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Silicon carbide film deposition at low temperatures using monomethylsilane gas
Surface & Coatings Technology 204 (2010) 1432–1437
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Silicon carbide film deposition at low temperatures using monomethylsilane gas Hitoshi Habuka ⁎, Hiroshi Ohmori, Yusuke Ando Department of Chemical and Energy Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 7 June 2009 Accepted in revised form 15 September 2009 Available online 25 September 2009 PACS: 81.15.Gh 71.20.Nr 81.15.Kk
a b s t r a c t A silicon carbide film is formed at low temperatures on a silicon surface by chemical vapor deposition using monomethylsilane gas along with hydrogen chloride gas in ambient hydrogen at atmospheric pressure. A 0.2-μm thick film, obtained at 1073 K and at a hydrogen chloride gas concentration greater than that of the monomethylsilane gas, possessed a specular surface having the root-mean-square microroughness of 0.7 nm. At temperatures lower than 900 K, the 0.1-μm thick silicon carbide film could be formed on the silicon surface, immediately after the surface cleaning in ambient hydrogen at 1373 K. Because the weight of the film formed at room temperature did not decrease after the etching using hydrogen chloride gas at 1073 K, the film obtained in this study is expected to be a tough coating film. © 2009 Elsevier B.V. All rights reserved.
Keywords: Silicon carbide Monomethylsilane Chemical vapor deposition Low temperature
1. Introduction A silicon carbide (SiC) thin film is expected to be a suitable coating material because it is very stable and tough even in a hazardous environment, such as the coating film of a susceptor of a silicon epitaxial reactor [1]. However, one of the problems of SiC as a coating material is the very high temperature often necessary for its film formation. A SiC film is formed using the chemical vapor deposition (CVD) method at various substrate temperatures [2–4]. The deposition temperature is often comparable to the melting point of silicon and may induce some damage due to the difference in the thermal expansion between the SiC film and the substrate material [5]. In order to reduce these problems, the SiC film coating at lower temperatures is expected. For the low temperature film formation, methylsilanes, such as monomethylsilane (MMS), dimethylsilane and trichloromethylsilane, are expected candidates, because these gases have a covalent bond between the silicon and carbon in their molecular structure. Many researchers have studied SiC film formation technology using monomethylsilane gas [4–13], as well as dimethylsilane [14]. The low temperature SiC film was formed on a silicon surface using MMS gas with hydrogen chloride (HCl) gas at 1073 K [15,16]. By this method, the excess silicon in the SiC film was reduced by hydrogen chloride gas, because HCl can react with SiHx in the gas phase and Si at the surface to form gaseous product of chlorosilanes. By this method, the
⁎ Corresponding author. Tel./fax: +81 45 339 3998. E-mail address: [email protected] (H. Habuka). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.09.044
SiC film thickness saturated within 60 s. Here, it should be noted that the high purity HCl gas did not corrode the stainless tube and quartz chamber in the CVD reactor. In order to develop the low-temperature SiC CVD technology, further details should be obtained, particularly about the influence of the gas composition, the surface morphology, the film formation temperature and the substrate surface cleaning condition. The relatively low-temperature SiC film formation has been studied in the field of surface physics and surface science. Particularly, Nakazawa and Suemitsu [6] studied the thermal decomposition of MMS molecules chemisorbed on the silicon surface. In their study, MMS was chemisorbed at room temperature on the silicon surface, which was cleaned in a high vacuum environment at high temperature immediately before the chemisorption. This indicates that SiC film or its intermediate compound film can be formed at low temperatures using MMS gas at the clean silicon surface which has dangling bonds or silicon dimers. In order to prepare an atomically clean silicon surface, the industrial silicon epitaxial growth process employs the annealing technique in ambient hydrogen. Additionally, the hydrogen-annealed silicon surface has been known to contain monohydrides with silicon dimers [17–19]. These dimers are expected to help the formation of chemical bonds between the silicon surface and precursors, such as MMS, at low temperatures. Thus, the SiC deposition from MMS gas on the silicon surface combined with the hydrogen annealing technique should be studied. Therefore, in this study, the SiC film deposition on a silicon surface is further studied using MMS gas at 1073 K and lower temperatures, particularly about the influence of the gas composition, the surface
