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The effect of substrate orientation on WSi2 formation
Thin Solid Films, 238 (1994) 155-157
The effect of substrate orientation Awatar
155
on WSi 2
formation
S i n g h a n d W . S. K h o k l e
Microelectronics Area, CEERI, Pilani, Raj, 333031 (India)
K. Lal Materials Characterisation Division, N.P.L., Dr. K. S. Krishnan Marg, New Delhi, 110012 (India) (Received March 2, 1993; accepted July 21, 1993)
Abstract The effect of substrate orientation on tungsten silicide formation has been studied. It has been found that the silicide on Si(100) forms at low temperature, has fine grained surface morphology and has low sheet resistance as compared with that formed on Si(l I 1). We attribute this to the difference in microstructure of the films, due mainly to the difference in the nucleation process for the two substrates.
1. Introduction It has recently been shown by Singh et al. [1] that the lateral growth of nickel silicide depends on the nature of the silicon substrate. It is much greater for Si(100) than for Si( 11 I). This behaviour is attributed to the difference in microstructures [1, 2] o f the silicide formed. Motivated by this, we set out to study this effect on WSi2 formation by the reaction between tungsten and silicon. This paper reports the first results of our study of the effect o f substrate orientation on WSi2 formation.
2. Experimental details P-silicon wafers 2 inches in diameter with (100) and (111) orientations and resistivity o f 4 - 6 fi cm were used as the starting material. The wafers were cleaned using the standard Radio Corporation o f America (RCA) cleaning process followed by an H F dip. In this process, the silicon wafers are subjected to 10 min boiling in a solution of N H 4 O H + H202 + H 2 0 (1:1:5). This is followed by thorough rinsing in distilled water. The wafers are once again boiled for 10min in a solution of HCI + H202 + H 2 0 (1:1:5) and are subsequently rinsed in distilled water. A tungsten film 1600/~ thick was deposited on the wafers by r.f. magnetron sputtering at 500 W with an r.f. supply frequency of 13.56 MHz. The target was 3 inches in diameter and the target-wafer spacing was 2 inches. The system vessel was evacuated to 10 -6 T o r r before admitting argon as the discharge gas for sputtering at a pressure of 3 x 10 -z Torr. The tungsten deposition rate was 160/~, m i n - k After deposition, the r.f. supply was switched off and the main valve was closed. Argon gas was admitted to the bell jar
0040-6090/94/$7.00
and the system was opened. The wafers were then transferred to the r.f. target. The system vessel was once again evacuated to 10 -6 T o r r before admitting argon as the discharge gas for sputtering near the sputter threshold with optimum r.f. power at a pressure of 3 x 10 - 2 T o r r . This sputtering near the sputter threshold is designated as discharge treatment [3]. This treatment was given for 30 min and was found essential for promoting W - S i adhesion and silicidation [4]. The system vessel was switched off and argon gas was admitted. The wafers were taken out and were arranged in the sandwich configuration, i.e. each wafer was placed between two cleaned silicon wafers. These were then loaded into the tungsten filament fitted into another vacuum system. The sandwich configuration [5] protects the wafer coated with tungsten film from the degassed contaminants. The vacuum system was evacuated to 1 0 - 6 T o r r and the wafers in the sandwich configuration were vacuum annealed at 400-1000 °C for 30 rain by passing current in the tungsten filament. The temperature of the sandwich configuration was monitored by a thermocouple attached to it. The characterisation, both electrical and structural, was done by four probe, SEM and X-ray diffraction.
3. Results and discussion Figure 1 shows the sheet resistance of the tungsten silicide as a function of vacuum anneal temperature. It is found to increase with annealing temperature, reaching its maximum value at 650 °C and decreasing thereafter in the case o f Si(100) samples. The lowest sheet resistance is 6 t2 []-1; this is uniform across the wafer. The thickness of the silicide thus formed was 4000/~.
© 1994 - - Elsevier Sequoia. All rights reserved
156
Awatar Singh et al. /Substrate orientation effect on WSi 2 formation
g I--
-// / I I
I I
50
40
I
/
/
b /
i °
W Six a"
C) cv.
