Thin Solid Films 519 (2011) 3401–3406

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Sputter deposited LiPON thin films from powder target as electrolyte for thin filmbattery applications

C.S. Nimisha a,⁎, K. Yellareswar Rao a, G. Venkatesh b, G. Mohan Rao a, N. Munichandraiah b

a Department of Instrumentation, Indian Institute of Science, Bangalore 560012, Indiab Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560012, India

⁎ Corresponding author. Fax: +91 8023600135.E-mail address: nimishacs@yahoo.com (C.S. Nimisha

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.01.087

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 May 2010Received in revised form 24 December 2010Accepted 4 January 2011

Keywords:Powder targetSolid electrolyteLiPON thin filmsX-ray photoelectron spectroscopy

Lithium phosphorus oxynitride (LiPON) thin films as solid electrolytes were prepared by reactive radiofrequency (rf) magnetron sputtering from Li3PO4 powder compact target. High deposition rates and ease ofmanufacturing powder target compared with conventional ceramic Li3PO4 targets offer flexibility in handlingand reduce the cost associated. Rf power density varied from 1.7 Wcm−2 to 3 Wcm−2 and N2 flow from 10 to30 sccm for a fixed substrate to target distance of 4 cm for best ionic conductivity. The surface chemicalanalysis done by X-ray photoelectron spectroscopy showed incorporation of nitrogen into the film as bothtriply, Nt and doubly, Nd coordinated form. With increased presence of Nt, ionic conductivity of LiPON wasfound to be increasing. The electrochemical impedance spectroscopy of LiPON films confirmed an ionicconductivity of 1.1×10−6 Scm−1 for optimum rf power and N2 flow conditions.

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1. Introduction

In recent years, much importance has been given to the researchand development of Li-ion batteries, which are promising candidatesfor micro-power applications [1–4]. Various research groups areinvolved in the development of different kinds of Li-ion batteries, inwhich thin film battery (TFB), is a fast growing category. Thesebatteries make use of a thin solid electrolyte layer between cathodeand anode sequentially deposited on to a substrate by variousdeposition processes, matching to modern semiconductor technolo-gies. Thin films deposited by physical vapor deposition techniques(PVD) can greatly improve reaction areas of rechargeable TFB as theyprovide cleaner and intimate interface between cathode, electrolyteand anode layers. For any lithium based battery, the electrolyte layerpermits the repeated and rapid transfer of Li+ ions between the anodeand cathode over an expected range of operating conditions. A thinlayer of solid electrolyte provides considerable savings in terms ofvolume and mass of the battery compared to liquid or polymerelectrolyte.

Widely used solid electrolytes are either glassy (oxides or sulfides)or ceramic type. Glassy or amorphous electrolytes have severaladvantages compared to crystalline counterparts; such as wide rangeof selection in composition, isotropic properties, no grain boundaries,easy film formation etc. Altogether solid electrolytes exhibit higherionic conductivities with single ion conduction, in which only Li+ ionsare mobile while the counter anions and other cations form a rigid

network [5]. This effectively suppresses the undesirable side reactionsor decomposition of electrolyte due to anionic concentration gradientacross the electrolyte. Different solid electrolyte chemistries includeLi2S–SiS2 [6], Li2S–SiS2–Li4SiO4 [7], Li2S–SiS2–Li3PO4 [8], Li2S–P2S5 [9],Li2O–Al2O3–TiO2–SiO2–P2O5 [10] etc. Ionic conductivities in the range10−3–10−4 Scm−1 have been reported from sulfur based and ceramicelectrolytes. Sulfur based electrolytes are moisture sensitive and tendto corrode the deposition equipment making it difficult to handle.High temperature annealing requirement for ceramic electrolyteslimits their applications when multi-layered stacks made of materialswith varying thermal expansion coefficients are used. As an alterna-tive, amorphous lithium phosphorus oxynitride (LiPON) thin filmsformedby sputtering fromLi3PO4 target in pureN2,with a compositionof Li3.3 PO3.9 N0.17 and a Li+-ion conductivity of 2×10−6 Scm−1 at25 °C [11] has been integrated as an electrolyte layer of TFB [2–4].Although LiPON has lower ionic conductivity compared to other solidelectrolytes, stability with Li anode, better durability, ease ofintegration with TFB with a matching potential window of 0–5.5 Vfavor this oxinitride glassy electrolyte. A recent review on fast Li-ionconductors by Knauth provides more insight in to the state-of-the-artknowledge of solid electrolytes [12].

