E L S E V I E R Thin Solid Films 305 (1997) 103-109

thfin o

Effects of deposition parameters on composition, structure, resistivity and step coverage of TiN thin films deposited by electron cyclotron

resonance plasma-enhanced chemical vapor deposition

J o n g - S e o k K i m a, E u n g - J i k L e e a, J o n g - T a e B a e k b, W o n - J o n g L e e a

a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KA1ST), Taejon, South Korea b Semiconductor Technology Division, Electronics and Telecommunications Research Institute (ETRI), Taejon, South Korea

Received 2I August 1996; accepted I7 February 1997

Abstract

TiN thin films of high quality--low impurity content, high crystallinity and low resistivity--were prepared by electron cyclotron resonance (ECR) plasma-enhanced chemical vapor deposition (PECVD) at low temperatures using TIC14, N 2 and H 2. The effects of gas flow rate, microwave power and temperature on the composition, structure, resistivity and step coverage of the film were studied. Proper control of the H 2 flow rate was important in lowering the resistivity and impurity content of the films and enhancing the step conformality of the deposits. Increases in deposition temperature and in microwave power decreased the deposition rate, resistivity and impurity content of the film. Chlorine was below the detection limit of Auger electron spectroscopy (approximately 0.1 at.%), even at a deposition temperature of 30(310) °C. TiN film deposited at 250(400) °C showed a resistivity of 96 ixl2 cm and a bottom coverage of 47%. © 1997 Elsevier Science S.A.

Keywords: Chemical vapour deposition (CVD); Plasma processing and deposition; Titanium nitride

1. Introduction

TiN film is used as a diffusion barrier layer for contact holes in the metallization process of microelectronics ow- ing to its high thermal stability, low electrical resistivity (approximately 20 i~f~ cm for bulk material) and good diffusion barrier properties [1]. In most cases TiN film is prepared by sputtering. As the manufacture of submicrom- eter generation devices requires the metallization of small and deep contact and via holes with high aspect ratio (AR), the limited conformality of sputter-deposited TiN films produces problems; the limited coverage of the barrier at the contact base may not yield adequate protection and the self-shadowing of deposits creates a re-entrant profile of the barrier layer at the opening of the contact hole, thus restricting complete filling with conducting materials [2]. Therefore, there is a substantial interest in replacing sput- tering with chemical vapor deposition (CVD) which offers good step coverage.

Low pressure chemical vapor deposition (LPCVD) us- ing TiC14 and NH 3 can improve the conformality and lower the film resistivity but it requires a high deposition temperature of 600 °C [3]. Metal-organic (MO) CVD

using TDMAT and TDEAT can lessen the stress in the film and is a low temperature process, but it also has drawbacks such as high impurity contents, high resistivity and poor conformality [3,4]. In this context, plasma-en- hanced CVD (PECVD) using TiC14 is suitable for the deposition of TiN films which have good conformality and low impurity content at a low deposition temperature [5].

The conventional r.f. PECVD process, however, suffers from particle problems arising from homogeneous reac- tions in the gas phase, and cannot provide films of low resistivity below the deposition temperature of 500 °C [6]. A deposition process utilizing electron cyclotron resonance (ECR) plasma, which has a high plasma density (ap- proximately 1011 cm-3), low energy (10-40 eV) ions bombarding the substrate and high electron temperature (5-10 eV) at low pressure (0.1-10 mTorr), compared with r.f. plasma, prevents homogeneous reactions and can solve the particle problem [7]. Moreover, the high degree of gas decomposition and high concentration of excited species and radicals enable the deposition of low resistivity TiN films at a lower temperature.

We prepared TiN films by ECR-PECVD using TIC14, N 2 and H 2 as reactants with variations in the H 2 flow rate,

0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0040-6090(97)00116-8

104 J.-S. Kim et aL / Tttin Solid Fibns 305 (1997) i03--109

microwave power and deposition temperature. The effects of the deposition parameters on the composition, structure, resistivity and step coverage of TiN films were studied.

