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Atomic-layer chemical-vapor-deposition of silicon dioxide films with an extremely low hydrogen content
Applied Surface Science 130–132 Ž1998. 202–207
Atomic-layer chemical-vapor-deposition of silicon dioxide films with an extremely low hydrogen content Kei-ichi Yamaguchi a , Shigeru Imai a,) , Naoto Ishitobi a , Masashi Takemoto a , Hidejiro Miki a , Masakiyo Matsumura b a
Department of Electrical and Electronic Engineering, Ritsumeikan UniÕersity, 1-1-1, Noji-Higashi, Kusatsu-shi, Shiga 525-77, Japan b Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152, Japan Received 1 September 1997; accepted 10 December 1997
Abstract We achieved the atomic-layer-deposition of SiO 2 with an extremely low H content for the first time by using SiŽNCO.4 and NŽC 2 H 5 . 3. The deposition rate was independent of the exposure times of both sources. The saturated deposition rate ˚ was about 1.2 Arcycle at 1508C. AFM observation revealed that the increase of roughness after 50 deposition cycles was ˚ in root mean square. The FT-IR spectra showed that the deposited film was SiO 2 without Si–H, Si–OH or only about 0.4 A NCO bond. q 1998 Elsevier Science B.V. All rights reserved. PACS: 81.15.Gh; 34.50.Dy; 77.55.q f Keywords: Atomic-layer chemical-vapor deposition; Extremely low hydrogen content; Silicon dioxide; Tetra-iso-cyanate-silane; Tri-ethylamine
1. Introduction
˚ have Recently, insulating films as thin as 100 A been required for ULSI applications, for example, DRAM capacitor. Furthermore, the precise control of film thickness on an atomic scale is required for a tunnel barrier used in non-volatile memory transistors or single electron transistors. This is because a tunneling current flowing through the barrier depends exponentially on the reciprocal barrier thickness. Atomic layer epitaxy ŽALE. w1–5x, which was
)
Corresponding author. Tel.: q81-77-561 2883; fax: q81-77561 2663; e-mail: [email protected].
developed for compound semiconductors at first, or atomic-layer chemical-vapor-deposition ŽAL-CVD. w6–12x is one of the most promising technologies for this requirement, because it can precisely control deposition layer thickness on an atomic scale. Thus, various studies on the AL-CVD of insulating films have been reported. Gasser et al. have successfully demonstrated the AL-CVD of SiO 2 films by alternating the exposures of tetra-iso-cyanate-silane ŽTICS: SiŽNCO.4 . and H 2 O w7x. It should be noted that they adopted TICS as a source gas. It is significantly different from other sources Ž for example, SiH 4 , TEOS ŽSiŽOC 2 H 5 .4 ., etc.. because of its ‘H-free’ characteristics. However, the films deposited in their study
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 0 5 1 - 8
K. Yamaguchi et al.r Applied Surface Science 130–132 (1998) 202–207
contained a lot of hydrogen which deteriorates film quality, because they used H 2 O as a partner gas. On the other hand, Uchida et al. w13x have proposed a new gas combination, i.e., TICS and tri-alkyl-amine ŽTAA: NR 3 ; R5CH 3 , C 2 H 5 . . . .. They have demonstrated H-free chemical-vapor-deposition ŽCVD. of SiO 2 from a mixture of TICS and TAA, though not in a layer-by-layer manner. The deposited films had an extremely low H content since the C–H bonds in TAA molecules are stable. In this paper, we attempted an AL-CVD of SiO 2 with an extremely low H content by using tertiaryethyl-amine ŽTEA: NŽC 2 H 5 . 3 .. TEA is a liquid with a boiling point of 89.68C and a vapor pressure of 57 Torr at 258C. A substrate was alternately exposed to TICS and TEA, instead of H 2 O. Deposition characteristics, surface flatness and film composition will be presented. 2. Experiments Fig. 1 shows the hot-wall type CVD system used in our experiments. A chamber was evacuated by a mechanical booster pump ŽMBP. backed by a rotary pump ŽRP.. The background pressure was about 20 mTorr. It can supply TICS and TEA alternately by the opening and closing of computer-controlled valves. An N2 line was also attached for purging. The supplying of source gas was carried out in a batch manner. That is, a sample was exposed to one of the source gases by confining the gas within a chamber and the gas was exhausted after the exposure duration. N2 purging was also carried out in a batch manner. Source gases were supplied without a carrier gas, and their chamber pressures were their vapor pressures at their reservoir temperatures. TICS
Fig. 1. Schematic view of the AL-CVD system.
