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Fine structure of Si LVV and N KLL Auger signals for thermally nitrided SiO2 films
Thin Solid Films, 115 (1984) 135-139 PREPARATION AND CHARACTERIZATION
135
F I N E S T R U C T U R E O F Si LVV AND N K L L A U G E R SIGNALS FOR T H E R M A L L Y N I T R I D E D SiO2 FILMS YUTAKA YORIUME Ibaraki Electrical Communication Laboratory, Ntppon Telegraph and Telephone Public" Corporation. Tokai. lbaraki-ken 319-11 (Japan) (Received September 9, 1983; accepted March 20, 1984)
The fine structure of Si LVV and N K L L Auger signals was determined at various depths in thermally nitrided SiO2 films. The peak near 80 eV is shifted to a slightly higher energy at the surface and at the film-silicon interface, where the nitrogen concentration is greater. It is a little lower in the interior, where the nitrogen concentration is less. The N K L L peak was shifted to a slightly higher energy at the interface than at the surface and in the interior.
i. INTRODUCTION
Silicon nitride is a dielectric material that has been found to be useful for a number of applications in the semiconductor device industry. While it is most commonly prepared by chemical vapour deposition or reactive plasma deposition, the silicon oxynitride film obtained by the thermal nitriding of SiO2 film has recently become a centre of attention and interest as a result of the work of Ito et al. t and its potential application to very-large-scale integration technology 2"3. Numerous published studies are available on the Auger electron spectroscopy (AES) analysis of silicon oxynitride films. Some have concerned depth profiles for nitrogen, oxygen and silicon 1'4- 6. For example, nitrogen depth profiles have been measured and it has been shown that nitrogen penetrates deeply through the SiO 2 film and piles up at the film-silicon interface 4"7. Amano and Ekstedt s have also observed the nitrogen pile-up at the interface in addition to the highly nitrided surface layer using a backscattering method. Other studies have concerned the chemical structure of the silicon oxynitride films prepared by high temperature or plasma chemical vapour deposition 9-13 or the thermal nitriding of silicon 1'*'I~ However, chemical structure details have not been reported for thermally nitrided SiO2 films. In this investigation the structure of the Si LVV and N K L L AES signals for thermally nitrided SiO2 films was determined. The AES signals were shown to have a dependence on the depth in the thermally nitrided SiO 2 films. 2. EXPERIMENTAL DETAILS
Czochralski-grown p-type (100)-oriented silicon wafers with resistivities ranging from 1 to 4 f~ cm were used for preparation of the samples. After the wafers had 0040-6090/84/$3.00
f(~ ElsevierSequoia/Printedin The Netherlands
136
v. YORIUME
been chemically cleaned in hot aqueous N H 4 O H - H z O 2 and H C I - H e O 2 solutions, followed each time by rinsing in deionized water, and had been given a slight SiO z etch in dilute hydrofluoric acid, they were thermally oxidized at temperatures of 900, 950 and 1000 +'C in dry oxygen. The oxide film thickness ranged from 100 to 1000 A. The wafers were then subjected to thermal nitriding in anhydrous ammonia gas. The nitriding details are described in ref. 4, For Auger analysis the samples were mounted in an SAM 590 system from Physical Electronics. The vacuum system was degassed to a base pressure of about 5 × 10 7 Pa. The primary electron beam was incident at an angle of 60 + with respect to the normal to the surface of the specimen. The primary beam energy Ep was 5.0 keV. The peak-to-peak modulation for the first derivative recording was held at 1 eV. The resolution of the cylindrical mirror analyser was set at 0.3~',i, i.e. the energy difference between the positive and negative peaks of reflected primary electrons was about 0.3'.',/0of the primary beam energy. Auger analyses were carried out at various film depths, with many repetitions of analysis and sputtering. During the sputtering the chamber was filled with argon gas to a pressure of about 6.7 x 10 - a Pa, and the samples were bombarded with 1 keV argon ions. The sputtering rate was varied by adjusting the ion beam raster area such that the sputtering time for each sputtering step was several minutes or several tens of minutes. The pressure in the vacuum chamber was also kept at about 6.7 x 10 - 3 Pa during the analysis. 3. RESULTS AND DISCUSSION Figure I shows a typical Auger signal from the surface of a sample 500/~ thick that was nitrided at 1100 ~C for 2 h, N K L L peaks ranging from 360 to 390 eV, in addition to Si LVV (40-1 l0 eV) and O K L L (480-520 eV) peaks, were detected. The carbon signal is due to the adsorbate, which disappears if the surface is sputtered slightly.