LiPON electrolyte films have been synthesized by different vapordeposition techniques like rf magnetron sputtering [13–16], ion-beamassisted deposition (IBAD) [17], pulsed laser deposition (PLD) [18], ande-beam evaporation [19]. Among rf sputtering, IBAD, PLD and e-beamevaporation, a maximum ionic conductivity of 3.3×10−6 Scm−1 hasbeen reported from rf sputtered LiPON films [20]. Rf sputtering ispreferred over other PVD processes due to its high repeatability formulti-elemental compounds, formation of pin hole free filmswith goodcontacts and high particle energy leading to dense layers. The major

Fig. 1. Schematic illustration of magnetron sputtering system using Li3PO4 powdertarget for LiPON deposition.

Fig. 3. XRD patterns of pure Li3PO4 powder target and LiPON film deposited.(Deposition conditions: rf power density of 3 Wcm−2, N2 flow of 30 sccm).

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limiting factor of rf magnetron sputtering from Li3PO4 ceramic target isthe low deposition rate (typically 2 nm/min) [20] and a non-uniformerosion which eventually leads to non stoichiometry of the multi-elemental target. Lee et al. [21] for rf sputtered LiPONfilms andKimet al.[22] for plasmaassisted PVDof LiPONfilmshave reportedhigher growthrates with an ionic conductivity in the range of 10−7–10−9 Scm−1.

Typically lithium phosphate targets are prepared by a series ofsteps which includes calcinations of Li3PO4 powder, binder addition,drying, sieving and ball milling followed by hot/cold press to increasethe density of the target. This disc has to be sintered for long hours tocomplete the target making process. But after few depositions, βLi3PO4 transforms to polymorphic γ Li3PO4 due to temperatureincrease in the target which finally leads to cracking of the sinteredtargets [23]. Considering the material loss associated and cost ofproduction of ceramic sintered targets, sputtering from powder targetis a straight forward and cost effective technique. This avoids crackingof brittle targets due to low thermal conductivity. Sputtering frompowder target has been reported for multi-component films such asYBCO, CrB–MoSx and CrB–TiC–MoSx with good repeatability [24,25].In the present study we have combined the merits of rf sputterdeposition and a cost effective powder compact Li3PO4 target in N2

plasma. The goals of this study are to,

i. Explain the synthesis of LiPON films in N2 plasma from apowder compact Li3PO4 target with a high deposition rate.

ii. Characterize the properties of resulting films in terms ofstructure, surface morphology, N2 incorporation by elementalcomposition study and Li+-ion conductivity.

iii. Optimize the deposition conditions in terms of rf power and N2

flow during deposition.

Fig. 2. Schematic layout of metal–insulator–metal (MIM) structure of Pt/LiPON/Al forimpedance measurement.

2. Experimental details

Stoichiometric Li3PO4 powder targets were made by filling Li3PO4

powder (99.99% Sigma Aldrich ) in a 3 inch diameter Cu disk with atrench of 3 mm depth and was packed tightly by pressing with a flatmetal plate. This powder target was fixed on to rf magnetron in asputter-up configuration. The schematic representation of thedeposition system with powder target arrangement is shown in Fig. 1.

Fig. 4. (a) SEM image of LiPON film surface on Pt coated Si substrate and (b) cross-section of Pt/LiPON/Al (MIM) structure. (Deposition conditions are, Rf power density of3 Wcm−2, N2 flow of 30 sccm.)

Fig. 5. Impedance spectra of LiPON thin films deposited from powder target at different rf power densities (a) 1.7 Wcm−2 (b) 2.2 Wcm−2, (c) 2.6 Wcm−2, (d) 3 Wcm−2 from Pt/LiPON/Al sandwich structure.