203
Fig. 2. Time sequence of atomic layer deposition.
and TEA reservoirs were kept at 408C and room temperature Žabout 20 to 258C., respectively. Gas lines were kept at 908C in order to prevent the condensation of sources within lines. The time sequence of atomic layer deposition is shown in Fig. 2. At first, TICS was supplied to the chamber at about 1 Torr which is its vapor pressure at 408C. Though the detailed mechanism is not clear, we expect that TICS molecules are partially decomposed at the chemically active surface to terminate the surface and that the surface covered by partially decomposed TICS molecules becomes inactive to other TICS molecules. After exhausting TICS, a number of N2 batch purgings were done. Next, TEA was supplied at 57 Torr, which is its vapor pressure at room temperature. Here, we expect that TEA completely decomposes TICS molecules on the surface to form SiO 2 monolayer and that the surface becomes active to TICS again. After exhausting TEA, a number of N2 batch purgings were done again. Si chips were chemically cleaned by using organic Žmethanol. and inorganic ŽNH 4OH:H 2 O 2 :H 2 O s 1:1:4. solutions, and then dipped in a diluted HF Ž5%. solution just before loading into the chamber. Fifty cycles of deposition were done. A set of standard parameters is tabulated in Table 1. These values were used throughout the experiment unless otherwise mentioned. The dependence of deposited film thickness on each parameter was investigated. Deposited film thickness was measured by using ellipsometry. The flatness of the surface before and after the deposition was observed by using atomic force microscopy ŽAFM.. The composition of deposited films was
204
K. Yamaguchi et al.r Applied Surface Science 130–132 (1998) 202–207
Table 1 Standard conditions TICS exposure time Temperature of TICS reservoir TEA exposure time Temperature of TEA reservoir Deposition temperature Evacuation time N2 purging time N2 purging number Background pressure Temperature of gas lines Cycle number
180 s 408C 180 s room temperature 1508C 10 s 7s 5 about 20 mTorr 908C 50 cycles Fig. 4. Dependence of the film thickness on the exposure time of TICS.
investigated by using Fourier transform infrared absorption ŽFT-IR..
3. Results and discussion 3.1. Deposition characteristics Fig. 3 shows the dependence of the film thickness on the number of cycles under the standard conditions. The film thickness linearly increased with the cycle number. The deposition rate was about 1.2 ˚ Arcycle. Morishita et al. w9x have estimated 1 mono˚ We layer ŽML. of amorphous SiO 2 to be 2 A. followed Morishita et al., though various definitions of 1 ML have been reported. Therefore, the deposition rate corresponds to 0.6 MLrcycle. The dependence of the film thickness on the exposure time of TICS and TEA is shown in Figs. 4 and 5, respectively. The film thickness depended on the exposure time of TICS being shorter than 120 s.
Fig. 3. Dependence of the film thickness on the number of cycles.
But over the exposure time of 120 s, the film thickness was saturated. Here, the whole surface was inactive to TICS. We also confirmed, experimentally, that the deposition rate was independent of the TICS pressure. The adsorption rate of TICS before saturation was much lower, in spite of a higher TICS pressure, than in the work of Gasser et al. w7x. One of the possible reasons is that the higher deposition temperature decreased the adsorption rate. Another is the difference in the surface conditions. In Gasser’s work, the surface was covered by O–H bond, which is very active to TICS at room temperature. However, we suppose that in our work there is no O–H bond on the surface because our deposition was almost H-free, as will be described later. The film thickness increased with the exposure time of TEA and became saturated when exposure
Fig. 5. Dependence of the film thickness on the exposure time of TEA.
