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Oxynitridation of Si(100) surface with thermally excited N2O gas
Thin Solid Films 500 (2006) 129 – 132 www.elsevier.com/locate/tsf
Oxynitridation of Si(100) surface with thermally excited N2O gas Y. Enta *, K. Suto, S. Takeda, H. Kato, Y. Sakisaka Faculty of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561, Japan Received 10 March 2005; received in revised form 12 October 2005; accepted 21 November 2005 Available online 28 December 2005
Abstract Silicon oxynitride films have been grown with thermally excited N2O gas, which has a low toxicity in comparison with other oxynitridation agents. Dependences of reaction rates on excitation temperature and substrate temperature have been investigated by Auger electron and photoelectron spectroscopies. These results show that the thermal excitation of N2O obviously promotes the oxynitridation of the silicon surface, especially the oxidation reaction. At higher substrate temperatures, the nitridation of the silicon surface increases and the oxidation is reduced. By mass analysis of the residual gas in the reaction chamber, it was also found that the thermal excitation of N2O causes N2O to be decomposed into N2 and O. This is consistent with the obtained effect that the thermal excitation of N2O promotes especially the oxidation reaction, because atomic oxygen (O) acts as a strong oxidant. D 2005 Elsevier B.V. All rights reserved. Keywords: Silicon oxynitride; N2O gas; Thermal excitation; Si(100)
1. Introduction Silicon oxynitride has been extensively studied as an alternative to the conventional silicon dioxides in highly integrated metal-oxide-semiconductor devices, because it has superior abilities to suppress boron diffusion and improve hot carrier residence [1– 7]. One of the most popular methods to form the oxynitride film on a silicon surface is to utilize thermal reaction with chemical gases such as NO [8,9] and NH3 [3,10]. However, these gases have a high toxicity. In this study, we focus on N2O as a chemical gas for the oxynitridation, because N2O has a relatively low toxicity. However, N2O has also a low reactivity in comparison with the other gases mentioned above. Furthermore, it is generally believed that N2O mainly acts as an oxidant in spite of the compound of nitrogen and oxygen atoms [2,11– 15]. In order to enhance the reactivity of N2O and activate the nitridation reaction, we employed thermally excited N2O, which is generated by passing the gas through a ceramic cell heated above 1000 -C before introducing it to the silicon surface. Thermally excited N2O may be decomposed into its component pieces and/or excited to some radical states, resulting in the * Corresponding author. E-mail address: [email protected] (Y. Enta). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.11.058
enhancement of its reactivity. In this study, in order to assess the effect of the thermal excitation of N2O, the oxynitride films were grown on the silicon surface under several growth conditions, and then the growth rates, nitrogen concentrations and chemical bondings [16 –20] of the grown films were investigated by Auger electron spectroscopy (AES) and Si 2p core-level photoelectron spectroscopy (PES). Mass analysis of residual gases in the reaction chamber was also carried out during the growth. The results reveal the mechanism of the promoting of oxynitridation effect derived from the thermal excitation of N2O. 2. Experimental AES measurements were performed with a cylindrical mirror analyzer and a 3 kV electron gun as an excitation source. Silicon 2p core-level PES measurements were performed with a hemispherical electron analyzer and synchrotron radiation (SR) from the beam line BL-3B at the Photon Factory, the High Energy Accelerator Research Organization (KEK-PF). The incident angle of the SR light, whose energy was set at 135eV, was 45- to the surface normal and normalemission photoelectrons were detected. The overall instrumental energy resolution for the PES measurement was 0.3eV. A mirror-polished, B-doped Si(100) wafer cut to the size of
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30 5 0.35 mm3 was used as the silicon substrate. The wafer was chemically treated with the RCA cleaning method [21] and then annealed by resistive heating to 1000 -C in an ultrahigh vacuum reaction chamber with the base pressure of 1 10 8 Pa. No surface contamination was detected by AES. The N2O gas with a purity of 99.999% was introduced into the heating ceramic cell to be thermally excited before being exposed to the silicon surface. The ceramic cell was made of an alumina tube and heated by tungsten filaments. The temperatures of the silicon substrate and the ceramic cell were monitored by an infrared radiation thermometer. The distance between the cell and the silicon is about 15 cm. 3. Results and discussion As mentioned in the Introduction, N2O has a low reactivity as compared with the other oxynitridation gases such as NO, NO2 and NH3. This indicates the low rate of the oxynitridation. Moreover, previous works [2,11 –15] concluded that nitrogen concentration in the thermal oxynitride film grown with N2O was a few percent or less. In this study, this conclusion was reexamined at the substrate temperatures below 800 -C. As a result, it was found that the growth rate of the oxynitridation with non-excited N2O is one-hundredth or less of that of the oxidation with O2. No nitrogen in the grown film was detected by AES as shown in bottom spectrum in Fig. 1. Therefore, nonexcited N2O was found to act as only oxidant under our growth condition. Next, the effect of N2O thermally excited by the heating cell was investigated. Fig. 1 shows representative AES spectra of
Fig. 1. Auger electron spectra of oxynitrides on Si(100) grown with N2O gas thermally excited at cell temperatures of 1200 -C, 1100 -C and 1000 -C, and without the excitation (from top to bottom). The substrate temperature, N2O pressure and growth time are 500 -C, 1 10 4 Pa and 32 min, respectively.