30
t I
20
60
I
I
40
I
20
20 (DEGREES) Fig. 2. XRD patterns of W-Si(100) samples (a) after discharge treatment, and after 30 min anneal at (b) 500 °C, (e) 700 °C, (d) 800 °C and (e) 1000 °C. 10
I 200
I ~00 - - - -
I 600
I 800
I 1~00
T(*C)
Fig. 1. Tungsten silicide sheet resistance us. vacuum anneal temperature for discharge-treated (a) W-Si(100) and (b) W-Si(111).
The X-ray diffractograms of discharge-treated W Si(100) samples followed by 30 min vacuum anneal at different temperatures in the range 400-1000 °C are shown in Fig. 2. It is obvious that the discharge treatment of the W-Si(100) samples produces an amorphous tungsten silicide. This is because the discharge treatment causes mixing of tungsten and silicon. The mixed silicide phases of tetragonal and hexagonal WSi2 are observed to form at a vacuum anneal temperature of 500 °C for 30 min. The tungsten peak is not observed at all: this indicates its full reaction with silicon. The
WGSi3 phase is also absent, although it appears in the conventionally [6, 7] annealed samples of W-Si(100). We attribute this to the suppression of WGSi3 formation in our process and the production of WSi2 as the first phase at a temperature of 500 °C, much lower than that normally used in conventional processing of WSi2 by furnace annealing of W-Si(100) samples [6, 7]. The end phase is always tetragonal WSi2. The sheet resistance of the tungsten silicide in the case of W - S i ( l l l ) samples does not increase with anneal temperature up to 500 °C. Thereafter it increases, attaining the maximum value at 750 °C and decreasing at higher anneal temperatures. The minimum sheet resistance in this case is about double that obtained in the W-Si(100) samples. This is because of the difference in the microstructures of the films, due mainly to the difference in the nucleation process [1, 2] for the two substrates. The SEM images (Fig. 3) confirm this. The native oxide thickness difference [8-10] on two types of substrate is not the main cause, as its removal by presputter cleaning before tungsten deposition does not change the resistivity behaviour (Fig. 1).
Awatar Singh et al./ Substrate orientation effect on WSiz formation
157
Si(100) (i) forms at lower temperature, (ii) has a finegrained surface morphology and (iii) has a low sheet resistance as compared with tungsten silicide formed on S i ( l l l ) . We attribute this to the difference in the microstructures of the films, due mainly to differences in the nucleation process for the two substrates. This is just a preliminary finding, which, it is hoped, will stimulate further work in this direction. (a)
Acknowledgments We are thankful to Microwave Tube Area, C E E R I , for the target mounting and to R. Bhattacharya of the National Physical Laboratory, New Delhi for sputtering work.
References (b) Fig. 3. SEM images of surface morphology of WSi2(T) on (a) Si(100) and (b) Si(111). The stable silicide phase in this case also is tetragonal WSi2. SEM reveals rougher silicide surface m o r p h o l o g y than that obtained with W - S i ( 1 0 0 ) samples, as shown in Fig. 3. The X R D spectra obtained in this case were identical to those shown in Fig. 2. The silicidation shows differences only from Fig. 1.
4. Conclusions It has been found that substrate orientation greatly affects the tungsten silicide formation. The silicide on
1 A. Singh, W. S. Khokle, M. Prudenziati, G. Majni and B. Morten, J. Appl. Phys., 66(1989) 1190. 2 J. O. Olowolafe, M. A. Nicolet and J. W. Mayer, Thin Solid Films, 38 (1976) 143. 3 A. Singh, Microelectron. Reliab., 23 (1983) 185. 4 A. Singh, P. D. Vyas, W. S. Khokle, C. Singh and K. C. Nagpal, Proc. 5th Int. Workshop on Physics of Semiconductor Devices, New Delhi, Dec. 2-6, 1991. Tata McGraw-Hill, New Delhi,
p. 460. 5 A. Singh and W. S. Khokle, Microelectron. J., 20 (1989) 11. 6 M. Siegel, J. J. Santiago and J. Von der Spiegel, Proc. Mater. Res. Soc., 52 (1986) 289. 7 S. P. Murarka, Silicides for VLSI Applications, Academic Press, New York, 1983. 8 R. Rutter, Solid State Electron., 23 (1980) 998. 9 T. I. Kamins, S. S. Laderman, D. J. Coulman and J. E. Turner, J. Electrochem. Soc., 133 (1986) 1438. 10 Y. Pauleau, P. Lami, A. Tissier, R. Pantel and J. C. Oberlin, Thin Solid Films, 143 (1986) 259.