K. Yamaguchi et al.r Applied Surface Science 130–132 (1998) 202–207
time was over 60 s ŽFig. 5.. Here, the whole surface was changed and became active to TICS. Both of the ˚ saturated rates were about 1.2 Arcycle. These are evidence of self-limited deposition. However, the deposition rate depended on the deposition temperature as shown in Fig. 6. The film thickness increased as the deposition temperature ˚ decreased and it reached 2 Arcycle, that is, 1 MLrcycle at 808C. Unfortunately, we could not confirm if the saturation occurred below 808C because measured deposition rates below 808C were extremely scattered and not reliable. In order to confirm the effect of N2 purging during the deposition sequence, the dependence of the film thickness on purging processes was investigated. The thickness was saturated over the N2 purging number of 3 as shown in Fig. 7. Thus, N2 purging was found to work adequately under the standard purging number of 5. In other words, residual source gases do not affect the self-limitation under standard conditions. We discuss here the reason why the deposition rate decreased with increasing deposition temperature. Gasser et al. w7x explained that steric hindrance ˚ caused their non-ideal deposition rate of 1.7 Arcycle at room temperature, but this effect is supposed to be independent of deposition temperature. There was no unwanted excess deposition by residual source gases, so the excess deposition which depends on the deposition temperatures, is not the answer. Insufficient adsorption of TICS or TEA during the exposure time at higher temperatures is also excluded from the answer because the deposition rate is saturated for the exposure time.
Fig. 6. Dependence of the film thickness on the deposition temperature.
205
Fig. 7. Dependence of the film thickness on the N2 purging number.
One of the remaining possible reasons is that some kind of contaminant and a source gas competitively terminated the surface. Though the whole surface was saturated by terminators due to differences in the temperature dependence of the adsorption rate, the amount of the source adsorbed changed with the temperature. The origin of the contaminants is unknown, but the contaminants must be removed from the source before the next source exposure so that the deposition can continue. The contaminants may have originated from the decomposition of TICS and may have been removed from the surface by TEA. Another possible reason is that there are two kinds of adsorption mechanism of TEA. In the first mechanism, the two ethyl groups of a TEA molecule react with two surface sites and the remaining one ethyl group is kept as a reactive site for TICS in the next cycle. In the second mechanism, one ethyl group reacts with the surface site and two ethyl groups remain as reactive sites for TICS. As a result, the number of new sites for TICS in the second mechanism is four times greater than those in the first mechanism. The adsorption rate of TEA, however, has different temperature dependences for the two adsorption mechanisms; therefore, the number of surface sites for TICS changes with the temperature. The third possible reason is that the thickness of 1 monolayer changed with the temperature, that is, the density of the film changed with the temperature. Fig. 8 shows AFM images of the surface after 50 cycles of deposition. The original surface roughness ˚ in root mean square and the roughness was 1.71 A
206
K. Yamaguchi et al.r Applied Surface Science 130–132 (1998) 202–207
Fig. 8. AFM image of a surface after 50 cycles of deposition.