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Fig. l. Typical Auger signal from an SiO 2 film nitrided at ! 1 0 0 C for 2 h (film thickness, 500/~}. Fig. 2. Depth profiles of the N KLL, Si LVV and O K L L Auger peak heights from an SiO 2 film nitrided at 1100"C for 2 h (film thickness, 500 ,~}.
Si AND N AUGER SIGNALS FOR N1TRIDED S i O 2 FILMS
137
The depth profiles of the N KLL, Si LVV and O K L L Auger peak heights are plotted in Fig. 2 for the sample described above. The nitrogen concentration is increased at the surface of the film and at the interface of the film and the silicon substrate. In the interior of the film the nitrogen concentration is lower although the nitrogen signal was easily detectable. Corresponding to the highly nitrided layer, the oxygen signal is decreased. The structures of the Si LVV and N K L L signals in the first-order derivative for dN/dE are shown in Fig. 3. The approximate sputter-etched thickness (in ~ngstr6ms) from the initial surface before measurement is given to the right of each curve. The major peak in the Si LVV signal, called the first peak, occurs at 80 or 81 eV for the initial sputtering stage. The first peak shifts to about 79 eV in the interior but it returns to 80 or 81 eV near the interface, although a major peak, called the second peak, appears at about 92 eV. r~second pesIc~ ~
first peak-
"~
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~
initial
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(420)
/"
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~'----v~'~.j.-~-
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'
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40
60
380
400
Auger electron energy (eV) Fig. 3. Si LVV and N KLL Auger signals from an SiO z film nitrided at 1100~C for 2 h (film thickness, 500,~). The film was repeatedly processed while undergoing AES measurement and argon ion sputter etching. The numerals on the right of the curves are the etched depth (in :~ngstr6ms) from the initial surface.
The depth dependence of the first peak energy corresponds to that of the nitrogen concentration. At both the surface and the interface, where the nitrogen concentration is greater, the first peak energy shifts to a higher value. In the interior, where the nitrogen concentration is less, the first peak energy decreases to the value for SiO2. This is more pronounced for less-nitrided samples. Signals are shown in Fig. 4 for samples nitrided at 900 "C for 2 h. The nitrogen concentration is less in the
138
Y. Y O R I U M E
interior of the film. The first peak energy is reduced to 78 eV, which is equivalent to the value for an SiO2 film. For intensely nitrided samples, such as films 100 or 200/1, thick nitrided at 1200 °C for 2 h or nitrided at 1100°C for 5 h, the first peak energy does not become less than 80 eV. The energy shift appears to correspond to the nitrogen concentration in the oxynitride films.
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initial
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SiO2 film n i t r i d e d at 900 C for 2 h (film thickness, 500 ~). The film was repeatedly processed while undergoing AES measurement and argon ion sputter etching. The numerals on the right of the curves are the etched depth (in ~ngstr6ms) from the initial surface. Fig. 4. Si L V V a n d N K L L A u g e r signals f r o m a n
The nitrogen peak energy is 382 or 383 eV at the surface and in the interior. At the interface it shifts to about 385 eV. This does not correspond to the nitrogen concentration. Amano and Ekstedt 8 have reported that nitrogen or a complex of nitrogen and hydrogen migrates to the SiO2-Si interface during high temperature thermal nitriding, where they react with interfacial silicon atoms. This will cause a shift in the nitrogen peak energy. Nitrogen at the surface or in the interior is also bound to a silicon atom, but there are many more oxygen atoms around the silicon atoms so that the other bonds to the silicon atom may be occupied by oxygen. Accordingly, the nitrogen peak energy does not depend on its concentration at the surface and in the interior. Amano and Ekstedt have also stated in ref. 8 that nonregistered silicon atoms at the interface are increased for thermally nitrided samples. This may be caused by the formation of silicon nitride, which is denser than SiO2 and has more silicon atoms per molecule. This is consistent with the observed nitrogen pile-up 4.
