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UV–visible absorption spectra of silicon CVD intermediates
Available online at www.sciencedirect.com
Thin Solid Films 516 (2008) 517 – 520 www.elsevier.com/locate/tsf
UV–visible absorption spectra of silicon CVD intermediates Seigo Nakamura ⁎, Akira Matsugi, Akio Susa, Mistuo Koshi Department of Chemical System Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656, Japan Available online 13 August 2007
Abstract Transient absorption in 394–435 nm wavelength range following 193 nm photolysis of disilane has been measured by using cavity ring-down spectroscopy (CRDS). A broad and continuum absorption band was observed. Time profiles of the absorption measured at several wavelengths were similar and found to have at least two components. The decaying part of the absorption can be attributed to Si(H2)Si based on the kinetic consideration and available information from the literature. The absorption was also measured in the hot wire CVD (HW-CVD) of SiH4. A broad and continuum band was observed. © 2007 Elsevier B.V. All rights reserved. Keywords: UV–vis absorption; CRDS; Silane; Si2H2
1. Introduction Identification of the precursor molecules for the film growth is important to control film quality. In the silicon HW-CVD process, a source gas SiH4 is mainly decomposed to Si and H atoms on the hot filament. H and Si atoms readily react with SiH4 as follows. H þ SiH4 ⇒H2 þ SiH3
ðR1Þ
Si þ SiH4 ⇒products
ðR2Þ
SiH3 produced by the reaction (R1) has high reactivity to the surface and it is thought to be an important precursor in HWCVD of SiH4 [1,2]. Contrary to the R1, there is no direct experimental information on the products of the R2. According to the quantum mechanical calculations of Holt et al. [3], silylsilylene, HSiSiH3 in triplet manifold is produced at first because of the spin conservation, and then this triplet HSiSiH3 will be converted to singlet manifold by the inter-system crossing. This HSiSiH3 has enough energy to isomerize to H2SiSiH2 and further decomposes to Si(H2)Si + H2. Kinetic life time of Si(H2)Si in the gas phase can be long because it does not react with closed shell molecules such as SiH4 [4]. On the other hand, a recent quantum calculation indicates that the Si(H2)Si ⁎ Corresponding author. E-mail address: [email protected] (S. Nakamura). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.205
will react with dangling bonds on the Si surface without any energy barrier [5]. It is predicted that Si(H2)Si is an important precursor in the HW-CVD because of its kinetic stability in the gas phase and high reactivity on the surface. The only method to detect Si(H2)Si so far is the photoionization mass spectroscopy [4,6,7]. Nakajima et al. [4] detected Si2H2 in the 193 nm photolysis of Si2H6 by using a VUV-PIMS (Vacuum Ultra Violet-Photo-ionization Mass Spectrometry) and investigated the reactivity of Si2H2 in the gas phase. They assumed that Si2H2 observed in their experiment was Si(H2)Si because this is a most stable isomer [8]. However, a spectrometric detection of Si(H2)Si is desirable to confirm this assumption. In order to understand the role of Si(H2)Si in the HW-CVD processes, a spectroscopic detection of this species is required since the isomer of Si2H2 has to be distinguished. Maier et al. reported UV–visible absorption of Si(H2)Si in the low temperature Ar matrix [9]. They found an absorption band around 409 nm. This wavelength is in good agreement with the prediction of a quantum chemical calculation. In the present study, the absorption of Si(H2)Si in the gas phase was searched in the wavelength region of 394–435 nm in the 193 nm photolysis of Si2H6 for establishing the spectrometric detection method. Search for the absorption band is also tried in the HW-CVD of SiH4. 2. Experimental Transient species were detected with a CRDS method. Schematic diagram of the experimental apparatus is illustrated
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Fig. 1. Schematic diagram of the experimental apparatus (laser photolysis).
