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Synthesis of buried silicon nitride layers by rapid thermal annealing
Thin Solid Films, 164 (1988) 429-434
429 SYNTHESIS OF BURIED SILICON NITRIDE LAYERS BY RAPID THERMAL ANNEALING* C. M. S. RAUTHAN, AMI CHAND, SUDHIR CHANDRA AND G. BOSE
Centre for Applied Research in Electronics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016 (India)
Buried layers of silicon nitride (Si3N4) were synthesised by rapid thermal annealing (RTA) of high-dose (10 ~8 ions cm -2) nitrogen implanted (at 150 keV) silicon. The influence of RTA temperature and time on the formation of buried layers of silicon nitride was studied by IR transmission and X-ray diffraction techniques. It is found that RTA at 1200°C for 150 s or 1250°C for 100 s results in buried polycrystalline silicon nitride layers. The results are compared with those obtained by traditional furnace annealing.
l. INTRODUCTION
Silicon-on-insulator (SOI) technology has attracted much interest in the last decade, having the advantages of greater packing density, higher speed, improved immunity to CMOS latch-up and increased radiation tolerance ~'2. Among the most promising ideas are those having to do with the synthesis of buried dielectric layers by high-dose nitrogen or oxygen implantation. Formation of buried silicon nitride layers by ion implantation has been reported by several workers 3-7. These workers have used conventional furnace annealing (FA) after implantation to achieve deviceworthy silicon-on-insulator substrates. In general, a post-implant thermal annealing at a temperature higher than 1200 °C for a duration of more than 2 h is necessary to synthesise buried silicon nitride layers. The processing time can be reduced significantly by employing rapid thermal annealing (RTA) techniques after implantation. Recent work by Reeson e t al. 8 has shown that an improved quality of SO1 structure can be obtained after a post-implant annealing at 1405 °C for 30 min by flash halogen lamps. The structure and composition of rapid thermally annealed low-dose nitrogen implanted silicon have been studied by Liliental e t al. 9 In this paper, we report our investigations on the feasibility of synthesising the buried silicon nitride layers by the RTA process after high-dose nitrogen implantation. The analysis has been made with infrared (IR) transmission spectra and X-ray diffractograms. The results are compared with those of conventional furnace annealed samples. * Paper presentedat the 7th InternationalConferenceon ThinFilms,New Delhi,India,December7-11, 1987. 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netherlands
430
c.M.S. RAUTHANet al.
2. EXPERIMENTAL DETAILS
p-type single crystal (100), polished silicon wafers of diameter 50ram and resistivity 22-45 fl cm were implanted with atomic nitrogen ions (14N+) to a total dose of 1018 ions cm - 2 at 150 keV using a medium current ion implanter (Varian D F 3000). Since the ion beam was raster scanned over an area of diameter I00 ram, three wafers were implanted together to make efficient use of the implantation time. The implants were carried out at 7 ° from the major axis normal to the surface in a vacuum of 3 × 1 0 - 6 Torr. The ion beam current density varied from 5.5 to 6.5 ~tA c m - 2. The temperature of the wafers monitored during implantation was found in the range of 25(~340 °C due to ion beam heating. After implantation, wafers were cut into pieces for further processing. Post-implant annealing treatment was performed on an RTA system (Heatpulse 210T, AG Associates) by using focused halogen lamps in dry nitrogen ambient. The temperature ramp up rate was 150 c'C s-~ until the desired peak temperature was reached and was held for the required length of time. Furnace annealing was carried out on a sample for 3 h at 1200 °C in dry nitrogen ambient. Structural transformations due to annealing were monitored by IR absorption and glancing angle X-ray diffraction analysis. A Perkin-Elmer (model 580B) double beam spectrophotometer was used to record IR spectra. An identical unimplanted silicon wafer which had undergone the same cleaning and annealing steps was used in the reference beam. Samples were also analysed with an X-ray diffractometer (Rigaku D / m a x 7B-RU 200 H) using Cu Kct radiation (2 = 1.54 ~) at a glancing angle of~ = 5 ° and a scanning angle of 20 (where 0 is the Bragg angle), from 20 ° to 90 °. 3.