Fig. 2. Time evolutions of O/Si intensity ratios in AES spectra of oxynitrides. Symbols ?, r and > stand for the cell temperatures of 1200 -C, 1100 -C and 1000 -C, respectively. Symbol q stands for no excitation of N2O. The substrate temperature and N2O pressure are the same as in Fig. 1. The solid lines are a guide to the eyes.
Si(100) surface exposed to N2O for 32 min at the substrate temperature of 500 -C and N2O pressure of 1 10 4 Pa. The temperatures of the heating cell are denoted at each spectrum in Fig. 1. Distinct nitrogen and larger oxygen peaks can be seen in the upper three spectra. The most interesting result in Fig. 1 is that the oxygen peak intensities strongly depend on the temperature of the heating cell while the nitrogen ones are almost constant, showing a characteristic of the thermal excitation of N2O. Namely, the oxidation reaction is selectively promoted by the thermally excited N2O. The time evolution of the oxygen peak intensity in the AES spectrum was obtained for various cell temperatures and the results are shown in Fig. 2. As found in Fig. 1, the ratio depends on the cell temperature and, in particular, the oxidation is strongly promoted at the cell temperature of 1200 -C. Therefore, it is shown that N2O is effectively activated as an oxidant at the cell temperature above 1200 -C. Fig. 3 illustrates substrate temperature dependences of the AES intensities for the oxynitride films, which are grown for 32 min under N2O pressure of 1 10 4 Pa, for the two cases of
Fig. 3. Substrate temperature dependence of O/Si and N/Si intensity ratios. Open and closed symbols stand for the cell temperatures of 1100 -C and 1200 -C, respectively. The N2O pressure and growth time are the same as in Fig. 1. The solid and broken lines are a guide to the eyes.
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the cell temperatures of 1100 -C and 1200 -C. At the substrate temperature below 550 -C, both intensity ratios of O/Si and N/ Si are almost constant, while above 550 -C they show the opposite behaviors: the drop in O/Si ratio and rise in N/Si ratio, as the temperature is elevated. Most importantly, at the substrate temperatures below 550 -C, the oxidation reaction is dominant, while at the temperatures above 550 -C the nitridation reaction is dominant. For both cell temperatures of 1100 -C and 1200 -C, a similar substrate temperature dependence is observed. Nitrogen-rich oxynitride with NO [22] and N2O [23] has been also reported in the higher substrate temperature region. Fig. 4 shows representative Si 2p core-level spectra of the oxynitride on Si(100) grown for various substrate temperatures of (a) 300 -C, (b) 500 -C and (c) 800 -C. Basically, they are composed of two major components. One is ascribed to a bulk component, which can be seen as spin-orbit-split two peaks at relative binding energies (BE) of 0 and 0.6 eV. The other is ascribed to oxynitride component at the relative BE of 3– 4 eV, which is rather broad due to the presence of various intermediate states of Si– O bondings. What has to be noticed is the peak position of the oxynitride component. The position for the oxynitride grown at 800 -C obviously shifts to lower binding energy by ¨ 1 eV as compared with those for 300 -C and 500 -C. This result shows that the nitrogen concentration in the oxynitride grown at 800 -C is higher than that in the other oxynitrides, because the chemical shift due to Si– N bonding is somewhat smaller than that due to Si –O bonding. Although AES results in Fig. 3 do not include information on
Fig. 4. Representative Si 2p core-level spectra recorded at 135 eV photon energy from oxynitride on Si(100) grown at the substrate temperatures of (a) 300 -C, (b) 500 -C and (c) 800 -C. The N2O pressure, cell temperature and growth time are 5 10 4 Pa, 1200 -C and 32 min, respectively.