in Fig. 1. Transients were formed by the laser photolysis of Si2H6 diluted in He buffer gas in a stainless steel photolysis cell operated under slowly flowing conditions. The gas handling system consisted of electronic mass flow controllers, a mechanical pump and a capacitance manometer. The total flow was typically 400–600 sccm, the total pressure in the cell was 5 Torr and the partial pressure of Si2H6 was typically 8– 18 mTorr. The ring-down cavity was 77 cm long with a pair of high reflectance mirrors (Los Gatos Research, R N 0.99995 at 415 nm). A typical ring-down time of the present CRD system was 20 μs. The absorbance was calculated from the time history of the ring-down profile which shows an exponential decay. The system employed two pulsed lasers controlled by a pulse generator (Stanford Research Systems DG535). The unfocused output of the pulsed ArF excimer laser (193.3 nm, Lambda Physik COMPex 110) was used for the photolysis. The excimer laser repetition rate was 10 Hz. It was considered that the repetition rate provided sufficient time between excimer laser pulses for the cell to be completely flushed of
Fig. 2. Typical absorption spectra after the 193 nm laser photolysis of Si2H6 taken at 20 μs and 10 ms. 15 mTorr of Si2H6 diluted in 5 Torr helium. The spectrum measured in the hot wire decomposition of SiH4 is also depicted: 0.68 mTorr of SiH 4 diluted in 3.9 Torr helium, tungsten filament temperature = 2000 K, residence time = 0.3 s.
photolysis products before the next laser pulse. Typical photolysis laser power was about 30 mJ cm− 2 pulse− 1 . The third harmonics of Nd:YAG laser (Spectra Physics GCR 230) pumped dye laser (Lambda Physik LPD 3002) was used to probe absorption of the photolysis products. The photolysis laser light entered the flow cell at a right angle to the cavity and overlapped the probe laser beam at the center of the flow cell. Leakage of light was monitored by a photomultiplier tube (Hamamatsu R1527) mounted behind the end mirror. The absorption was also measured for the products of hot wire decomposition of SiH4. A tungsten hot wire (0.2 mm ϕ and 40 mm long) was placed near the center of the flow cell along with the cavity. The wire was resistively heated by a stabilized DC power supply. The temperature of the tungsten wire was measured by an infrared radiation thermometer. Typical filament temperature was 2000 K. No signal was detected without SiH4 (i.e., only He) at this filament temperature. This confirms that the effect of the thermal lens is negligible. 3. Results and discussion Typical absorption spectra between 394 and 435 nm wavelength range are shown in Fig. 2. Two spectra taken at different delay times (20 μs and 10 ms) between photolysis laser and probe laser are depicted in the figure. A broad and continuum band was observed in both cases. The sharp peak at 410.3 nm can be attributed to the spin-forbidden transition of Si (3s23p2 1S ⇒ 3s23p4s 3P). Time profiles (1 ms time scale) of the transient absorption observed at three wavelengths (395, 409 and 434 nm) are shown in Fig. 3. The absorption immediately increases without any time delay to the photolysis laser pulse. It decays slowly until 500 μs and remains constant. The decay rates and intensities at longer times seem to depend on the wavelength. The absorption intensities immediately after the photolysis laser are found to be proportional to the photolysis laser power, as shown in Fig. 4.
Fig. 3. Time profiles of the absorption at 395, 409 and 434 nm (each time profile has been shifted in Y-axis direction): 15 mTorr of Si2H6 diluted in 5 Torr helium, ArF laser power 30 mJ cm− 2 pulse− 1.
S. Nakamura et al. / Thin Solid Films 516 (2008) 517–520
Fig. 4. Intensity of the absorption taken at t = 20 μs as a function of the ArF laser power: 15 mTorr of Si2H6 diluted in 5 Torr helium.
