/K has been measured at room temperature for a 1 /zm-thick boron-doped polysilicon film with a sheet resistance of 60 Q/D. For n-type polysilicon an a8 of -240 /iV/K for a 600 ft/D phosphorus-doped polysilicon film and an aa of -176/iV/K for a lOOfl/D film have been obtained.

Amorphous silicon The Seebeck coefficient of amorphous silicon films has been measured by Jones et al. [3.17] for a-Si:H films and by Kodato et al. [3.18] for a-Si:H:F films. For amorphous silicon films doped with phosphorus a Seebeck coefficient between -0.7 and -2 mV/K, depending on temperature and resistivity, has been measured in the temperature range from 200 to 700 K. Higher-doped samples have a lower value of a8, which increases less sharply with decreasing temperature. For the highly conductive a-Si:H:F films of both p-type and n-type values for a8 of 180 to 210/iV/K and -120 to -210/W/K, respectively, have been obtained. The Seebeck coefficients of these films are proportional to the temperature, while those of low-conductive a-Si:H films are inversely proportional to the temperature, just as is the case for highly and lower-doped monocrystalline silicon. At room temperature, a temperature coefficient of the Seebeck coefficient has been measured in the order of 0.8%/K for the n-type films and 0.7%/K for the p-type films.

3.2.5 Figure of merit

An important criterion in the selection of a material for thermoelectric applications is the figure of merit. This parameter has to be maximized over the temperature range of interest in order to maximize the signal-to-noise ratio. The figure of merit Z of a thermocouple made of materials a and b is defined as:

Z = ( a a - a b ) 2 / ( ( p a K a ) ± + ( p b K b ) ± ) 2 ( 3 - 1 7 )

but in comparing different thermoelectric materials it is more convenient to use the figure of merit z for a single material defined as:

z = as2/PK (3.18)

where ag is the Seebeck coefficient, p the electrical resistivity and /c the thermal conductivity of the material. For two materials whose individual figures of merit z and zn are similar and whose Seebeck coefficients are opposite in sign but similar in

33

magnitude, the figure of merit of the couple is approximately equal to the average of the individual figures of merit. In general Z must be regarded as a rather complicated average of zp and zn.

Table 3.3: The figures of merit of some thermoelectric materials.

Material Electrical resistivity

(fim)

Positive thermoelements

Ge (thin film) InAs ZnSb PbTe PbSe Sb2Te3 Bi2Te3 Bi2Te3 Bi2Te3-25%Bi2Se3 Bi2Te3-10%Bi2Se3

8.3 x l0~ 4

2.0 x KT5

5.0 x 10"6

1.2 x 10"5

Negative thermoelements

Ge (thin film) InAs Si InP 2As 9 Pbfe Bi2Te3-25°/oSb2Te3 Bi2Te3 Bi2Te3 Bi2Te3-50%Sb2Te3 Bi2Te3-74%Sb2Te3

6.9 x 10"3

2.0 x 10"5

3.5 x 10"5

7.7 x 10"6

8.2 x 10"6

Seebeck coeff.

(/iV/K)

420 200

130 190

-548 -180 -450

-210

Figure of merit z ( K - i )

3.3 x 10"6

8.0 x 10"5

1.0 x 10"3

1.2x 10"3

1.2x 10"3

1.2x 10"3

1.8x 10"3

2.2 x 10-3

2.7 x 10"3

2.8 x 10"3

6.8 x 10-7

2.7 x 10"5

4.0 x 10"5

6.0 x 10-4

1.5x 10"3

2.2 x lO" 3

2.3 x 10"3

2.6 x 10"3

2.8 x 10-3

3.0 x 10"3

Ref.

[3.16] [3.21-3.22]

[3.7] [3.7] [3.7] [3.7] [3.7] [3.1] [3.1] [3.1]

[3.16] [3.21-3.22]

[3.10] [3.21] [3.7] [3.1] [3.7] [3.1] [3.1] [3.1]

It is desirable to find materials with the highest possible z in each temperature region. This involves simultaneously controlling three macroscopic parameters: the Seebeck coefficient, thermal conductivity and electrical resistivity. The Wiedemann-Franz law states that the ratio of the thermal to the electric conductivity is the same for all metals at a given

34

temperature. The maximum values of the figure of merit for metals are, therefore, obtained when the Seebeck coefficient is the highest. However, a differential Seebeck coefficient of more than about 0.1 mV/K cannot be realized in a metallic thermocouple [3.7]. In semiconductors, absolute Seebeck coefficients of up to one or more mV/K may be obtained. Therefore, the value of z that may be obtained is higher for some semiconductors than for any metals.

The figures of merit for a number of thermoelectric materials are shown in Table 3.3 [3.1,3.7,3.10,3.16,3.21,3.22]. While the differential Seebeck coefficient for a chromel-constantan couple is about 70 M V / K , for a bismuth-antimony couple it is about 110/iV/K. By using an alloy of 91% bismuth and 9% antimony instead of pure bismuth as the negative thermoelement, Z is increased to about 0.23 x 10~3 K - 1 . This figure is close to the maximum which may be achieved using metals or metallic alloys.

Up to 1950 the best positive thermoelectric material was zinc antimonide. When the compound contained small quantities of tin and silver its Seebeck coefficient, measured against constantan, was found to be 250 /xV/K. The highest figures of merit have nowadays been achieved with the lead compounds having group VI elements (PbTe, PbSe, PbS) or the V-VI compounds, bismuth telluride and antimony telluride. In Table 3.3 values of z for germanium, for silicon and for some III-V compounds are also shown. Although the figures of merit for these semiconductors are one or more orders of magnitude smaller, these materials are rather interesting for applications in the sensor field, because of their well-established technology. This is certainly the case for silicon, whose value reported in the table has been calculated using the resistivity and Seebeck coefficient values which are optimal for thermopile design (see the following section).

3.3 Integrated silicon thermopiles

3.3.1 Thermopile performance

The performance of Si-Al thermopiles, whose structure is schematically shown in Fig. 3.7, depends on the doping concentration or resistivity of the silicon strips and on the layout geometry. The silicon thermopile fabricated by an IC process, namely a bipolar process, is 10 nm deep or less. This means that only the first 10 fim of the silicon wafer - - whose total thickness is 300 ^ m - - are actively used. The silicon substrate underneath the thermopile acts as a thermal shunt, drastically reducing the thermal resistance of the silicon piece containing the thermopile and

35

consequently reducing the thermopile-output voltage. By removing the unnecessary silicon mass, the thermal resistance is significantly increased (if the silicon thickness is reduced from 300 /zm to 10 /urn, an increase by a factor of 30 is obtained) and so is the device performance [3.25]. This explains why thermal sensors using thermopiles generally have a cantilever beam or a membrane structure.

With Eq. (3.9) we will now estimate the performance of a Si-Al thermopile for a thin cantilever beam containing a thermopile between the tip and the base, and for a round membrane containing a thermopile with the hot junctions in the middle and the cold junctions on the periphery. It is assumed that the thermopile strips and the membrane or beam thickness D are of the order of 10 pm. We shall ignore the fact that for electrical isolation some separation between neighboring strips is necessary.

Cantilever beam For a rectangular thermopile of width W, length X, length/width ratio A = X/W and with N strips (see Fig. 3.10) we find:

N'-k. < 3 I 9 )

where Rtp is the thermopile internal resistance, and jRse is the electrical sheet resistance. The output voltage of the thermopile then becomes:

tftp-Ata,*lt/M (3.20)

where P is the heat flowing through the thermopile and Rst is the thermal sheet resistance of the beam. We can use the following relations:

*8e=f; * . t - ^ - ; *th = ̂ . t ; a* = ^ r l n ( ^ ° ) (3.20

where the electrical resistivity p is related to the measured value of ae of the strips through the last expression, E is the equivalent electrical sheet thickness and K is the thermal conductivity. Then the following expression for the thermopile output voltage can be formulated:

TT _ mkP U in — t p ' QK

r*tPM D P Mn(p/p0) (3.22)

36

where ƒ = E/D. The maximum is found for ln(/>/p0) = 2. If only thermal noise (4kTRtpp is present in the thermopile, the signal-to-noise ratio (SNR) for the 0-1 Hz band then becomes:

SNR = mk qKe(p0kT)± m 4.8x l0 6 C^)* /W (3.23)

The factor in brackets is typically in the order of 10s for D = 10 urn. Thus the signal-to-noise ratio SNR, Noise-Equivalent Power NEP and Noise-Equivalent Temperature Difference NETD are typically of the order:

S N R * 109/W NEP * 1 nW NETD « 1 /iK (3.24)

INTERACTION AREA THERMOPILE

Figure 3.10: Schematic cross-section of an electrochemically etched cantilever-beam thermal sensor.

Round membrane The equations found above are applicable to a thermopile on a round membrane as well (see Fig. 3.11). If the hot junctions of the pile lie on an inner circle with diameter Rx and the cold junctions on the outer circle of the membrane R0 , then the equations hold for round membranes if A is replaced by the effective length-width ratio for membranes AR:

AR= [iniRJR^/Hr (3.25)

37

This factor is practically limited to 0.5, restricting the thermal resistance of membrane devices to 0.5 x RRt.

THERMOPILE

Figure 3.11: Schematic cross-section of an electrochemically etched round-membrane thermal sensor.

3.3.2 Use of thermopiles in thermal sensors

Several interesting thermal sensors using integrated thermopiles have already been developed [3.26]. Their ease of operation - - for measuring the output only a multimeter is required - - makes them very attractive to work with. In most thermal sensors some area will be needed for the desired physical interaction with the ambient. Considerations of the yield, time constant, beam bending (in the case of cantilever beams only!) and heat loss through the ambient gas usually set a limit on the maximum dimensions of the sensor. In that case the thermopile and the interaction area compete for room, and an optimum division can be arranged. For different types of sensors the optimum will, in general, be different. We can, for example, calculate the optimum for the case in which the power exchange P with the physical quantity to be measured is proportional to the interaction area. First, let us look into a cantilever-beam type of device. If the interaction area has a length E, then P = EWI, with / a constant determined by the physics of the sensor. If F is the full length of the beam, then F = E+X = E + AW. Using Eq. (3.22) and ln(p/p0) = 2 we find:

38

£/t =C/M±~ 0.38 xClW^F1* (3.26)

for the optimum E = 2F/3, X = F/3, with C as a constant.

For a thermal sensor based on a round membrane the power exchange P = nRi ƒ, leading to:

Utp = C / M R 2* 0.38 x CIR20 (3.27)

for the optimum \n{R0/Rl) = 0.25 and C as the same constant as in the previous formula.

Comparing Eq. (3.27) with Eq. (3.26) leads to the conclusion that a square cantilever-beam sensor of typical dimensions F is - in this case -approximately 4 x as sensitive as a round membrane sensor with typical dimensions 2R0 = F. This difference is modified somewhat by the larger heat loss of a cantilever beam to the ambient gas. For a 4 mm long beam of Rst = 700 K / W D the loss is in the order of 33% in air, compared to only a few percent for a round membrane of the same Rat and size.

3.4 The silicon thermopile infrared detector

3.4.1 The working principle

A very interesting thermal sensor based on the integrated silicon thermopile is the infrared detector. For this device we chose a cantilever beam structure (see Fig. 3.10), since this structure, as shown by the analysis presented in the previous section, offers a higher thermal resistance - - and consequently a higher sensitivity - - than a membrane type device of the same thickness and comparable dimensions. The interaction area of the sensor is, in this case, the absorbing or active area of the detector (/4D), and it is coated with an infrared radiation absorbing material.

The radiation absorbed will generate heat which will flow through the beam to the heat sink. A temperature difference over the beam will be generated in this way and will be measured by the integrated thermopile. The thermopile output voltage Utp will be proportional to the incident

39

power, i.e.:

Utp = Na8RthP (3.28)

where N is the number of strips, ag is the Seebeck coefficient of silicon, P is the heat generated by the absorbed incident radiation and Rth is the thermal resistance of that part of the beam containing the thermopile.

If the device is not in vacuum the presence of gas in the gap between the beam and the heat sink will reduce the effective temperature difference across the thermopile. This is due to the fact that part of the heat generated by the absorbed radiation will flow to the heat sink through the gas molecules present in the gap. The thermal resistance of the beam and the thermal conductance G of the gas volume in the gap determine the degree of reduction of the temperature difference across the thermopile. In the presence of gas, the thermopile output is approximately given by [3.27]:

Utp = NaBRstPA(l-LGRatL2) (3.29)

which in vacuum (i.e for G = 0) reduces to Eq(3.28). This expression, which results from a simple one-dimensional model, gives a good estimation of the pressure effect and it shows that the higher the thermal conductivity of the gas, the larger the deviation of the thermopile output from its vacuum value will be.

3.4.2. Design criteria

Let us see now how the responsivity R, the relative detectivity D* and the time constant r are related to the geometry of the device. This is necessary in order to define the layout of the device. The responsivity R is defined as the ratio of the device output and the radiation input. By using Eqs. (3.20) and (3.21) it can be expressed as:

R = NaaRetX/W (3.30)

which shows that the responsivity benefits from a large length/width ratio of the thermopile.

40

The relative detectivity is defined as:

D* (3.31) NEP

where AD is the active area of the detector, B is the frequency bandwidth and NEP is the noise equivalent power, which represents the minimum detectable power of the detector. Assuming that only Johnson noise is present (which is generally the case for thermopile IR detectors) NEP can be written as follows:

P(4kTRtpB)± NEP = (3.32)

' tP

Combining Eqs. (3.31) and (3.32), for a frequency band of 1 Hz, we can express D* as follows:

D* t p D , (3.33) P(4kTRtp)±

By using the expression (3.28) for Ut and deriving Rtp from Eq.(3.19), the dependence of the relative detectivity on the geometry of the device can be explicitly shown:

D* = (3.34) (4*77*se)±

which has a maximum for X = E. This means that in order to maximize the relative detectivity D* the thermopile and the absorbing area have to be equal in length.