2. Experimental details

A schematic diagram of the ECR-PECVD system used in this study is presented in Fig. 1. It consists of a magnetron system, a magnetic system, a gas feeding sys- tem, a discharge chamber, a reaction chamber and a high pumping system. TiC14 and N 2 gases were used as reac- tants and H 2 gas as reducing agent. N 2 and H 2 gases were introduced into the discharge chamber to generate the plasma, and TiC14 gas was introduced into the reaction chamber through a ring-type gas distributor. The TiC14 container was kept at room temperature (equilibrium vapor pressure of about 9 Tort) and the TiC14 gas flow rate was controlled precisely by a vapor source mass flow controller (VFC). The substrate was heated by a graphite resistive heater and the temperature on the surface of the substrate was measured using a SensArray S / N 6396. The substrate temperature increased with the deposition time because of the heating effect generated by energetic ion bombard- ment. Fig. 2 shows the changes in substrate temperature as a function of plasma exposure time. In this study, TiN films were deposited for 5 min. The substrate temperature increased during the deposition process and was almost saturated after 8 -9 min as shown in Fig. 2. Therefore the deposition temperature is represented by A ( B ) °C, where A refers to the initial wafer temperature before exposure to the plasma and B refers to the wafer temperature after exposure to plasma for 5 min. The deposition conditions are summarized in Table 1.

The chemical composition of the film was analyzed

2.45 GHz Mlcl'OWaV¢

Disdm'ge | / Q u ~ liner e . ~ Qlkartz window

I * , u u , ' n " n I S~l't°~ - ' t , . l

TiCl4l I I h i . . I load-lock Reaction

! I system

Fig. 1. A schematic diagram of the ECR-PECVD system for TiN film formation.

800 , , 1 . . . .

v 500 ~ , . _ _ _ _ ~ _ _ _ . - - - - m ~ m O.___._.--~ ~ H £3

° "

400

3oo °-- - - -°

200 / I o . ~ o .

100 ,o I i

0 2 4 6 8 10

Exposure time (rain)

Fig. 2. Changes in the substrate temperature as a function of plasma exposure time.

using Auger electron spectroscopy (AES, Perkin-Elmer SAM 4300) with an incident electron energy of 5 keV and modulation voltage of 4 V. The samples were sputtered at about 12 nmmin -1 with an Ar ion beam acceleration voltage and current density of 3 keV and 300 /.LAcm -2 respectively. The compositional ratio of Ti and N and the impurity concentrations within the film were calculated using the relative sensitivity factor of each element with respect to nitrogen. The peak-to-peak height of N around 383 eV was extracted from the Ti + N overlapped peak in the AES spectra adopting the method proposed by Hof- mann [8]. From the AES spectra of standard Ti and TiN samples, the relative sensitivity factor of Ti vs. N, STi/SN, can be calculated. 'The standard TiN sample used for the quantitative analysis in this study was measured to contain about 5% oxygen, meaning that the standard TiN sample is N deficient. On the assumption that oxygen exists in the form of TiO 2 within the standard TiN sample, the peak- to-peak height of N was calibrated and S . n / S ~ was calcu- lated; the STi /S N was 0.65. The relative sensitivity factor of C1 vs. N, Sa/Sr~ = 0.50, was calculated from Ruther- ford backscattering spectroscopy (RBS, NEC 5SDH-II) compositional analysis. The relative sensitivity factors of O and C, S o / S N = 1.82 and S c / S ~ = 0.63, were obtained from the AES handbook [9]. The thickness of the TiN film was measured using an c~-step profilometer. The resistivity of the TiN film was calculated as thickness times sheet resistance measured by a four-point probe. To minimize the variation of the resistivity with the thickness of the film, the thickness of the film was fixed in the range

Table 1 Deposition conditions of TiN thin films

Pressure (mTorr) 2.5-5 Deposition time (min) 5 Deposition temperature ( ° C ) 30(310)-450(520) Microwave power (W) 400, 700, 1000, 1300 TiC14 flow rate (standardcm 3 rain- t) 2 N z flow rate (standardcm 3 rain- 1) 90 H 2 flow rate (standardem 3 rain- 1) 0, 25, 50, 75

J.-S. Kim et al. / Thin Solid Films 305 (1997) 103-]09 105

60

50

o

40 e-

8 : g

10

, = - ~ - r - - - - = 0 25 50 75

H 2 f low rate ( sccm )

Fig. 3. Chemical composit ion of TiN films as a function of H 2 f low rate.