The time profiles with longer time scale (up to 10 ms) are shown in Fig. 5. In this figure, time profiles were taken with four different initial concentrations of Si2H6 (8, 12, 15 and 18 mTorr) at constant photolysis laser power. The absorption intensity immediately after the photolysis (20 μs) is found to be proportional to the partial pressures of Si2H6, as shown in Fig. 6. Absorption intensities at longer times than 2 ms are constant with the Si2H6 partial pressures of 8 and 12 mTorr, whereas it increases when the partial pressures are 15 and 18 mTorr. The transient absorption observed in the present work has no observable delay to the photolysis laser pulse. In addition, the absorption intensity immediately after the photolysis is proportional to the photolysis laser power and the initial partial pressure of Si2H6, as can be seen in Figs. 4 and 6. The time resolution of the present CRD system is limited by the ringdown time (20 μs). However, within this dead time, only few collisions between photolysis products and Si2H6 are possible. Therefore secondary reactions can be neglected in the rise time of the absorption signal. Those facts lead to the conclusion that the absorbent at t = 0 is the primary product of the Si2H6
Fig. 5. Time profiles of the absorption at 409 nm with various initial pressures of Si2H6 (each time profiles have been shifted in Y-axis direction): total pressure = 5 Torr (diluted in He), ArF laser power = 30 mJ cm− 2 pulse− 1.
519
photolysis. Photolysis of Si2H6 at 193 nm is extremely complex because absorption of one 193 nm photon (148 kcal/mol) provides enough energy to break any single bond in Si2H6. It is estimated [10], based on energetics, that 21 distinct photodissociation pathways are available and at least 14 distinct photoproducts are possible [11]. The list of possible photoproducts includes all of the possible mono-silicon hydrides SiHn (n = 1–4). However, those species do not have absorption bands in the wavelength region of the present study, except for SiH. Although SiH has the absorption band of A-X transition at around 410–415 nm, SiH is not responsible to the broad continuum absorption, since the A–X transition of SiH is a bound–bound transition. Another possible photoproducts are transient species of the type Si2Hm (m = 2–5) which are virtually uncharacterized experimentally. Most of these species may exhibit novel bonding and multiple isomeric forms. A few studies had been performed to detect those di-silicon species. Jasinski [11] observed the transient absorption at the wavelength of 333.6, 351.1 and 363.8 nm. He speculated that the transient absorption immediately after the photolysis was due to H2SiSiH2. Tada et al. [7] tried to detect products of 193 nm photolysis by using LIF and VUV-PIMS. They detected H atom and SiH2 by LIF. In their VUV-PIMS experiments, they employed the photo-ionization energy of 8.9, 9.5 and 10.2 eV. Although those ionization energies are enough to ionize molecules of SiH3, Si(H2)Si, SiSiH3, H2SiSiH2, HSiSiH3 and H3SiSiH2, only the photoproduct they could detect was the Si (H2)Si with the experimental conditions similar to the present study. On the basis of this observation, the absorbent for the decaying part of the transient absorption observed in the present study is most likely attributable to Si(H2)Si. Experiments to detect Si(H2)Si by Maier et al. [9] also support this assignment. In their experiments, Si(H2)Si was produced in the Ar matrix by the UV-photolysis of H2SiSiH2. They observed the broad continuum absorption centered at 409 nm. They also performed the time dependent DFT (density functional theory) calculation at the B3LYP/6-311+G** level of theory and found that the
Fig. 6. Intensity of the absorption taken at t = 20 μs (●) and 10 ms (○) as a function of the initial pressures of Si2H6: total pressure = 5 Torr (diluted in He), ArF laser power = 30 mJ cm− 2 pulse− 1.
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wavelength for the excitation to the 1B2 state corresponds to 408 nm. The identification of the absorption at long time scale is difficult because there are too many possibilities for the absorbent. The absorption spectrum taken at t = 10 ms is also a broad continuum and its spectral feature is similar to the spectrum at t = 20 μs as shown in Fig. 2. It may be caused by higher silanes produced by the consecutive reactions of photoproducts. Indeed, in Fig. 6, the delayed absorbance at t = 10 ms increases non-linearly against the partial pressures of Si2H6. This suggests that consecutive reactions with Si2H6 may occur. Furthermore, Tada et al. [7] detected Si3H8 as a product of the SiH2 + Si2H6 reaction followed by the 193 nm photolysis of Si2H6. Production of much higher silane may be possible by consecutive reactions to give absorption in the wavelength region of this work. The absorption spectrum observed in the hot wire decomposition of SiH4 is also depicted in Fig. 2. Strong continuum absorption was observed. Structureless feature of the spectrum and the lack of the time-dependent information make the identification difficult. The kinetic simulations have been performed for the prediction of possible absorbent. The chemical kinetic model for the hot wire decomposition of SiH4 has been developed [5] and used here. Most abundant product of the reaction is Si(H2)Si and therefore, it is expected that the part of absorption is due to Si(H2)Si.
