Extended Abstracts of the 1992 International Conference on Solid State Devices and Materials, Tsukuba, 1992, pp.184-186

0.4 pm Contact with Lower Resistivity Metallization Technologyfor ULSI Applications

M.Sekine, N.Itoh, T.Akimoto,T.Shinmura, *D.T.C. Huo,Y.Kakuhara, K.Kaj iyana, Y. Yamada, K.Yam azaki, and Y. Murao

VLSI Development Division NEC CorporarionI 1 20 S hi mokuzawa S agamihara, Kanagaw a 229, Japan

Phone: (M27)71-0672

600 Mounta'" oT*: "Jlf"i?liiftn Jersey 07 s7 4Phone: 908-582-7892

This paper describes one of the most promising technology for 0.4pm contact with lower contactresistance using blanket W-CVD. Coherent Ti sputtering follo*ed by rapid thermal nirization(RTN) wasapplied as a glue layer for blanket CVD-W. Even for 0.4pm contai:t with aspect ratio of four, coherentsputtering keeps 20Vo bottom coverage of Ti and reduces the p+Si contact resistance to 200Q/via ascompared-with conventional sputtering which results in open failure. Furthermore, we applied specialchemical $V etchigg-technique to remove damaged Si layei which was induced by window 6tching-. Thisdamaged luygl will degrade the reacrion at the metal/Si interface and cause a hilher contact resiJtance.Combined with coherent sputtering, contact resistance can be reduced to 130C/via and contact resistivityof 1.3x10-7Ocm2 for p+5i contact was realized. This value is in good agreement with the theoreticalvalue for TiSix/p+Si. Thus, t!1s pcllnglggy can be readily implement to the manufacturing of quartermicron IJLSI devices such as 256Mbit DRAM.

PA4.4

Conditions of W-CVD for nucleation were asfollowing: WFO/S iHa flow ratio=3, totalpressure=300mTorr, and substratetemperature=400oC. Conditions of W-CVD for contactfilling were as following: H2|TVF6 flow ratio=6, totalpre s SUre= 3 0Torr, and su b strate temperanrre=4OOoC.

The TiN glue layer for the blanket W-CVD wereapplied by coherent sputtering of Ti and then followedby the rapid thermal nitrization(RTN). During coherentsputtering, a collimator was placed between the targetand the deposited wafer to minimize the angledistribution of sputtered material. The conventionalsputtering method was also used to prepare Ti and TiNfilms for comparison with those films prepared by ourimproved processes described in this paper.

Chemical dry etching was used to remove thedamaged Si layers caused by window etching.Chemical dry etching following window etching wasperformed usin g CF a,/ Oz chemistry.

RESULTS AND DISCUSSIONFig. 1 shows the bottom coverage versus the

contact diameter for Ti glue layers prepared by thecoherent spultering and the conventional spuffaringtechniques. The thickness of dielectric layeri used inthis experiment was 1.6pm. Even for the fabrication of9.+pry contact with an aspecr ratio of four, the gluelayer bottom coverage is about four times larger in the

INTRODUCTIONDeep sub-half micron contact filling is the most

lmpgrtant technology for ULSI manufacruring. BlanketW-CVD is one of the most promising candidate forsub-half micron contact because of its excellent stepcoverage. W-CVD needs a glue layer which is usuallydeposited by PVD to suppress the reacrion between Siand the filling material and it also serves as a contact tothe diffusion layer. Unfortunately, the contactresistance increases as the aspect ratio in windowincreases because the conventional PVD technique cannot provide a sufficiently thick glue layer on the bottomof the contact. Furthermore, damaged layer at Sisurface induced by the contact fabrication piocess candegrade the reaction between metaVsi interface and willincrease the contact resistance as the window sizedecreasesl)'2).

In this study, we have developed a noveltechnology using a combination of two techniques:(l)the coherent Ti sputtering technique to improve thebottom coverage of Ti glue layer and (2)the Si surfacecleaning technique using chemical dry etching(CDE) toremove the damaged Si layers. These techniques canyigl4 a lower contact resistance even for the 0.4pmwindow with an aspect ratio of four.