RESULTS AND DISCUSSION
The IR transmission spectra were used to study the structural transformations of silicon nitride as a function of anneal temperature and time. Figure 1 shows the IR spectra of an as-implanted sample and of RTA samples at different temperatures for 30 s. A broad absorption band centred at 820 c m - ~ appears for the as-implanted sample. This IR absorption band is associated with one of the Si - N stretching bond frequencies 1o. The absorption band shifts slightly towards higher wavenumbers on annealing up to 1150~C and starts sharpening at 1200°C. This shift in the IR absorption peak towards higher wavenumbers may be attributed to the release of bond strains and disorder introduced during implantation 1°'~1. The effect of annealing time on the IR spectra is depicted in Fig. 2. It is seen that RTA at 1200 °C for 20 s results in the sharpening of the broad band only, while 30 s RTA (Fig. 1) transforms it into a complex spectrum with three peaks. Sharpness of these peaks in the complex spectrum increases with annealing time up to 150s. No further modification in IR spectra is observed when annealing is done for more than 150 s. Formation of this complex spectrum is linked with the transformation of a m o r p h o u s silicon nitride into crystalline silicon nitride 1°. The peaks in the complex spectrum are positioned at wavenumbers 845, 890 and 945 cm-1. The values are consistent with those obtained on furnace annealed samples as shown in Fig. 2 and also reported by other workers 3 - 5. This suggests that the crystalline silicon nitride layer is obtained by RTA at 1200 °C for 150 s.
SYNTHESIS OF BURIED S i N BY RTA
431
RTA 1250" %30S
R'iA 1200"% 30S
J .6
RrA 1150"C.,305
IJJ
~
z
R'[A 1100'C,30S
~.. ~ . . _ _ .As . ~ -~ ,mptantecl i.n z rr" er
I
1400
I
1200
I
1000
I
I
800
600
400
WAVE NUMBER (cm -1)
Fig. 1. IR transmission spectra of as-implanted sample and RTA samples for 30s at various temperatures.
X-ray diffractograms for as-implanted and RTA samples at different temperatures for 30 s have been shown in Fig. 3. A broad peak of silicon (311 ) plane at 56.5 ° observed for the as-implanted sample remains even after annealing up to 1250 °C. Peaks linked to silicon start emerging at 28.4 ° and 47.3 ° when anneal temperature is increased and become pronounced at 1250°C. Appearance of these peaks after annealing is attributed to the recrystallisation of top damaged silicon into polysilicon. Crystallization of silicon nitride starts only after annealing at 1200 °C, as indicated by occurrence of several other weak intensity peaks, This is consistent with our IR transmission studies (Fig. 1). Moderate peaks associated with Si3N 4 are seen with RTA at 1200°C for 250 s and at 1250°C for 100 s as shown in Fig. 4. It is obvious from the figure that the broad peak at 56.5 ° (Fig. 3) gets resolved in two peaks, one associated with Si ( 3 1 1 ) plane at 56.2 ° and the other with Si3N 4 (222) plane at 57.8 ° . It is also seen that the broad peak is well resolved into two peaks when the sample is annealed at 1200 °C for 250 s while it starts resolving at 1250 °C for 100 s, Experimental and reference values (calculated from ASTM standard data) of 20 peak positions are shown in Table I. It may be mentioned that Bourguet e t al. 4 have also observed the same planes (as indicated in Table I) of ~-Si3N 4 and silicon with Xray diffraction pattern obtained at a glancing angle of 15 °. This suggests that polycrystalline Si3N 4 formed upon RTA in our case is in ~t phase. However it could
432
c.M.S. RAUTHAN et al.
FA 1200 ° , 3 h r s
RTA 1200 ° , 150 $
\
u~
RTA 1200°p 60 S L O I,I Z
RTA 1200°C , 2 0 S ~E v3 Z < n~ I-el:
1400
I 1200
I I 1000 800 WAVE NUMBER ( cm- I )
l 600
400
Fig. 2. IR transmission spectra of RTA and furnace annealed samples at 1200 °C.
ul E :3
.d
RTA 1200°C ~ 30S
v >I--
z LU I--" Z
RTA 1100°C, 30 S
20
I
1
I,
4o
60
8o
9o
20 ( d e g r e e s )
Fig. 3. X-ray diffractograms of as-implanted sample and RTA samples for 30 s at various temperatures.