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morphology, the film formation temperature and the substrate surface cleaning condition. Through this study as an extension of our previous study [15,16], hydrogen chloride gas is introduced along with MMS gas, expecting the decrease of excess silicon in the SiC film. 2. Experimental In order to obtain the SiC film by the CVD method, the horizontal cold-wall reactor shown in Fig. 1 was used. This reactor consists of a gas supply system, a quartz chamber and six infrared lamps. A 30mm-wide × 40-mm-long (100) silicon substrate manufactured by the Czochralski method is horizontally placed on the bottom wall of the quartz chamber. The silicon substrate is heated by infrared rays from the six halogen lamps through the quartz chamber. Because quartz absorbs only a slight amount of infrared light having the wavelength of ca. 1 μm from the halogen lamps, the temperature of the quartz chamber wall remains low. Electric power provided to the infrared lamps is adjusted on the basis of the temperatures measured beforehand in ambient nitrogen. The gas flow channel above the substrate of this reactor has a low height and a small rectangular cross section in order to achieve a very high consumption efficiency of the reactive gases. The height and the width of the quartz chamber are 10 mm and 40 mm, respectively, similar to those in our previous studies [20–22]. The gas supply system has the function of introducing gases of hydrogen, nitrogen, MMS, hydrogen chloride and chlorine trifluoride. Hydrogen gas is used as the carrier gas as well as the cleaning gas for removing the silicon oxide film and organic contamination on the silicon surface [21]. Throughout the process, hydrogen gas is introduced into the reactor at atmospheric pressure at the flow rate of 2 slm. Water vapor in the hydrogen gas is removed by passing the hydrogen gas through a liquid nitrogen trap (77 K) at the entrance of the reactor. Chlorine trifluoride gas [23,24] is used in order to remove the SiC film, which is formed on the inner wall of the quartz chamber during the SiC CVD process. The typical process used in this study is Steps (A), (B) and (C) shown in Fig. 2. Step (A): cleaning of the silicon surface at 1373 K for 10 min in ambient hydrogen. Step (B): SiC film formation using a gas mixture of MMS and hydrogen chloride, at room temperature — 1073 K after Step (A). Step (C): etching by hydrogen chloride gas at 1073 K for 10 min.
Fig. 2. Process for SiC film deposition and etching using gases of MMS, hydrogen chloride and hydrogen. Step (A): cleaning silicon surface at 1373 K, Step (B) film formation after Step (A), and Step (C): etching of film surface using hydrogen chloride gas at 1073 K.
exposed to the gas mixture of hydrogen chloride and hydrogen at the flow rate of 100 sccm and 2 slm, respectively, at 1073 K, as Step (C) in Fig. 2. Here, it may be pointed out that silicon surface suffers significant etching by hydrogen chloride gas at 1073 K [20]. After finishing the film deposition, the quartz chamber is cleaned using chlorine trifluoride gas (Kanto Denka Kogyo Co., Ltd., Tokyo, Japan) at the concentration of 10% in ambient nitrogen at 673–773 K for 1 min at atmospheric pressure. This cleaning technique is an application of the SiC etching technique using chlorine trifluoride gas [23,24]. In order to evaluate the chemical bond between silicon and carbon in the SiC film, the X-ray photoelectron spectra (XPS) were obtained using a PHI 5400MC spectrometer (Perkin-Elmer Co., Ltd., USA) which had an aluminium X-ray source. The incident angle of X-ray was 90° for the film having the thickness larger than 0.1 μm. The surface morphology was observed using an optical microscope (USB Microscope, Scalar Corp., Tokyo), a laser microscope (VK-9500, Keyence Corp., Tokyo), a scanning electron microscope (SEM) (VE-8800, Keyence, Tokyo) and an atomic force microscope (AFM) (VN-8000, Keyence Corp., Tokyo and SPA400 and SPI3800N, Seiko Instruments Co., Ltd., Tokyo). Surface microroughness was evaluated by AFM. In order to observe the surface morphology and the film thickness, a transmission electron microscope (TEM) (HD-2700, Hitachi High-Technologies, Tokyo) was taken. The XPS and TEM measurements were ex situ performed at the Foundation of Promotion of Material Science and Technology of Japan (Tokyo). 3. Results and discussion
The average thickness of the SiC film is evaluated from the increase in the substrate weight. In our preliminary experiment, a polycrystalline 3C–SiC wafer produced by the CVD method (Admap Inc., Tokyo) was found to suffer from no etching by the 100% hydrogen chloride gas at 1273 K for over 40 min. This clearly indicates that SiC film is not etched by the hydrogen chloride gas. Thus, in order to quickly determine the formation of SiC, some of the films obtained by Steps (A) and (B) were
Fig. 1. Horizontal cold-wall chemical vapor deposition reactor used for SiC film deposition.