4. Concluding remarks Continuous absorption was observed at around 394–435 nm in the photolysis of Si2H6 and in the HW-CVD of SiH4. On the basis of the kinetic consideration, this absorption is speculated to be responsible to Si(H2)Si species. More detailed discussion for the identification of this continuous absorption is still open for future work. References [1] Y. Nozaki, M. Kitazoe, K. Horii, H. Umemoto, A. Matsuda, H. Matsumura, Thin Solid Films 395 (2001) 47. [2] A. Matsuda, K. Nomoto, Y. Takeuchi, A. Suzuki, A. Yuuki, J. Perrin, Surf. Sci. 227 (1990) 50. [3] J.K. Holt, M. Swiatek, D.G. Goodwin, R.P. Muller, W.A. Goddard III, H.A. Atwater, Thin Solid Films 395 (2001) 29. [4] Y. Nakajima, K. Tonokura, K. Sugimoto, M. Koshi, Int. J. Chem. Kinet. 33 (2001) 136. [5] S. Nakamura, K. Matsumoto, A. Susa, M. Koshi, J. Non-Cryst. Solids 352 (2006) 919. [6] R. Ruscic, J. Berkowitze, J. Chem. Phys. 95 (1991) 2407. [7] N. Tada, K. Tonokura, K. Matsumoto, M. Koshi, A. Miyoshi, H. Matsui, J. Phys. Chem., A 103 (1999) 322. [8] B.T. Colegrove, H.F. Schaefer, J. Phys. Chem. 94 (1990) 5593. [9] G. Maier, H.P. Reisenauer, J. Glatthaar, Chem. Eur. J. 8 (2002) 4383. [10] H. Stafast, Appl. Phys., A 45 (1988) 93. [11] J.M. Jasinski, Chem. Phys. Lett. 183 (1991) 55.
Thin Solid Films 516 (2008) 517 – 520 www.elsevier.com/locate/tsf
UV–visible absorption spectra of silicon CVD intermediates Seigo Nakamura ⁎, Akira Matsugi, Akio Susa, Mistuo Koshi Department of Chemical System Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656, Japan Available online 13 August 2007
Abstract Transient absorption in 394–435 nm wavelength range following 193 nm photolysis of disilane has been measured by using cavity ring-down spectroscopy (CRDS). A broad and continuum absorption band was observed. Time profiles of the absorption measured at several wavelengths were similar and found to have at least two components. The decaying part of the absorption can be attributed to Si(H2)Si based on the kinetic consideration and available information from the literature. The absorption was also measured in the hot wire CVD (HW-CVD) of SiH4. A broad and continuum band was observed. © 2007 Elsevier B.V. All rights reserved. Keywords: UV–vis absorption; CRDS; Silane; Si2H2
1. Introduction Identification of the precursor molecules for the film growth is important to control film quality. In the silicon HW-CVD process, a source gas SiH4 is mainly decomposed to Si and H atoms on the hot filament. H and Si atoms readily react with SiH4 as follows. H þ SiH4 ⇒H2 þ SiH3
ðR1Þ
Si þ SiH4 ⇒products
ðR2Þ
SiH3 produced by the reaction (R1) has high reactivity to the surface and it is thought to be an important precursor in HWCVD of SiH4 [1,2]. Contrary to the R1, there is no direct experimental information on the products of the R2. According to the quantum mechanical calculations of Holt et al. [3], silylsilylene, HSiSiH3 in triplet manifold is produced at first because of the spin conservation, and then this triplet HSiSiH3 will be converted to singlet manifold by the inter-system crossing. This HSiSiH3 has enough energy to isomerize to H2SiSiH2 and further decomposes to Si(H2)Si + H2. Kinetic life time of Si(H2)Si in the gas phase can be long because it does not react with closed shell molecules such as SiH4 [4]. On the other hand, a recent quantum calculation indicates that the Si(H2)Si ⁎ Corresponding author. E-mail address: [email protected] (S. Nakamura). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.205
will react with dangling bonds on the Si surface without any energy barrier [5]. It is predicted that Si(H2)Si is an important precursor in the HW-CVD because of its kinetic stability in the gas phase and high reactivity on the surface. The only method to detect Si(H2)Si so far is the photoionization mass spectroscopy [4,6,7]. Nakajima et al. [4] detected Si2H2 in the 193 nm photolysis of Si2H6 by using a VUV-PIMS (Vacuum Ultra Violet-Photo-ionization Mass Spectrometry) and investigated the reactivity of Si2H2 in the gas phase. They assumed that Si2H2 observed in their experiment was Si(H2)Si because this is a most stable isomer [8]. However, a spectrometric detection of Si(H2)Si is desirable to confirm this assumption. In order to understand the role of Si(H2)Si in the HW-CVD processes, a spectroscopic detection of this species is required since the isomer of Si2H2 has to be distinguished. Maier et al. reported UV–visible absorption of Si(H2)Si in the low temperature Ar matrix [9]. They found an absorption band around 409 nm. This wavelength is in good agreement with the prediction of a quantum chemical calculation. In the present study, the absorption of Si(H2)Si in the gas phase was searched in the wavelength region of 394–435 nm in the 193 nm photolysis of Si2H6 for establishing the spectrometric detection method. Search for the absorption band is also tried in the HW-CVD of SiH4. 