EXPERIMENTALContact resistance and junction leakage were

measured using contact,iurays with diameters rangingfrom 0.4 to 1.2pm. Contact holes having depthsbetween 0.3 and 1.5pm were defined i; BPSGdielectrics on p* and p+ junctions. Additionalimplantation of P and BF2 were performed after contacrhole definition. The main process conditions aresummarized in table 1. Both the excess W and gluelayers were etched away by the etchback process uiingSF6 and Cl2 chemistry3).

184

As,70keV,5E15cm-2

P,70keV,5E15cm-2

BF2,70keV,5E15cm-2

BF2,70keV,5E1Scm-2

coherent sputtered Ti films thanthe conventional sputtering.

in those prepared by

Contact Depth=l.6UmI Coherent sputteringO Conventional

I

frtII

ooo

-to0.2 0.4 0.6 0.8 1.0 1.2 1.4

CONTACT DIAMETER (trm)

Fig. I Dependence of Ti bottom coverage on diflercnt conuctdiametcr.

-f- Conventional

--'-l- CoherentSputtering

.2 0.4 0.6 0.E 1.0 r.2 l.CONTACT DIAMETER (pm)

-lr- Conventional

-*- CoherentSputtering

.2 0.4 0.6 0.8 1.0 1.2 1.4CONTACT DIAMII'I'B,R (pm)

Fig.2 Contact resistance measurement. results for n+ and p+ g1

using coherent sputtering compared with conventional sputtering.

Fig. 2 shows the dependence of contact resistanceon window size using coherent sputtering andconventional sputtering techniconventional sputtering techniques. The Ti films were1000A thick. The TiN layers were formed by RTN offormed by RTN of

sufficient bottom coverage of Ti glue layers to stabilizethe contact resistance even for this small feature size.

The junction leakage for n+Si contact measurementresult is shown in fig. 3. While recent investigationshave reported that TiN film formed by nitrization of Tidoes not always have a good barrier property4),5), ourresults showed a consistently good barrier propertywith a very low junction leakage current. This can beexplained by our process optimization for W-CVDnucleation6).

10'4

1o-6

1o-8

1o'10

ro'11B|AS VOLTAGE (V)

Fig 3 Junction leakage measurement result. for n+Si usingcoherent sputtering.

p+ Si contact

. Without CDEI With CDE

0 100 200 300 400 500 600^700tloTToM rHrcKNEss oF Ti (A)

Fig. 4 Dependence of 0.4pm contact resisrance on thickness of Tiat he bottom of contact. and on surface treatment using CDE.

Fig. 4 shows the dependence of contact resistanceon thickness of Ti glue layer at rhe bottom of 0.4pmcontacts with and without chemical dry etch(CDE)treatment of Si surfaces. Contact resistance increased asthe Ti thickness decreased. This explains why thecontact resistance was decreased by using coherentsputtering as compared with the conventionalsputtering. Chemical dry e.tching(CDE) stabilizes thecontact resistance for 100A Ti films but the effect isminimal for the much thicker, e.g. 600A, films. Fig. 5shows the density of state(DOs) XPS specrra from Sisurface after the window etching and CDE followingwindow etching. Density of near Fermi level(Ep) wasdecreased by the window etching and increased by theCDE. This is the reason why contact resistance can beimproved by" using CDE. This result shows that in thecase of 100A thick Ti glue layers, the differences incontact resistance were caused by the differentelectronic states of Si surfaces similar to the case of

G' 80

;rli

Y60t4.Ir-'l-l!

b40U

c20FFca0

't

,_r 200

zF-cn

H loo&FOuo0U

SF2utGE=()ul(tvr!