3.1. SiC film thickness First, the SiC film thickness was evaluated at various gas compositions of MMS and hydrogen chloride for 5 min at 1073 K, as shown in Fig. 3 and as listed in Table 1. The hydrogen gas flow rate was 2 slm; the hydrogen chloride gas flow rate was 100 sccm (circle), 150 sccm (square) and 200 sccm (triangle). In Fig. 3, the film thickness entirely decreases with the increasing hydrogen chloride gas flow rate. The square and triangle show that the SiC film thickness was very small but it gradually increased with the increasing MMS gas flow rate between 50 and 200 sccm. In contrast to this, the SiC film thickness obtained at the hydrogen chloride gas flow rate of 100 sccm, indicated by the circle, showed a significant increase at the MMS gas flow rate greater than 100 sccm. Here, the surface appearance of the film having such a significant thickness increase was dark and very rough. Here, it should be noted that the silicon substrate surface was significantly etched by hydrogen chloride gas at its flow rate of 100 sccm for 60 s, without MMS gas. This indicates that the silicon– silicon bond present at the film surface can be removed by hydrogen chloride gas. Thus, based on the results in this study and following our
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Fig. 3. SiC film thickness produced for 60 s at 1073 K at various gas flow rates of MMS and hydrogen chloride. Hydrogen chloride gas flow rate was 100 sccm (circle), 150 sccm (square) and 200 sccm (triangle). Hydrogen gas flow rate was 2 slm.
previous studies [15,16], the amount of excess silicon in the SiC film was considered to be decreased, however, the insufficient amount of hydrogen chloride gas could not sufficiently suppress the incorporation of excess silicon. From Fig. 3, the amount of hydrogen chloride gas, comparable to or greater than that of the MMS gas, is considered to be necessary for the effective removal of the excess silicon. Because the HCl flow rate larger than 150 sccm was sufficient for the MMS gas flow rate between 50 and 200 sccm, the film thickness could linearly increase with the increasing MMS gas flow rate, as indicated using square and triangle in Fig. 3. 3.2. Surface morphology The surface morphology of the SiC film was evaluated by the AFM, because some of the SiC films obtained in this study showed a very smooth appearance by visual inspection. Fig. 4 shows the AFM photograph of (a) silicon surface before the film formation, and (b) SiC film surface with a thickness of 0.2 μm obtained at 1073 K for 5 min at the MMS gas flow rate of 92 sccm and hydrogen chloride gas flow rate of 150 sccm. The measured area was 0.2 × 0.2 μm. Fig. 4 (a) shows that the silicon substrate surface before the film formation was very smooth with the average roughness (Ra) and the root-mean-square roughness (RMS) of 0.2 nm and 0.3 nm, respectively. After the SiC film formation, the surface roughness slightly
Table 1 Conditions of film deposition and etching. Figure
Temperature (K)
Time (min)
MMS (sccm)
HCl (sccm)
H2 (slm)
3 3 3 3 3 3 3 3 3 3 3 3 6 6 7 7
1073 1073 1073 1073 1073 1073 1073 1073 1073 1073 1073 1073 300 1073 300 1073
5 5 5 5 5 5 5 5 5 5 5 5 1 10 1 10
46 92 138 184 46 92 138 184 46 92 138 184 92 0 69 0
100 100 100 100 150 150 150 150 200 200 200 200 150 100 0 100
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
and 4
and 8 and 8 (b) (a), 7 (c)
Fig. 4. AFM photograph of (a) silicon substrate surface and (b) SiC film surface with the thickness of 0.2 μm obtained at 1073 K for 5 min at the MMS gas flow rate of 92 sccm and HCl gas flow rate of 150 sccm. The Ra and RMS microroughness were 0.6 nm and 0.7 nm, respectively. The measured area was 0.2 × 0.2 μm.
increased due to the formation of short hillocks, as shown in Fig. 4 (b), which was formed following a grain nature. However, its surface appearance was still specular by visual inspection. The Ra and RMS microroughness were evaluated to be 0.6 nm and 0.7 nm, respectively. Some of the SiC films obtained from MMS gas at 1073 K showed a small grain-like surface [15,16], but the other films showed a very smooth surface. Because the specular surface is expected to have a higher coating quality than that of a grain-like surface, the condition for obtaining the smooth surface with a high reproducibility should be studied in future. 3.3. Low-temperature deposition In this section, the low-temperature SiC film formation is studied. For maintaining the gas condition over this study, hydrogen chloride gas was introduced with MMS gas, even at room temperature, at which hydrogen chloride gas hardly reacted with silicon. In the SiC film formation for 60 s at various temperatures between 1073 K and room temperature following Steps (A) and (B) in Fig. 2, the obtained film thickness was very small, around 0.1 μm, and their surface often had a grain-like morphology, as shown in Fig. 5 and a yellowish
Average film thickness (μm) 0.47 0.72 4.38 6.56 0.34 0.24 0.74 0.84 0.67 0.24 0.36 0.50 0.10 – b 0.1 –
Fig. 5. Surface morphology of the film formed at room temperature for 60 s using MMS gas (92 sccm) and hydrogen chloride gas (150 sccm), immediately after the surface cleaning in ambient hydrogen at 1373 K for 10 min.