˚ The increased surafter the deposition was 2.07 A. ˚ Thus, we face roughness was only about 0.4 A. concluded the surface flatness is kept after AL-CVD. 3.2. Film composition The FT-IR absorption spectrum of the film after 150 cycles of deposition is shown in Fig. 9. The sample was kept in a pure N2 ambient after unloading the sample from the CVD system and until completing the FT-IR measurement in order to exclude moisture because NCO groups remaining in the deposited film are very reactive to moisture at room temperature. The FT-IR absorption spectrum of ˚ thick thermal oxide film made by dry the 120 A oxidation at 10008C is also shown in Fig. 9 for reference. The central Si–O asymmetric stretching
mode appeared at around 1060 cmy1 in the spectrum of the as-deposited sample, but a peak at 840 cmy1 for Si–N bonds was absent. Therefore, the deposited film is SiO 2 , not Si 3 N4 . A peak at 2300 cmy1 for Si–H or NCO bonds was not found. Peaks for SiŽCH 3 . n , SiC 2 H 5 Žat about 1250 cmy1 . or for CO Žat 1870 to 1650 cmy1 were not found either, though we need to investigate more about C content using other techniques. For the purpose of comparison, the film deposited for 150 cycles was boiled in H 2 O for 60 min, and annealed in vacuum for 30 min at 7008C after boiling. The FT-IR spectra of the film after boiling and annealing are also shown in Fig. 9. The spectrum is very similar for the as-deposited sample and for the annealed sample. The strong peak at 930 cmy1 for Si–OH was found in the spectrum for the boiled sample. Therefore, we concluded that the as-deposited sample had only a few Si–OH bond. In other words, AL-CVD of SiO 2 with an extremely low H content was achieved. Finally, small reverse peaks from 1400 to 1800 cmy1 came from a reference data for removing background and they were due to the H 2 O remaining in the ambient. 4. Conclusion The AL-CVD of SiO 2 with an extremely low H content was achieved by alternating the exposures of TICS and TEA. The deposition rate was independent of gas exposure times, although that depended on the deposition temperature. The deposition rate was about ˚ 1.2 Arcycle at 1508C. AFM observation revealed that the increase in roughness after 50 cycles of the ˚ The FT-IR spectra deposition was only about 0.4 A. showed that the film was SiO 2 and had extremely low H content. We hope this method can be used for ultra-thin gate formation processing used in ULSIs such as flash memories. Acknowledgements TICS was a gift from Showa-Denko.
Fig. 9. FT-IR absorption spectra of a Ža. sample after 150 cycles of deposition, Žb. sample a after boiling in H 2 O for 60 min, Žc. sample b after annealing at 7008C for 30 min, and Žd. thermal oxide.
References w1x T. Suntola, M.J. Antson, US Patent, No. 4058430 Ž1977.. w2x T. Suntola, Mater. Sci. Rep. 4 Ž1989. 261.
K. Yamaguchi et al.r Applied Surface Science 130–132 (1998) 202–207 w3x J. Nishizawa, K. Aoki, S. Suzuki, K. Kikuchi, J. Cryst. Growth 99 Ž1990. 502. w4x S. Imai, T. Iizuka, O. Sugiura, M. Matsumura, Thin Solid Films 225 Ž1993. 168. w5x S. Sugahara, M. Kadoshima, T. Kitamura, S. Imai, M. Matsumura, Appl. Surf. Sci. 90 Ž1995. 349. w6x M. Ritala, M. Leskela, P. Soininen, L. Niinisto, ¨ E. Nykanen, ¨ ¨ Thin Solid Films 225 Ž1993. 288. w7x W. Gasser, Y. Uchida, M. Matsumura, Thin Solid Films 250 Ž1994. 213. w8x S.M. George, O. Sneh, A.C. Dillon, M.L. Wise, A.W. Ott, L.A. Okada, J.D. Way, Appl. Surf. Sci. 82–83 Ž1994. 460.
207
w9x S. Morishita, Y. Uchida, M. Matsumura, Jpn. J. Appl. Phys. 34 Ž1995. 5738. w10x S. Morishita, W. Gasser, K. Usami, M. Matsumura, J. NonCryst. Solids 187 Ž1995. 66. w11x H. Goto, K. Shibahara, S. Yokoyama, Appl. Phys. Lett. 68 Ž1996. 3257. w12x S. Morishita, S. Sugahara, M. Matsumura, Appl. Surf. Sci. 112 Ž1997. 198. w13x Y. Uchida, S. Takei, M. Matsumura, Jpn. J. Appl. Phys. 36 Ž1997. 1509.