The time constant or response time of a detector r is defined as the time required for the output signal to reach (1-1/e) of its final value. As in electrical circuits the time constant is given by the product of a resistance and a capacitance, in this case being the thermal or heat resistance Rth and the heat capacitance H, i.e.:

41

' = * t h " (3.35)

The heat capacitance is given by the product of the specific heat cp and the mass of the silicon cantilever beam, i.e.:

H = cPiSi8SiWDF (3.36)

where 5s i is the density of silicon. By multiplying Eq. (3.36) by the expression for Rth given by Eq. (3.21) we obtain:

T= ( CpS ] F2 (3.37) *- K } Si

The factor in parentheses is the inverse of the thermal diffusivity of silicon and is approximately equal to 1.1 x 104sec/m2 at room temperature. In evaluating the time constant of the detector we have only considered the silicon beam. In reality the thermal mass is larger due to the presence of the black coating. Due to a lack of data on the black coating, it was not possible to calculate its contribution to the thermal mass. As we will see from the experimental results shown in Chapter 5, the measured time constant is larger than the calculated one.

While the time constant depends essentially only on the length of the beam, the responsivity and the relative detectivity are determined by more parameters, as can be seen from Eqs. (3.30) and (3.34). From these expressions it is clear that a large thermal resistance and a small electrical one are desirable. However, they are not completely independent since they are interrelated through the geometry of the device. For example, while all three characteristic parameters of the detector benefit from a thinner beam, increasing the thermopile length will increase not only the thermal resistance, but the electrical one as well. Further, a longer device will also have a longer time constant. This means that a compromise has to be found. Since many configurations are possible, the choice of the fabrication process for the thermopile strips and the device layout will be determined by the type of application for which the detector is to be used.

With respect to the process used to fabricate the thermopile, we based our choice on the results of our investigation into the different types of silicon thermopiles (see Section 3.2.4). To fabricate the thermopile strips we used a p-type diffusion, which* gives a rather high value of the Seebeck coefficient (QS C± 0.7 mV/K) and a resistivity which is not too high

42

(RBe ~ 35n/D). The thermopile strips are 60 /im wide with a separation of 20 nm between adjacent strips. If we fix the dimensions of the whole IC (6 mm x 6 mm) and the membrane area (about 4 mm x 4 mm) several configurations are possible within these limits. A few example are reported in Table 3.4 for a) a single-beam device and b) a multi-beam device (array).

Table 3.4 Possible configurations of IR cantilever-beam detectors^

F X N W Rt Rth D* R NEP r (mm) (mm) (mm) (Kfi) (K/W) (10 8cmHzVw) (V/W) (nW) (msec)

single-beam device 4 3

* 4 3 4 3

multi-3 2 3 2 2 2 1

2 1.5 2 1.5 2 1.5

40 40 44 44 22 22

■beam device 1.5 1.0 1.5 1.0 1.0 1.0 0.5

5 5 5 5 2 1 1

3.5 3.5 3.8 3.8 2.0 2.0

.45

.45

.35

.35

.18

.10

.10

46 35 51 38 25 19

4.4 2.9 4.4 2.9 1.2 0.6 0.3

380 285 350 260 670 500

2220 1480 2860 1900 3700 6670 3330

1.01 0.76 1.02 0.77 1.00 0.75

0.75 0.52 0.85 0.57 0.50 0.47 0.24

10.6 7.8 10.8 8.1

10.2 7.7

7.8 5.2 9.9 6.7 5.2 4.7 2.3

2.6 3.0 2.7 3.1 2.0 2.3

1.1 1.3 0.8 1.1 0.8 0.7 0.9

180 100 180 100 180 100

100 45 100 45 45 45 11

t For the meaning of the parameters see list of symbols.

For our detector realization we chose the configuration (denoted by a * in the table) which uses the most of the available space, i.e. we opted for a beam which is as large as possible and a thermopile having the largest possible number of strips. As can be seen from the table this configuration offers the largest values for the responsivity and the relative detectivity. For a multi-beam device (or array) the beams have to be much smaller if several elements are to be combined within the same frame. An 8-element linear array has also been realized as an example of multi-beam configuration and will be illustrated in the upcoming chapters.

43

4. FABRICATION PROCESS

4.1 Introduction

Silicon micromachining is of great importance for the development of inexpensive, batch-fabricated, high-performance sensors which can easily be interfaced with microprocessors [4.1]. Silicon microstructures, such as membranes and cantilever beams, are particularly important for thermal sensors, since they reduce the thermal capacity of the device and, in the case of cantilever beams or bridges, provide thermally isolated regions, thus improving the device performance.

As seen in the previous chapter a cantilever-beam shaped device is preferable to a membrane one. In this chapter we will describe the etch process we developed to fabricate silicon cantilever beams containing integrated devices. (This etch process can also be used to fabricate membranes containing integrated devices if a membrane structure is desired for some application). While cantilever beams made of Si02 or Si02/Si3N4, as well as beams made of heavily doped p+ silicon (used mostly as supporting structures), have been realized for several applications [4.1], cantilever beam made of low, uniformly doped epilayer silicon and containing integrated devices have not been previously reported. These structures were realized using integrated-circuit fabrication technology and an electrochemically controlled etching (ECE) of silicon. This ECE technique [4.2], in which a voltage bias on the n-type epilayer is employed to prevent the epilayer from being etched, can be performed as the last step in the fabrication process, allowing devices to be fabricated within the cantilever beam by means of standard silicon planar technology. Other advantages offered by this combination of silicon planar technology and ECE are: batch fabrication (with a high yield and thus low cost) and the possibility of having on-chip interface electronics.

Two methods of preparing silicon cantilever beams are described. In one method electrically isolated regions {channels) were created on three sides of the membrane by the deep p-type (DP) isolation diffusion. In this way the etch stops at the epilayer/substrate junction, but continues through the isolated regions. In the other, a combination of ECE and plasma etching of silicon is used: membranes are first fabricated by ECE and a photoresist layer is then deposited and patterned on the front side of the wafer. The

45

channels are now etched with a plasma etch process, viz., reactive ion etching.

The effect of the oxide thickness and of the aluminum interconnection pattern on the flatness of the beam is analyzed. Further, the influence of the initial surface condition and of the etching solution temperature on the quality of the beam/membrane surface is investigated. A description of the process used to fabricate the infrared detector and the infrared sensing array will conclude this chapter.

4.2 The cantilever beam structure

4.2.1 The etch process

Chemical etchants for silicon are numerous [4.1]. They can be isotropic or anisotropic, dopant dependent or not and can have varying degrees of selectivity to silicon, which determines the appropriate masking materials. Micromachining makes more extensive use of the anisotropic etchants than of the isotropic ones. This is mainly due to the fact that anisotropic etchants create pits with well-defined side walls. The pits deepen without widening. Thus an anisotropic etchant can create closely spaced arrays of holes: the edges of the openings in the mask can be placed as close together as photolithography allows. In the vertical direction (thickness), the etching of silicon can be controlled in different ways. If etching time adjustments (or a time stop of the etch) is used, the thickness of the final diaphragm cannot be accurately controlled due to etch-rate variation and starting substrate-thickness variation. A better control of the diaphram thickness can be achieved with automatic etch-stop techniques, such as the etch-rate reduction due to highly doped p-type layers (boron concentration greater than 5 x 1019 at/cm3) [4.1] and the electrochemically controlled etching of silicon [4.2].

In this last method, unwanted silicon is removed chemically while the regions to be retained are passivated electrochemically. A small positive voltage applied to one side of an n/p junction will passivate the n side, protecting it from etching, while the p side of the junction - - the unpassivated region - - etches normally. (A reverse situation, i.e. preserving the p side and etching the n side of a junction is also possible. In this case, though, the maximum allowable cell voltage is reduced, because the junction is forward biased.) Hot alkaline solutions, such as KOH solutions, are useful etchants for this application because they are characterized by a relatively sharp active/passive transition and by a

46

large ratio of silicon etch rates between the unpassivated and passivated regions. For KOH solutions ratios greater than 200:1, depending on the temperature and concentration of the solution, are easily obtained [4.2].

The ECE method has the advantage of retaining all the anisotropic etching characteristics of KOH without needing a heavily doped p+ layer to stop the etch. Because of the high boron concentration required to stop the etch, such a p+layer introduces considerable mechanical strains in the remaining membrane. Moreover, the p+ layer is so heavily doped that it precludes the formation of electrical devices within it, and it is very difficult to grow a good monocrystalline epilayer on top of such a p+ layer. These disadvantages are not present in the electrochemically stopped, low doped membranes. These considerations, together with the possibility of being able to perform the etching as the final processing step, determined our choice of the ECE technique to realize our microstructures.

A schematic diagram of our experimental set-up is shown in Fig. 4.1.

+ Q

EZZZZZZZZ 5 ^

s

-V- — 0

////////////////^ry

i' m

7-7~r

W^

<STIRRER>

///A

\ \ \ \ W \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ K

hot water

U \ \ v \ \ ^ u ^ \ \ \ ^ \ ^ u \ \ \ v v ^ v \ ' v u w \ \ r r

Pt electrode

KOH/H2O

Figure 4.1: Experimental set-up used for the electrochemically controlled etching of silicon.

47

The etchant, a solution of KOH and water (250 gr KOH in 500 ml water), was continuously stirred by a magnetic stirrer and kept at the selected temperature by means of a hot-water thermostatic system. The silicon wafer, once completely processed, was mounted face down on a stainless steel holder with black wax. The anode contact was made on the n-type epilayer, the side to be preserved, by a spring-loaded contact, while a platinum foil was used as the cathode. Once the wafer was immersed in the solution the unmasked areas on the back side were etched. The etching continued until the n-epilayer/p-substrate junction was reached. If a positive voltage larger than 0.6 V is applied, the etching will stop at the junction. If the voltage is lowered or shut off, the epilayer is no longer passivated and the etching will start up again in less than a minute. Voltages between 0.8 and 1 V were generally used and the solution was kept at a temperature of 85 °C. In these conditions the etch rate of silicon was 1.4 jum/min.

One-step etch method In order to realize structures of the type shown in Fig. 3.10, the silicon wafer was processed in the following way. A 10 /xm-thick n-type epilayer was grown on a 20 ftcm p-type <100> silicon wafer. Deep p-type (DP) diffusion was used to create electrically isolated regions on three sides of the membrane. The main device contained in the beam was a p-type silicon thermopile which was realized by means of a 6 jum-deep p-type diffusion. Shallow p-type (SP) and shallow n-type (SN) diffusions were carried out successively. During the SN diffusion n+ islands were created to realize a good contact to the epilayer for the ECE. A 750 A low pressure chemical vapor deposition (LPCVD) Si3N4 layer, an excellent masking material against KOH, was grown on the back side of the wafer and patterned. Finally, contact opening and interconnection steps were carried out.

The completely processed wafer, ready to be etched, has the structure shown in Fig. 4.2a. At this point the wafer is mounted on the holder, immersed in the solution and the etching starts. The etching will stop once it reaches the junction epilayer/substrate, but will continue in those regions which are electrically isolated from the passivated epilayer by the DP channels, as shown in Fig. 4.2b. The final structure is shown in Fig. 4.2c.

Fig. 4.3 plots the cell current versus time. As the wafer is being etched (region 1), the cell resistance will slowly, but continuously decrease and simultaneously the current will increase (the same amount of power is supplied during this first phase). In the proximity of the junction substrate/epilayer (region 2), the change in cell resistance is very sharp

48

and more power has to be supplied in order to keep the epilayer at the proper voltage. The peak in the current represents the stop in the etching. At this point the process will reverse, i.e. less power has to be supplied and less current will flow through the cell. The etch continues only in the isolated channels (see region 3), so some current will still be flowing.

Q) n-epi \P/ ~ ^ ] T

4zP*- -*t> , fa^r

pSi

b)

c)

\ n « ^ _ Si02

3 in

Figure 4.2: Simplified process flow chart for the fabrication of a cantilever beam with the one-step etch method: a) the processed wafer, ready to be etched; b) the etch stops at the epilayer and continues into the isolated channels; c) the final structure.

49

1 5 -

1 0 -

5 -

o-

1 I

■ i * i » i

2 1 3

I 60 120

TIME (min)

180 2^0

Figure 4.3: The current flowing through the cell during the etching versus etching time.

Two-step etch method. An alternative way to fabricate cantilever beams is to combine ECE and plasma etching of silicon. Fig. 4.4 illustrates the simplified process flow chart for this two-step etch. The fabrication process is the same as in the previous case except that the DP isolation diffusion is not performed (Fig. 4.4a). (The DP mask will be used for the plasma etching step.) Once the membrane has been fabricated (see Fig. 4.4b) by stopping the etching at the epilayer/substrate junction as explained above, a photoresist layer is deposited and patterned on the front side of the wafer. The channels are etched with a plasma process, reactive ion etching (RIE). In a plasma etching process, etchant feed gas is introduced into a glow discharge where reactive species are formed. These species diffuse to the wafer surface and react, forming volatile products which are removed by sputtering. The product species are swept from the reactor by the gas flow. RIE is a low pressure process in which ion-induced etching processes are dominant [4.3]. It is generally used in IC fabrication to etch dielectric

50

films for passivation or insulation. In our case we want to use this method to etch silicon (10mm thick epilayer). This means that a set of conditions has to be found in which the etch rate of the Si02 and of the photoresist are not too high, or in which the ratio of the silicon etch rate and the oxide/photoresist etch rate is sufficiently high. To this end a group of experiments were carried out: several parameters, all of which influence the etch rate, were varied. These are namely the pressure, the gas composition and the rf power. These experiments were performed in a plasma deposition system (PlasmaTherm), which has a planar electrode configuration, using a mixture of CF4 and 0 2 . Two types of wafer were prepared, one type using Si02 as the masking layer, and the other using photoresist (Hunt HPR-204).