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In order to verify the SiC film formation, Fig. 6 shows the XPS spectra of C 1s of the 0.1-μm thick deposited film which was obtained from MMS gas at room temperature, following the condition listed in Table 1, and further etched by hydrogen chloride gas at 1073 K for 10 min, following Steps (A), (B) and (C) shown in Fig. 2. Fig. 6 clearly shows the existence of the Si–C bond at 283 eV. Because silicon atom bonding with the carbon atom is consistently detected at 101 eV, the obtained film is concluded to contain the Si–C bond. In order to show the influence of hydrogen chloride gas at room temperature, the SiC film formation was performed only with MMS gas. This resulted in the same film formation when using MMS and hydrogen chloride gases. Thus, hydrogen chloride gas reasonably has less significant influence at room temperature. 3.4. Film formed at room temperature Fig. 6. XPS spectra of C 1s of the SiC film. The film was obtained from MMS gas and hydrogen chloride gas on silicon surface at room temperature after annealing in hydrogen ambient. This film was further exposed to hydrogen chloride gas at 1073 K for 10 min before the XPS measurement.
appearance indicating the existence of the SiC film. Thus, the film formation at the lowest temperature, that is, at room temperature, was further studied. The average film thickness obtained at room temperature, following Steps (A) and (B) in Fig. 2, was 0.1 μm, which was comparable to the thickness obtained at 1073 K. In order to quickly evaluate the coating quality of the SiC film, the film surface was further exposed to hydrogen chloride gas at 1073 K, following Step (C) in Fig. 2. Because the film surface showed no decrease in weight and because the surface appearance was maintained, the film formed at room temperature was expected to be SiC. In order to show the necessary condition for the film formation at room temperature, MMS gas was supplied to silicon substrate skipping the silicon surface cleaning (Step (A)) of the process shown in Fig. 2. This resulted in no weight increase to indicate no film formation; its surface was significantly etched by hydrogen chloride gas at 1073 K, by Step (C) in Fig. 2. Thus, the surface cleaning in ambient hydrogen (Step (A)) is shown to take an important role for the SiC film formation from MMS gas at low temperatures.
In order to evaluate the robustness of the SiC film formed at room temperature, the deposited film was exposed to hydrogen chloride gas at 1073 K, following Fig. 2; its surface was compared with that of a silicon substrate after exposed to hydrogen chloride gas. Fig. 7 shows SEM photograph of the silicon substrate surface and SiC film. Fig. 7 (a) is the silicon substrate surface after etching using hydrogen chloride gas at the flow rate of 100 sccm diluted by hydrogen gas of 2 slm, at the substrate temperature of 1073 K for 10 min, without SiC film formation. This figure shows the existence of many pits indicating the occurrence of etching by hydrogen chloride gas. Fig. 7 (b) shows the SiC film surface formed using MMS gas of 69 sccm at room temperature for 1 min. This figure shows that there was no large pit at the deposited film surface. Next, this surface was exposed to hydrogen chloride gas at the flow rate of 100 sccm diluted in hydrogen gas of 2 slm at 1073 K for 10 min. This condition was exactly the same as that performed for the silicon surface, shown in Fig. 7 (a). As shown in Fig. 7 (c), a considerable morphology change was not observed at the deposited film surface, except of particles intentionally taken in order to clearly focus the surface for SEM observation. Fig. 8 is the TEM micrograph of the cross section of the SiC film obtained in this study. The film, shown in Fig. 8, was obtained from MMS gas and hydrogen chloride gas on silicon surface at room
Fig. 7. Surface of (a) silicon substrate after etching using hydrogen chloride gas at 1073 K for 10 min, (b) deposited film using MMS gas of 69 sccm at room temperature, and (c) the film of (b) further etched using hydrogen chloride gas at 1073 K for 10 min.