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Pre-cleaned platinum deposited silicon substrates (Silicon ValleyInc.USA) were held at a distance of 4 cm away from the target. Thesubstrate holder was provided with a to and fro movement above thetarget for homogeneous deposition of the film over the substrate. A

Fig. 6. Arrhenius plot of ionic conductivity of LiPON thin film vs. temperature.(Deposition conditions: rf power density of 3 Wcm−2, nitrogen flow of 30 sccm).

base vacuum of 1×10−6 mbar was obtained with a turbo molecularpump backed by a rotary pump. The powder compact target was pre-sputtered for an hour to remove any hydrocarbons present and also tosinter the top layers of target locally. Substrate temperature was notcontrolled during deposition, but was observed to rise up to 110 °Ctowards the end of deposition due to plasma heating. The depositiontime of 40 min was kept constant for all the depositions used in this

Fig. 7. XPS survey spectra of LiPON film and Li3PO4 powder used as target. (Depositionconditions are, rf power density of 3 Wcm−2, N2 flow of 30 sccm).

Fig. 8. (a) Core level spectra of P 2p region showing the shift in binding energy (B.E.) ofP 2p for film and Li3PO4 powder used as target, (b) C 1s region of LiPON film and Li3PO4

powder.

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study. Rf power density was varied from 1.7 Wcm−2 to 3 Wcm−2 andN2 flow from 10 to 40 sccm in order to optimize the processingconditions.

The crystal structure of Li3PO4 powder and films was characterizedby X-ray diffraction (XRD) using Bruker D8 Advance (Cu-Kα radiation,λ=1.5405 A0). A SIRION 200 field emission scanning electronmicroscope (FESEM) was employed to investigate the surfacemorphology and cross sectional microstructure of LiPON films. Allthe samples were coated with a thin Pd/Au layer to reduce surfacecharging effects.

X-ray photoelectron spectroscopy (XPS) analysis was performedwith SPECS GmbH spectrometer (Phoibos 100 MCD Energy Analyzer)using MgKα radiation (1253.6 eV). The residual pressure inside theanalysis chamber was in 10−10 mbar range. The spectrometer wascalibrated using photoemission lines of Ag (Ag 3d3/2=367 eV withreference to Fermi level). Peaks were recorded with constant passenergy of 40 eV. Because of surface charge induced peak shifts, C 1s at284.6 eV was taken as a reference energy position to correct the shift.The N 1s peaks were resolved using a peak synthesis program inwhich a non-linear background was assumed. The synthetic peakswere defined with a combination of Gaussian and Lorentziandistributions with a fixed FWHM of 1.8 eV.

The LiPON film thickness was determined from Dektak 150 stylusprofilometer. Ionic conductivity of the films was obtained from theAC-impedance measurement of Pt/LiPON/Al sandwich structures(Fig. 2) fabricated on Pt coated silicon substrates with a LiPONthickness of 1 μm and Al top layer deposited by thermal evaporationto an area of 2×2 mm. The test cells showed an open circuit voltage of

1.6 V. The AC-impedance measurements were performed at roomtemperature and at higher temperature (27 °C to 130 °C) using a Bio-Logic SA potentiostat/Galvanostat (model: VPM3) at frequencies from1 Hz to 100 KHz.

3. Results and discussion

X-ray diffraction patterns of pure Li3PO4 powder and LiPON filmdeposited on silicon wafer are shown in Fig. 3. While Li3PO4 powderhas various crystalline phases, LiPON film did not exhibit any peaks,indicating amorphous nature of the film. This is advantageous forbattery applications since the ionic conductivity of amorphous films isgenerally more isotropic and higher than that of single crystal ortextured polycrystalline films [26].

Fig. 4(a) shows the surface micrograph of as deposited LiPON filmon Pt coated silicon and the cross-sectional view of Pt/LiPON/Alsandwich structure in Fig. 4(b). The as deposited LiPON film is smoothwithout any cracks or pin holes on the surface. Also at the interface itmakes a clean contact with the top and bottom metal layers,minimizing interfacial resistance between the layers.