2. Experimental Transient species were detected with a CRDS method. Schematic diagram of the experimental apparatus is illustrated
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Fig. 1. Schematic diagram of the experimental apparatus (laser photolysis).
in Fig. 1. Transients were formed by the laser photolysis of Si2H6 diluted in He buffer gas in a stainless steel photolysis cell operated under slowly flowing conditions. The gas handling system consisted of electronic mass flow controllers, a mechanical pump and a capacitance manometer. The total flow was typically 400–600 sccm, the total pressure in the cell was 5 Torr and the partial pressure of Si2H6 was typically 8– 18 mTorr. The ring-down cavity was 77 cm long with a pair of high reflectance mirrors (Los Gatos Research, R N 0.99995 at 415 nm). A typical ring-down time of the present CRD system was 20 μs. The absorbance was calculated from the time history of the ring-down profile which shows an exponential decay. The system employed two pulsed lasers controlled by a pulse generator (Stanford Research Systems DG535). The unfocused output of the pulsed ArF excimer laser (193.3 nm, Lambda Physik COMPex 110) was used for the photolysis. The excimer laser repetition rate was 10 Hz. It was considered that the repetition rate provided sufficient time between excimer laser pulses for the cell to be completely flushed of
Fig. 2. Typical absorption spectra after the 193 nm laser photolysis of Si2H6 taken at 20 μs and 10 ms. 15 mTorr of Si2H6 diluted in 5 Torr helium. The spectrum measured in the hot wire decomposition of SiH4 is also depicted: 0.68 mTorr of SiH 4 diluted in 3.9 Torr helium, tungsten filament temperature = 2000 K, residence time = 0.3 s.
photolysis products before the next laser pulse. Typical photolysis laser power was about 30 mJ cm− 2 pulse− 1 . The third harmonics of Nd:YAG laser (Spectra Physics GCR 230) pumped dye laser (Lambda Physik LPD 3002) was used to probe absorption of the photolysis products. The photolysis laser light entered the flow cell at a right angle to the cavity and overlapped the probe laser beam at the center of the flow cell. Leakage of light was monitored by a photomultiplier tube (Hamamatsu R1527) mounted behind the end mirror. The absorption was also measured for the products of hot wire decomposition of SiH4. A tungsten hot wire (0.2 mm ϕ and 40 mm long) was placed near the center of the flow cell along with the cavity. The wire was resistively heated by a stabilized DC power supply. The temperature of the tungsten wire was measured by an infrared radiation thermometer. Typical filament temperature was 2000 K. No signal was detected without SiH4 (i.e., only He) at this filament temperature. This confirms that the effect of the thermal lens is negligible. 3. Results and discussion Typical absorption spectra between 394 and 435 nm wavelength range are shown in Fig. 2. Two spectra taken at different delay times (20 μs and 10 ms) between photolysis laser and probe laser are depicted in the figure. A broad and continuum band was observed in both cases. The sharp peak at 410.3 nm can be attributed to the spin-forbidden transition of Si (3s23p2 1S ⇒ 3s23p4s 3P). Time profiles (1 ms time scale) of the transient absorption observed at three wavelengths (395, 409 and 434 nm) are shown in Fig. 3. The absorption immediately increases without any time delay to the photolysis laser pulse. It decays slowly until 500 μs and remains constant. The decay rates and intensities at longer times seem to depend on the wavelength. The absorption intensities immediately after the photolysis laser are found to be proportional to the photolysis laser power, as shown in Fig. 4.