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Fig. 5. N2O pressure dependence of average growth rates of oxynitride on Si(100) surface. The growth rate was estimated after N2O dose of 7680 L. The substrate and cell temperatures are 500 -C and 1100 -C, respectively. The solid line is a guide to the eyes.
the chemical bonding state, they are qualitatively consistent with the results of Si 2p core-level PES results in Fig. 4. Next, N2O pressure dependence of the growth rate of the oxynitride was investigated to clarify what controlled the growth rate. Average growth rates of the oxynitride, which are estimated from the O/Si Auger peak intensity ratio after dose of 7680 Langmuir (L) (1 L= 10 6 TorrIs) at each N2O pressure, is shown in Fig. 5. The substrate and cell temperatures are 500 -C and 1100 -C, respectively. Until the pressure of 2 10 4 Pa, the growth rate continues to increase proportionally with the N2O pressure and, thereafter, it looks constant. Although this is not conclusive because of a lack of data points, we can consider that there are two regions, where the mechanism of the growth switches at a critical pressure around 2 10 4 Pa; the control by N2O supply in the lower pressure region and the control by surface-reaction rate in the higher pressure region. These behaviors of the growth rates have been frequently observed on surface reactions using gas sources [24]. In order to elucidate the cause of the enhancement of the growth rate by the thermally excited N2O, we carried out the mass analysis of residual gases in the reaction chamber during growth. Fig. 6 shows the intensities of mass number 28, which correspond to N2 or CO species, and the intensities of mass number 30, which correspond to NO species, before and after
Fig. 6. Mass analysis of residual gases in the reaction chamber before and after thermal excitation of N2O.
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thermal excitation of N2O. It is clear that the N2 intensity remarkably increases by the thermal excitation of N2O, while the NO intensity is almost unchanged. These results indicate that the N2O is hardly decomposed to NO and N species, but to N2 and O species during the gas phase reaction. Consequently, the oxidation reaction is selectively promoted by the thermal excitation of N2O as shown in Figs. 1 and 3, because atomic oxygen (O) acts as a strong oxidant while the nitrogen molecule (N2) is rather stable without acting as a nitriding agent. A slight nitridation is probably due to a little existence of the decomposition to NO and N, which appears to be independent of the excitation temperature. At the substrate temperature above 600 -C, the nitridation is promoted as shown in Fig. 3. Although the mechanism of the nitridation promotion is an unsettled question and more studies are needed to clarify it, one of the most likely explanations we considered is as follows. It is well known that the oxidation reaction of silicon causes surface etching with SiO desorption under higher substrate temperatures and lower oxygen pressures condition. In this growth condition, the oxide growth is suppressed and, consequently, more dangling bonds, which act as active sites, appear on the surface. On the other hand, it is supposed that nitride remains on the surface even at higher temperatures, because SiN is much less volatile than SiO. Therefore, the increase of the surface dangling bonds causes the promotion of the nitridation. 4. Conclusion The growth rates, nitrogen concentrations and chemical bondings of the oxynitride on Si(100) grown with thermally excited N2O gas were investigated by AES and Si 2p core-level PES. The thermal excitation of N2O selectively promotes the oxidation reaction, resulting in increasing the growth rate. By mass analysis of the residual gas in the reaction chamber, it was found that this promotion effect of the oxidation is due to atomic oxygen generated by the thermal excitation of N2O. The higher substrate temperature above 600 -C contributes to the promotion of the nitridation and the suppression of the oxidation. In this study, the oxynitride film was grown only with N2O gas, which has a lower toxicity than the other oxynitridation gases such as NO, NO2 and NH3, and the method is quite simple. Therefore, it is expected to be easy to control the growth rate and its elemental composition by optimizing the excitation temperature of N2O and the substrate temperature.