Q) n-Gpi

*W> P +

—"W-—•*%#> 1

pSi

Si 0 ;

b) -^g^- - ^ ̂—4̂ -1 C D

J \ n +

Si3N4

. Si 02

SUN 3 114

X £ -**&*- -*>*>•. 5 ^ ^ _ S i 0 2

S i 3 N 3'H

Figure 4.4: Simplified process flow chart for the fabrication of a cantilever beam with the two-step etch method: a) the processed wafer, ready to be etched; b) the etch stops at the epilayer, thus forming the membrane; c) the final structure after the plasma etching of the channels.

51

The etch rates of the oxide and the phoresist were obtained by measuring the thickness of the layer before and after the etch by ellipsometry measurements. The silicon etch rate was estimated first by optical microscope measurements and, later, more accurately by scanning electron microscopy (SEM) measurements; The results are summarized in Table 4.1

Table 4.1: Silicon, silicon oxide and photoresist etch rates in different plasma etching conditions.

% 0 ,

4 4 4 4 4 6 8

Power (W)

50 50

100 150 150 150 150

Pressure (mTorr)

11 30 30 30 50 50 50

Si (/im/min)

0.03 . 0.05

0.06 0.08 0.10 0.13 0.15

Etch rates Si02 (A/min) HPR204 (A/min)

110 50 120 55 160

The best results were achieved under the following conditions. The etching gas composition was CF4 + 6% 0 2 . The pressure of the gas in the reaction chamber was maintained at 50 * 1 mTorr. The rf power was 150 W and the temperature of the chamber was set at 50 °C. In these conditions etch rates of ^0.1 /xm/min for silicon, ~ 50 A/min for silicon dioxide and a 120 A/min for the photoresist were measured. Considering that the thickness of the photoresist was approximately 13000 A, these etch rates allow us to etch the 10^m of silicon while still preserving a thin layer of photoresist, or, in other words, the insulation oxide is still covered by a layer of photoresist and, consequently, will not be damaged.

This plasma etching process is of the isotropic type. A lateral etching equal to 70% of the depth of the channel was generally observed. Figure 4.4c illustrates the final structure of a cantilever beam fabricated by the second method.

Silicon cantilever beams have been successfully fabricated in both ways. In Fig. 4.5 the front and back sides of one of these devices are shown. (We note that this device is a vacuum sensor, one of the thermal sensors utilizing the cantilever beam structure [4.4].)

52

a)

I ■ . •. \<t ***. » l' «**«**•* ' ^*'*:

&m&&$ b)

■"■■..,

> ■ . * . "

Figure 4.5: An electrochemically etched cantilever-beam device: a) front and b) back sides. The beam is approximately 3.6 mm long, 2.8 mm wide and 10 \im thick; the whole IC is 6 mm x 6mm.

53

The first method we used to fabricate the beams is a one-step anisotropic etch. The use of DP channels to electrically isolate some regions from the passivated epilayer proved to be a very useful tool. In fact, the desired structure was fabricated by simply using this standard step of the bipolar process. One can even envision that it will be easy to create structures other than beams, as one only has to re-design the DP mask. Fig. 4.6 shows an SEM micrograph of a beam fabricated in this way. The anisotropic nature of the etch is clearly visible in the shape of the membrane and the channels.

Figure 4.6: SEM micrograph of a beam fabricated with the one-step etch method. The magnification for the left side of the photograph is 10 times that on the left side. The long white strip is equivalent to 10 nm.

In the two-step etch process, the ECE is used only to fabricate the membranes. Therefore, less precaution has to be taken in mounting the sample onto the holder, since no solution will reach the surface of the wafer. However, in this case, although the membrane is etched anisotropically, the channels are etched isotropically. The contrast can be seen in Fig. 4.7, where an SEM micrograph of a sample prepared by this method is shown.

54

Figure 4.7: SEM micrograph of a beam fabricated with the two-step etch method. The magnification for the rigth side of the photograph is 5 times that on the left side. The long white strip is equivalent to 20 y,m.

4.2.2 Influence of the oxide thickness

In a first run the beams were bent up to 200 /xm downwards. Such samples, being fabricated in a standard bipolar process, had a rather thick silicon dioxide layer (~ 4000 A). The oxide was thermally grown and, in cooling the wafer from the growth temperature to the ambient temperature, the difference in the expansion coefficients of silicon and silicon dioxide introduced stresses (the oxide was in a state of compressive stress), which resulted in the bending of the beam [4.5]. However, the flatness of the beam is a rather important requirement for the good performance of certain types of sensors. Therefore, some devices had small bridges (approximately 80 to 300 urn in width and 200//m in length) between the tip of the cantilever beam and the surrounding thick rim (see Fig. 4.8), to keep the beam fairly flat.

55

Figure 4.8: Photograph of a device with bridges.

In order to realize beams which were flat without bridges, the effect of the oxide thickness and of the aluminum interconnection pattern was investigated. A new set of samples with different oxide thicknesses was prepared. These samples were classified as type (a), (b), or (c), according to the oxide thickness. The samples were processed in an identical way, except for the drive-in step following the SN diffusion, the last diffusion in the bipolar process. For the samples of type (a) the standard drive-in step (temperature = 1000 °C, ambient = 0 2 + 80 °C H 2 0 , time = 60 minutes was used, resulting in a 4000 A-thick oxide layer. For the samples of types (b) and (c) the drive-in step was divided into two parts. After the first part in which the standard conditions were used (although only for 40 minutes), the oxide layer was etched away and a thinner layer was regrown in the remaining 20 minutes of the drive-in step. For the samples of type (b) this was done under standard conditions of temperature and ambient, resulting in a 1150 A-thick oxide layer, while for the samples of type (c) the oxide was regrown at the same temperature but in a different ambient, i.e. 0 2 atmosphere, resulting in a 540 A-thick layer. The samples were then electrochemically etched and mounted for SEM analysis. Figure 4.9 illustrates the effect of the oxide thickness on the bending of the beam. The samples of type (a) were bent downwards 200^m, while the samples of type (b) and (c) were curled slightly, the displacement being 16 and 20 /im, respectively.

56

Figure 4.9: SEM micrographs of the cross-section of a sample with a) 4000 A, b) 1150 A and c) 540 k-thick oxide layer. The black strip (rigth-hand top corner) is equivalent to 500 nm.

4.2.3 Influence of the aluminum interconnection pattern

The curling of the beams for the samples of types (b) and (c) is due to the aluminum interconnection pattern. Aluminum is evaporated on the Si02/Si structure. It is well known [4.5,4.6] that many thin-film materials deposited by vacuum evaporation are in a state of stress, generally tensile stress, which is generated in the process of condensation. In fact, when the film material is first condensed, it must be considerably hotter than the substrate; on cooling, thermal contraction will cause a tensile stress. In the case of evaporated aluminum film a small tensile stress, thus opposite to the one present between silicon and silicon dioxide, can be expected. This is indeed the case for our samples. In fact, once the aluminum was etched away and only the oxide was left, the beams bent downwards, 24 /im for the (b) samples and 16 /zm for the (c) samples. In both cases a variation of about 40 nm was measured once the aluminum was removed. Figure 4.10

57

illustrates the effect of the aluminum pattern for a sample of type (c). At the top there is an SEM micrograph of a completely processed sample and at the bottom the sample is shown after the aluminum metallization has been removed.

Figure 4.10: SEM micrographs of the cross-section of a type (c) sample. Top: with aluminum; bottom: with aluminum removed. The first white strip is equivalent to 100 /urn.

4.2.4 Influence of other parameters

Initial surface condition The influence of a few other parameters on the quality of the membrane/beam surface was also investigated. At first we looked at the effect of the initial surface condition. For this purpose we prepared samples from three different types of initial surface:

58

a) ■ ■:. - - " :

= ' v i - '

-tg-r-rr^rr' ■

l '

"'.-»-■' ' '

r i -

~ ™ r

, .̂̂ -y

y n ">„ ". / i , " < ■ - ' >

—

'?■

i ' -WJS-if "¥w^9-.!«*«-•>«»■—--TJM T*.*^*rrr

Si

b)

c)

i

Figure 4.11: The surface of electrochemically etched membranes. Initial surface condition: a) unpolished, b) chemically polished and c) mechanically and chemically polished (see text).

59

- unpolished; - chemically polished: the wafers were etched in a HF: HN0 3 = 2 : 5

solution at room temperature for 2 minutes (under these conditions ~ 40 jum of silicon is removed);

- mechanically polished: the wafers were polished with Carborundum powder 300 and 400, and subsequently etched for 1 minute in the HF: H N 0 3 = 2 :5 solution. (~ 60/im of silicon is removed in this way).

The results can be seen in Fig. 4.11 where photographs of the electrochemically etched membranes for all three types of initial surfaces are shown. The surface roughness has been estimated to be of the order of only a few hundred angstroms for all three samples. Since the quality of the membrane surface is not greatly affected by the initial surface preparation, we later used, for the sake of simplicity, unpolished wafers only.

Solution temperature The effect of the solution temperature on the quality of the membrane surface was also investigated. Samples were etched at temperatures varying between 70 and 85 °C. The etch rate of the silicon ranged from 0.8 jum/min to 1.4 /rni/min and the quality of the etched surface as well as the resistance of the masking layer were not visibly affected by the change in the solution temperature. Due to the thickness of the silicon wafers (=* 300 /mi) a temperature of 85 °C was chosen to reduce the etching time. Higher temperatures were not used in order to prevent softening of the wax used to mount the wafer onto the holder.

60

4.3 IR detector fabrication process

To fabricate the silicon thermopile IR detector a process had to be compiled, based on the results of the investigations into both the silicon thermopile performance (see previous chapter) and the etching process used to fabricate the cantilever beam structure. Of the two methods developed to realize these microstructures, we chose the two-step etch method. Our choice was motivated by the fact that in the one-step etch method too much care (especially when mass-production has to be feasible) has to be taken in mounting the sample during the etch, as otherwise the solution can come in contact with the front surface of the wafer and damage the aluminum interconnection (Al is etched very fast in hot KOH solutions). If the front surface of the processed, ready-to-etch wafer could be coated by a plasma-deposited layer of Si3N4, the one-step etch method would be preferred, because it is shorter than the other process. Although the plasma deposition of dielectrics is currently being investigated in the Departmental Workshop, it is not yet available for processing. Therefore, we opted for the two-step etch method, in which the solution never comes in contact with the front surface of the wafer. This prevents the devices from being damaged. (Moreover, in this method a vacuum-type holder can be used, thus eliminating the need for wax.)

The layout of the infrared detector (IS 452) and that of a linear array (IS 453) were combined in the same design and processed at the same time. The process used is a somewhat modified bipolar process (not all steps of a standard bipolar process are used), in which some extra steps have been added for the etching of the cantilever beam structures. A simplified process flow chart for the cantilever-beam infrared detectors is shown in Fig 4.12 and a description of the major steps in the wafer processing is given in Table 4.2. Each masking step represents a complete photolithographic procedure, i.e. photoresist deposition, patterning, window etching and removal. We note that some of the mask names used in this case do not refer to standard process steps. Whereas DP normally refers to deep - - o r isolation - - p-type diffusion, here it stands for plasma etching. Likewise, DN is used here to denote the special p-type diffusion for the thermopile strips, instead of deep - - o r collector - - n-type diffusion. Based on the results of the investigation into the effect of the oxide thickness and the aluminum interconnection pattern on the bending of the beam, an optimal oxide thickness was evaluated (~ 1500 A). This oxide thickness was obtained by dividing the drive-in step of the SN diffusion into two parts (see Table 4.2).

The completely processed wafers were sawn and mounted in standard 24-and 28-pin IC housings. At this point the last step in the fabrication process takes place, i.e. the coating of the interaction area with an absorbing layer (see Fig. 4.12d). A black paint from 3M, Nextel 3101,

61

Table 4.2: Major fabrication steps the infrared sensing array (IS 451

Processing step Mask

n-epitaxial layer

oxidation

p-type thermopile diffusion DN deposition drive-in

p-shallow diffusion SP deposition oxidation boron-glass removal drive-in

n-shallow diffusion SN deposition drive-in (first step) oxide removal drive-in (second step)

Si3N4 mask deposition patterning IN

Contact windows CO

Interconnection IC deposition annealing

Cantilever beam shaping Membrane shaping "Channel" etching DP

for the infrared detector and -IS 453).