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SiC film deposited at room temperature. For further development, the way to control the coating film thickness should be developed. 3.5. Film formation mechanism
Fig. 8. TEM micrograph of the cross section of the SiC film. The film was obtained from MMS gas and hydrogen chloride gas on silicon surface at room temperature after annealing in hydrogen ambient. This film was further exposed to hydrogen chloride gas at 1073 K for 10 min before the TEM measurement.
temperature after annealing at 1373 K in hydrogen ambient. This film was further exposed to hydrogen chloride gas at 1073 K for 10 min, before the TEM measurement. Fig. 8 shows that the entire silicon substrate surface was sufficiently covered with the SiC film consisted of arranged many grains, diameter of which was about 0.2–0.3 μm. The average film thickness in the observed area was about 0.3 μm Additionally, any etch pit and pin-hole caused due to etching by hydrogen chloride gas could not be found at the SiC–silicon interface in this figure. Thus, the SiC film deposited at room temperature in this study is recognized to be tough to a hazardous ambient including hydrogen chloride gas. Although the film thickness in Fig. 8 was larger than 0.1 μm, the area covered with quite thin film also existed, with no etched shape or pin-hole at SiC–silicon interface. Thus, the entire silicon substrate surface was protected from hydrogen chloride gas, by means of the
Based on the result that the surface cleaning in ambient hydrogen was necessary for producing the SiC film, the surface chemical process for the low temperature SiC formation using MMS gas is pictorially shown in Fig. 9 and discussed below. The SiC film formation is considered to be initiated by Step (a), as shown in Fig. 9. At Step (a), MMS molecule approaches to silicon dimer present at hydrogen-terminated silicon surface. The silicon dimer is assumed to be broken in order to accept the MMS molecule. Here, Step (a) is for an initiation of the surface chemical reaction; Steps (b) and (c) are for a repetition of the surface chemical reaction to produce multilayer film. After Step (a), Processes 1, 2 and 3, are expected to occur. At Step (b1) in Process 1, silicon atom in MMS is chemisorbed with silicon atom of the substrate. Here, hydrogen radicals are produced. The hydrogen radicals bond to the neighboring silicon atoms. At Step (c1) in Process 1, two hydrogen atoms are produced; dangling bonds remain at the neighboring silicon atoms. At Step (b2) in Process 2, silicon atom in MMS is chemisorbed with silicon atom of the substrate, similar to Process 1. Next, one of the 1hydrogen radicals produced can approach the hydrogen atom bonding with the carbon atoms in the chemisorbed MMS molecule. At Step (c2) in Process 2, two hydrogen atoms are produced; dangling bonds remain at the neighboring silicon atom and at the carbon atom in the MMS. At Step (b3) in Process 3, silicon atom in MMS is chemisorbed with silicon atom of the substrate, similar to Processes 1 and 2; one of the hydrogen radical produced can approach the hydrogen atom bonding
Fig. 9. Surface processes for low temperature SiC film growth. (a) approach of MMS to silicon dimer at hydrogen-terminated silicon surface. Process 1: (b1) chemisorption of MMS and production of hydrogen radicals, and (c1) production of hydrogen molecules, and silicon dangling bonds at the neighboring silicon. Process 2: (b2) chemisorption of MMS and production of hydrogen radicals, and (c2) production of hydrogen molecules, silicon dangling bond at the neighboring silicon, and carbon dangling bond in the chemisorbed MMS. Process 3: (b3) chemisorption of MMS and production of hydrogen radicals, and (c2) production of hydrogen molecules, and silicon dangling bonds at the neighboring silicon and in the chemisorbed MMS.
H. Habuka et al. / Surface & Coatings Technology 204 (2010) 1432–1437
with the silicon atom in the chemisorbed MMS molecule. At Step (c3) in Process 3, two hydrogen atoms are produced; dangling bonds remain at the neighboring silicon atom and at the silicon atom in the MMS. Because the dangling bonds formed after Steps (c1), (c2) and (c3) can accept more MMS molecules, chemisorption of MMS is expected to be spread and repeated over the substrate surface. When Process 2 is slower than Process 3, a larger amount of C–H bond can remain at the film surface. Because this induces the C–H termination over the entire surface, the SiC film formation finally saturates. Here, silicon dimer was reported to be very weak [25]; many research groups [6,26] reported the occurrence of the dissociative adsorption of organosilane on silicon dimer at room temperature. Additionally, Silvestrelli et al. [27] reported that SiH2CH3 can bond to silicon dimer, when the MMS molecule vertically approached the surface. Taking into account these previous studies, the surface process, shown in Fig. 9, is considered to be consistent with the results of the low temperature SiC film formation and its saturation, using MMS gas. 4. Conclusions A silicon carbide film is formed at a substrate temperature lower than 1073 K on a silicon surface by chemical vapor deposition using monomethylsilane gas along with hydrogen chloride gas in ambient hydrogen at atmospheric pressure. Some of the films, obtained at 1073 K and at the hydrogen chloride gas concentration greater than that of monomethylsilane gas, showed a specular surface having the root-mean-square microroughness of 0.7 nm, which was measured by AFM. At low temperatures, particularly, at room temperature, the 0.1–0.3-μm thick silicon carbide film could be formed on the silicon surface immediately after the hydrogen annealing at 1373 K. Following the SEM and TEM observation, the silicon carbide film formed at room temperature consisted of many small grains; hydrogen chloride gas at 1073 K caused no etch pit and pin-hole at its surface and at the interface between the silicon carbide film and the silicon substrate. Silicon dimers produced by the hydrogen annealing are considered to accept monomethylsilane molecules in order to produce covalent bonds. Because the obtained film suffered no etching by hydrogen chloride gas, the SiC film produced by the process
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developed in this study is expected to be a tough coating film applicable to a hazardous environment.