SYNTHESIS OF BURIED
SiN aY RTA
433
o
o'1 z kl.I I-.-Z
I 40
20
I 60 20 ( d e g r e e s )
I 80
90
Fig. 4. X-ray diffractograms of RTA and furnace annealed samples. TABLE I E X P E R I M E N T A L A N D REFERENCE V A L U E S OF 2 0 PEAK P O S I T I O N S F O R S I L I C O N A N D S I L I C O N N I T R I D E
Experimental 20 values
AS TM standard
RTA 1200 °C 250 s
RTA 1250 °C 100 s
FA 1200 °C 3h
20
hkl
Silicon 28.3 47.2 56.2
28.4 47.2 56.2
28.5 47.2 56.2
28.41 47.29 56.06
111 a 220 a 311"
Silicon nitride 20.6 22.8 26.5 30.9 34.5 35.3 38.9 41.9 43.5 -57.8 62.5
20.5 22.8 26.5 31.0 34.5 35.3 38.9 42.0 43.3 -57.7 --
20.5 22.9 26.4 31.0 34.5 35.3 39.0 42.0 43.4 51.6 57.7 --
20.53 22.89 26.42 30.87 34.47 35.19 38.78 41.81 43.39 51.54 57.69 62.42
101" 110" 200" 201" 102" 210" 211" 202" 301" 311 222 321
a Peaks observed by Bourguet et al.* not be determined
whether
the peaks linked with silicon are from the superficial
l a y e r o r f r o m t h e p o l y s i l i c o n e m b e d d e d in t h e b u r i e d n i t r i d e l a y e r . T h e r e s u l t s o f X r a y d i f f r a c t i o n s t u d i e s , a s s h o w n i n F i g . 4, d e m o n s t r a t e t h a t f u r n a c e a n n e a l i n g
434
c.M.S. RAUTHANet al.
produces several strong intensity peaks associated with polycrystalline silicon nitride, compared with weak peaks produced with RTA samples. The presence of the peaks corresponding to silicon nitride, though weak in intensity, suggest that the synthesis of buried layers of polycrystalline a-silicon nitride can be realised by RTA of high-dose nitrogen implanted silicon. 4. CONCLUSION The IR transmission spectra demonstrate that polycrystalline Si3N4 can be synthesised by RTA of nitrogen implanted silicon at 1200°C and above. The annealing time required is as low as 30 s at 1250 °C. This has been further confirmed by X-ray diffraction studies. RTA offers a potentially strong technique for synthesising buried nitride layers by high-dose implantation for SOI structures. It will be very interesting to investigate the formation of these structures at higher RTA temperatures. ACKNOWLEDGMENTS
The authors are especially grateful to Professor A. B. Bhattacharyya and Dr. Amitabh Jain for their continuing encouragement and valuable discussions. Thanks are due to Mr. R. S. Rastogi for assisting in X-ray diffraction studies. The financial support of the Department of Electronics, Government of India, is also acknowledged. REFERENCES 1 S.L. Partridge, IEEE Proc. 1, Solid State and Electron Devices, 133 (1986) 66. 2 H . W . Lam, A. F. Tasch, Jr., and R. F. Pinizzotto, in N. G. Einspruch (ed.), V L S I Electronics: Microstructure Science, Vol. 4, Academic Press, New York, 1982, pp. 1-54. 3 R.J. Dexter, S. B. Watelski and S. T. Picraux, Appl. Phys. Lett., 23 (1973) 455. 4 P. Bourguet, J. M. Dupart, E. Le Tiran, P. Auvray, A. Guivarch, M. Salvi, G. Pelous and P. Henoc, J. Appl. Phys., 51 (1980) 6169. 5 T. Tsujide, M. Nojiri and H. Kitagava, J. Appl. Phys. 51 (1980) 1605. 6 P . L . F . Hemment, R. F. Peart, M. F. Yao, K. G. Stephens, R. J. Chater, J. A. Kilner, D. Meekison, G. R. Booker and R. P. Arrowsmith, Appl. Phys. Lett., 46 (1985) 952. 7 G. Zimmer and H. Vogt, IEEE Trans. Electron Devices, 30 (1983) 1515. 8 K.J. Reeson, P. L. F. Hemment, C. D. Meekison, G. R. Booker, R. J. Chater, J. R. Davis and G. K. Celler, Appl. Phys. Lett., 50 (1987) 1882. 9 Z. Liliental, R. W. Carpenter and J. C. Kelly, Thin Solid Films, 138 (1986) 141. 10 C.D. Fung, J. L. Liao, K. R. Elsayed and J. J. Kopanski, Syrup. on Silicon Nitride Thin Insulating Films, in Electrochem. Soc. Proc., 83-8 (1983) 403. 11 A . D . Yadav and M. C. Joshi, Thin Solid Films, 59 (1979) 313.