Details

Phosphorus doped, SiHCl3, 1200 °C Thickness = 10 jum p = 0.7 ficm

60 min at 1100 °C in 0 2 + 95 °C H 2 0 45 min at 900 °C in 0 2 + 95 °C H 2 0

boron doped 70 min at 920 °C RD= 55-60 ÜD 60 min at 1100 °C R D = 3 5 n n

boron doped 22 min at 920 °C RD = 85-95 üa 30 min at 700 °C in 0 2 + 25 °C H 2 0

20 min at 1130°C in 0 2 30 min in 0 2 + 85 °C H 2 0

phosphorus diffusion 13 min at 1000 °C RD = 10-11 Qn 35 min at 1000 °C in 0 2 + 80 °C H 2 0

25 min at 1000 °C in 0 2 + 80 °C H 2 0

LPC VD at 7 5 0 °C thickness = 7 5 0 A Plasma etching (RIE) CF4 + 6%02, 15 min

Si02 etch (1%HF)

Aluminum evaporation, thickness = 1.2/xm 20 min at 450 °C in N2 + 25 °C H 2 0

ECE in KOH solution at 85 °C Plasma etching in CF4 + 6%02

62

was used. The paint was prepared by mixing the basic component with a hardener, Nextel-6018, in an 8 to 1 ratio. Then a 5-10% Nextel thinner 8061 was added. The absorbing area of the detector was coated with this mixture, and the samples were baked at 150 °C for 20 minutes. According to the manufacturer's specifications, the reflection of a 50-75 /zm thick layer prepared in such a way is less than 2% and its resistance to corrosive environments is rather good.

a)

b) *fr*> */&-&-n-epi I P+ 7

pSi

\ n +

S<3N4

Si02

Si3N,

d) £jr^l ffrg-Jfrq-

n-epi —I ( - \rn vu vi

J j \P+ T^nT pSi

SiO,

Si3N,

Figure 4.12: Simplified process flow chart for the fabrication of the detectors: a) the processed wafer; b) the first step of the etch process; c) the second step of the etch process and d) the final structure, after deposition of the black coating.

SEM photographs of (the front and back sides of) an infrared detector and of an array, before deposition of the black paint, are shown in Fig. 4.13 and Fig 4.14, respectively.

63

a)

b)

Figure 4.13: SEM micrographs of a cantilever-beam infrared detector: a) front and b) back sides.

64

a)

b)

Figure 4.14: SEM micrographs of an 8-element linear infrared sensing array: a) front and b) back sides.

65

Let us now conclude this chapter by summarizing the major advantages offered by the fabrication process we developed to realize the silicon cantilever beams containing integrated devices:

The devices can be fabricated in a standard IC fabrication process since the etch can be performed as the last step. The etch process is compatible with silicon planar technology. This is very important not only because it allows batch fabrication of the devices, with the obvious advantages of low cost and high yield, but also because it offers the possibility of having on-chip interface electronics. The fabrication process is very flexible, i.e. other types of thermopiles can be used and different geometries can be easily realized.

66

5. EXPERIMENTAL RESULTS

5.1 Introduction

With the fabrication process illustrated in the previous chapter both single infrared detectors and infrared sensing arrays have been realized. In order to characterize the silicon thermopile infrared detector, the basic parameters, i.e. responsivity, relative detectivity and time constant have been measured and compared with the estimated values of such parameters. Some other measurements, such as spatial homogeneity analysis and spectral response, were also carried out to achieve a better understanding of the performance, as well as the potentials and the limits, of these detectors. The experimental results for the single detector are reported in the first part of this chapter.

Measurements of the responsivity, the relative detectivity and the time constant have also been performed for an 8-element linear array. Spatial resolution measurements, to evaluate the cross-talk effect between the array elements, have been done. The results of these measurements are reported in the second half of this chapter, which will be concluded by the description of a possible application of this infrared sensing array: a monochromatic radiation sensor. This sensor utilizes the silicon thermopile infrared array in combination with a Fresnel zone plate (FZP) to resolve radiation into its wavelength components, to focus the components onto the elements of the array, and to simultaneously detect them without having to move any parts of the sensor. Preliminary measurements aiming at the evaluation of the sensor resolution have been carried out and are reported here.

5.2 The single detector

5.2.1 The detector layout

From the previous chapter it is clear that the technology used to fabricate the silicon thermopile infrared detector is very flexible, which means that very many configurations are possible. In general, the particular

67

application for which the detector is intended will determine the device layout. A schematic diagram of the cross-section of the cantilever-beam infrared detector we developed is shown in Fig. 5.1.

Figure 5.1: A schematic diagram of the cross-section of the cantilever-beam infrared detector.

The structure of the detector is as follows. The beam is 4 mm long, 3.8 mm wide and 10 /tm thick. A 50 /mi wide groove separates the beam from the supporting rim on three sides. The whole IC, i.e. the beam, the gap and the rim, is 6 mm x 6 mm. A photograph of a completed detector mounted in a standard 24 pin IC housing is shown in Fig. 5.2. One half of the beam is coated by the absorbing layer, while the other half contains the thermopile. This means that the absorbing area of the detector is approximately 2 mm x 3.8 mm. As far as the thermopile is concerned, 44 p-type silicon strips could be placed in the remaining space. Each silicon strip is 60 /mi wide, ^ 2 mm long and ~ 6 /mi deep. A 10 /mi wide Al strip interconnects two adjacent silicon strips, which are separated by a 20 /tm spacing. (Since the p-type strips are 6/tm deep - - see next section - - a minimum separation of 15 /Jin is required to avoid short circuit). A 100 jum wide thermocouple, fabricated by a shallow-p-type diffusion (SP), was placed in the middle of the thermopile. In this way the temperature gradient in the center of the beam can' be measured. A heating resistor was positioned at the top of the thermopile, thus mostly underneath the black coating. The =* 1.8 kn resistor, which is 3700 pm long and 740/im wide, was fabricated in the shallow p-type diffusion step. The purpose of this resistor is to test the thermopile. In fact, by applying a known voltage to the resistor and measuring the output voltage of the thermopile, the

68

thermopile sensitivity can be evaluated and compared with the expected value.

Figure 5.2: Photograph of a mounted detector.

Test structures Several test structures are placed on the thick supporting rim. They are used to obtain additional information about some of the steps in the fabrication process. In order to measure the sheet resistance of the three diffusion processes used (SP, SN and the "short" DP diffusion used for the thermopile strips and which will now be denoted by the symbol TP), three van der Pauw squares were included in the design. These structures are positioned at the top right, bottom left and bottom right corners (see Fig. 5.2). They allow us to measure the variation in the sheet resistance values between chips from the same wafers, as well as from different wafers. Thus, once the wafers are processed, before the sawing and mounting take place, the sheet resistance is measured by means of these van der Pauw squares [5.1]. The average values of the sheet resistance and the standard deviation, a, for the three diffusion processes are shown, for a few wafers, in Table 5.1.

69

Table 5.1: The average sheet resistance (and the standard deviation) of the SP, SN and TP diffusion processes for different wafers.

Wafer number

1 2 3 4

SP ^seWa)

271 258 256 274

o

4 4 4 4

SN RJfl/u)

8.4 7.7 7.2 8.4

a

0.5 0.3 0.7 0.7

TP *B.(n/D)

25.0 40.0 35.5 33.8

a

5 2 3 2

As mentioned in the previous chapter, a special diffusion was used for the thermopile strips (TP). In order to evaluate the depth of this - - non-standard - - diffusion, a test structure (see Fig. 5.3 and Fig. 5.4) was designed. The diffused rectangular area, denoted by A in Fig. 5.3, and the square structures, denoted by B, are separated by a progressively increasing (from 4 /zm to 20 ^m) distance s. The resistance between A and B (i=l ,...,9) is measured. For small values of s there is an overlap between the two diffused areas (see Fig. 5.4b), while for larger values of s (see Fig. 5.4a) this is not the case. Since the lateral diffusion is approximately equal to the vertical one, the depth of the TP diffusion can be evaluated by controlling the value of s for which a finite resistance between A and B (s) is measured. The depth will be about s/2. With this test structure a depth of 6 nm was evaluated, and a separation of 20 pm between two thermopile strips was chosen to avoid electrical shorting.

Finally, a third type of test structure, which consists of silicon strips fabricated by different diffusion processes (see Fig. 5.5), was used to evaluate the etch rate of silicon doped by diffusion as compared to that of epilayer silicon, in the plasma etching step. This test was carried out because if the etch rate of higher-doped silicon is not much lower than that of the epilayer, SN grooves can be used to serve as a contact to the epilayer during the ECE step and can later be etched away in the RIE step to form the beam. In this way a higher density of active devices can be obtained. Fig. 5.6 shows a SEM micrograph of the structure of Fig. 5.5. From the SEM analysis, it can be concluded that the (RIE) etch rate of silicon is not strongly dependent on the doping concentration (a variation of less than 0.5 yum in the depth of the grooves was measured).

70

//f

1

v9

WA

7Z<

VkZ^A

W^

— f§

^ ^ V/A

Figure 5.3: Top view of a test structure used to evaluate the depth of the thermopile strips' special diffusion process (TP). The dashed areas indicate the aluminum metalization, which partly overlaps the TP diffused regions.

V//////A SiO ';..,. F

TP TP

SiO, h SL z.

TP TP

Figure 5.4: Cross-section of the test structure shown in Fig. 5.3: a) along line a and b) along line b.

71

SP SN TP

I I I ' I

m rn i i

plasma etch mask diffusion mask

Figure 5.5: Test structures used to evaluate the etch rate of diffused silicon strips during the plasma etching step.

Figure 5.6: SEM micrograph of the structure shown in Fig. 5.5, after the plasma etch step. (The first white strip is equivalent to 10 \im.)

72

5.2.2 Responsivity to blackbody radiation

The measurement set-up The infrared detectors were first characterized by using blackbody radiation in the 300 to 400 K temperature range to measure the responsivity. For these measurements the detectors were mounted in a vacuum system together with a blackbody source. A schematic diagram of the measurement set-up is shown in Fig. 5.7.

Figure 5.7:The measurement set-up used for the responsivity measurements.

The vacuum chamber consisted of a 301 glass bell jar. The aluminum block inside the bell jar was used to mount the sample as well as for a heat sink. A 100 O platinum resistor (Fluke) was placed in the aluminum block in order to measure its temperature, which is the ambient temperature. The blackbody source, positioned at a distance of =± 5 cm from the sample, consisted of a 10 fl metal resistor coated by a thick layer of the same black paint that we used for the detectors. The area of the resistor/

73

blackbody source is ^ 6 cm2. On the back of the resistor, a two-terminal IC temperature transducer (AD590), utilized to measure the blackbody source temperature (see Fig. 5.8), was soldered. By supplying a voltage of + 5 V, the transducer acts as a high impedance, constant current regulator passing 1 /zA/K. The transducer is calibrated to 298.2 /uA output at 298.2 K. The load resistance RL was chosen such that VT = 1 mV/K. A current generator was used to let current (from 50 mA up to 500 mA) flow through the metal resistor so as to raise its temperature. The blackbody source temperature was, in this way, increased from the ambient temperature value up to 400 K. Higher temperatures could not be used, because the AD590 temperature transducer can only operate below 420 K (120 °C).

►5Vo

AD 590

o-

Figure 5.8: The electrical circuit used to measure the blackbody source temperature.

The measurement procedure The responsivity measurements were carried out using a Hewlett-Packard 86B computer. Via the parallel HP-IB bus, an HP3478A 5±-digit multimeter and a 20-channel Keithley scanner were controlled to measure all the relevant electric signals during a measurement run (see Fig. 5.9). The electrical measurement set-up also included two Delta power supplies, used to supply the + 5 V voltage to the AD590 transducer and the heating power to the blackbody source, and a floppy disk-drive unit, a plotter and a printer for, respectively, the storage, the plotting and the printing of the measured data.

A typical measurement run is divided into two phases. After evacuating the beel jar - - o r filling it with a known gas - - the device is tested and the resistance values of the thermopile and the thermocouple are measured.

74

Then, the detector response to the blackbody radiation is measured. First, while no current is flowing through the metal resistor, the thermopile and the test thermocouple output voltage (i.e. the offset), the blackbody temperature and the ambient temperature were measured. The blackbody temperature is increased in steps of about 10 degree each, and for each such value, all of the above-mentioned signals are measured and recorded. The only necessary manual operation of the whole run was to vary the blackbody temperature. At the end of each run the measurement data were processed, stored, plotted and printed.

t Digital HP-IB bus

T Analog voltages

Figure 5.9: The electrical measurement set-up for the responsivity measurements.

Responsivity in vacuum The responsivity of an infrared detector is defined as the ratio of the device output and the radiation input (see Chapter 3). To obtain the responsivity of our detector, we therefore need to measure the thermopile output voltage and to calculate the incident power absorbed by the detector. A first set of measurements was carried out in vacuum, (i.e. at a pressure of =; 6 x 10"5Torr). In Fig. 5.10 the thermopile output voltage is plotted versus blackbody temperature. The plot does not indicate the measured points individually, but shows instead a line interconnecting these points. Values of 30 to 50 /JV per one degree of temperature difference between the source and the detector were measured. A good agreement with Stefan's

75

law (Eq. 2.2) was found, as the output voltage of the device rises approximately with the fourth power of the blackbody temperature. To obtain the responsivity of the detector we now have to calculate the incident power absorbed by the detector. If P = oT4 is the power emitted by the source, the power incident on the detector active area, Pinc , will be given by:

^ B B ^ B B ^ D

7T d

where ABB is the area of the blackbody source, AD is the absorbing area of the detector and d is the distance between the source and the detector. The factor ir derives from the analysis of the emission of blackbody radiation from an extended source [5.2]. However, the detector reemits part of the absorbed radiation versus the source. Therefore, we should subtract this amount from the incident power to obtain the effective power, P e f f , absorbed from the detector:

^eff ~ 2 ( ^ B B - ^ D ) 7T d

where TD is the detector temperature. Since the area of the detector and the source-to-detector distance can vary, in the processing of the data the output voltage of the thermopile was only divided by the factor ( r B B - TD ). The normalized output voltage obtained in this way is a quantity proportional to the responsivity. Fig. 5.11 shows a normalized thermopile output voltage versus blackbody temperature. The SP thermocouple output voltage is also plotted. The ratio of the TP thermopile output to the SP thermopile output is given by ^ as,TP A7ya8 S P AT, which is about 32 if the AT" is the same. The ratio

of the measured output voltages is just about this value, confirming that the temperature difference in the center of the beam, where the thermocouple is positioned, is almost the same as that across the whole beam.