Si AND N AUGER SIGNALS FOR NITRIDED S i O 2 FILMS
139
4. CONCLUSION
Si LVV and N KLL Auger signals were determined at various depths in thermally nitrided SiO 2 films. The silicon Auger peak energy is shifted in correspondence to the nitrogen concentration. The nitrogen Auger peak energy does not depend on the nitrogen concentration but is shifted at the film-silicon interface. REFERENCES
1 T. lto, T. Nozaki and H. ishikawa, J. Electrochem. Soc., 127 (1980) 2053. 2 T. Ito, H. Arakawa, T. Nozaki and H. lshikawa, J. Electroehem. Soc., 127(1980) 2248. 3 M . L . Naiman, F. L. Terry, J. A. Burus, J. I. Raffel and R. Aucoin, Tech. Dig. Int. Electron Det, ices Meet., 1980, IEEE, New York, 1980, p. 562. 4 Y. Yoriume, J. Vac. Sci. Technol. B, 1 (1983) 67. 5 J.S. Johannessen and W. E. Spicer, J. Vac. Sci. Technol., 13 (1976) 849. 6 T. lto, H. lshikawa and Y. Fukukawa, Proc. 12th Solid State Devices Conj,, in Jpn. J. ,4ppl. Phys., Suppl. 1, 18(1981)33. 7 H. Houston and M. G. Lagary, J. Vac. Sci. Technol., 13 (1976) 361. 8 J. A m a n o and T. Ekstedt, Appl. Phys. Lett., 41 (1982) 81. 9 T . N . Wittberg, J. R. Hoenigman, W. E. Moddeman, C. R. Cothern and M. R. Gulett, J. Vae. Sci. Technol., 15 (1978) 348. 10 H . H . Madden and P. H. Holloway, J. Vac. Sci. Technol., 16 (1978) 618. 11 A. van Oostrom, L. Augustus, F. H. P. M. Habraken and A. E. T. Kniper, J. Vac. Sci. Teehnol.. 20 (1982) 953. 12 H . H . Madden, J. Electrochem. Soc.. 128(1981) 625. 13 S. T h o r n a s a n d R.J. Mattox, J. Vac. Sci. Technol., 124(1977) 1942. 14 R. Fteckingbottom and P. R. Wood, Sur]~ Sci., 36 (1973) 594. [5 J . F . Delord, A . G . S c h r o t t a n d S . C. Fain. Jr.,J. Vac. Sci. Technol.,17 (1980) 517.
135
F I N E S T R U C T U R E O F Si LVV AND N K L L A U G E R SIGNALS FOR T H E R M A L L Y N I T R I D E D SiO2 FILMS YUTAKA YORIUME Ibaraki Electrical Communication Laboratory, Ntppon Telegraph and Telephone Public" Corporation. Tokai. lbaraki-ken 319-11 (Japan) (Received September 9, 1983; accepted March 20, 1984)
The fine structure of Si LVV and N K L L Auger signals was determined at various depths in thermally nitrided SiO2 films. The peak near 80 eV is shifted to a slightly higher energy at the surface and at the film-silicon interface, where the nitrogen concentration is greater. It is a little lower in the interior, where the nitrogen concentration is less. The N K L L peak was shifted to a slightly higher energy at the interface than at the surface and in the interior.
i. INTRODUCTION
Silicon nitride is a dielectric material that has been found to be useful for a number of applications in the semiconductor device industry. While it is most commonly prepared by chemical vapour deposition or reactive plasma deposition, the silicon oxynitride film obtained by the thermal nitriding of SiO2 film has recently become a centre of attention and interest as a result of the work of Ito et al. t and its potential application to very-large-scale integration technology 2"3. Numerous published studies are available on the Auger electron spectroscopy (AES) analysis of silicon oxynitride films. Some have concerned depth profiles for nitrogen, oxygen and silicon 1'4- 6. For example, nitrogen depth profiles have been measured and it has been shown that nitrogen penetrates deeply through the SiO 2 film and piles up at the film-silicon interface 4"7. Amano and Ekstedt s have also observed the nitrogen pile-up at the interface in addition to the highly nitrided surface layer using a backscattering method. Other studies have concerned the chemical structure of the silicon oxynitride films prepared by high temperature or plasma chemical vapour deposition 9-13 or the thermal nitriding of silicon 1'*'I~ However, chemical structure details have not been reported for thermally nitrided SiO2 films. In this investigation the structure of the Si LVV and N K L L AES signals for thermally nitrided SiO2 films was determined. The AES signals were shown to have a dependence on the depth in the thermally nitrided SiO 2 films. 2. EXPERIMENTAL DETAILS
Czochralski-grown p-type (100)-oriented silicon wafers with resistivities ranging from 1 to 4 f~ cm were used for preparation of the samples. After the wafers had 0040-6090/84/$3.00
f(~ ElsevierSequoia/Printedin The Netherlands
136
v. YORIUME