Atomic-layer chemical-vapor-deposition of silicon dioxide films with an extremely low hydrogen content Kei-ichi Yamaguchi a , Shigeru Imai a,) , Naoto Ishitobi a , Masashi Takemoto a , Hidejiro Miki a , Masakiyo Matsumura b a
Department of Electrical and Electronic Engineering, Ritsumeikan UniÕersity, 1-1-1, Noji-Higashi, Kusatsu-shi, Shiga 525-77, Japan b Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152, Japan Received 1 September 1997; accepted 10 December 1997
Abstract We achieved the atomic-layer-deposition of SiO 2 with an extremely low H content for the first time by using SiŽNCO.4 and NŽC 2 H 5 . 3. The deposition rate was independent of the exposure times of both sources. The saturated deposition rate ˚ was about 1.2 Arcycle at 1508C. AFM observation revealed that the increase of roughness after 50 deposition cycles was ˚ in root mean square. The FT-IR spectra showed that the deposited film was SiO 2 without Si–H, Si–OH or only about 0.4 A NCO bond. q 1998 Elsevier Science B.V. All rights reserved. PACS: 81.15.Gh; 34.50.Dy; 77.55.q f Keywords: Atomic-layer chemical-vapor deposition; Extremely low hydrogen content; Silicon dioxide; Tetra-iso-cyanate-silane; Tri-ethylamine
1. Introduction
˚ have Recently, insulating films as thin as 100 A been required for ULSI applications, for example, DRAM capacitor. Furthermore, the precise control of film thickness on an atomic scale is required for a tunnel barrier used in non-volatile memory transistors or single electron transistors. This is because a tunneling current flowing through the barrier depends exponentially on the reciprocal barrier thickness. Atomic layer epitaxy ŽALE. w1–5x, which was
)
Corresponding author. Tel.: q81-77-561 2883; fax: q81-77561 2663; e-mail: [email protected].
developed for compound semiconductors at first, or atomic-layer chemical-vapor-deposition ŽAL-CVD. w6–12x is one of the most promising technologies for this requirement, because it can precisely control deposition layer thickness on an atomic scale. Thus, various studies on the AL-CVD of insulating films have been reported. Gasser et al. have successfully demonstrated the AL-CVD of SiO 2 films by alternating the exposures of tetra-iso-cyanate-silane ŽTICS: SiŽNCO.4 . and H 2 O w7x. It should be noted that they adopted TICS as a source gas. It is significantly different from other sources Ž for example, SiH 4 , TEOS ŽSiŽOC 2 H 5 .4 ., etc.. because of its ‘H-free’ characteristics. However, the films deposited in their study
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 0 5 1 - 8
K. Yamaguchi et al.r Applied Surface Science 130–132 (1998) 202–207
contained a lot of hydrogen which deteriorates film quality, because they used H 2 O as a partner gas. On the other hand, Uchida et al. w13x have proposed a new gas combination, i.e., TICS and tri-alkyl-amine ŽTAA: NR 3 ; R5CH 3 , C 2 H 5 . . . .. They have demonstrated H-free chemical-vapor-deposition ŽCVD. of SiO 2 from a mixture of TICS and TAA, though not in a layer-by-layer manner. The deposited films had an extremely low H content since the C–H bonds in TAA molecules are stable. In this paper, we attempted an AL-CVD of SiO 2 with an extremely low H content by using tertiaryethyl-amine ŽTEA: NŽC 2 H 5 . 3 .. TEA is a liquid with a boiling point of 89.68C and a vapor pressure of 57 Torr at 258C. A substrate was alternately exposed to TICS and TEA, instead of H 2 O. Deposition characteristics, surface flatness and film composition will be presented. 2. Experiments Fig. 1 shows the hot-wall type CVD system used in our experiments. A chamber was evacuated by a mechanical booster pump ŽMBP. backed by a rotary pump ŽRP.. The background pressure was about 20 mTorr. It can supply TICS and TEA alternately by the opening and closing of computer-controlled valves. An N2 line was also attached for purging. The supplying of source gas was carried out in a batch manner. That is, a sample was exposed to one of the source gases by confining the gas within a chamber and the gas was exhausted after the exposure duration. N2 purging was also carried out in a batch manner. Source gases were supplied without a carrier gas, and their chamber pressures were their vapor pressures at their reservoir temperatures. TICS
Fig. 1. Schematic view of the AL-CVD system.