- 2 2 For a distance d = 5cm and a detector active area of 7.3 x 10 cm , a responsivity of =; 10.5 * 1 V/W was found for most of the samples. This is in a good agreement with the expected value (see Table 3.4). Values for three batches of samples are shown in Table 5.2.

76

IR 452-309 TEMP 294.58 (K)

/-*\ 1 8 ÜJ < _l O > 1 4 i — O

THER

MOPI

LE

OJ

^^

]0 350 BLACKBODY TEMPERATURE (K)

400

Figure 5.10: The thermopile output voltage versus blackbody temperature.

. 0

THER

MOPI

LE O

UTPUT

VOLT

AGE

(NORMALIZED)

, O,

O,

C co

no

~

1U3C

452-309 TEMP 298.

]0

78 (K)

BLACKBODY

1 : Thermopile 2 : SP couplg

350 TEMPERATURE (K)

4C ]0

Figure 5.11: The normalized thermopile and SP thermocouple output voltage versus blackbody temperature.

77

Table 5.2: Responsivity (in vacuum) to blackbody radiation and electrical resistance for samples of three batches.

Sample R (V/W) # t P ( k n )

452-202 452-203 452-204 452-205 452-212

452-303 452-309 452-311

452-403 452-407 452-410 452-411 452-412

9.7 10.0 10.7 10.3 10.4

9.4 11.3 11.1

9.4 11.4 9.3 9.3 9.1

59.0 55.3 53.6 53.7 52.9

52.8 46.1 44.4

47.7 43.1 45.8 43.7 42.3

The variation in the responsivity value for different samples is due to several factors. If we look at Eq. (3.30), we see that the Seebeck coefficient, the length of the beam and/or the beam thickness can differ from sample to sample. In estimating the responsivity of the detector, a beam thickness of 10 /xm, a sheet resistance of 35 ft/D and a beam length of 4 mm were assumed.

As far as the cantilever-beam thickness is concerned, we should remember that it has the same value as the epilayer thickness. In the growth of the epilayer, variations of * 1 /mi are not unusual. Thinner beams result in higher responsivity than thicker ones. (A variation of 1 fim in D translates into a variation of 10% of the responsivity.)

During the fabrication process, a misalignment between the mask on the front side of the wafer and the one on the back side can take place. This misalignment generally occurs in the direction of the length of the beam (due to the particular configuration of the mask aligner equipment) and can be as large as 100 /mi. This means that the effective beam length can be as small as 3.9 mm or as long as 4.1 mm. A maximum variation of 5% of the responsivity can be attributed to the beam length variations. The different responsivity values for samples coming from different wafers are mostly due to the two above-mentioned effects.

78

For samples coming from the same wafer, the variation in responsivity is essentially due to the deviation from the average value of the sheet resistance of the silicon thermopile strips, which translates (see Eq. (3.9)) into a variation of the Seebeck coefficient value. Of course, this effect can be responsible for samples coming from different wafers as well.

In general, these three effects might work in opposite directions, thus explaining why the variation of the measured responsivity values are generally in the order of 10% or less.

Another thing that should be considered is that the measured values are obtained by dividing the thermopile output voltage by the absorbed power. While the thermopile output voltage is a measured value, the absorbed power is calculated with Eq. (5.2). In this expression the active area of the detector (as the deposition of the black coating takes place after mounting the sample, it can vary from sample to sample) and the distance between the source and the detector can also be slightly inaccurate, introducing an inaccuracy in the calculated absorbed power.

One last thing to be mentioned at this point is that the responsivity of sample 452-407 is about 20% higher than the other samples from the same wafer, because this sample was completely coated by black paint. However, the time constant of such a device, due to the increase in thermal mass, increases substantially, making the improvement in responsivity not worthwhile. Therefore, for most of the samples, only the active area was blackened.

Responsivity in argon and air If the housing in which the detector is mounted is evacuated, then the maximum value of responsivity is measured. However, if evacuation is not possible, filling the housing with a gas of a lower thermal conductivity than air will give a better responsivity than in air [5.3]. Therefore, we measured the responsivity of the infrared detector in air and in a gas ambient. In our laboratory, nitrogen and argon were available, but argon was preferred because its thermal conductivity - - which is about 0.73 times the thermal conductivity of air - - i s lower than that of nitrogen. For these measurements the evacuated chamber was filled with argon, by letting the gas into the chamber through a Balzer needle valve, or filled with air through the venting valve until atmospheric pressure is reached. In Fig.5.12 the thermopile output voltage in vacuum, argon and air is plotted versus blackbody temperature, while the normalized output voltages in the three ambients are shown in Fig.5.13. A decrease in output voltage of about 20% in argon and 25% in air with respect to the values in vacuum was observed.

79

Figure 5.12: The thermopile output voltage in different ambients.

IR 452-309 Vacuum NORM FACTOR = T - T — - A r 9 0 n

BB DET A i r

,n ° 0 ÜJ ( J < 1—

>

Nio"1

3 l CL a z c t UJ IE \— ,n"2

1 0 3C

<

)0 350 BLACKBODY TEMPERATURE (K)

, 4C )0

Figure 5.13: The normalized thermopile output voltage in different ambients.

5.2.3 Relative detectivity, NEP and time constant

In order to obtain the relative detectivity, whose expression is given by Eq. (3.31), we first need to measure the noise equivalent power or NEP. This, as can be seen from Eq. (3.32), is given by the ratio of the noise voltage to the responsivity. This means that both the responsivity and the noise voltage have to be measured.

The relative detectivity D* of the detector was measured in air, using a wide-band blackbody source kept at a temperature of 500 K. The experimental set-up used for this measurement is shown in Fig. 5.14. The blackbody source has a circular opening of 0.2 inches in diameter and the distance between the source and the detector is 10 cm. The (104 times) amplified output voltage of the detector is fed into a HP spectrum analyzer with a bandwidth of 0.72 Hz.

Blackbody Source

Chopper Detector Amplifier

Spectrum Analyzer

Radiation path

Electrical signal

Figure 5.14: Block diagram of the set-up used for the detectivity measurements.

The measurement procedure is as follows. First the incident power is calculated by using a slightly modified Eq. (5.1). In fact, due to the presence of the chopper, the value given by Eq. (5.1) has to be multiplied by a factor which, for the type of chopper we used, is equal to 0.45. Then the output voltage of the thermopile is measured both when the radiation is incident upon the detector and when a shutter is positioned between the source and the detector. In this way the signal and the noise (generally around 20 nV/Hz^) are measured, respectively. This was done for several frequencies. In Fig. 5.15, the normalized responsivity (i.e. normalized to the responsivity at the lowest frequency at which the measurement system

81

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Figure 5.16: Amplified detector response to chopped radiation from a 500 K blackbody radiation source.

82

could operate) is plotted versus frequency, while the amplified output voltage of the detector is shown in Fig. 5.16. A noise equivalent power of about 5 nW was obtained for the lowest frequency value (about 200 mHz), which translates into a relative detectivity of 5 x 107 cmHzfyw.

From Fig. 5.16, a time constant (in air) of about 300 msec can be estimated. This is more than one and a half times the calculated value (in vacuum). This discrepancy is due to the fact that in the calculation of the time constant, the effect of the black paint was not considered. The black coating increases the thermal mass, thus slowing the detector response.

5.2.4 Spectral response

For the spectral response measurement, the experimental set-up shown in Fig. 5.17 was used. The source is a silicon carbide rod (Globar) which, when a 12V voltage and 6.5 A ac current are supplied, reaches a temperature of about 1600 °C. The emitted radiation travels through three slits and a monochromator to reach alternatively the reference detector or the thermopile infrared detector. The reference used is a Golay cell.

Source Chopper

Power Supply

Slit

_D

Lock in Amplifier

Monochromafor —

Slit

0

Pre-Amplifier

Sample or

Reference Detecfor

Radiation path

Electrical signal

Figure 5.17: Block diagram of the set-up used for the spectral response measurements.

The relative response of the thermopile infrared detector is shown in Fig. 5.18 for wavelengths between 2 and 10 /zm. The response to longer wavelengths was not measured, because of the extremely low power emitted by the source for wavelengths longer than 10 /xm. The detector response varies between 0.85 and 1.0 for most of the measured spectrum,

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and its shape is typical of the spectral response which might be expected from thermopile detectors, assuming the Golay cell to have a flat response [5.4].

2.5 5 7.5

WAVELENGTH (|im)

Figure 5.18: Relative response of the detector as a function of wavelength.

5.2.5 Spatial homogeneity

The silicon thermopile infrared detectors also have been tested for their spatial homogeneity by using a He-Ne laser. A block diagram of the experimental set-up used is shown in Fig. 5.19. The diaphragm used is a circular one, 1 mm in diameter, and the distance between the diaphragm and the detector could be changed, since the detector was mounted on a holder which could be moved along the optical axis. The light spot was scanned across the absorbing area of the detector (3.7 mm x 2mm) in both the x and y directions. The x and y values give the center of the light spot. The hot junctions of the thermopile are located at y = - 500 nm and the absorbing area between y = - 500 nm and y = + 1500 /zm. Figure 5. 20 shows a three-dimensional drawing of the spatial dependence of the detector response (not to scale, since each plot is shifted in both the x and y directions an arbitrary amount in order to distinguish the different scanning plots). From the measured data, a deviation of about 5% - - o r less - - from the average value has been observed, implying that, despite

84

the relatively large absorbing area, a fairly homogeneous response of the detector is obtained.

lens diaphragm

He-Ne laser h-4-i Detector Amplifier

Computer

X-Y Recorder

Optical path

Electrical signal

Figure 5.19: Block diagram of the experimental set-up used for the spatial homogeneity measurements.

_L_ _1_ X-POSITION LASER SPOT

Figure 5.20: Three-dimensional drawing of the spatial dependence of the detector response (not to scale), with the hot junctions of the thermopile located on the line y = -SOOpm and the absorbing area between y = -500 and y = +1500 fj,m.

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5.3 The infrared sensing array

Infrared sensing arrays are required for many applications, since they can provide thermal imaging with a higher speed and better focal-plane sensitivity than a single detector and they can simplify or even avoid complexity due to mechanical scanning. It is therefore interesting to investigate the performance of infrared sensing arrays based on integrated silicon thermopiles. By using the same fabrication process developed for the fabrication of the single detector, arrays can also be realized. An 8-element linear array was designed and characterized in order to obtain a first evaluation of the potentials of such arrays. The description of the device, the experimental results and a possible application in a monochromatic radiation sensor will be the topics of this section.

5.3.1 The array layout

The 8-element linear array we realized consists of 8 cantilever beams, each 10 nm thick, 3 mm long and 440 /um wide, surrounded by a thick rim which acts as a heat sink. A schematic diagram of the cross-section of the array is shown in Fig. 5.21.

Figure 5.21: A schematic drawing of the cross-section of the silicon thermopile infrared sensing array.

As for the single detector, one half of the beam is coated by an absorbing layer and the other half contains a thermopile. The thermopile, which in this case consists of 5 p-Si strips diffused (special TP diffusion process) in an n-type epilayer, has an electrical resistance of about 4 kft. The silicon

86

strips are each 60 fim wide and 1.5 mm long, with a 20 /tm spacing, and they are interconnected by 10 fim wide aluminum strips. A heating resistor is positioned at the top of the thermopile. The ~1.2kO resistor, which is 420 nm long and 86 /xm wide, was fabricated in the shallow p-type diffusion step. By applying a known voltage to this resistor, the sensitivity of the thermopile can be evaluated and the cross-talk effect between the array elements can be measured. The array elements are separated by a 20 nm wide groove, except for the separation between beams 3 and 4 and between beams 5 and 6, which is 50 fim.

A photograph of an IC containing one 8-element array is shown in Fig. 5.22.

Figure 5.22; Photograph of the IC containing the 8-element infrared sensing array.

On the top part of the IC, some other beams are visible, which are used for testing the performance of different thermopile length/thermopile width ratios. In fact, the two beams positioned on the left side are both 2 mm in length and, respectively, 440 fxm (beam A) and 340 fim (beam B) in width. (This means that the thermopile length/thermopile width ratio is 2.3 for beam A and 2.9 for beam B, while for the array elements it is 3.4.) These beams have the same structure as the array elements, i.e. half of the beam is coated by black paint and the other half contains a 5-strip p-Si/Al thermopile. However, the thermopile (and the beam) length is only two-thirds that of the array elements, and for beam B the thermopile strips are smaller (only 40 fim wide).

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5.3.2 Responsivity to blackbody radiation

The responsivity measurements were performed using the same experimental set-up which was utilized for the single detector characterization, with a similar measurement procedure. Fig. 5.23 shows the thermopile output voltage (in vacuum) for the 8 elements of the array versus blackbody temperature. Values of approximately 4 /iV output voltage per one degree of temperature difference between the source and the detector were measured.

Figure 5.23: Output voltage of the array elements versus blackbody temperature (in vacuum).