Fig. 3. Time profiles of the absorption at 395, 409 and 434 nm (each time profile has been shifted in Y-axis direction): 15 mTorr of Si2H6 diluted in 5 Torr helium, ArF laser power 30 mJ cm− 2 pulse− 1.
S. Nakamura et al. / Thin Solid Films 516 (2008) 517–520
Fig. 4. Intensity of the absorption taken at t = 20 μs as a function of the ArF laser power: 15 mTorr of Si2H6 diluted in 5 Torr helium.
The time profiles with longer time scale (up to 10 ms) are shown in Fig. 5. In this figure, time profiles were taken with four different initial concentrations of Si2H6 (8, 12, 15 and 18 mTorr) at constant photolysis laser power. The absorption intensity immediately after the photolysis (20 μs) is found to be proportional to the partial pressures of Si2H6, as shown in Fig. 6. Absorption intensities at longer times than 2 ms are constant with the Si2H6 partial pressures of 8 and 12 mTorr, whereas it increases when the partial pressures are 15 and 18 mTorr. The transient absorption observed in the present work has no observable delay to the photolysis laser pulse. In addition, the absorption intensity immediately after the photolysis is proportional to the photolysis laser power and the initial partial pressure of Si2H6, as can be seen in Figs. 4 and 6. The time resolution of the present CRD system is limited by the ringdown time (20 μs). However, within this dead time, only few collisions between photolysis products and Si2H6 are possible. Therefore secondary reactions can be neglected in the rise time of the absorption signal. Those facts lead to the conclusion that the absorbent at t = 0 is the primary product of the Si2H6
Fig. 5. Time profiles of the absorption at 409 nm with various initial pressures of Si2H6 (each time profiles have been shifted in Y-axis direction): total pressure = 5 Torr (diluted in He), ArF laser power = 30 mJ cm− 2 pulse− 1.
519
photolysis. Photolysis of Si2H6 at 193 nm is extremely complex because absorption of one 193 nm photon (148 kcal/mol) provides enough energy to break any single bond in Si2H6. It is estimated [10], based on energetics, that 21 distinct photodissociation pathways are available and at least 14 distinct photoproducts are possible [11]. The list of possible photoproducts includes all of the possible mono-silicon hydrides SiHn (n = 1–4). However, those species do not have absorption bands in the wavelength region of the present study, except for SiH. Although SiH has the absorption band of A-X transition at around 410–415 nm, SiH is not responsible to the broad continuum absorption, since the A–X transition of SiH is a bound–bound transition. Another possible photoproducts are transient species of the type Si2Hm (m = 2–5) which are virtually uncharacterized experimentally. Most of these species may exhibit novel bonding and multiple isomeric forms. A few studies had been performed to detect those di-silicon species. Jasinski [11] observed the transient absorption at the wavelength of 333.6, 351.1 and 363.8 nm. He speculated that the transient absorption immediately after the photolysis was due to H2SiSiH2. Tada et al. [7] tried to detect products of 193 nm photolysis by using LIF and VUV-PIMS. They detected H atom and SiH2 by LIF. In their VUV-PIMS experiments, they employed the photo-ionization energy of 8.9, 9.5 and 10.2 eV. Although those ionization energies are enough to ionize molecules of SiH3, Si(H2)Si, SiSiH3, H2SiSiH2, HSiSiH3 and H3SiSiH2, only the photoproduct they could detect was the Si (H2)Si with the experimental conditions similar to the present study. On the basis of this observation, the absorbent for the decaying part of the transient absorption observed in the present study is most likely attributable to Si(H2)Si. Experiments to detect Si(H2)Si by Maier et al. [9] also support this assignment. In their experiments, Si(H2)Si was produced in the Ar matrix by the UV-photolysis of H2SiSiH2. They observed the broad continuum absorption centered at 409 nm. They also performed the time dependent DFT (density functional theory) calculation at the B3LYP/6-311+G** level of theory and found that the
Fig. 6. Intensity of the absorption taken at t = 20 μs (●) and 10 ms (○) as a function of the initial pressures of Si2H6: total pressure = 5 Torr (diluted in He), ArF laser power = 30 mJ cm− 2 pulse− 1.