Li+-ion conductivity of LiPON films was measured by electro-chemical impedance spectroscopy. Complex impedance of each of thetest pads was measured in 1–105 Hz frequency range at roomtemperature. The impedance obtained is a characteristic of a single-phase ionic conductor with blocking electrode configuration. Theionic conductivity was calculated from the electrolyte resistance Rel

(which is the real part of impedance ZRe value at selected frequency inwhich −Zim goes through a local minimum-Yu method) using therelation,

σ = 1= Rel × d = A

whered is the thickness andA is the surface area of contact of LiPONthinfilms [27]. There have been contradictory reports on the effect of rfpower density on ionic conductivity of LiPON film. Earlier Choi et al.[28] reported that ionic conductivity of sputter deposited LiPON filmswas inversely proportional to rf power density, whereas, studies byRoh et al. supported a directly proportional relation of ionicconductivity with rf power density [29]. Our experiments showed anincreased ionic conductivity with increase in rf power density inconformitywith the study of Roh et al. The bode plot representation ofionic conductivity obtained from LiPON films deposited with differentrf powers is shown in Fig. 5(a–d). It can be seen that as the rf powerdensity is increased from1.7 Wcm−2 to 3 Wcm−2, there is an increasein the ionic conductivity from 2.3×10−9 Scm−1 to 1.1×10−6 Scm−1.According to linear fit of Arrhenius equation, impedance analysisshowed an increased ionic conductivitywith increase inmeasurementtemperature (Fig. 6). The activation energy Ea of LiPON has beencalculated using the equation,

ln σTð Þ = ln σ0Tð Þ−Ea = kT

where Ea is the activation energy, σ is ionic conductivity, T istemperature in Kelvin and k is Boltzmann constant. Activation energywas found to be 0.44 eV for the LiPON film deposited at optimizedconditions.

In some of the reported studies on LiPON thin films, the rate ofdepositionwas very low from the ceramic target and hence long hoursof deposition were employed [20,30]. But we have observed that filmsof 1.2 μm thick can be deposited from powder target of Li3PO4 in N2

plasmawith a deposition time of 40 min. Here the rate of deposition isaround 30 nm/min, which is 15 times higher than the sputtering ratefrom ceramic Li3PO4 target. In order to remove an atom from a targetsurface, energy greater than surface binding energy (Esurf) has to besupplied. Since for the powder target, atoms are loosely confined tosurrounding atoms, the energy required to remove it from the lattice

Fig. 9. N 1s XPS spectra with component analysis showing triply coordinated nitrogen as Nt and doubly coordinated nitrogen as Nd of LiPON films deposited with 4 different rfpowder densities, (a) 1.7 Wcm−2, (b) 2.2 Wcm−2, (c) 2.6 Wcm−2, (d) 3 Wcm−2.The solid symbols represent the raw data and smooth curve represents the fitted data.

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site is less compared with sintered target, which is relatively tightpacked. Thus, the higher deposition rate obtained from the powdertarget compared to a ceramic target can be due to less binding energyassociated with the powder target. Also the microscopic unevennessof top surface layer leads to higher effective surface area of powderparticles which contributes to high deposition rates.

Fig. 10. Ionic conductivity and Nt/Nd ratio of LiPON thin films as a function of rf powerdensity.

To investigate the chemical nature of deposited films, XPS analysisof LiPON thin films deposited from powder target was done to provideinformation about elemental bonding environment. Estimated errorin the calculated chemical composition can be around 10% for multi-elemental compounds [31]. Survey scans of Li3PO4 powder and LiPONfilms done from 1100 to 10 eV clearly depict the incorporation of

Fig. 11. Ionic conductivity and Nt/Nd ratio of LiPON thin films as a function of nitrogenflow.

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nitrogen in to LiPON film, which was otherwise absent in powdersample (Fig. 7).

Studies with XPS showed a P 2p peak shift of Li3PO4 from 134.5 to132.8 eV for LiPON films due to nitrogen incorporation [32]. Thisreduction in binding energy is attributed to the replacement of P–Obonds by P–N bonds which change the charge distribution aroundphosphorus in thin films. Core level spectra of P 2p for both film andpowder samples are shown in Fig. 8(a) for comparison. Fig. 8(b)shows the corresponding C 1s peaks from both Li3PO4 powder andLiPON film. Also the study of amorphous phosphorous nitrides byVeprek et al. suggested nitrogen incorporation as doubly (–N=) andtriply coordinated (–N≤) state [33]. Our studies on LiPON films,revealed nitrogen incorporation in to Li3PO4 as both doublycoordinated ‘Nd’ (peak at, 399.4 eV), or triply coordinated ‘Nt’, (peakat, 400.8 eV) manner. Resolving N 1s spectrum of LiPON filmsdeposited with different rf power densities, into two componentsand measuring the area of Nt and Nd gives a quantitative measure ofeach. Fig. 9(a–d) shows N 1s spectrum and its component analysis forthe films deposited with different rf power densities of 1.7, 2.2, 2.6and 3 Wcm−2. A clear trend on increase in Nt is observed withincrease in rf power.