429 SYNTHESIS OF BURIED SILICON NITRIDE LAYERS BY RAPID THERMAL ANNEALING* C. M. S. RAUTHAN, AMI CHAND, SUDHIR CHANDRA AND G. BOSE
Centre for Applied Research in Electronics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016 (India)
Buried layers of silicon nitride (Si3N4) were synthesised by rapid thermal annealing (RTA) of high-dose (10 ~8 ions cm -2) nitrogen implanted (at 150 keV) silicon. The influence of RTA temperature and time on the formation of buried layers of silicon nitride was studied by IR transmission and X-ray diffraction techniques. It is found that RTA at 1200°C for 150 s or 1250°C for 100 s results in buried polycrystalline silicon nitride layers. The results are compared with those obtained by traditional furnace annealing.
l. INTRODUCTION
Silicon-on-insulator (SOI) technology has attracted much interest in the last decade, having the advantages of greater packing density, higher speed, improved immunity to CMOS latch-up and increased radiation tolerance ~'2. Among the most promising ideas are those having to do with the synthesis of buried dielectric layers by high-dose nitrogen or oxygen implantation. Formation of buried silicon nitride layers by ion implantation has been reported by several workers 3-7. These workers have used conventional furnace annealing (FA) after implantation to achieve deviceworthy silicon-on-insulator substrates. In general, a post-implant thermal annealing at a temperature higher than 1200 °C for a duration of more than 2 h is necessary to synthesise buried silicon nitride layers. The processing time can be reduced significantly by employing rapid thermal annealing (RTA) techniques after implantation. Recent work by Reeson e t al. 8 has shown that an improved quality of SO1 structure can be obtained after a post-implant annealing at 1405 °C for 30 min by flash halogen lamps. The structure and composition of rapid thermally annealed low-dose nitrogen implanted silicon have been studied by Liliental e t al. 9 In this paper, we report our investigations on the feasibility of synthesising the buried silicon nitride layers by the RTA process after high-dose nitrogen implantation. The analysis has been made with infrared (IR) transmission spectra and X-ray diffractograms. The results are compared with those of conventional furnace annealed samples. * Paper presentedat the 7th InternationalConferenceon ThinFilms,New Delhi,India,December7-11, 1987. 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netherlands
430
c.M.S. RAUTHANet al.
2. EXPERIMENTAL DETAILS
p-type single crystal (100), polished silicon wafers of diameter 50ram and resistivity 22-45 fl cm were implanted with atomic nitrogen ions (14N+) to a total dose of 1018 ions cm - 2 at 150 keV using a medium current ion implanter (Varian D F 3000). Since the ion beam was raster scanned over an area of diameter I00 ram, three wafers were implanted together to make efficient use of the implantation time. The implants were carried out at 7 ° from the major axis normal to the surface in a vacuum of 3 × 1 0 - 6 Torr. The ion beam current density varied from 5.5 to 6.5 ~tA c m - 2. The temperature of the wafers monitored during implantation was found in the range of 25(~340 °C due to ion beam heating. After implantation, wafers were cut into pieces for further processing. Post-implant annealing treatment was performed on an RTA system (Heatpulse 210T, AG Associates) by using focused halogen lamps in dry nitrogen ambient. The temperature ramp up rate was 150 c'C s-~ until the desired peak temperature was reached and was held for the required length of time. Furnace annealing was carried out on a sample for 3 h at 1200 °C in dry nitrogen ambient. Structural transformations due to annealing were monitored by IR absorption and glancing angle X-ray diffraction analysis. A Perkin-Elmer (model 580B) double beam spectrophotometer was used to record IR spectra. An identical unimplanted silicon wafer which had undergone the same cleaning and annealing steps was used in the reference beam. Samples were also analysed with an X-ray diffractometer (Rigaku D / m a x 7B-RU 200 H) using Cu Kct radiation (2 = 1.54 ~) at a glancing angle of~ = 5 ° and a scanning angle of 20 (where 0 is the Bragg angle), from 20 ° to 90 °. 3.