3' 4oo

; 3oo

71

Fe 2oorrl&

b roo

F7-8o

1512

-3 4oo

Ff 3ooz,'ct 200o

. 100FU

-20o0

Ti in a nitrogen ambient ar 700oC. The depth of contacrwas 1.5pm. During fabrication of 0.4prm conracts,coherent sputtering of glue layers resulted in significantimprovement in the contact resistance of about200Q/via fo1 p+Si while rhe conventional sputteringprocess resulted in open failure. This result suggeststhat the coherent sputtering process has achieved a

I I t I

/,/

-/-/

185

il-G'

F-(nzt{F.zzct)(t)

rd

selective W-CVDI),2). These phenomena suggest thatTi silicidation at Ti/Si interface was degraded by thedamaged Si layer, and a part of Ti was consumed toreduce the damaged Si layer ar the initial stage ofsilicidation during RTN. Thus, the surface statQ effectwas more profound ar thinner Ii thickness(100A) thanthe thicker ti thickness(6004.). The chemicat aryetching developed in this experiment can improve the Tisilicidation at the Tilsi interface and stabilize the contactresistance. It can also reduce the required critical gluelayer thickness which will simplify the depositionprocess.

Fig. 6 shows the improvement in the contactresistance for p+Si by a combining of coherentsputtering and chemical dry etching techniques. Thecontact resistivity was calculated to be 1.3x10-7cm2from the slope of linear relationship between contactresistance and the area of contact window. Thisresistivity value is in good agreement with thetheoretical value of TiSix/p+Si.7)

20 16 12 8 4 O(En)

BINDING ENERGY (eV)

Fig.5 Density of valence electron spectra from Si surfacemeasured by using XPS.

p+ Si---t- Without CDE

--# WithCDE

.2 0.4 0.6 0.8 1.0 t.2 L.4CONTACT DIAMETER (pm)

Fig. 6 Improvement of p+Si contact resistance usingcoherent sputtering and CDE.

CONCLUSIONIn conclusion, we have achieved a stable contact

resistance near the theoretically lowest value in 0.4pmcontacts by an innovative use of (1)coherent sputteringof Ti glue layers and (2)chemical dry etching inremoving the damaged Si layers. The coherentsputtering process improved the bottom coverage of Tiglue layers and reduced contact resistance. Chemicaldry etching improved the Ti silicidation ar rhe TVSiinterface and stabilized the contact resistance. Thecombination of these two techniques solved thedegradation of contact resistance for small contacts witha large aspect ratio. This technology can improving thecontact reliability and it can be readily implemented tothe manufacturing of quarter-micron ULSI devices suchas 256Mbit DRAM.

ACKNOWLEDGMENTAuthors wish to thank Drs. M.Kamosida,

A.Morino, M.Ogawa and O.Kudoh from NECCorporation and P.T.Panousis and J.T.Clemens fromAT&T Bell Laboratories for their encouragementthrough this work. They also wish to thank Mr. K.Yamanaka for lithography.

REFERENCES1) M. Sekine et al., Proceedings 8th International IEEEVLSI Multilevel Interconnection Conference, p.335,1991.2) M. Sekine et al., Proceedings of the 1991 Workshopon Advanced Metallization for ULSI Applications,p.255, 1991.3) T. Akimoto et aI., Proceedings of the fall meeting ofthe Japan Applied Physics Society Conference, 1990,28a-SZD-21.4) M. Rutten et al., Proceedings of the 1991 Workshopon Advanced Metallization for ULSI Applications,p.277, 1991.5)R. C. Ellwanger et al., Proceedings of the 1991Workshop on Advanced Metallizarion for ULSIApplications, p.50L, L991.6) M. Sekine et al., IEICE Technical Report, 9lNo.332 (1991) p.19.7)A. Y C. Yu, Solid-State Electronics 13(1970)p.239.*Work was performed at VLSI Development Division,NEC Corporation while D.T.C.Huo srationed there asan exchange engineer during the NEC-AT&TMicroelectronics Joint Development Program.

300

't

; 2oozFqon L00F-OE-z8o

0

WITHOUT CDE

186