The effect of substrate orientation Awatar
155
on WSi 2
formation
S i n g h a n d W . S. K h o k l e
Microelectronics Area, CEERI, Pilani, Raj, 333031 (India)
K. Lal Materials Characterisation Division, N.P.L., Dr. K. S. Krishnan Marg, New Delhi, 110012 (India) (Received March 2, 1993; accepted July 21, 1993)
Abstract The effect of substrate orientation on tungsten silicide formation has been studied. It has been found that the silicide on Si(100) forms at low temperature, has fine grained surface morphology and has low sheet resistance as compared with that formed on Si(l I 1). We attribute this to the difference in microstructure of the films, due mainly to the difference in the nucleation process for the two substrates.
1. Introduction It has recently been shown by Singh et al. [1] that the lateral growth of nickel silicide depends on the nature of the silicon substrate. It is much greater for Si(100) than for Si( 11 I). This behaviour is attributed to the difference in microstructures [1, 2] o f the silicide formed. Motivated by this, we set out to study this effect on WSi2 formation by the reaction between tungsten and silicon. This paper reports the first results of our study of the effect o f substrate orientation on WSi2 formation.
2. Experimental details P-silicon wafers 2 inches in diameter with (100) and (111) orientations and resistivity o f 4 - 6 fi cm were used as the starting material. The wafers were cleaned using the standard Radio Corporation o f America (RCA) cleaning process followed by an H F dip. In this process, the silicon wafers are subjected to 10 min boiling in a solution of N H 4 O H + H202 + H 2 0 (1:1:5). This is followed by thorough rinsing in distilled water. The wafers are once again boiled for 10min in a solution of HCI + H202 + H 2 0 (1:1:5) and are subsequently rinsed in distilled water. A tungsten film 1600/~ thick was deposited on the wafers by r.f. magnetron sputtering at 500 W with an r.f. supply frequency of 13.56 MHz. The target was 3 inches in diameter and the target-wafer spacing was 2 inches. The system vessel was evacuated to 10 -6 T o r r before admitting argon as the discharge gas for sputtering at a pressure of 3 x 10 -z Torr. The tungsten deposition rate was 160/~, m i n - k After deposition, the r.f. supply was switched off and the main valve was closed. Argon gas was admitted to the bell jar
0040-6090/94/$7.00
and the system was opened. The wafers were then transferred to the r.f. target. The system vessel was once again evacuated to 10 -6 T o r r before admitting argon as the discharge gas for sputtering near the sputter threshold with optimum r.f. power at a pressure of 3 x 10 - 2 T o r r . This sputtering near the sputter threshold is designated as discharge treatment [3]. This treatment was given for 30 min and was found essential for promoting W - S i adhesion and silicidation [4]. The system vessel was switched off and argon gas was admitted. The wafers were taken out and were arranged in the sandwich configuration, i.e. each wafer was placed between two cleaned silicon wafers. These were then loaded into the tungsten filament fitted into another vacuum system. The sandwich configuration [5] protects the wafer coated with tungsten film from the degassed contaminants. The vacuum system was evacuated to 1 0 - 6 T o r r and the wafers in the sandwich configuration were vacuum annealed at 400-1000 °C for 30 rain by passing current in the tungsten filament. The temperature of the sandwich configuration was monitored by a thermocouple attached to it. The characterisation, both electrical and structural, was done by four probe, SEM and X-ray diffraction.
3. Results and discussion Figure 1 shows the sheet resistance of the tungsten silicide as a function of vacuum anneal temperature. It is found to increase with annealing temperature, reaching its maximum value at 650 °C and decreasing thereafter in the case o f Si(100) samples. The lowest sheet resistance is 6 t2 []-1; this is uniform across the wafer. The thickness of the silicide thus formed was 4000/~.
© 1994 - - Elsevier Sequoia. All rights reserved
156
Awatar Singh et al. /Substrate orientation effect on WSi 2 formation
g I--
-// / I I
I I
50
40
I
/
/
b /
i °
W Six a"
C) cv.
30
t I
20
60
I
I
40
I
20
20 (DEGREES) Fig. 2. XRD patterns of W-Si(100) samples (a) after discharge treatment, and after 30 min anneal at (b) 500 °C, (e) 700 °C, (d) 800 °C and (e) 1000 °C. 10
I 200
I ~00 - - - -
I 600
I 800
I 1~00
T(*C)
Fig. 1. Tungsten silicide sheet resistance us. vacuum anneal temperature for discharge-treated (a) W-Si(100) and (b) W-Si(111).