RESULTS AND DISCUSSION
The IR transmission spectra were used to study the structural transformations of silicon nitride as a function of anneal temperature and time. Figure 1 shows the IR spectra of an as-implanted sample and of RTA samples at different temperatures for 30 s. A broad absorption band centred at 820 c m - ~ appears for the as-implanted sample. This IR absorption band is associated with one of the Si - N stretching bond frequencies 1o. The absorption band shifts slightly towards higher wavenumbers on annealing up to 1150~C and starts sharpening at 1200°C. This shift in the IR absorption peak towards higher wavenumbers may be attributed to the release of bond strains and disorder introduced during implantation 1°'~1. The effect of annealing time on the IR spectra is depicted in Fig. 2. It is seen that RTA at 1200 °C for 20 s results in the sharpening of the broad band only, while 30 s RTA (Fig. 1) transforms it into a complex spectrum with three peaks. Sharpness of these peaks in the complex spectrum increases with annealing time up to 150s. No further modification in IR spectra is observed when annealing is done for more than 150 s. Formation of this complex spectrum is linked with the transformation of a m o r p h o u s silicon nitride into crystalline silicon nitride 1°. The peaks in the complex spectrum are positioned at wavenumbers 845, 890 and 945 cm-1. The values are consistent with those obtained on furnace annealed samples as shown in Fig. 2 and also reported by other workers 3 - 5. This suggests that the crystalline silicon nitride layer is obtained by RTA at 1200 °C for 150 s.
SYNTHESIS OF BURIED S i N BY RTA
431
RTA 1250" %30S
R'iA 1200"% 30S
J .6
RrA 1150"C.,305
IJJ
~
z
R'[A 1100'C,30S
~.. ~ . . _ _ .As . ~ -~ ,mptantecl i.n z rr" er
I
1400
I
1200
I
1000
I
I
800
600
400
WAVE NUMBER (cm -1)
Fig. 1. IR transmission spectra of as-implanted sample and RTA samples for 30s at various temperatures.
X-ray diffractograms for as-implanted and RTA samples at different temperatures for 30 s have been shown in Fig. 3. A broad peak of silicon (311 ) plane at 56.5 ° observed for the as-implanted sample remains even after annealing up to 1250 °C. Peaks linked to silicon start emerging at 28.4 ° and 47.3 ° when anneal temperature is increased and become pronounced at 1250°C. Appearance of these peaks after annealing is attributed to the recrystallisation of top damaged silicon into polysilicon. Crystallization of silicon nitride starts only after annealing at 1200 °C, as indicated by occurrence of several other weak intensity peaks, This is consistent with our IR transmission studies (Fig. 1). Moderate peaks associated with Si3N 4 are seen with RTA at 1200°C for 250 s and at 1250°C for 100 s as shown in Fig. 4. It is obvious from the figure that the broad peak at 56.5 ° (Fig. 3) gets resolved in two peaks, one associated with Si ( 3 1 1 ) plane at 56.2 ° and the other with Si3N 4 (222) plane at 57.8 ° . It is also seen that the broad peak is well resolved into two peaks when the sample is annealed at 1200 °C for 250 s while it starts resolving at 1250 °C for 100 s, Experimental and reference values (calculated from ASTM standard data) of 20 peak positions are shown in Table I. It may be mentioned that Bourguet e t al. 4 have also observed the same planes (as indicated in Table I) of ~-Si3N 4 and silicon with Xray diffraction pattern obtained at a glancing angle of 15 °. This suggests that polycrystalline Si3N 4 formed upon RTA in our case is in ~t phase. However it could
432
c.M.S. RAUTHAN et al.
FA 1200 ° , 3 h r s
RTA 1200 ° , 150 $
\
u~
RTA 1200°p 60 S L O I,I Z
RTA 1200°C , 2 0 S ~E v3 Z < n~ I-el:
1400
I 1200
I I 1000 800 WAVE NUMBER ( cm- I )
l 600
400
Fig. 2. IR transmission spectra of RTA and furnace annealed samples at 1200 °C.
ul E :3
.d
RTA 1200°C ~ 30S
v >I--
z LU I--" Z
RTA 1100°C, 30 S
20
I
1
I,
4o
60
8o
9o
20 ( d e g r e e s )
Fig. 3. X-ray diffractograms of as-implanted sample and RTA samples for 30 s at various temperatures.