Deposi t ion conditions: 450(520) °C, 1300 W, TiC14:N 2 = 2:90 s tandardcm 3 m i n - t

85-100 nm. The crystallinity of the TiN film was investi- gated using an X-ray diffractometer (XRD, Rikagu Inc.) with Cu Kc~ radiation source. The X-ray beam was fixed at a glancing angle of 2 ° from the specimen surface. The step coverage of the TiN film was observed with scanning electron spectroscopy (SEM, JEOL JSM 5200) after the TiN film was deposited onto a 1.5 I~m X 0.4 I~m (AR = 3.8) contact hole.

3. Results and discussion

TiN films were deposited at various H 2 flow rates ranging from 0 to 75 standard cm 3 rain -1. The deposition temperature, microwave power, and gas flow rates were 450(520) °C, 1300 W and TiC14:N 2 = 2:90 standard cm 3 min - t respectively. The chemical composi- tion of TiN films was analyzed by AES and the results with respect to the H 2 flow rate are shown in Fig. 3. The compositional ratio of N to Ti is between 1.01 and 1.03 for the H a flow rates. Without H 2, the oxygen and carbon contents within the films are 1.5 and 1.6 at.% respectively. As the H a flow rate increases, the oxygen and carbon contents within the films decrease to the detection limit. It has been reported that ECR hydrogen plasma helps the removal of oxygen and carbon [10,11]. In the present work, it is likely that H 2 in the plasma reduces the oxygen and carbon contents. However, a further increase in the H o_ flow rate to 75 standard cm 3 min-1 causes an increase in the oxygen and carbon contents to 1.0 and 1.6 at.% respectively. To understand this phenomenon, the charac- teristic emission of the ECR plasma at each H 2 flow rate was examined using optical emission spectroscopy (OES). The OES spectra are shown in Fig. 4. The peak at 656 mn is from hydrogen and the peaks around 330-430 nm are from nitrogen. The hydrogen peak intensity rises with the hydrogen flow rate while all the other peaks decrease. This indicates that hydrogen cools the plasma. The increased impurity contents at a high H 2 flow rate result from the cooling effect of hydrogen. With increasing hydrogen in-

N2 N+ (a) H: = 0 sccm

I

=. N2 N + ~ CO) H 2 = 25 s c c m

..... ,~ ~.~,~. N2 (c) H 2 = 75 sccm

300 400 500 ' 600 700

Wavelength (nm)

Fig. 4. Optical emission spectra of ECR plasmas as a function of H 2 flow

rate,

put, the plasma becomes less activated, which results in insufficient decomposition of the reactants. These two effects of hydrogen, reducing the impurity content and cooling the plasma, constrain the appropriate H 2 flow rate. The chlorine content is below the AES detection limit both with and without H 2 flow.

The resistivity of TiN films as a function of the H 2 flow rate is shown in Fig. 5(a). When the H 2 flow rate is 0 standard cm 3 min -1, the resistivity of TiN films is 80 txf~cm. As the H 2 flow rate increases to 25 and 50 standard cm 3 min -1, the resistivity decreases to 46 txf~ cm then increases to 55 ixf~cm when the H 2 flow rate increases further to 75 standard cm 3 min - I . The impurity content and film stoichiometry are the most important factors determining the resistivity of TiN thin film; other factors such as the thickness, preferred orientation, mi- crostructure, and porosity are also related [1,12,13]. Fig. 6 shows the XRD patterns of TiN films for various H 2 flow rates. TiN films deposited with H 2 are crystallized with strong (200) preferred orientation, whereas the film de- posited without H 2 is also crystallized but grows with random orientation. Fig. 7(a) and (b) shows SEM and TEM images of the TiN films deposited without and with

150 ~ ~ ~ , 25

120

E O

90

• .= 60 t/)

30

° ~ o (b) ° ~ °

e

J

20 C2

O

S" 3

0 25 50 75

H 2 Flow Rate (sccm)

Fig. 5. (a) Resistivity and (b) deposition rate of TiN films as a function of

H 2 flow rate. Deposit ion conditions: 450(520) °C, I300 W, TiCI4:N 2 = 2:90 s tandardcm 3 m i n - 1.