203
Fig. 2. Time sequence of atomic layer deposition.
and TEA reservoirs were kept at 408C and room temperature Žabout 20 to 258C., respectively. Gas lines were kept at 908C in order to prevent the condensation of sources within lines. The time sequence of atomic layer deposition is shown in Fig. 2. At first, TICS was supplied to the chamber at about 1 Torr which is its vapor pressure at 408C. Though the detailed mechanism is not clear, we expect that TICS molecules are partially decomposed at the chemically active surface to terminate the surface and that the surface covered by partially decomposed TICS molecules becomes inactive to other TICS molecules. After exhausting TICS, a number of N2 batch purgings were done. Next, TEA was supplied at 57 Torr, which is its vapor pressure at room temperature. Here, we expect that TEA completely decomposes TICS molecules on the surface to form SiO 2 monolayer and that the surface becomes active to TICS again. After exhausting TEA, a number of N2 batch purgings were done again. Si chips were chemically cleaned by using organic Žmethanol. and inorganic ŽNH 4OH:H 2 O 2 :H 2 O s 1:1:4. solutions, and then dipped in a diluted HF Ž5%. solution just before loading into the chamber. Fifty cycles of deposition were done. A set of standard parameters is tabulated in Table 1. These values were used throughout the experiment unless otherwise mentioned. The dependence of deposited film thickness on each parameter was investigated. Deposited film thickness was measured by using ellipsometry. The flatness of the surface before and after the deposition was observed by using atomic force microscopy ŽAFM.. The composition of deposited films was
204
K. Yamaguchi et al.r Applied Surface Science 130–132 (1998) 202–207
Table 1 Standard conditions TICS exposure time Temperature of TICS reservoir TEA exposure time Temperature of TEA reservoir Deposition temperature Evacuation time N2 purging time N2 purging number Background pressure Temperature of gas lines Cycle number
180 s 408C 180 s room temperature 1508C 10 s 7s 5 about 20 mTorr 908C 50 cycles Fig. 4. Dependence of the film thickness on the exposure time of TICS.
investigated by using Fourier transform infrared absorption ŽFT-IR..
3. Results and discussion 3.1. Deposition characteristics Fig. 3 shows the dependence of the film thickness on the number of cycles under the standard conditions. The film thickness linearly increased with the cycle number. The deposition rate was about 1.2 ˚ Arcycle. Morishita et al. w9x have estimated 1 mono˚ We layer ŽML. of amorphous SiO 2 to be 2 A. followed Morishita et al., though various definitions of 1 ML have been reported. Therefore, the deposition rate corresponds to 0.6 MLrcycle. The dependence of the film thickness on the exposure time of TICS and TEA is shown in Figs. 4 and 5, respectively. The film thickness depended on the exposure time of TICS being shorter than 120 s.
Fig. 3. Dependence of the film thickness on the number of cycles.
But over the exposure time of 120 s, the film thickness was saturated. Here, the whole surface was inactive to TICS. We also confirmed, experimentally, that the deposition rate was independent of the TICS pressure. The adsorption rate of TICS before saturation was much lower, in spite of a higher TICS pressure, than in the work of Gasser et al. w7x. One of the possible reasons is that the higher deposition temperature decreased the adsorption rate. Another is the difference in the surface conditions. In Gasser’s work, the surface was covered by O–H bond, which is very active to TICS at room temperature. However, we suppose that in our work there is no O–H bond on the surface because our deposition was almost H-free, as will be described later. The film thickness increased with the exposure time of TEA and became saturated when exposure
Fig. 5. Dependence of the film thickness on the exposure time of TEA.