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wavelength for the excitation to the 1B2 state corresponds to 408 nm. The identification of the absorption at long time scale is difficult because there are too many possibilities for the absorbent. The absorption spectrum taken at t = 10 ms is also a broad continuum and its spectral feature is similar to the spectrum at t = 20 μs as shown in Fig. 2. It may be caused by higher silanes produced by the consecutive reactions of photoproducts. Indeed, in Fig. 6, the delayed absorbance at t = 10 ms increases non-linearly against the partial pressures of Si2H6. This suggests that consecutive reactions with Si2H6 may occur. Furthermore, Tada et al. [7] detected Si3H8 as a product of the SiH2 + Si2H6 reaction followed by the 193 nm photolysis of Si2H6. Production of much higher silane may be possible by consecutive reactions to give absorption in the wavelength region of this work. The absorption spectrum observed in the hot wire decomposition of SiH4 is also depicted in Fig. 2. Strong continuum absorption was observed. Structureless feature of the spectrum and the lack of the time-dependent information make the identification difficult. The kinetic simulations have been performed for the prediction of possible absorbent. The chemical kinetic model for the hot wire decomposition of SiH4 has been developed [5] and used here. Most abundant product of the reaction is Si(H2)Si and therefore, it is expected that the part of absorption is due to Si(H2)Si.
4. Concluding remarks Continuous absorption was observed at around 394–435 nm in the photolysis of Si2H6 and in the HW-CVD of SiH4. On the basis of the kinetic consideration, this absorption is speculated to be responsible to Si(H2)Si species. More detailed discussion for the identification of this continuous absorption is still open for future work. References [1] Y. Nozaki, M. Kitazoe, K. Horii, H. Umemoto, A. Matsuda, H. Matsumura, Thin Solid Films 395 (2001) 47. [2] A. Matsuda, K. Nomoto, Y. Takeuchi, A. Suzuki, A. Yuuki, J. Perrin, Surf. Sci. 227 (1990) 50. [3] J.K. Holt, M. Swiatek, D.G. Goodwin, R.P. Muller, W.A. Goddard III, H.A. Atwater, Thin Solid Films 395 (2001) 29. [4] Y. Nakajima, K. Tonokura, K. Sugimoto, M. Koshi, Int. J. Chem. Kinet. 33 (2001) 136. [5] S. Nakamura, K. Matsumoto, A. Susa, M. Koshi, J. Non-Cryst. Solids 352 (2006) 919. [6] R. Ruscic, J. Berkowitze, J. Chem. Phys. 95 (1991) 2407. [7] N. Tada, K. Tonokura, K. Matsumoto, M. Koshi, A. Miyoshi, H. Matsui, J. Phys. Chem., A 103 (1999) 322. [8] B.T. Colegrove, H.F. Schaefer, J. Phys. Chem. 94 (1990) 5593. [9] G. Maier, H.P. Reisenauer, J. Glatthaar, Chem. Eur. J. 8 (2002) 4383. [10] H. Stafast, Appl. Phys., A 45 (1988) 93. [11] J.M. Jasinski, Chem. Phys. Lett. 183 (1991) 55.