106 J.-S. Kim et al. / Thin Solid Fihns 305 (]997) 103--109

(200) H 2 = 0 sccm 1

(1~1) t Si (220)

H 2 = 25 scc rn

__>, ao

H 2 = 50 sccm

e"

H 2 = 75 sccrn

20 30 40 50 60 70 80

Diffraction angle (29) Fig. 6. XRD patterns of TiN films as a function of H 2 flow rate. Deposition conditions: 450(520) °C, 1300 W, TiC14:N 2 = 2:90 standard cm 3 min - ?.

1 , 5

(um)

'>[0,4 ~.

,:L! q

H 2 respectively. The TiN film deposited without H 2 has a larger grain size (approximately 40 nm) and seems to have some porosity. The film deposited with H 2 has finer grain size (approximately 20 nm) and a dense structure. The low resistivity of TiN films deposited at H 2 flow rates of 25 and 50 standard cm 3 min-1 is ascribed to the low impurity content and the dense structure.

Fig. 5(b) shows deposition rate vs. H 2 flow rate. When the H2 flow rate increases from 0 to 50

Fig. 8. Magnified views of TiN fih-ns deposited at the bottom of the contact hole (t.5 ~m×0.4 p.m, AR = 3.8): (a) H 2 0 standardcm 3 min-~, (b) H, 25 standardcm "~ min -k

Fig. 7. SEM and TEM images of TiN films deposited (a) without H 2 and (b) with H 2 (25 standardcm 3 rain-i).

standardcm 3 rain - t , the deposition rate decreases from 19 to 17 nmmin -1. This is because dissociated chlorine [14,15] has an etching effect and because dense films with little impurity are formed. When the H~ flow rate in- creases to 75 standardcrn "~ rain -~, the deposition rate in- creases again to 20 nm rain -~ . This indicates the formation of less dense :films which results from the insufficient activation of precursors caused by the plasma cooling effect as shown in Fig. 4.

Fig. 8 shows magnified views of TiN films deposited at the bottom of the contact hole (1.5 b~m × 0.4 /xm, AR = 3.8). The effecl of hydrogen on the conformality of TiN films is ctearly shown in the figure. Without H 2, TiN film is hardly deposited onto the side wall. The conformality around the step edge is very poor. With H 2, the bottom coverage and side coverage are conformal, and conformal- ity at the step edge is very good. Raaijmakers and Sherman [1] also reported that the conformality of TiN films de- posited by LPCVD using H 2 as a dilution gas was good.

TiN thin fihns were deposited at microwave powers ranging from 4(?.0 to 1300 W at the deposition temperature,

J.-5, Kim et al. / Thin Solid Films 305 (1997) 103- ]09 Z07

60

5o

t - O

40 r-"

r'- O

lO

400 700 1000 1300 Microwave Power ( Watt )

Fig. 9. Chemical composition of TiN films as a function of rmcrowave power. Deposition c0flditions: 450(520) °C, TiCI4:N2:Hz = 2:90:25 stan- dard cm 3 min- t.

E 0

._z, >

rY

8OO

7OO

6OO

5OO

40O

300

200

100

0

i

)

O - . ~

~ O •

400 700 1000 1300

Microwave Power (Watt)

30

25 [D

2o O

15 .-, ®

10 "5" B

5 B &

Fig. 11. (a) Resistivity and (b) deposition rate of TiN films as a function of microwave power. Deposition conditions: 450(520) °C, TiC14:N2:H~ " = 2:90:25 standardcm 3 rain- i.

pressure and gas flow rates of 450(520) °C, 3 mTorr and

TiC14:N a:H 2 = 2:90:25 s tandardcm 3 m i n - I respectively. The chemical composit ion of the TiN films as a function of microwave power is shown in Fig. 9. The N to Ti composit ion ratio is 0 .97-1 .02 independent of the mi- crowave power. The chlorine content within the films is 2.5 at.% at 400 W and decreases below the detection limit above 700 W. The oxygen content within the films is 6.8 at.% at 400 W and 2.3 at.% at 700 W and is not detected above 1000 W. Carbon is not detected at all microwave power variations. As the decomposit ion and reaction of TiC14 and N 2 become more active and the reducing power of H 2 increases with microwave power, the amount of chlorine and oxygen impurities is reduced. Fig. 10 shows optical emission spectra of the ECR plasma at various microwave powers. The increase in plasma activation at higher microwave power is revealed as an increase in the intensity of the emission peaks in the figure.