Acknowledgment The authors thank Dr. Masahiko Aihara of Yokohama National University for his helpful suggestions for this study.
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Silicon carbide film deposition at low temperatures using monomethylsilane gas Hitoshi Habuka ⁎, Hiroshi Ohmori, Yusuke Ando Department of Chemical and Energy Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 7 June 2009 Accepted in revised form 15 September 2009 Available online 25 September 2009 PACS: 81.15.Gh 71.20.Nr 81.15.Kk
a b s t r a c t A silicon carbide film is formed at low temperatures on a silicon surface by chemical vapor deposition using monomethylsilane gas along with hydrogen chloride gas in ambient hydrogen at atmospheric pressure. A 0.2-μm thick film, obtained at 1073 K and at a hydrogen chloride gas concentration greater than that of the monomethylsilane gas, possessed a specular surface having the root-mean-square microroughness of 0.7 nm. At temperatures lower than 900 K, the 0.1-μm thick silicon carbide film could be formed on the silicon surface, immediately after the surface cleaning in ambient hydrogen at 1373 K. Because the weight of the film formed at room temperature did not decrease after the etching using hydrogen chloride gas at 1073 K, the film obtained in this study is expected to be a tough coating film. © 2009 Elsevier B.V. All rights reserved.
Keywords: Silicon carbide Monomethylsilane Chemical vapor deposition Low temperature
1. Introduction A silicon carbide (SiC) thin film is expected to be a suitable coating material because it is very stable and tough even in a hazardous environment, such as the coating film of a susceptor of a silicon epitaxial reactor [1]. However, one of the problems of SiC as a coating material is the very high temperature often necessary for its film formation. A SiC film is formed using the chemical vapor deposition (CVD) method at various substrate temperatures [2–4]. The deposition temperature is often comparable to the melting point of silicon and may induce some damage due to the difference in the thermal expansion between the SiC film and the substrate material [5]. In order to reduce these problems, the SiC film coating at lower temperatures is expected. For the low temperature film formation, methylsilanes, such as monomethylsilane (MMS), dimethylsilane and trichloromethylsilane, are expected candidates, because these gases have a covalent bond between the silicon and carbon in their molecular structure. Many researchers have studied SiC film formation technology using monomethylsilane gas [4–13], as well as dimethylsilane [14]. The low temperature SiC film was formed on a silicon surface using MMS gas with hydrogen chloride (HCl) gas at 1073 K [15,16]. By this method, the excess silicon in the SiC film was reduced by hydrogen chloride gas, because HCl can react with SiHx in the gas phase and Si at the surface to form gaseous product of chlorosilanes. By this method, the
⁎ Corresponding author. Tel./fax: +81 45 339 3998. E-mail address: [email protected] (H. Habuka). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.09.044
SiC film thickness saturated within 60 s. Here, it should be noted that the high purity HCl gas did not corrode the stainless tube and quartz chamber in the CVD reactor. In order to develop the low-temperature SiC CVD technology, further details should be obtained, particularly about the influence of the gas composition, the surface morphology, the film formation temperature and the substrate surface cleaning condition. The relatively low-temperature SiC film formation has been studied in the field of surface physics and surface science. Particularly, Nakazawa and Suemitsu [6] studied the thermal decomposition of MMS molecules chemisorbed on the silicon surface. In their study, MMS was chemisorbed at room temperature on the silicon surface, which was cleaned in a high vacuum environment at high temperature immediately before the chemisorption. This indicates that SiC film or its intermediate compound film can be formed at low temperatures using MMS gas at the clean silicon surface which has dangling bonds or silicon dimers. In order to prepare an atomically clean silicon surface, the industrial silicon epitaxial growth process employs the annealing technique in ambient hydrogen. Additionally, the hydrogen-annealed silicon surface has been known to contain monohydrides with silicon dimers [17–19]. These dimers are expected to help the formation of chemical bonds between the silicon surface and precursors, such as MMS, at low temperatures. Thus, the SiC deposition from MMS gas on the silicon surface combined with the hydrogen annealing technique should be studied. Therefore, in this study, the SiC film deposition on a silicon surface is further studied using MMS gas at 1073 K and lower temperatures, particularly about the influence of the gas composition, the surface