Acknowledgements This work was partially performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2003G296). References [1] H. Hwang, W. Ting, B. Maiti, D. Kwong, J. Lee, Appl. Phys. Lett. 57 (1990) 1010. [2] E.C. Carr, R.A. Buhrman, Appl. Phys. Lett. 63 (1993) 54. [3] E. Cartier, D.A. Buchanan, G.J. Dunn, Appl. Phys. Lett. 64 (1994) 901. [4] K.S. Krish, L. Manchanda, F.H. Baumann, M.L. Green, D. Brasen, L.C. Feldman, A. Ourmazd, IEDM Tech. Dig., 1994, p. 325. [5] B. Maiti, P.J. Tobin, V. Misra, R.I. Hegde, K.G. Reid, C. Gelatos, IEDM Rech. Dig., 1997, p. 651. [6] H. Fukuda, T. Arakawa, S. Ohno, Jpn. J. Appl. Phys. 29 (1990) L2333. [7] Y.H. Ha, S. Kim, S.Y. Lee, J.H. Kim, D.H. Baek, H.K. Kim, D.W. Moon, Appl. Phys. Lett. 74 (1999) 3510. [8] M. Bhat, L.K. Han, D. Wristers, J. Yan, D.L. Kwong, J. Fulford, Appl. Phys. Lett. 66 (1995) 1225. [9] Z.-Q. Yao, H.B. Harrison, S. Dimitrijev, D. Sweatman, Appl. Phys. Lett. 64 (1994) 3584. [10] F.H.P.M. Habraken, A.E.T. Kuiper, Y. Tamminga, J.B. Theeten, J. Appl. Phys. 53 (1982) 6996. [11] E.C. Carr, K.A. Ellis, R.A. Buhrman, Appl. Phys. Lett. 66 (1995) 1492. [12] A.E.T. Kuiper, H.G. Pomp, P.M. Asveld, W. Arnold Bik, F.H.P.M. Habraken, Appl. Phys. Lett. 61 (1992) 1031. [13] G. Weidner, D. Kru¨ger, Appl. Phys. Lett. 62 (1993) 294. [14] Y. Okada, P.J. Tobin, R.I. Hegde, J. Liao, P. Rushbrook, Appl. Phys. Lett. 61 (1992) 3163. [15] T.Y. Chu, W. Ting, J.H. Ahn, S. Lin, D.L. Kwong, Appl. Phys. Lett. 59 (1991) 1412. [16] G. Rangelov, J. Stober, B. Eisenhut, Th. Fauster, Phys. Rev. B44 (1991) 1954. [17] Z.H. Lu, S.P. Tay, R. Cao, P. Pianetta, Appl. Phys. Lett. 67 (1995) 2836. [18] Z.-Q. Yao, J. Appl. Phys. 78 (1995) 2906. [19] J.P. Chang, M.L. Green, V.M. Donnelly, R.L. Opila, J. Eng Jr., J. Sapjeta, H.C. Le, T. Gustafsson, E. Garfunkel, J. Appl. Phys. 87 (2000) 4449. [20] J.W. Kim, H.W. Yeom, Y.D. Chung, K. Jeong, C.N. Whang, M.K. Lee, H.J. Shin, Phys. Rev. B66 (2002) 035312. [21] W. Kern, D.A. Puotinen, RCA Rev. 31 (1970) 187. [22] E.P. Gusev, H.C. Lu, T. Gustafsson, E. Garfunkel, M.L. Green, D. Brasen, J. Appl. Phys. 82 (1997) 896. [23] M.L. Green, D. Brasen, K.W. Evans-Lutterodt, L.C. Feldman, K. Krisch, W. Lennard, H.-T. Tang, L. Manchanda, M.-T. Tang, Appl. Phys. Lett. 65 (1994) 848. [24] F. Hirose, M. Suemitsu, N. Miyamoto, Jpn. J. Appl. Phys. 28 (1989) L2003.