The resistivity of the TiN films as a function of mi- crowave power is shown in Fig. 1 l(a). There is a sudden fall in the resistivity from 750 to 46 ~ f / c m when the

(,9 t "

t -

N2 (a) 700 Watt

N +

N2 (b) 1300 Watt

300 400 500 600 700 Wavelength (nm)

Fig. I0. Optical emission spectra of ECR plasmas as a function of microwave power.

microwave power increases from 400 to 1300 W. How- ever, the X-ray diffraction patterns in Fig. 12 indicate that the crystallinity of TiN fihns is not changed within this range of microwave power. Therefore, the decrease in resistivity at higher microwave powers can be explained in terms of the decreased impurity level and film densifica- tion.

Fig. 1 l(b) shows the deposition rate of TiN films as a function of microwave power. The decrease in deposition rate at higher microwave power is because of the densifi- cation of the films as a result of active decomposit ion of TiC14 and N a and reaction of the reactants on the sub- strate. I'n addition to densification, etching by chloric species formed during the deposition process contributes to reducing the deposition rate.

TiN thin films were deposited at deposition tempera- tures ranging from 30(310) °C to 450(520) °C. The mi-

v

t - ' o c-

m

(2OO)

,,~~ 400 watt

s~ (220)

700 watt

1300 watt

20 3'0 ' 4'0 ' 5~0 6~0 7'0 80

Diffraction angle (20)

Fig. 12. XRD patterns of TiN films as a function of microwave power. Deposition conditions: 450(520) °C, TiC14:Nz:H 2 = 2:90:25 standard cm s min - ~.

108 J.-S. Kim et a l . / Thin Solid Films 305 (1997) 103- t09

60

"-~ 5o

~" 4o 0

e - -

8 10 ¢-.

O o

'VT--T--' ' 'v---V---7~' ¥ ~ V " - ~ v 0 0 100 200 300 4 0 0 500

(a'lo) (4'00) (4~0) (5~_0) D e p o s i t i o n temperature (°C)

Fig. 13. Chemical composition of TiN films as a function of deposition temperature. Deposition conditions: 1300 W, TiC14:N2:H 2 =2:90:25 standard cm 3 rain- 1.

crowave power, pressure and gas flow rate were kept at 1300 W, 3 mTorr and TiCI4:N2:H 2 = 2:90:25 standard cm 3 rain -1. The chemical composition of TiN films as a function of deposition temperature is shown in Fig. 13. The N to Ti compositional ratio decreases from 1.08 to 1.01 and then becomes close to stoichiometry as the deposition temperature increases. Chlorine is not de- tected even at 30(310) °C. Chlorine is easily eliminated by the active reaction of reactants at 1300 W microwave power. Chlorine is reported to be rich in TiN films pre- pared by CVD methods below 500 °C [16]. When a TiN film was deposited at 30(310) °C, the oxygen content within the film was 1.2 at.% but a large amount of oxygen (approximately 8 at.%) was detected at the TiN-Si inter- face. Some microcracks were formed in the film deposited at 30(310) °C, probably owing to the large stress induced by the sharp change in the substrate temperature during deposition. The oxygen content is below the detection limit above 400(490) °C. Oxygen impurity, which comes mostly from outgassing of water vapor, is effectively eliminated at high microwave power (1300 W) and high deposition temperature (above 400(490) °C).