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morphology, the film formation temperature and the substrate surface cleaning condition. Through this study as an extension of our previous study [15,16], hydrogen chloride gas is introduced along with MMS gas, expecting the decrease of excess silicon in the SiC film. 2. Experimental In order to obtain the SiC film by the CVD method, the horizontal cold-wall reactor shown in Fig. 1 was used. This reactor consists of a gas supply system, a quartz chamber and six infrared lamps. A 30mm-wide × 40-mm-long (100) silicon substrate manufactured by the Czochralski method is horizontally placed on the bottom wall of the quartz chamber. The silicon substrate is heated by infrared rays from the six halogen lamps through the quartz chamber. Because quartz absorbs only a slight amount of infrared light having the wavelength of ca. 1 μm from the halogen lamps, the temperature of the quartz chamber wall remains low. Electric power provided to the infrared lamps is adjusted on the basis of the temperatures measured beforehand in ambient nitrogen. The gas flow channel above the substrate of this reactor has a low height and a small rectangular cross section in order to achieve a very high consumption efficiency of the reactive gases. The height and the width of the quartz chamber are 10 mm and 40 mm, respectively, similar to those in our previous studies [20–22]. The gas supply system has the function of introducing gases of hydrogen, nitrogen, MMS, hydrogen chloride and chlorine trifluoride. Hydrogen gas is used as the carrier gas as well as the cleaning gas for removing the silicon oxide film and organic contamination on the silicon surface [21]. Throughout the process, hydrogen gas is introduced into the reactor at atmospheric pressure at the flow rate of 2 slm. Water vapor in the hydrogen gas is removed by passing the hydrogen gas through a liquid nitrogen trap (77 K) at the entrance of the reactor. Chlorine trifluoride gas [23,24] is used in order to remove the SiC film, which is formed on the inner wall of the quartz chamber during the SiC CVD process. The typical process used in this study is Steps (A), (B) and (C) shown in Fig. 2. Step (A): cleaning of the silicon surface at 1373 K for 10 min in ambient hydrogen. Step (B): SiC film formation using a gas mixture of MMS and hydrogen chloride, at room temperature — 1073 K after Step (A). Step (C): etching by hydrogen chloride gas at 1073 K for 10 min.
Fig. 2. Process for SiC film deposition and etching using gases of MMS, hydrogen chloride and hydrogen. Step (A): cleaning silicon surface at 1373 K, Step (B) film formation after Step (A), and Step (C): etching of film surface using hydrogen chloride gas at 1073 K.
exposed to the gas mixture of hydrogen chloride and hydrogen at the flow rate of 100 sccm and 2 slm, respectively, at 1073 K, as Step (C) in Fig. 2. Here, it may be pointed out that silicon surface suffers significant etching by hydrogen chloride gas at 1073 K [20]. After finishing the film deposition, the quartz chamber is cleaned using chlorine trifluoride gas (Kanto Denka Kogyo Co., Ltd., Tokyo, Japan) at the concentration of 10% in ambient nitrogen at 673–773 K for 1 min at atmospheric pressure. This cleaning technique is an application of the SiC etching technique using chlorine trifluoride gas [23,24]. In order to evaluate the chemical bond between silicon and carbon in the SiC film, the X-ray photoelectron spectra (XPS) were obtained using a PHI 5400MC spectrometer (Perkin-Elmer Co., Ltd., USA) which had an aluminium X-ray source. The incident angle of X-ray was 90° for the film having the thickness larger than 0.1 μm. The surface morphology was observed using an optical microscope (USB Microscope, Scalar Corp., Tokyo), a laser microscope (VK-9500, Keyence Corp., Tokyo), a scanning electron microscope (SEM) (VE-8800, Keyence, Tokyo) and an atomic force microscope (AFM) (VN-8000, Keyence Corp., Tokyo and SPA400 and SPI3800N, Seiko Instruments Co., Ltd., Tokyo). Surface microroughness was evaluated by AFM. In order to observe the surface morphology and the film thickness, a transmission electron microscope (TEM) (HD-2700, Hitachi High-Technologies, Tokyo) was taken. The XPS and TEM measurements were ex situ performed at the Foundation of Promotion of Material Science and Technology of Japan (Tokyo). 3. Results and discussion
The average thickness of the SiC film is evaluated from the increase in the substrate weight. In our preliminary experiment, a polycrystalline 3C–SiC wafer produced by the CVD method (Admap Inc., Tokyo) was found to suffer from no etching by the 100% hydrogen chloride gas at 1273 K for over 40 min. This clearly indicates that SiC film is not etched by the hydrogen chloride gas. Thus, in order to quickly determine the formation of SiC, some of the films obtained by Steps (A) and (B) were
Fig. 1. Horizontal cold-wall chemical vapor deposition reactor used for SiC film deposition.