K. Yamaguchi et al.r Applied Surface Science 130–132 (1998) 202–207
time was over 60 s ŽFig. 5.. Here, the whole surface was changed and became active to TICS. Both of the ˚ saturated rates were about 1.2 Arcycle. These are evidence of self-limited deposition. However, the deposition rate depended on the deposition temperature as shown in Fig. 6. The film thickness increased as the deposition temperature ˚ decreased and it reached 2 Arcycle, that is, 1 MLrcycle at 808C. Unfortunately, we could not confirm if the saturation occurred below 808C because measured deposition rates below 808C were extremely scattered and not reliable. In order to confirm the effect of N2 purging during the deposition sequence, the dependence of the film thickness on purging processes was investigated. The thickness was saturated over the N2 purging number of 3 as shown in Fig. 7. Thus, N2 purging was found to work adequately under the standard purging number of 5. In other words, residual source gases do not affect the self-limitation under standard conditions. We discuss here the reason why the deposition rate decreased with increasing deposition temperature. Gasser et al. w7x explained that steric hindrance ˚ caused their non-ideal deposition rate of 1.7 Arcycle at room temperature, but this effect is supposed to be independent of deposition temperature. There was no unwanted excess deposition by residual source gases, so the excess deposition which depends on the deposition temperatures, is not the answer. Insufficient adsorption of TICS or TEA during the exposure time at higher temperatures is also excluded from the answer because the deposition rate is saturated for the exposure time.
Fig. 6. Dependence of the film thickness on the deposition temperature.
205
Fig. 7. Dependence of the film thickness on the N2 purging number.
One of the remaining possible reasons is that some kind of contaminant and a source gas competitively terminated the surface. Though the whole surface was saturated by terminators due to differences in the temperature dependence of the adsorption rate, the amount of the source adsorbed changed with the temperature. The origin of the contaminants is unknown, but the contaminants must be removed from the source before the next source exposure so that the deposition can continue. The contaminants may have originated from the decomposition of TICS and may have been removed from the surface by TEA. Another possible reason is that there are two kinds of adsorption mechanism of TEA. In the first mechanism, the two ethyl groups of a TEA molecule react with two surface sites and the remaining one ethyl group is kept as a reactive site for TICS in the next cycle. In the second mechanism, one ethyl group reacts with the surface site and two ethyl groups remain as reactive sites for TICS. As a result, the number of new sites for TICS in the second mechanism is four times greater than those in the first mechanism. The adsorption rate of TEA, however, has different temperature dependences for the two adsorption mechanisms; therefore, the number of surface sites for TICS changes with the temperature. The third possible reason is that the thickness of 1 monolayer changed with the temperature, that is, the density of the film changed with the temperature. Fig. 8 shows AFM images of the surface after 50 cycles of deposition. The original surface roughness ˚ in root mean square and the roughness was 1.71 A
206
K. Yamaguchi et al.r Applied Surface Science 130–132 (1998) 202–207
Fig. 8. AFM image of a surface after 50 cycles of deposition.