The normalized output voltage, which is proportional to the responsivity, is plotted in Fig. 5.24. The responsivity can be obtained by dividing the output voltage of each array element by the incident power. The latter is calculated by using Eq. (5.2), with AB being equal to 0.66 x 10"6m2. A responsivity of ~ 7.5 * 0.5 V/W was found for most of the samples. This value is in good agreement with the calculated value (7.9 V/W). In Fig. 5.24 the normalized output for beams A and B is also shown. Responsivity 'values of ~ 5.1 V/W and =* 6.4V/W were generally measured for beams A and B, respectively.

88

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Figure 5.24: Normalized output voltage of the array elements and of beams A and B versus blackbody temperature (in vacuum).

From Fig. 5.23 and Fig. 5.24, a slight variation in the output of the array elements can be observed. This variation, which is generally between 5 and 8% of the average value, is mostly due to the fact that the array elements are individually blackened, which means that the effective absorbing area of each element is somewhat different. This, in combination with variations in the resistance of the thermopile contained in each element, explains the difference in the output for elements of the same array. Variations in the responsivity between different samples are due to the same effects analyzed in the previous section for the single detector.

Measurements were carried out in argon and air as well and, again, a decrease of 20% in argon and one of 25% in air were observed.

89

Cross-talk effect In order to evaluate the spatial resolution of this 8-element linear array, the cross-talk between the array elements was measured by using the heating resistors integrated into each beam. By supplying voltage to the heating resistor of one beam of the array and measuring the output voltage of all the beams, the cross-talk effect can be measured. While no cross-talk was observed in vacuum, the effect could be measured in the presence of argon or air, due to heat conduction through the gas. In Fig.5.25 the relative response for the 8 array elements in argon (solid line) and in air (dashed line) is shown when heat is being dissipated into beam number 5.

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Figure 5.25: Relative output of the array elements to the output of beam number 5, where heat is being dissipated: in argon (solid line) and in air (dashed line).

The response of the two adjacent beams is slightly different due to the fact that the separation between beam numbers 5 and 6 is 50 /zm, while the separation between beam numbers 5 and 4 is only 20 /zm. The effect in air is larger, as expected, due to the higher thermal conductivity of air compared to argon. Very similar results are obtained by dissipating heat in the other elements of the array. In all cases, the cross-talk effect is limited to the first two beams on both sides of the beam into which heat is being dissipated. The other beams have an output voltage which is less than 1% of the output voltage of the heated beam.

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5.3.3 Relative detectivity, NEP and time constant

The relative detectivity of the array elements was measured in air only. Again, the same set-up and the same measurement procedure which were used for the single detector have been utilized.

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In Fig: 5.26 the relative response is plotted versus the frequency, while the amplified response of an array element to chopped 500 K blackbody radiation is shown in Fig. 5.27. From this figure, a time constant of =i 180 msec can be estimated. A NEP of ~ 1 nW and a relative detectivity D* of 5xl0 7 cmHzfyw are typical values for these arrays under the measurement conditions specified in Section 5.2.3.

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Figure 5.27: Amplified response of an array element to chopped radiation from a 500 K blackbody source.

5.3.4 The array as part of a monochromatic radiation sensor

In this last section we will describe an application of the thermopile sensing array. It is a monochromatic radiation sensor which utilizes the silicon thermopile detector array in combination with a Fresnel Zone Plate (FZP). Incident radiation with a wide spectral range from visible to infrared is made monochromatic and simultaneously focused by the FZP, and detected by the thermopile detector.

The FZP is a circular grating characterized by a constant A = ƒ A, where ƒ is the focal length of the FZP, and A is the radiation wavelength [5.5]. Thus the focal length of the FZP is inversely proportional to the wavelength and is determined by /=A/A. The FZP resolves the incident radiation into monochromatic components. Since each component has its own focal point, it will be projected onto one element of the sensing array (see Fig. 5.28). This means that the 8-element linear array can detect 8 different wavelength components simultaneously.

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Fresnel Zone Infrared sensing Plate array

Figure 5.28: Schematic diagram showing the principle of the monochromatic radiation sensor.

A FZP with 200 transparent arcs was fabricated by a pattern generator. The Cr coating was etched only at those 200 arcs on the photomask. Each arc (see Fig. 5.29) has an inner radius {(2n-l)AP and an outer radius (2nA)^ (for n=l-200). The minimum arc width is 4.3/mi. The radius of the FZP is 3.464 mm, and its focal length, for 1 /xm wavelength radiation, is 30 mm. This FZP is effective for radiation from 0.36/xm to 2.7 fim. In this wavelength range the substrate glass is transparent. This range can be extended easily from 0.2 /mi to 4/xm by using a different substrate material, such as fused quartz, which is transparent for a broader spectral range. The FZP is fan-shaped in order to eliminate unnecessary radiation components.

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93

In order to test the sensor, preliminary experiments were carried out using visible light. An Ar+ laser, which operates at 5 different wavelengths, was used as a source. The array was mounted onto a holder which can translate in both directions and rotate so that the sample can be tilted to a certain angle 6 with respect to the optical axis. The measurement set-up is shown in Fig. 5.30, while a photograph of the FZP and the array during a measurement is shown in Fig. 5.31.

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Figure 5.3 J: Photograph of the FZP and the array during a measurement.

94

A first set of measurements was carried out to evaluate the sensor resolution, which is determined by the FZP resolution (depending essentially on the number of arcs) and the dimensions of the array. For the FZP we used a resolution of 0.5% is expected [5.6]. When the laser beam is projected on the FZP, 6% of the incident power is focused at the focal point (50% is reflected and 44% is transmitted but not focused), which is determined by the wavelength. The sensing array was placed along the optical axis of the FZP (i.e. 8 = 0), with the beams of the array perpendicular to the optical axis. The resolution of this sensor was estimated by the measurement results shown in Fig. 5.32, where it can be seen that the 476.5 nm line - - detected by beam number 5 - - is completely separated from the 488.0 nm line - - detected by beam number 2.

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Between these two peaks there are two more beams of the array. This implies that the resolution of this sensor is 0.8%. This value is higher than the expected value of 0.5% because of the width of the array elements. In fact, if we consider two wavelengths X1 and A2 focused, respectively, in

95

fx and in f2, \± can be distinguished from A2 only if f2 - / i is larger than the width of the array elements. So, in this case, it is the array dimensions which limit the resolution. Better resolutions can be achieved by using an array with narrower beams or by using a different FZP. With the same array and an FZP with different arc spacings and arc widths (which means larger spacings between the focal points of the wavelengths), a resolution of 0.43% was obtained. This can be seen in Fig. 5.33, where two successive wavelength components are focused on two beams - - beam number 8 and beam number 4 - - separated by three other beams.

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In order to prove that the sensor can detect more radiation components simultaneously, the array was inclined an angle 6 with respect to the optical axis. The measurement results for a sample inclined 0 = 5° are shown in Fig. 5.34. Due to the inclination of the sample, only one beam - - i n this case beam number 2 - - is located on the optical axis, where the radiation is in focus, while the other beams are slightly out of focus. This explains the sharpest peak for beam number 2, corresponding to the

96

514.5 nm component. Although the other wavelength components are out of focus on the sensing array, the 5 laser wavelengths are clearly resolved.

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Figure 5.34: The five lines of the Ar+ laser detected by the sensor, without any mechanical movements.

A significant advantage of this sensor is that it can detect a number of different spectral components equal to the number of the thermopile array elements without having to move any parts of the sensor. Due to the flexibility of the technological process and the properties of integrated silicon thermopiles, other arrays, with a larger number of elements and dimensions properly selected - - and thus capable of simultaneously

97

detecting, with very good resolution, many radiation components - - can easily be designed. This monochromatic radiation sensor, presented here mainly as an example of an application of the thermopile infrared sensing array, can potentially be utilized in various application areas, such as spectroscopy and remote temperature measurements.

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6. DISCUSSION AND CONCLUSIONS

The main aim of the research work presented in this thesis was to investigate the feasibility of using integrated silicon thermopiles for infrared sensing. In order to fulfill this objective, an extensive investigation of the Seebeck effect in silicon and a comprehensive analysis of the thermopile performance was necessary. The knowledge acquired in doing this formed a necessary base for the further development of the project, i.e. applications of the integrated silicon thermopile for thermal sensing. Any type of signal which can generate an on-chip temperature difference (or alter an existing known temperature difference) across the thermopile can be detected. Devices based on the integrated silicon thermopile and capable of measuring mechanical and electrical signals, such as a vacuum sensor [6.1], an RMS converter [6.2] and a flow sensor [6.3], have already been successfully realized. The infrared detector presented in this thesis proved to be another interesting application of the integrated silicon thermopile.

Let us now summarize the most important results of the research work performed.

Based on the investigation of the Seebeck effect in silicon and its dependence on the doping concentration and on the temperature, the most suitable type of thermopile was selected. A p-type silicon/aluminum thermopile, in which the silicon strips are fabricated by a diffusion process, was chosen because of the rather high value of the Seebeck coefficient (as ~ 0.7 mV/K) and the not-too-high sheet resistance (Rae a 35Q/D). Another reason for selecting this type of thermopile was the low temperature coefficient of as (0.17%/K).

Two possible structures for the infrared detector have been analyzed in detail: a cantilever beam and a circular membrane. It was concluded from this investigation that a cantilever-beam structure is preferable because of its higher sensitivity. In fact, a square cantilever beam of typical dimensions F is approximately 4 times as sensitive as a round membrane of comparable dimensions (2R0 = F). This difference is somewhat modified by the larger heat loss of a cantilever beam to the ambient gas. However, if the devices are sealed in vacuum or in a low thermal conductivity gas, the heat loss is reduced and the larger sensitivity of the cantilever-beam structure can be fully utilized. Further, the relation between the infrared

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detector parameters and the geometry of the device was used to define the device layout. A maximum in the relative detectivity was obtained for a device in which the thermopile and the absorbing area are equal in length.

Once the most suitable type of thermopile and device structure were selected, the fabrication process was compiled. The cantilever-beam structures containing integrated devices were realized by an etching process which is fully compatible with silicon planar technology. It is a combination of an electrochemically controlled etching (ECE) and a plasma etching (RIE) of silicon. This etching process offers the following advantages: - The devices can be fabricated in a standard IC fabrication process,

since the etch can be performed as the last step. The compatibility of the etching with silicon planar technology is very important, not only because it allows batch-fabricated, high-performance devices to be realized, but also because it offers the possibility of having on-chip interface electronics and/or read-out structures. This etching process allows the fabrication of microstructures containing integrated devices, which are very important for sensors, and particularly for thermal sensors, where the thermal resistance plays a central role in the device performance. By being able to have the sensor fabricated in a very-thin cantilever beam, much smaller devices with a higher sensitivity can be fabricated and, consequently, many more applications can be pursued.

With the fabrication process developed, a cantilever-beam infrared detector and an infrared sensing array, based on integrated silicon thermopiles, have been realized and extensively characterized. The basic parameters, i.e. responsivity, relative detectivity and time constant, have been measured and compared to the calculated values. A good agreement was found between measured and calculated values, and an analysis of the possible causes of discrepancies was also performed. From the experimental results, sufficient information is available to design detectors with a different layout, according to the specific requirements dictated by the particular application for which the detector has to be used. For example, due to the large responsivity (> 10 V/W in vacuum) measured for the single detector, it is possible to make use of it without having to amplify the output signal. In fact, an on-chip temperature difference across the thermopile of less than one degree generates an output voltage of several millivolts. If for certain applications a shorter time constant is desired, shorter devices have to be used. Shorter devices are less sensitive, but the noise - - which, for this type of detector, is only thermal noise, thus essentially determined by the thermopile electrical resistance - - will be reduced. This means that the use of an amplifier will not significantly affect the

100

signal-to-noise ratio. The amplifier can be either integrated on the same chip or used in a hybrid configuration due to the very small number of wires (only two) needed to connect the detector output (which is the thermopile output voltage) to the amplifier.

Finally, one application of the infrared sensing array has been described. It is a monochromatic radiation sensor in which the infrared sensing array is combined with a Fresnel zone plate (FZP). Our current sensor, which has a resolution of 0.4%, can detect 8 different spectral lines simultaneously, without having to move any part of the sensor. Several types of FZP can be designed and, depending on its characteristics, the array layout can be defined. This means that a better resolution can be achieved and the same number of radiation components as array elements can be detected. If a large number of elements is desired, the signal read-out - - which can be a severe problem due to the large number of leads involved - - i s not a serious problem, since the array is fabricated by using IC technology and the read-out structure can be integrated on the same chip.

In conclusion, infrared detectors and infrared sensing arrays based on integrated silicon thermopiles have been successfully realized by using standard IC technology and silicon micromachining. These devices have a good responsivity and a relative detectivity which is comparable to most of the other thermopile type infrared detectors reported in literature. Further, the great flexibility of the fabrication process makes it possible to realize many different structures, so that different applications can be pursued. In fact, the ease of operation (no chopping of the incident radiation or external bias are needed), the possibility of batch fabrication of the devices and of having on-chip interface electronics and/or read-out structures, make these devices very attractive for applications where reliable, easy-to-handle and inexpensive detectors are needed.

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REFERENCES

Chapter 1

[1.1] S. Middelhoek, D. J. W. Noor lag, Silicon micro-transducers, J.Phys. E: Sci. Instrum., 14 (1981) 1343-1352.

[1.2] S. Middelhoek, A. C .Hoogerwerf, Classifying solid-state sensors: The sensor effect cube, Sensors and Actuators, 10 (1986) 1-8.

[1.3] S. Middelhoek, S. Kordic, D. W. de Bruin, Silicon: a promising material for sensors, Bull. SEV/VSE 76, 5 (1985) 253-257.

[1.4] S. Middelhoek, Integrated sensors, Proc. 3rd Sensor Symp.,Japan 1983, 1-10.