Oxynitridation of Si(100) surface with thermally excited N2O gas Y. Enta *, K. Suto, S. Takeda, H. Kato, Y. Sakisaka Faculty of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561, Japan Received 10 March 2005; received in revised form 12 October 2005; accepted 21 November 2005 Available online 28 December 2005
Abstract Silicon oxynitride films have been grown with thermally excited N2O gas, which has a low toxicity in comparison with other oxynitridation agents. Dependences of reaction rates on excitation temperature and substrate temperature have been investigated by Auger electron and photoelectron spectroscopies. These results show that the thermal excitation of N2O obviously promotes the oxynitridation of the silicon surface, especially the oxidation reaction. At higher substrate temperatures, the nitridation of the silicon surface increases and the oxidation is reduced. By mass analysis of the residual gas in the reaction chamber, it was also found that the thermal excitation of N2O causes N2O to be decomposed into N2 and O. This is consistent with the obtained effect that the thermal excitation of N2O promotes especially the oxidation reaction, because atomic oxygen (O) acts as a strong oxidant. D 2005 Elsevier B.V. All rights reserved. Keywords: Silicon oxynitride; N2O gas; Thermal excitation; Si(100)
1. Introduction Silicon oxynitride has been extensively studied as an alternative to the conventional silicon dioxides in highly integrated metal-oxide-semiconductor devices, because it has superior abilities to suppress boron diffusion and improve hot carrier residence [1– 7]. One of the most popular methods to form the oxynitride film on a silicon surface is to utilize thermal reaction with chemical gases such as NO [8,9] and NH3 [3,10]. However, these gases have a high toxicity. In this study, we focus on N2O as a chemical gas for the oxynitridation, because N2O has a relatively low toxicity. However, N2O has also a low reactivity in comparison with the other gases mentioned above. Furthermore, it is generally believed that N2O mainly acts as an oxidant in spite of the compound of nitrogen and oxygen atoms [2,11– 15]. In order to enhance the reactivity of N2O and activate the nitridation reaction, we employed thermally excited N2O, which is generated by passing the gas through a ceramic cell heated above 1000 -C before introducing it to the silicon surface. Thermally excited N2O may be decomposed into its component pieces and/or excited to some radical states, resulting in the * Corresponding author. E-mail address: [email protected] (Y. Enta). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.11.058
enhancement of its reactivity. In this study, in order to assess the effect of the thermal excitation of N2O, the oxynitride films were grown on the silicon surface under several growth conditions, and then the growth rates, nitrogen concentrations and chemical bondings [16 –20] of the grown films were investigated by Auger electron spectroscopy (AES) and Si 2p core-level photoelectron spectroscopy (PES). Mass analysis of residual gases in the reaction chamber was also carried out during the growth. The results reveal the mechanism of the promoting of oxynitridation effect derived from the thermal excitation of N2O. 2. Experimental AES measurements were performed with a cylindrical mirror analyzer and a 3 kV electron gun as an excitation source. Silicon 2p core-level PES measurements were performed with a hemispherical electron analyzer and synchrotron radiation (SR) from the beam line BL-3B at the Photon Factory, the High Energy Accelerator Research Organization (KEK-PF). The incident angle of the SR light, whose energy was set at 135eV, was 45- to the surface normal and normalemission photoelectrons were detected. The overall instrumental energy resolution for the PES measurement was 0.3eV. A mirror-polished, B-doped Si(100) wafer cut to the size of
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30 5 0.35 mm3 was used as the silicon substrate. The wafer was chemically treated with the RCA cleaning method [21] and then annealed by resistive heating to 1000 -C in an ultrahigh vacuum reaction chamber with the base pressure of 1 10 8 Pa. No surface contamination was detected by AES. The N2O gas with a purity of 99.999% was introduced into the heating ceramic cell to be thermally excited before being exposed to the silicon surface. The ceramic cell was made of an alumina tube and heated by tungsten filaments. The temperatures of the silicon substrate and the ceramic cell were monitored by an infrared radiation thermometer. The distance between the cell and the silicon is about 15 cm. 3. Results and discussion As mentioned in the Introduction, N2O has a low reactivity as compared with the other oxynitridation gases such as NO, NO2 and NH3. This indicates the low rate of the oxynitridation. Moreover, previous works [2,11 –15] concluded that nitrogen concentration in the thermal oxynitride film grown with N2O was a few percent or less. In this study, this conclusion was reexamined at the substrate temperatures below 800 -C. As a result, it was found that the growth rate of the oxynitridation with non-excited N2O is one-hundredth or less of that of the oxidation with O2. No nitrogen in the grown film was detected by AES as shown in bottom spectrum in Fig. 1. Therefore, nonexcited N2O was found to act as only oxidant under our growth condition. Next, the effect of N2O thermally excited by the heating cell was investigated. Fig. 1 shows representative AES spectra of
Fig. 1. Auger electron spectra of oxynitrides on Si(100) grown with N2O gas thermally excited at cell temperatures of 1200 -C, 1100 -C and 1000 -C, and without the excitation (from top to bottom). The substrate temperature, N2O pressure and growth time are 500 -C, 1 10 4 Pa and 32 min, respectively.