The resistivity of TiN films as a function of deposition temperature is shown in Fig. 14(a). As the deposition temperature increases from 30(310) to 250(400) °C, the resistivity decreases from 195 to 96 Ixf~ cm and is below 60 txO cm above 300(430) °C. The resistivity of TiN films deposited by ECR-PECVD is much lower than that of TiN films deposited by other methods: using collimated sputter- ing [17], LPCVD [3] and MOCVD [3,4], p is 100-120 ~f~ cm for collimated TiN, about 70 ~fZ cm for LPCVD TiN prepared using TiC14-NH 3, and 500-1000 Ixf~ cm for MOCVD TiN prepared using TDMAT-NH 3. LPCVD TiN has relatively low resistivity but requires a high process temperature, typically above 650 °C. Fig. 15 shows XRD patterns of the TiN films vs. deposition temperature. The crystallinity of TiN films does not vary with the

250 = , , ~ 25

20 2 0 0 ~ ~ ( ( ~ ) o 5 150 15 e

• loo (a) " -%. . . . . lo "s 3

n , 50 5 E]

0 - - ' I , I , I , I , 0 0 100 200 300 4 0 0 5 0 0

_ _ 1 ~ I r

(310) (400) (460) (520)

Deposition Temperature (°C) Fig. 14. (a) Resistivity and (b) deposition rate of TiN films as a function of deposition temperature. Deposition conditions: 1300 W, TiC14:N 2 :H 2 = 2:90:25 standardcm 3 rain -1 .

deposition temperature. The decrease in resistivity at higher deposition temperature can be explained by the decrease in impurity content within the films. TiN thin films deposited by LPCVD [13] and r.f. PECVD [6] using TiC14 are reported not to be fully crystallized below 500 °C. How- ever, TiN thin films deposited using an ECR plasma, in which the decomposition and reaction of reactants are very active and the elimination of impurities is effective, are thought to have strong crystallinity even at a low deposi- tion temperature of 30(310) °C. The deposition rate of the TiN films as a function of deposition temperature is shown in Fig. 14(b). As the deposition temperature increases from 30(310) to 450(520) °C, the deposition rate decreases from 20 to 17 nmmin - t .

Fig. 16 shows a cross-sectional SEM photograph of TiN film deposited onto a 1.5 Ixm X 0.4 /xm (AR = 3.8) con- tact hole at 250(400) °C. The conformality around the step edge is very good. The bottom coverage and the side

450(520) ~C

~" ~ as°(4ss)°c

(200/I 30(31 O) °C • "n si (220)

2o 3'0 4; 5; & 7'o 80 Diffraction angle (2e)

Fig. 15. XRD patterns of TiN films as a function of deposition tempera- ture. Deposition conditions: 1300 W, TiC14:N2:H 2 = 2:90:25 standard cm 3 rain- ~.

J.-S. Kim er al. /Thin Solid Fihns 305 (1997) 103-109 109

appropr ia te H 2 flOW rate was needed for the decrease in resistivity of the films and impurity content within the films and conformal deposition. The increase in deposition temperature and microwave power resulted in a decrease in deposition rate, resistivity and impurity content. Chlo- rine was not detected even at 30(310) °C. Chlorine was easily eliminated by the active reaction of reactants at 1300 W microwave power. TiN film deposited at 250(400) °C showed a resistivity of 96 txf~ cm and a bottom cover- age of 47%. The step coverage was better on the bottom than on the side wall, because of the anisotropy of the ECR plasma.

Acknowledgements

This research was supported by Korean Electronics and Telecommunications Research Institute (ETRI) through the future microelectronics project, 'Studies on Advanced Ba- sic Technology'.

Fig. I6. Cross-sectional SEM photograph of TiN fiim deposited onto a 1.5 ~xm×0.4 ~zm (AR=3.8) contact hole. Deposition conditions: 250(400) "C, I300 W, TiC14:N2:H 2 = 2:90:25 standardcm 3 min- i.

coverage of TiN film are 47% and 25% respectively. The bottom coverage of TiN films deposited by collimated sputtering is below 30% for a similar deposition rate and contact hole size [17]. The step coverage is better on the bottom than on the side wall, because of the anisotropy of the ECR plasma. It is considered that the thinner the TiN film on the side wall, the easier the contact hole filling in the following process.

4. Conclusion

TiN thin films, which are used in the very large scale integrated circuit (VLSI) process as a diffusion barrier, were prepared by electron cyclotron resonance (ECR) plasma-enhanced chemical vapor deposition (PECVD) us- ing TiC14, N 2 and H a as reactants. The effects of gas flow rate, microwave power and temperature on composition, structure, resistivity and step coverage were studied. An

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