been chemically cleaned in hot aqueous N H 4 O H - H z O 2 and H C I - H e O 2 solutions, followed each time by rinsing in deionized water, and had been given a slight SiO z etch in dilute hydrofluoric acid, they were thermally oxidized at temperatures of 900, 950 and 1000 +'C in dry oxygen. The oxide film thickness ranged from 100 to 1000 A. The wafers were then subjected to thermal nitriding in anhydrous ammonia gas. The nitriding details are described in ref. 4, For Auger analysis the samples were mounted in an SAM 590 system from Physical Electronics. The vacuum system was degassed to a base pressure of about 5 × 10 7 Pa. The primary electron beam was incident at an angle of 60 + with respect to the normal to the surface of the specimen. The primary beam energy Ep was 5.0 keV. The peak-to-peak modulation for the first derivative recording was held at 1 eV. The resolution of the cylindrical mirror analyser was set at 0.3~',i, i.e. the energy difference between the positive and negative peaks of reflected primary electrons was about 0.3'.',/0of the primary beam energy. Auger analyses were carried out at various film depths, with many repetitions of analysis and sputtering. During the sputtering the chamber was filled with argon gas to a pressure of about 6.7 x 10 - a Pa, and the samples were bombarded with 1 keV argon ions. The sputtering rate was varied by adjusting the ion beam raster area such that the sputtering time for each sputtering step was several minutes or several tens of minutes. The pressure in the vacuum chamber was also kept at about 6.7 x 10 - 3 Pa during the analysis. 3. RESULTS AND DISCUSSION Figure I shows a typical Auger signal from the surface of a sample 500/~ thick that was nitrided at 1100 ~C for 2 h, N K L L peaks ranging from 360 to 390 eV, in addition to Si LVV (40-1 l0 eV) and O K L L (480-520 eV) peaks, were detected. The carbon signal is due to the adsorbate, which disappears if the surface is sputtered slightly.
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o~ •
SiLV v
~ o
• £
t
200
l
i
400
Electron energy (eV}
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£
600
0
,
,
5
10
NKLL
15
o
t
20
25
30
Sputtering Time (rain)
Fig. l. Typical Auger signal from an SiO 2 film nitrided at ! 1 0 0 C for 2 h (film thickness, 500/~}. Fig. 2. Depth profiles of the N KLL, Si LVV and O K L L Auger peak heights from an SiO 2 film nitrided at 1100"C for 2 h (film thickness, 500 ,~}.
Si AND N AUGER SIGNALS FOR N1TRIDED S i O 2 FILMS
137
The depth profiles of the N KLL, Si LVV and O K L L Auger peak heights are plotted in Fig. 2 for the sample described above. The nitrogen concentration is increased at the surface of the film and at the interface of the film and the silicon substrate. In the interior of the film the nitrogen concentration is lower although the nitrogen signal was easily detectable. Corresponding to the highly nitrided layer, the oxygen signal is decreased. The structures of the Si LVV and N K L L signals in the first-order derivative for dN/dE are shown in Fig. 3. The approximate sputter-etched thickness (in ~ngstr6ms) from the initial surface before measurement is given to the right of each curve. The major peak in the Si LVV signal, called the first peak, occurs at 80 or 81 eV for the initial sputtering stage. The first peak shifts to about 79 eV in the interior but it returns to 80 or 81 eV near the interface, although a major peak, called the second peak, appears at about 92 eV. r~second pesIc~ ~
first peak-
"~
'
~
initial
~",..j~,~ z w ,.r,,
(420)
/"
(4401
~'----v~'~.j.-~-
SiLvv
(49O) (5001
NKLL
'
'
'
'
40
60
380
400
Auger electron energy (eV) Fig. 3. Si LVV and N KLL Auger signals from an SiO z film nitrided at 1100~C for 2 h (film thickness, 500,~). The film was repeatedly processed while undergoing AES measurement and argon ion sputter etching. The numerals on the right of the curves are the etched depth (in :~ngstr6ms) from the initial surface.
The depth dependence of the first peak energy corresponds to that of the nitrogen concentration. At both the surface and the interface, where the nitrogen concentration is greater, the first peak energy shifts to a higher value. In the interior, where the nitrogen concentration is less, the first peak energy decreases to the value for SiO2. This is more pronounced for less-nitrided samples. Signals are shown in Fig. 4 for samples nitrided at 900 "C for 2 h. The nitrogen concentration is less in the
138
Y. Y O R I U M E
interior of the film. The first peak energy is reduced to 78 eV, which is equivalent to the value for an SiO2 film. For intensely nitrided samples, such as films 100 or 200/1, thick nitrided at 1200 °C for 2 h or nitrided at 1100°C for 5 h, the first peak energy does not become less than 80 eV. The energy shift appears to correspond to the nitrogen concentration in the oxynitride films.