SYNTHESIS OF BURIED
SiN aY RTA
433
o
o'1 z kl.I I-.-Z
I 40
20
I 60 20 ( d e g r e e s )
I 80
90
Fig. 4. X-ray diffractograms of RTA and furnace annealed samples. TABLE I E X P E R I M E N T A L A N D REFERENCE V A L U E S OF 2 0 PEAK P O S I T I O N S F O R S I L I C O N A N D S I L I C O N N I T R I D E
Experimental 20 values
AS TM standard
RTA 1200 °C 250 s
RTA 1250 °C 100 s
FA 1200 °C 3h
20
hkl
Silicon 28.3 47.2 56.2
28.4 47.2 56.2
28.5 47.2 56.2
28.41 47.29 56.06
111 a 220 a 311"
Silicon nitride 20.6 22.8 26.5 30.9 34.5 35.3 38.9 41.9 43.5 -57.8 62.5
20.5 22.8 26.5 31.0 34.5 35.3 38.9 42.0 43.3 -57.7 --
20.5 22.9 26.4 31.0 34.5 35.3 39.0 42.0 43.4 51.6 57.7 --
20.53 22.89 26.42 30.87 34.47 35.19 38.78 41.81 43.39 51.54 57.69 62.42
101" 110" 200" 201" 102" 210" 211" 202" 301" 311 222 321
a Peaks observed by Bourguet et al.* not be determined
whether
the peaks linked with silicon are from the superficial
l a y e r o r f r o m t h e p o l y s i l i c o n e m b e d d e d in t h e b u r i e d n i t r i d e l a y e r . T h e r e s u l t s o f X r a y d i f f r a c t i o n s t u d i e s , a s s h o w n i n F i g . 4, d e m o n s t r a t e t h a t f u r n a c e a n n e a l i n g
434
c.M.S. RAUTHANet al.
produces several strong intensity peaks associated with polycrystalline silicon nitride, compared with weak peaks produced with RTA samples. The presence of the peaks corresponding to silicon nitride, though weak in intensity, suggest that the synthesis of buried layers of polycrystalline a-silicon nitride can be realised by RTA of high-dose nitrogen implanted silicon. 4. CONCLUSION The IR transmission spectra demonstrate that polycrystalline Si3N4 can be synthesised by RTA of nitrogen implanted silicon at 1200°C and above. The annealing time required is as low as 30 s at 1250 °C. This has been further confirmed by X-ray diffraction studies. RTA offers a potentially strong technique for synthesising buried nitride layers by high-dose implantation for SOI structures. It will be very interesting to investigate the formation of these structures at higher RTA temperatures. ACKNOWLEDGMENTS
The authors are especially grateful to Professor A. B. Bhattacharyya and Dr. Amitabh Jain for their continuing encouragement and valuable discussions. Thanks are due to Mr. R. S. Rastogi for assisting in X-ray diffraction studies. The financial support of the Department of Electronics, Government of India, is also acknowledged. REFERENCES 1 S.L. Partridge, IEEE Proc. 1, Solid State and Electron Devices, 133 (1986) 66. 2 H . W . Lam, A. F. Tasch, Jr., and R. F. Pinizzotto, in N. G. Einspruch (ed.), V L S I Electronics: Microstructure Science, Vol. 4, Academic Press, New York, 1982, pp. 1-54. 3 R.J. Dexter, S. B. Watelski and S. T. Picraux, Appl. Phys. Lett., 23 (1973) 455. 4 P. Bourguet, J. M. Dupart, E. Le Tiran, P. Auvray, A. Guivarch, M. Salvi, G. Pelous and P. Henoc, J. Appl. Phys., 51 (1980) 6169. 5 T. Tsujide, M. Nojiri and H. Kitagava, J. Appl. Phys. 51 (1980) 1605. 6 P . L . F . Hemment, R. F. Peart, M. F. Yao, K. G. Stephens, R. J. Chater, J. A. Kilner, D. Meekison, G. R. Booker and R. P. Arrowsmith, Appl. Phys. Lett., 46 (1985) 952. 7 G. Zimmer and H. Vogt, IEEE Trans. Electron Devices, 30 (1983) 1515. 8 K.J. Reeson, P. L. F. Hemment, C. D. Meekison, G. R. Booker, R. J. Chater, J. R. Davis and G. K. Celler, Appl. Phys. Lett., 50 (1987) 1882. 9 Z. Liliental, R. W. Carpenter and J. C. Kelly, Thin Solid Films, 138 (1986) 141. 10 C.D. Fung, J. L. Liao, K. R. Elsayed and J. J. Kopanski, Syrup. on Silicon Nitride Thin Insulating Films, in Electrochem. Soc. Proc., 83-8 (1983) 403. 11 A . D . Yadav and M. C. Joshi, Thin Solid Films, 59 (1979) 313.