Fig. 2. Time evolutions of O/Si intensity ratios in AES spectra of oxynitrides. Symbols ?, r and > stand for the cell temperatures of 1200 -C, 1100 -C and 1000 -C, respectively. Symbol q stands for no excitation of N2O. The substrate temperature and N2O pressure are the same as in Fig. 1. The solid lines are a guide to the eyes.
Si(100) surface exposed to N2O for 32 min at the substrate temperature of 500 -C and N2O pressure of 1 10 4 Pa. The temperatures of the heating cell are denoted at each spectrum in Fig. 1. Distinct nitrogen and larger oxygen peaks can be seen in the upper three spectra. The most interesting result in Fig. 1 is that the oxygen peak intensities strongly depend on the temperature of the heating cell while the nitrogen ones are almost constant, showing a characteristic of the thermal excitation of N2O. Namely, the oxidation reaction is selectively promoted by the thermally excited N2O. The time evolution of the oxygen peak intensity in the AES spectrum was obtained for various cell temperatures and the results are shown in Fig. 2. As found in Fig. 1, the ratio depends on the cell temperature and, in particular, the oxidation is strongly promoted at the cell temperature of 1200 -C. Therefore, it is shown that N2O is effectively activated as an oxidant at the cell temperature above 1200 -C. Fig. 3 illustrates substrate temperature dependences of the AES intensities for the oxynitride films, which are grown for 32 min under N2O pressure of 1 10 4 Pa, for the two cases of
Fig. 3. Substrate temperature dependence of O/Si and N/Si intensity ratios. Open and closed symbols stand for the cell temperatures of 1100 -C and 1200 -C, respectively. The N2O pressure and growth time are the same as in Fig. 1. The solid and broken lines are a guide to the eyes.
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the cell temperatures of 1100 -C and 1200 -C. At the substrate temperature below 550 -C, both intensity ratios of O/Si and N/ Si are almost constant, while above 550 -C they show the opposite behaviors: the drop in O/Si ratio and rise in N/Si ratio, as the temperature is elevated. Most importantly, at the substrate temperatures below 550 -C, the oxidation reaction is dominant, while at the temperatures above 550 -C the nitridation reaction is dominant. For both cell temperatures of 1100 -C and 1200 -C, a similar substrate temperature dependence is observed. Nitrogen-rich oxynitride with NO [22] and N2O [23] has been also reported in the higher substrate temperature region. Fig. 4 shows representative Si 2p core-level spectra of the oxynitride on Si(100) grown for various substrate temperatures of (a) 300 -C, (b) 500 -C and (c) 800 -C. Basically, they are composed of two major components. One is ascribed to a bulk component, which can be seen as spin-orbit-split two peaks at relative binding energies (BE) of 0 and 0.6 eV. The other is ascribed to oxynitride component at the relative BE of 3– 4 eV, which is rather broad due to the presence of various intermediate states of Si– O bondings. What has to be noticed is the peak position of the oxynitride component. The position for the oxynitride grown at 800 -C obviously shifts to lower binding energy by ¨ 1 eV as compared with those for 300 -C and 500 -C. This result shows that the nitrogen concentration in the oxynitride grown at 800 -C is higher than that in the other oxynitrides, because the chemical shift due to Si– N bonding is somewhat smaller than that due to Si –O bonding. Although AES results in Fig. 3 do not include information on
Fig. 4. Representative Si 2p core-level spectra recorded at 135 eV photon energy from oxynitride on Si(100) grown at the substrate temperatures of (a) 300 -C, (b) 500 -C and (c) 800 -C. The N2O pressure, cell temperature and growth time are 5 10 4 Pa, 1200 -C and 32 min, respectively.