˚ The increased surafter the deposition was 2.07 A. ˚ Thus, we face roughness was only about 0.4 A. concluded the surface flatness is kept after AL-CVD. 3.2. Film composition The FT-IR absorption spectrum of the film after 150 cycles of deposition is shown in Fig. 9. The sample was kept in a pure N2 ambient after unloading the sample from the CVD system and until completing the FT-IR measurement in order to exclude moisture because NCO groups remaining in the deposited film are very reactive to moisture at room temperature. The FT-IR absorption spectrum of ˚ thick thermal oxide film made by dry the 120 A oxidation at 10008C is also shown in Fig. 9 for reference. The central Si–O asymmetric stretching
mode appeared at around 1060 cmy1 in the spectrum of the as-deposited sample, but a peak at 840 cmy1 for Si–N bonds was absent. Therefore, the deposited film is SiO 2 , not Si 3 N4 . A peak at 2300 cmy1 for Si–H or NCO bonds was not found. Peaks for SiŽCH 3 . n , SiC 2 H 5 Žat about 1250 cmy1 . or for CO Žat 1870 to 1650 cmy1 were not found either, though we need to investigate more about C content using other techniques. For the purpose of comparison, the film deposited for 150 cycles was boiled in H 2 O for 60 min, and annealed in vacuum for 30 min at 7008C after boiling. The FT-IR spectra of the film after boiling and annealing are also shown in Fig. 9. The spectrum is very similar for the as-deposited sample and for the annealed sample. The strong peak at 930 cmy1 for Si–OH was found in the spectrum for the boiled sample. Therefore, we concluded that the as-deposited sample had only a few Si–OH bond. In other words, AL-CVD of SiO 2 with an extremely low H content was achieved. Finally, small reverse peaks from 1400 to 1800 cmy1 came from a reference data for removing background and they were due to the H 2 O remaining in the ambient. 4. Conclusion The AL-CVD of SiO 2 with an extremely low H content was achieved by alternating the exposures of TICS and TEA. The deposition rate was independent of gas exposure times, although that depended on the deposition temperature. The deposition rate was about ˚ 1.2 Arcycle at 1508C. AFM observation revealed that the increase in roughness after 50 cycles of the ˚ The FT-IR spectra deposition was only about 0.4 A. showed that the film was SiO 2 and had extremely low H content. We hope this method can be used for ultra-thin gate formation processing used in ULSIs such as flash memories. Acknowledgements TICS was a gift from Showa-Denko.
Fig. 9. FT-IR absorption spectra of a Ža. sample after 150 cycles of deposition, Žb. sample a after boiling in H 2 O for 60 min, Žc. sample b after annealing at 7008C for 30 min, and Žd. thermal oxide.
References w1x T. Suntola, M.J. Antson, US Patent, No. 4058430 Ž1977.. w2x T. Suntola, Mater. Sci. Rep. 4 Ž1989. 261.
K. Yamaguchi et al.r Applied Surface Science 130–132 (1998) 202–207 w3x J. Nishizawa, K. Aoki, S. Suzuki, K. Kikuchi, J. Cryst. Growth 99 Ž1990. 502. w4x S. Imai, T. Iizuka, O. Sugiura, M. Matsumura, Thin Solid Films 225 Ž1993. 168. w5x S. Sugahara, M. Kadoshima, T. Kitamura, S. Imai, M. Matsumura, Appl. Surf. Sci. 90 Ž1995. 349. w6x M. Ritala, M. Leskela, P. Soininen, L. Niinisto, ¨ E. Nykanen, ¨ ¨ Thin Solid Films 225 Ž1993. 288. w7x W. Gasser, Y. Uchida, M. Matsumura, Thin Solid Films 250 Ž1994. 213. w8x S.M. George, O. Sneh, A.C. Dillon, M.L. Wise, A.W. Ott, L.A. Okada, J.D. Way, Appl. Surf. Sci. 82–83 Ž1994. 460.
207
w9x S. Morishita, Y. Uchida, M. Matsumura, Jpn. J. Appl. Phys. 34 Ž1995. 5738. w10x S. Morishita, W. Gasser, K. Usami, M. Matsumura, J. NonCryst. Solids 187 Ž1995. 66. w11x H. Goto, K. Shibahara, S. Yokoyama, Appl. Phys. Lett. 68 Ž1996. 3257. w12x S. Morishita, S. Sugahara, M. Matsumura, Appl. Surf. Sci. 112 Ž1997. 198. w13x Y. Uchida, S. Takei, M. Matsumura, Jpn. J. Appl. Phys. 36 Ž1997. 1509.