The X-ray diffractograms of discharge-treated W Si(100) samples followed by 30 min vacuum anneal at different temperatures in the range 400-1000 °C are shown in Fig. 2. It is obvious that the discharge treatment of the W-Si(100) samples produces an amorphous tungsten silicide. This is because the discharge treatment causes mixing of tungsten and silicon. The mixed silicide phases of tetragonal and hexagonal WSi2 are observed to form at a vacuum anneal temperature of 500 °C for 30 min. The tungsten peak is not observed at all: this indicates its full reaction with silicon. The
WGSi3 phase is also absent, although it appears in the conventionally [6, 7] annealed samples of W-Si(100). We attribute this to the suppression of WGSi3 formation in our process and the production of WSi2 as the first phase at a temperature of 500 °C, much lower than that normally used in conventional processing of WSi2 by furnace annealing of W-Si(100) samples [6, 7]. The end phase is always tetragonal WSi2. The sheet resistance of the tungsten silicide in the case of W - S i ( l l l ) samples does not increase with anneal temperature up to 500 °C. Thereafter it increases, attaining the maximum value at 750 °C and decreasing at higher anneal temperatures. The minimum sheet resistance in this case is about double that obtained in the W-Si(100) samples. This is because of the difference in the microstructures of the films, due mainly to the difference in the nucleation process [1, 2] for the two substrates. The SEM images (Fig. 3) confirm this. The native oxide thickness difference [8-10] on two types of substrate is not the main cause, as its removal by presputter cleaning before tungsten deposition does not change the resistivity behaviour (Fig. 1).
Awatar Singh et al./ Substrate orientation effect on WSiz formation
157
Si(100) (i) forms at lower temperature, (ii) has a finegrained surface morphology and (iii) has a low sheet resistance as compared with tungsten silicide formed on S i ( l l l ) . We attribute this to the difference in the microstructures of the films, due mainly to differences in the nucleation process for the two substrates. This is just a preliminary finding, which, it is hoped, will stimulate further work in this direction. (a)
Acknowledgments We are thankful to Microwave Tube Area, C E E R I , for the target mounting and to R. Bhattacharya of the National Physical Laboratory, New Delhi for sputtering work.
References (b) Fig. 3. SEM images of surface morphology of WSi2(T) on (a) Si(100) and (b) Si(111). The stable silicide phase in this case also is tetragonal WSi2. SEM reveals rougher silicide surface m o r p h o l o g y than that obtained with W - S i ( 1 0 0 ) samples, as shown in Fig. 3. The X R D spectra obtained in this case were identical to those shown in Fig. 2. The silicidation shows differences only from Fig. 1.
4. Conclusions It has been found that substrate orientation greatly affects the tungsten silicide formation. The silicide on
1 A. Singh, W. S. Khokle, M. Prudenziati, G. Majni and B. Morten, J. Appl. Phys., 66(1989) 1190. 2 J. O. Olowolafe, M. A. Nicolet and J. W. Mayer, Thin Solid Films, 38 (1976) 143. 3 A. Singh, Microelectron. Reliab., 23 (1983) 185. 4 A. Singh, P. D. Vyas, W. S. Khokle, C. Singh and K. C. Nagpal, Proc. 5th Int. Workshop on Physics of Semiconductor Devices, New Delhi, Dec. 2-6, 1991. Tata McGraw-Hill, New Delhi,
p. 460. 5 A. Singh and W. S. Khokle, Microelectron. J., 20 (1989) 11. 6 M. Siegel, J. J. Santiago and J. Von der Spiegel, Proc. Mater. Res. Soc., 52 (1986) 289. 7 S. P. Murarka, Silicides for VLSI Applications, Academic Press, New York, 1983. 8 R. Rutter, Solid State Electron., 23 (1980) 998. 9 T. I. Kamins, S. S. Laderman, D. J. Coulman and J. E. Turner, J. Electrochem. Soc., 133 (1986) 1438. 10 Y. Pauleau, P. Lami, A. Tissier, R. Pantel and J. C. Oberlin, Thin Solid Films, 143 (1986) 259.