•••
initial
(lo) (40)
f
(250) ~,t---,.-,--~--~.p-.J~ (390) (420) (430)
Z
uJ
F
(450)
f
(470)
pf
1490)
(5O0)
SiLvv
NKLt_
l
t
I
l
I
;
I
40
60
80
100
360
380
400
Auger electron energy (eV)
SiO2 film n i t r i d e d at 900 C for 2 h (film thickness, 500 ~). The film was repeatedly processed while undergoing AES measurement and argon ion sputter etching. The numerals on the right of the curves are the etched depth (in ~ngstr6ms) from the initial surface. Fig. 4. Si L V V a n d N K L L A u g e r signals f r o m a n
The nitrogen peak energy is 382 or 383 eV at the surface and in the interior. At the interface it shifts to about 385 eV. This does not correspond to the nitrogen concentration. Amano and Ekstedt 8 have reported that nitrogen or a complex of nitrogen and hydrogen migrates to the SiO2-Si interface during high temperature thermal nitriding, where they react with interfacial silicon atoms. This will cause a shift in the nitrogen peak energy. Nitrogen at the surface or in the interior is also bound to a silicon atom, but there are many more oxygen atoms around the silicon atoms so that the other bonds to the silicon atom may be occupied by oxygen. Accordingly, the nitrogen peak energy does not depend on its concentration at the surface and in the interior. Amano and Ekstedt have also stated in ref. 8 that nonregistered silicon atoms at the interface are increased for thermally nitrided samples. This may be caused by the formation of silicon nitride, which is denser than SiO2 and has more silicon atoms per molecule. This is consistent with the observed nitrogen pile-up 4.
Si AND N AUGER SIGNALS FOR NITRIDED S i O 2 FILMS
139
4. CONCLUSION
Si LVV and N KLL Auger signals were determined at various depths in thermally nitrided SiO 2 films. The silicon Auger peak energy is shifted in correspondence to the nitrogen concentration. The nitrogen Auger peak energy does not depend on the nitrogen concentration but is shifted at the film-silicon interface. REFERENCES
1 T. lto, T. Nozaki and H. ishikawa, J. Electrochem. Soc., 127 (1980) 2053. 2 T. Ito, H. Arakawa, T. Nozaki and H. lshikawa, J. Electroehem. Soc., 127(1980) 2248. 3 M . L . Naiman, F. L. Terry, J. A. Burus, J. I. Raffel and R. Aucoin, Tech. Dig. Int. Electron Det, ices Meet., 1980, IEEE, New York, 1980, p. 562. 4 Y. Yoriume, J. Vac. Sci. Technol. B, 1 (1983) 67. 5 J.S. Johannessen and W. E. Spicer, J. Vac. Sci. Technol., 13 (1976) 849. 6 T. lto, H. lshikawa and Y. Fukukawa, Proc. 12th Solid State Devices Conj,, in Jpn. J. ,4ppl. Phys., Suppl. 1, 18(1981)33. 7 H. Houston and M. G. Lagary, J. Vac. Sci. Technol., 13 (1976) 361. 8 J. A m a n o and T. Ekstedt, Appl. Phys. Lett., 41 (1982) 81. 9 T . N . Wittberg, J. R. Hoenigman, W. E. Moddeman, C. R. Cothern and M. R. Gulett, J. Vae. Sci. Technol., 15 (1978) 348. 10 H . H . Madden and P. H. Holloway, J. Vac. Sci. Technol., 16 (1978) 618. 11 A. van Oostrom, L. Augustus, F. H. P. M. Habraken and A. E. T. Kniper, J. Vac. Sci. Teehnol.. 20 (1982) 953. 12 H . H . Madden, J. Electrochem. Soc.. 128(1981) 625. 13 S. T h o r n a s a n d R.J. Mattox, J. Vac. Sci. Technol., 124(1977) 1942. 14 R. Fteckingbottom and P. R. Wood, Sur]~ Sci., 36 (1973) 594. [5 J . F . Delord, A . G . S c h r o t t a n d S . C. Fain. Jr.,J. Vac. Sci. Technol.,17 (1980) 517.