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Fig. 5. N2O pressure dependence of average growth rates of oxynitride on Si(100) surface. The growth rate was estimated after N2O dose of 7680 L. The substrate and cell temperatures are 500 -C and 1100 -C, respectively. The solid line is a guide to the eyes.
the chemical bonding state, they are qualitatively consistent with the results of Si 2p core-level PES results in Fig. 4. Next, N2O pressure dependence of the growth rate of the oxynitride was investigated to clarify what controlled the growth rate. Average growth rates of the oxynitride, which are estimated from the O/Si Auger peak intensity ratio after dose of 7680 Langmuir (L) (1 L= 10 6 TorrIs) at each N2O pressure, is shown in Fig. 5. The substrate and cell temperatures are 500 -C and 1100 -C, respectively. Until the pressure of 2 10 4 Pa, the growth rate continues to increase proportionally with the N2O pressure and, thereafter, it looks constant. Although this is not conclusive because of a lack of data points, we can consider that there are two regions, where the mechanism of the growth switches at a critical pressure around 2 10 4 Pa; the control by N2O supply in the lower pressure region and the control by surface-reaction rate in the higher pressure region. These behaviors of the growth rates have been frequently observed on surface reactions using gas sources [24]. In order to elucidate the cause of the enhancement of the growth rate by the thermally excited N2O, we carried out the mass analysis of residual gases in the reaction chamber during growth. Fig. 6 shows the intensities of mass number 28, which correspond to N2 or CO species, and the intensities of mass number 30, which correspond to NO species, before and after
Fig. 6. Mass analysis of residual gases in the reaction chamber before and after thermal excitation of N2O.
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thermal excitation of N2O. It is clear that the N2 intensity remarkably increases by the thermal excitation of N2O, while the NO intensity is almost unchanged. These results indicate that the N2O is hardly decomposed to NO and N species, but to N2 and O species during the gas phase reaction. Consequently, the oxidation reaction is selectively promoted by the thermal excitation of N2O as shown in Figs. 1 and 3, because atomic oxygen (O) acts as a strong oxidant while the nitrogen molecule (N2) is rather stable without acting as a nitriding agent. A slight nitridation is probably due to a little existence of the decomposition to NO and N, which appears to be independent of the excitation temperature. At the substrate temperature above 600 -C, the nitridation is promoted as shown in Fig. 3. Although the mechanism of the nitridation promotion is an unsettled question and more studies are needed to clarify it, one of the most likely explanations we considered is as follows. It is well known that the oxidation reaction of silicon causes surface etching with SiO desorption under higher substrate temperatures and lower oxygen pressures condition. In this growth condition, the oxide growth is suppressed and, consequently, more dangling bonds, which act as active sites, appear on the surface. On the other hand, it is supposed that nitride remains on the surface even at higher temperatures, because SiN is much less volatile than SiO. Therefore, the increase of the surface dangling bonds causes the promotion of the nitridation. 4. Conclusion The growth rates, nitrogen concentrations and chemical bondings of the oxynitride on Si(100) grown with thermally excited N2O gas were investigated by AES and Si 2p core-level PES. The thermal excitation of N2O selectively promotes the oxidation reaction, resulting in increasing the growth rate. By mass analysis of the residual gas in the reaction chamber, it was found that this promotion effect of the oxidation is due to atomic oxygen generated by the thermal excitation of N2O. The higher substrate temperature above 600 -C contributes to the promotion of the nitridation and the suppression of the oxidation. In this study, the oxynitride film was grown only with N2O gas, which has a lower toxicity than the other oxynitridation gases such as NO, NO2 and NH3, and the method is quite simple. Therefore, it is expected to be easy to control the growth rate and its elemental composition by optimizing the excitation temperature of N2O and the substrate temperature.
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