The effect of increasedNt/Nd ratio on ionic conductivity of LiPONfilmwith the increase in rf power density is plotted in Fig. 10. Due to thehigher ionic radius of N3− comparedwithO2−, the nitrogen substitutionin LiPON film for oxygen in Li3PO4 induces structural distortions, whichin fact improves the ionic conductivity and stability of LiPON. Thereduction in electrostatic energy, once the P–O bond is replaced by amore covalent P–Nbond, lowers the activation energy of Li±mobility inthe defect lattice. It was suggested that more structural distortioninduced by cross linked Nt than Nd in the LiPON films, consequentlyimproving ionic conductivitywith increased triply coordinated nitrogenNt [29,34]. For an rf power density of 1.7 Wcm−2, Nt/Nd ratio obtainedwas 0.27 with an ionic conductivity of 2.3×10−9 Scm−1. But as the rfpower density increased to 3 Wcm−2, Nt/Nd ratio improved to 1.42 andthe ionic conductivity also increased to 1.1×10−6 Scm−1. Howeverincrease of rf power density beyond 3Wcm−2 was difficult as thepowder particles of the target begin to splash to growing thin filmsurface.

Other than the rf power density, nitrogen flow rate is an importantprocess parameter that governs ionic conductivity of LiPON films [18].We have selected four different flow rates of 10, 20, 30 and 40 sccm ofnitrogen to fine tune the conductivity obtainedwith rf power density of3 Wcm−2. The dependence of ionic conductivity onN2flow rate andNt /Nd ratio obtained from XPS analysis is shown in Fig. 11. Initially ionicconductivity increases from 7.2×10−9 Scm−1 to 1.1×10−6 Scm−1 fora flow rate increase of 10 to 30 sccm. But for 40 sccm of N2, thedeposition rate itself reduces due to increased scattering of sputteredspecies and conductivity reduces to 4.7×10−7 Scm−1. With theincrease in the nitrogen flow, the incorporation of nitrogen into thelithiumphosphatematrix in thefilmaswell as the source also increases.Further addition of nitrogen into the source results in a reduction in thesputtering rate. It was observed that the deposition rate was increasedfrom15 nm/min to30 nm/min as thenitrogenflow increased from10 to30 sccm.At aflowrate of 40 sccm, thedeposition rate reduced to 20 nm/min and this effect was seen in the incorporation of nitrogen in the film.It has been also confirmed by XPS studies that a film deposited with40 sccm of nitrogen flow has lesser Nt/Nd ratio. Themaximum obtainedionic conductivity from this study is 1.1×10−6 Scm−1, which is lowcompared to the best reported ionic conductivity (3.3×10−6 Scm−1 ofLiPON films by Hu et al. [20]). However, LiPON films with an ionicconductivity of 4.5×10−7 Scm−1 has already been demonstrated assolid electrolyte of a LiCoO2/LiPON/Li TFB with a discharge capacity of59 μAh cm−2μm−1 and good capacity retention [35].

4. Conclusions

In this study we have shown that, sputtering from powder targetcan be useful for certain compounds like Li3PO4 in which breaking ofceramic target and material loss are severe problems. The ionicconductivity of LiPON films formed was in relative good agreementwith previously reported values with a higher deposition rate. Theeffects of rf power change and N2 flow during deposition are studiedin detail. It is seen that with increase in rf power density, ionicconductivity is increased and for increased nitrogen flow, there is anincreased ionic conductivity for 10 to 30 sccm but reduces for higherN2 flow of 40 sccm. Amorphous nature of the films during depositionprocess has been ensured and verified through XRD. Incorporation oftriply coordinated nitrogen enhances the ionic conductivity which hasbeen confirmed by XPS and AC impedance analysis. A maximum ionicconductivity of 1.1×10−6 Scm−1 was obtained for an Nt/Nd ratio of1.42 for 3 Wcm−2 and N2 flow of 30 sccm.

Acknowledgement

The authors acknowledge DRDO, Govt. of India for funding thiswork.

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