[1.5] A. W. v. Herwaarden, P. M. Sarro, Thermal sensors based on the Seebeck effect, Sensors and Actuators, 10 (1986) 321-345.

[1.6] J. F. Cros, La mesure des infrarouges: oü? quand? comment?, Mesures, 12 December 1983, 7-26.

[1.7] Y. Ueda, New trend in infrared sensors, Proc. 1st Sensor Symp., Japan 1981,199-204.

[1.8] R. J. Keyes, Recent advances in optical and infrared detector technology, in 'Optical and Infrared Detectors', R. J. Keyes Ed., Springer-Verlag, Berlin, 1980, 301-315.

Chapter 2

[2.1] E. H. Putley, History of infrared detection - Part I. The first detectors of thermal radiation, Infrared Phys. 22 (1982) 125-131.

[2.2] E. H. Putley, Thermal detectors, in 'Optical and Infrared Detectors', R.J.Keyes Ed., Springer-Verlag, Berlin, 1977, 71-100.

[2.3] W. G. Driscoll, W. Vaughan (Eds.), Handbook of Optics, McGraw-Hill Book Co., New York, 1978.

[2.4] R. Eisberg, R. Resnick, Quantum Physics, J.Wiley & Sons Inc., New York, 1974.

[2.5] P. W. Kruse, The photon detection process, in 'Optical and Infrared Detectors', R.J.Keyes Ed., Springer-Verlag, Berlin, 1977, 5-69.

[2.6] W. Budde, Physical Detectors of optical radiation, Academic Press, New York, 1983.

[2.7] G. R. Lahiji and K. D. Wise, A batch-fabricated silicon thermopile infrared detector, IEEE Trans. Electron Devices, ED-29 (1982) 14-22.

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[2.8] M. Kimura, Microbridge type infrared detector, Proc. 1st Sensor Symp., Japan 1981, 227-232.

[2.9] C. Shibata, C. Kimura and K. Mikami, Far infrared sensor with thermopile structure, Proc. 1st Sensor Symp., Japan 1981, 221-225. Th. Elbel, J. E. Muller, F. Volklein, Miniaturisierte thermische Strahlungssensoren: Die neue Thermosaule TS-50.1, Feingerate-technik, 34 (1985) 113-115. L. R. Wollman, The thermopile: the commercial infrared detector, Electro-Opto Syst. Des. (Sept 1979), 37-44. I.L.Choi and K. D. Wise, A silicon-thermopile-based infrared sensing array for use in automated manufacturing, IEEE Trans. Electron Devices, ED-33 (1986) 72-79. A. Tebo, Review of infrared detector technology, Laser Focus/Electro-Optics (Apr 1984) 46-60. K. C. Liddiard, Thin-film resistance bolometer IR detectors, Infrared Phys. 24 (1984) 57-64. D. P. Neikirk, D. B. Rutledge, Air-bridge microbolometer for far-infrared detection, Appi.Phys.Lett. 44 (1984) 153-155. H. Meixner, G. Mader, P. Kleinschmidt, Infrared sensors based on the pyroelectric polymer polyvinylidene fluoride (PVDF), Siemens Forsch.-u.Entwickel.-Ber. Bd. 15 (1986) nr.3, 105-114. M. Murata, S. Ito, Pyroelectric infrared sensor with modified PZT ceramics, Proc. 2nd Sensor Symp., Japan 1982, 45-47. A. Kaneko et al., Pyroelectric linear array infrared detector, Proc. 5th Sensor Symp., Japan 1985, 253-256. U. Korn, Z. Rav-Noy, S. Shtrikman, Pyroelectric PVF2 infrared detector arrays, Appl. Opt., 20 (1981) 1980-1982. J. Maserijan, A thin-film capacitive bolometer, Appl. Opt., 9 (1970)307-315. R. M. Walser, R. W. Bene', R. E. Caruthers, Radiation detection with the pyromagnetic effect, IEEE Trans. Electron Devices ED-18 (1971) 309-315. B.Paul, H.Weiss, Anisotropic InSb-NiSb as an infra-red detector, Solid State Electron. 11 (1968) 979-981. H. J. Goldsmid, N. Savvides, C. Uher, The Nernst effect in Cd3As2-NiAs, J.Phys.D, 5 (1972) 1352-1357. R. A. Smith, F. E. Jones, R. P. Chasmer, The detection and measurement of infrared radiation, Claredon, Oxford, 1968. W.L.Wolfe, G. H. Zissis (Eds.), The infrared handbook, The information and analysis (IRIA) Center, Environmental Research Institute of Michigan (1978).

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Chapter 3

[3.1] R. R. Heikes and R. W. Ure, Thermoelectricity: Science and Engineering, Interscience Publishers, New York-London, 1961.

[3.2] P. M. Sarro and A. W. van Herwaarden, Inhomogeneity effects in silicon thermopiles, Proc. 2nd Sensors and Actuators Symp., Enschede, The Netherlands, November 1-2, 1984, 129-135.

[3.3] L.Onsager, Phys. Rev., 37 (1931) 405, 38 (1931) 2265. [3.4] F. J. Blatt, P. A. Schroeder, C. L. Foiles and D. Greig, Thermoelectricpower

of metals, Plenum Press, New York, 1976. [3.5] R. D. Barnard, Thermoelectricity in metals and alloys, Taylor &

Francis Ltd., London, 1972. [3.6] T. J. Quinn, Temperature, Academic Press, London, 1983. [3.7] H. J. Goldsmid, Applications of thermoelectricity, Butler & Tanner

Ltd., London, 1960. [3.8] T. H. Geballe and G. W. Hull, Seebeck effect in silicon, Phys. Rev.,

98(1955)940-947. [3.9] H. G. Kerkhoff and G. C. M. Meijer, An integrated electrothermal

amplitude detector using the Seebeck effect, Proc. ESSCIRC, Southampton, U.K., 1979, 31-33.

[3.10] A. W. van Herwaarden, The Seebeck effect in silicon ICs, Sensors and Actuators, 6 (1984) 245-254.

[3.11] P. M. Sarro and A. W. van Herwaarden, unpublished. [3.12] G. D. Nieveld, Thermopiles fabricated using silicon planar

technology, Sensors and Actuators, 3 (1983) 179-183. [3.13] G. R. Lahiji and K. D. Wise, A batch-fabricated silicon thermopile

infrared detector, IEEE Trans. Electron Devices, ED-29 (1982) 14-22.

[3.14] T. H. Geballe and G.W.Hull, Seebeck effect in germanium, Phys. Rev., 94 (1954) 1134-1140

[3.15] H. P. R. Frederikse, Thermoelectric power of germanium below room temperature, Phys. Rev., 92 (1953) 248-252.

[3.16] Y. Onuma, Thermoelectric power of a vacuum deposited germanium thin film, Electrical Eng. in Japan, 89 (1969) 72-77.

[3.17] D.I.Jones, P. G. Le Comber and W. E. Spear, Thermoelectric power in phosphorous doped amorphous silicon, Phil. Mag., 36 (1977) 541-551.

[3.18] S. Kodato, S.Nishida, M. Konagai and K.Takahashi, High-conductivity a-Si:H:F film and its performance for a power sensor and a strain gauge, J. Non-Cryst. Solids, 59 & 60 (1983) 1207-1210.

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[3.19] M. Kimura, Microbridge type infrared detector, Proc. 1st Sensor Symp., Japan 1981, 227-232.

[3.20] G. Ahuja, H.C.Gupta and L.M.Tiwari, Thermoelectric power of III-V ternary mixed crystals, / . Phys. Soc. Jap., 52 (1983) 4283-4285.

[3.21] N. P. Kekelidge, Z. N. Kuinkadze, Z. F. Davitaya, G. N. Eritsyan, V. A. Saakyan and E. K. Karapetya, Studies of thermoelectric characteristics of M^As^x alloys, Phys. Status Solidi (a), 38 (1976)K49-K52.

[3.22] E. F. Hocking et al., Thermal and electrical transport in InAs-GaAs alloys, / . Appl. Phys., 37 (1966) 2879-2887.

[3.23] L. N. Kurbatov, I.M. Froimson and S. S. Shakhidzhanov, Thermoelectric power of single-crystal p-type silicon, Sov. Phys. Semicond., 11 (1977)697-698.

[3.24] G. C. M. Meijer, Thermal sensors based on transistors, Sensors and Actuators, 10 (1986) 103-125.

[3.25] P.M.Sarro and A. W. van Herwaarden, Silicon cantilever beams fabricated by electrochemically controlled etching (ECE) for sensor applications, J. Electrochem. Soc, 133 (1986) 1724-1729.

[3.26] A. W. v. Herwaarden, P. M. Sarro, Thermal sensors based on the Seebeck effect, Sensors and Actuators, 10 (1986) 321-346.

[3.27] A. W. v. Herwaarden, P.M.Sarro, H.C.Meijer, Integrated vacuum sensor, Sensors and Actuators, 8 (1985) 187-196.

Chapter 4

[4.1] K. E. Petersen, Silicon as mechanical material, Proc. IEEE, 70 (1982), 420-457.

[4.2] H. A. Waggener, Electrochemically controlled thinning of silicon, Bell Syst. Tech. J., 50 (1970) 473-475.

[4.3] H. H. Sawin, A review of plasma processing fundamentals, Solid-State Technoly, April 1985, 211-216.

[4.4] P.M.Sarro, A. W. van Herwaarden, Silicon cantilever beams fabricated by electrochemically controlled etching for sensor applications, J. Electroch. Soc, 133 (1986) 1724-1729.

[4.5] L. I.Maissel and R.Glang (Eds.), Handbook of Thin film Technology, McGraw Hill Book Co., New York, 1970.

[4.6] A. Ennos, Stresses developed in optical film coatings, Appl. Optics, 5 (1966), 51-56.

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Chapter 5

[5.1] L. J. v. d. Pauw, A Method for measuring specific resistivity and Hall effect of discs of arbitrary shape, Philips Research Rep., 13 (1958) 1-9.

[5.2] W. J. Smith, Modern Optical Engineering, McGraw Hill Book Co., New York, 1966, 182-188.

[5.3] W. Budde, Physical detectors of optical radiation, Academic Press, New York, 1983.

[5.4] R. W. Astheimer, S. Weiner, Solid-blacked evaporated thermopile radiation detectors, Appl. Optics, 3 (1964) 493-500.

[5.5] M. Born, E. Wolf, Principle of Optics, Pergamon Press, Oxford 1965.

[5.6] P.M. Sarro, H. Yashiro, A. W. v. Herwaarden, S. Middelhoek, An integrated silicon thermopile infrared sensing array, Proc. Fourth Int. Solid-State Sensors and Actuators Conf., Tokyo, Japan, June 3-5, 1987, 227-230.

Chapter 6

[6.1] A. W. v. Herwaarden, P.M. Sarro, H.C.Meijer, Integrated Vacuum sensor, Sensors and Actuators, 8 (1985) 187-196.

[6.2] A. W. v. Herwaarden, H. P. Hochstenbach, C. J. P. M. Harmans, Integrated true RMS converter, IEEE Trans. lustrum. Meas., IM-35 (1986) 224-225.

[6.3] B. W. v. Oudheusden, J. H. Huijsing, Integrated flow friction sensor, Proc. Fourth Int. Solid-State Sensors and Actuators Conf., Tokyo, Japan, June 3-5, 1987, 368-371.

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LIST OF SYMBOLS

Definition Units

Length/width ratio of thermopile Blackbody source area Detector active area Length/width ratio of circular thermopile Frequency bandwidth Light speed 2.9979 x 108

Specific heat at constant pressure Distance between blackbody source and detector Beam thickness Relative detectivity Length of interaction area Equivalent electrical sheet thickness Conduction-band-edge energy Fermi energy level Energy gap Impurity ionization energy Valence-band-edge energy Focal length Beam length Thermal conductance Planck's constant 6.63 x lO" 3 4

Thermal mass or thermal capacitance Electrical current Current density Boltzmann's constant 1.38 x 10~23

Constant for Seebeck coefficient ~2.6 Electron concentration Number of strips in a thermopile Conduction-band density of states Valence-band density of states Noise equivalent power Hole concentration Pyroelectric coefficient Power Elementary charge 1.60 x 10"19

Peltier heat Thomson heat Responsivity Inner radius junctions round-membrane thermopile

m* m2

□ m2

m/s J /kgK m2

m m Hztyw m m eV eV eV eV eV m m J / sK Js J/K A A/m2

J/K

elec./m3

m~3

m~3

W holes/m-3

C/Km 2

W As W/m2

W/m3

V/W

m

RT(y)

BB

V W X z z a as TTh 5Si <̂ F

K,

X V n p dv,T) a a T

Outer radius junctions round-membrane thermopile Electrical sheet resistance Thermal sheet resistance Thermal resistance Thermopile internal resistance Spectral radiancy Relaxation-time exponent n-type material Relaxation-time exponent p-type material Absolute temperature Temperature of blackbody source Detector temperature Thermopile output voltage Voltage Thermopile and beam width Thermopile length Figure of merit of a thermoelectric material Figure of merit of a thermocouple Temperature coefficient Seebeck coefficient Thomson coefficient Density of silicon 2329 Electrochemical potential <f>F = EF/q Phonon-drag-effect factor n-type material Phonon-drag-effect factor p-type material Thermal conductivity of solids Wavelength of electromagnetic radiation Frequency of electromagnetic radiation Peltier coefficient Electrical resistivity Blackbody radiation distribution function Electrical conductivity (a=l/p) Stefan-Boltzmann constant 5.67 x 10"8

Time constant

m n/D K/wa K/W n W/m2Hz

K K K V V m m K 1

K 1

K - i

V/K V/K Kg/m3

V

W/mK m s-1

V nm J/m3Hz S/m W/m2K4

s

SUMMARY

The aim of the research work presented in this thesis is to investigate the possibility of realizing a thermal type detector of infrared radiation, based on integrated silicon thermopiles and fabricated by using silicon planar technology. The device we developed uses silicon not only as a supporting structure, but also as one of the two thermocouple materials. Thus it benefits from both the advantages offered by silicon IC technology and from the large value of the Seebeck coefficient in silicon.

After a short description of the infrared radiation characteristics, both the photon-type and the thermal-type detection processes are reviewed. The most important types of thermal detectors are surveyed and their characteristics and their limitations are pointed out. Further, the advantages and disadvantages of both optical and thermal detectors are discussed.

In order to describe the working principle of the detector, some important properties of the integrated silicon thermopile, the basic element of the detector, are discussed. This is done by first reviewing the Seebeck effect, the physical effect exploited by the thermopile. This effect is a self-generating effect, that is the power for the output signal is supplied by the input signal itself, instead of an auxiliary power supply. This means that the thermopile is offsetless (no output signal is present without an input signal), which is a very attractive feature for a sensor to have. Next, the performance of several types of integrated silicon thermopiles and the influence of the structure of the device on its performance is analyzed. Based on the information obtained in this investigation, the design criteria of the device are formulated.

A cantilever beam structure was chosen for the infrared detector, due to the higher sensitivity of this structure compared to other structures. A fabrication process for realizing silicon cantilever beams containing integrated devices is therefore compiled and described in detail. Such structures are fabricated by two different methods. In one method only electrochemically controlled etching (ECE) of silicon is used to form the beams; in the other method, a combination of ECE and a plasma etching process, viz. reactive ion etching, is used. Both methods are analyzed and the advantages and problems inherent in them stated. A detailed description of the fabrication process used for the infrared detector is presented as well.

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Infrared detectors and infrared sensing arrays are realized and characterized in order to evaluate the basic parameters, i.e. responsivity, relative detectivity and time constant, and to compare them with the estimated values of these parameters. Some other measurements, such as spatial homogeneity and spectral response, are also carried out to achieve a better understanding of the performance as well as the limitations of these detectors. Finally, a possible application of the infrared sensing array is described: a monochromatic radiation sensor. This sensor utilizes the silicon thermopile infrared array in combination with a Fresnel zone plate to resolve radiation into its wavelength components, to focus the components onto the elements of the array, and to simultaneously detect them without having to move any parts of the sensor. Preliminary measurements aiming at the evaluation of the sensor resolution are performed and reported here.

In conclusion, infrared detectors and infrared sensing arrays based on integrated silicon thermopiles have been successfully realized by using standard IC technology and silicon micromachining. These devices have a good responsivity and a relative detectivity which is comparable to most of the other thermopile-type infrared detectors reported in literature. Further, the great flexibility of the fabrication process makes it possible to realize many different structures, so that different applications can be pursued. In fact, the ease of operation (no chopping of the incident radiation or external bias are needed), the possibility of batch fabrication of the devices and of having on-chip interface electronics and/or read-out structures make these devices very attractive for applications where reliable, easy to handle and inexpensive detectors are needed.

I l l

SAMENVATTING

Het doel van het in dit proefschrift beschreven onderzoek is een infrarooddetector van het thermische type te maken, die gebaseerd is op geintegreerde silicium thermozuilen en die gefabriceerd kan worden met behulp van de planaire silicium technologie. De ontwikkelde sensor gebruikt silicium niet alleen als substraat, maar ook als één van de twee materialen van de thermozuil. Hierdoor profiteert de detector zowel van de voordelen van de silicium IC-technologie als van de hoge waarde van de Seebeck coefficient van silicium.

Na een korte inleiding over de eigenschappen van infraroodstraling volgt een beschrijving van de optische en thermische detectieprocessen. Daarna wordt een overzicht gegeven van de belangrijkste types van thermische infrarooddetectoren, met een uiteenzetting van hun eigenschappen en beperkingen. Vervolgens wordt ingegaan op de voor- en nadelen van optische en thermische infrarooddetectoren.

Alvorens dieper in te gaan op de werking van de detector wordt eerst een aantal belangrijke eigenschappen behandeld van de in silicium geintegreerde thermozuil, het basiselement van de detector. Hiertoe wordt een beschrijving gegeven van het Seebeck effect, het fysisch effect waarop de thermozuil is gebaseerd. Dit effect is een zelf-genererend effect, dat wil zeggen dat het vermogen voor het uitgangssignaal geleverd wordt door het ingangssignaal zelf. Voor de werking van de detector is een externe voeding zodoende niet nodig. Dit betekent tevens dat de thermozuil geen offset vertoont, wat een erg aantrekkelijk kenmerk voor een sensor is. Vervolgens wordt er een analyse gemaakt van de prestaties van verschillende soorten van geintegreerde silicium thermozuilen en van de invloed die de structuur van de sensor op deze prestaties heeft. Op grond van de resultaten van deze analyse zijn criteria voor het ontwerp van de sensor geformuleerd.

Voor wat betreft de structuur van de infrarooddetector is gekozen voor een zogenaamde "cantilever-beam", vanwege de hogere gevoeligheid in vergelijking met andere structuren. Het fabricageproces dat voor het maken van zo'n silicium structuur met geintegreerde thermozuilen nodig is wordt in detail beschreven. Voor de fabricage van deze structuren worden twee verschillende methoden gebruikt. De eerste methode berust op het electrochemische etsen (ECE) van silicium; de tweede methode is gebaseerd op een combinatie van ECE en plasma etsen. Van beide methoden wordt

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een analyse gemaakt, en worden hun voor- en nadelen worden opgesomd. Hierna volgt een gedetailleerde beschrijving van het fabricageproces dat voor de infrarooddetector is gebruikt.

Er zijn infrarooddetectoren met één of met meer elementen op een rij gemaakt. Van deze detectoren zijn de waarden van diverse basisparameters bepaald, zoals gevoeligheid, relatieve detectiviteit en tijdconstante. Deze resultaten worden vergeleken met de berekende waarde van deze parameters. Daarnaast zijn metingen gedaan van de plaats- en golflengteafhankelijkheid van de gevoeligheid, om een beter inzicht te krijgen in de prestaties en de beperkingen van deze detectoren. Tenslotte wordt een mogelijke toepassing van een infrarooddetector met meerdere elementen beschreven: een monochromatische stralingssensor. Deze sensor bestaat uit een op silicium thermozuilen gebaseerde infrarooddetector samen met een Fresnel Zone Plate (een cirkelvormig tralie). De FZP focusseert licht van verschillende golflengten op verschillende elementen van de detector. Zodoende kan gelijktijdig straling van verschillende golflengtes worden gemeten, zonder hiervoor delen van de sensor te bewegen. Er zijn metingen verricht om de werking van deze sensoren te onderzoeken en de resultaten worden toegelicht.

Concluderend kan gezegd worden dat met succes infrarooddetectoren gebaseerd op in silicium geintegreerde thermozuilen zijn gefabriceerd. Hierbij is gebruik gemaakt van conventionele IC-technologie en silicium etstechnieken. Deze detectoren hebben een gevoeligheid en relatieve detectiviteit die gelijkwaardig zijn aan die van in de literatuur beschreven thermozuil infrarooddetectoren. De grote flexibiliteit van het fabricageproces maakt het mogelijk om vele verschillende structuren te realiseren, toegesneden op specifieke toepassingen. De eenvoudige werking (modulatie van de inkomende straling en een externe voeding zijn niet nodig), de mogelijkheid van massafabricage en de mogelijkheid om electronische schakelingen op de sensorchip mee te kunnen integreren maken de detectoren erg aantrekkelijk voor toepassingen waarin betrouwbare, gemakkelijk te gebruiken en goedkope detectoren gewenst zijn.

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ACKNOWLEDGMENTS

This thesis is really the result of the work, the advice, the assistance and the support, both technical and moral, of many colleagues, friends and relatives. To them I would like to express my deepest appreciation and thanks.

In particular, I would like to thank:

Professor S. Middelhoek, my thesis advisor, for his guidance and counsel as well as for his warmth and understanding throughout these past four years;

Professor F. P. Califano of University of Rome, Italy, and Professor J. J. Loferski of Brown University, Providence, USA, for early guidance, encouragement and career inspiration;

the members of the Departmental IC Workshop: J. Groeneweg, E. J. G. Goudena, F. J. de Jong, W. de Koning, E.J. Linthorst, Ir. P.K. Nauta, E. Smit, Ir. J.M. G. Teven, W. Verveer and L. Wubben, for their skillful fabrication of the devices, their indispensable cooperation and assistance at several stages of this work, as well as for their help with my learning Dutch;

all the members of the Electronic Instrumentation Laboratory for the many bits of help and for the very pleasant working atmosphere;

Dr. A. W. van Herwaarden, who first started the investigation on the Seebeck effect in silicon and its applications to sensors, for the fruitful cooperation which resulted in several publications, and for his valuable contributions to many aspects of this study;

Dr. H. C. G. Ligtenberg of Sentron v.o.f., Roden, The Netherlands, and Ir. M. J. J. Theunissen of Philips Research Laboratories, Eindhoven, The Netherlands, for the helpful discussions about the chemical etching of silicon;

Ir. T.Bakker, J. Winkel and M. Deutekom of TNO Research Laboratory, The Hague, The Netherlands, for making their relative detectivity and spectral response measurement equipment available and for their assistance during the measurements;

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Dr. J. de Vreede of the Van Swinden Laboratory of the Dutch National Service of Metrology, Delft, The Netherlands, for help in performing the spatial homogeneity measurements;

Frans Schneider for his invaluable technical assistance at different stages of the work;

The members of the Departmental Drafting Room for the careful reproduction of the figures and of the Photographic Laboratory for making the photographs of the devices;

Susan Massotty for improving the English of this thesis and for her friendship and support during the past four years;

The Dutch Foundation for Fundamental Research on Matter (FOM) for financial support (Project THFE-DI-3);

My husband, Rene', for his thoughtfulness, constant encouragement and infinite patience, and my son, Marco. To them and to my parents and grandmother, I dedicate this thesis.

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ABOUT THE AUTHOR

Pasqualina M. Sarro was born in Piedimonte Matese, Italy, in 1957. She received her "Laurea" degree (cum laude) in Solid-State Physics from the University of Naples, Naples, Italy, in 1980, with her thesis work concerning the fabrication and characterization of planar multijunction solar cells.

From 1981 to 1983, she was a Post-Doctoral Fellow in the Photovoltaic Research Group of the Division of Engineering, Brown University, Providence, Rhode Island, USA, where she was engaged in a research program on Cu-ternary photovoltaic cell fabrication by chemical spray pyrolysis (the program was supported by the Standard Oil Company of Ohio SOHIO).

From October 1983 to September 1987, she was a F.O.M. (Foundation for Fundamental Research on Matter) Research Assistant in the Electronic Instrumentation Laboratory at the Delft University of Technology, Delft, The Netherlands, working towards her Ph.D. degree, her thesis dealing with infrared sensors based on an integrated silicon thermopile.

In the fall of 1987 she will join the Delft Institute for Microelectronics and Submicron Technology (DIMES), at the Delft University of Technology, as Process Line Manager, where she will perform research on IC technology and silicon sensor technology.

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PUBLICATIONS AND PRESENTATIONS RELATED TO THE THESIS WORK

The research work described in this thesis has been, in part, published in scientific journals and presented at international conferences. The major papers and presentations are listed below.

P.M. Sarro, A.W. van Herwaarden, Inhomogeneity Effects in Silicon Thermopiles, 2nd National Symp. on Sensors and Actuators, Enschede, The Netherlands, 1-2 Nov. 1984, Proc. pp. 129-135.

P.M. Sarro, A.W. van Herwaarden, Silicon Cantilever Beams Fabricated by Electrochemically Controlled Etching for Sensor Applications, 15th ESSDERC'85, Aachen, West Germany, 9-12 Sept. 1985, Proc. pp. 90-91.

A.W. van Herwaarden, P.M. Sarro, H.C. Meijer, Integrated Vacuum Sensor, Sensors and Actuators, 8 (1985) 187-196.

P.M. Sarro, A.W. van Herwaarden, Silicon Cantilever Beams Fabricated by Electrochemically Controlled Etching for Sensor Applications, J.Electrochem. Soc, 133 (1986) 1724-1729.

P.M. Sarro, A.W. van Herwaarden, An Integrated Silicon Thermopile Infrared Detector, 16th ESSDERC'86, Cambridge, UK, 8-11 Sept 1986, Proc. pp. 99-100.

A.W. van Herwaarden, P.M. Sarro, Thermal Sensors Based on the Seebeck Effect, Sensors and Actuators, 10 (1986) 321-345.

i

A.W. van Herwaarden, P.M. Sarro, Double Beam Integrated Thermal Vacuum Sensor, / . Vac. Sci. Techn. A, Jul-Aug IV (1987).

P.M. Sarro, A.W. van Herwaarden, IR Detector Based on an Integrated Silicon Thermopile, 4th Int. Symp. on Opt. and Optoel. Appl. Sci. and Eng., The Hague, The Netherlands, 30 March - 3 April 1987.

P.M. Sarro, H. Yashiro, A.W. van Herwaarden, S. Middelhoek, An Infrared Sensing Array Based on Integrated Silicon Thermopiles, 4th Int. Conf. on Solid-State Transd., Tokyo, Japan, 2-5 June 1987. Proc. pp. 227-230.

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