3.1. SiC film thickness First, the SiC film thickness was evaluated at various gas compositions of MMS and hydrogen chloride for 5 min at 1073 K, as shown in Fig. 3 and as listed in Table 1. The hydrogen gas flow rate was 2 slm; the hydrogen chloride gas flow rate was 100 sccm (circle), 150 sccm (square) and 200 sccm (triangle). In Fig. 3, the film thickness entirely decreases with the increasing hydrogen chloride gas flow rate. The square and triangle show that the SiC film thickness was very small but it gradually increased with the increasing MMS gas flow rate between 50 and 200 sccm. In contrast to this, the SiC film thickness obtained at the hydrogen chloride gas flow rate of 100 sccm, indicated by the circle, showed a significant increase at the MMS gas flow rate greater than 100 sccm. Here, the surface appearance of the film having such a significant thickness increase was dark and very rough. Here, it should be noted that the silicon substrate surface was significantly etched by hydrogen chloride gas at its flow rate of 100 sccm for 60 s, without MMS gas. This indicates that the silicon– silicon bond present at the film surface can be removed by hydrogen chloride gas. Thus, based on the results in this study and following our
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Fig. 3. SiC film thickness produced for 60 s at 1073 K at various gas flow rates of MMS and hydrogen chloride. Hydrogen chloride gas flow rate was 100 sccm (circle), 150 sccm (square) and 200 sccm (triangle). Hydrogen gas flow rate was 2 slm.
previous studies [15,16], the amount of excess silicon in the SiC film was considered to be decreased, however, the insufficient amount of hydrogen chloride gas could not sufficiently suppress the incorporation of excess silicon. From Fig. 3, the amount of hydrogen chloride gas, comparable to or greater than that of the MMS gas, is considered to be necessary for the effective removal of the excess silicon. Because the HCl flow rate larger than 150 sccm was sufficient for the MMS gas flow rate between 50 and 200 sccm, the film thickness could linearly increase with the increasing MMS gas flow rate, as indicated using square and triangle in Fig. 3. 3.2. Surface morphology The surface morphology of the SiC film was evaluated by the AFM, because some of the SiC films obtained in this study showed a very smooth appearance by visual inspection. Fig. 4 shows the AFM photograph of (a) silicon surface before the film formation, and (b) SiC film surface with a thickness of 0.2 μm obtained at 1073 K for 5 min at the MMS gas flow rate of 92 sccm and hydrogen chloride gas flow rate of 150 sccm. The measured area was 0.2 × 0.2 μm. Fig. 4 (a) shows that the silicon substrate surface before the film formation was very smooth with the average roughness (Ra) and the root-mean-square roughness (RMS) of 0.2 nm and 0.3 nm, respectively. After the SiC film formation, the surface roughness slightly
Table 1 Conditions of film deposition and etching. Figure
Temperature (K)
Time (min)
MMS (sccm)
HCl (sccm)
H2 (slm)
3 3 3 3 3 3 3 3 3 3 3 3 6 6 7 7
1073 1073 1073 1073 1073 1073 1073 1073 1073 1073 1073 1073 300 1073 300 1073
5 5 5 5 5 5 5 5 5 5 5 5 1 10 1 10
46 92 138 184 46 92 138 184 46 92 138 184 92 0 69 0
100 100 100 100 150 150 150 150 200 200 200 200 150 100 0 100
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
and 4
and 8 and 8 (b) (a), 7 (c)
Fig. 4. AFM photograph of (a) silicon substrate surface and (b) SiC film surface with the thickness of 0.2 μm obtained at 1073 K for 5 min at the MMS gas flow rate of 92 sccm and HCl gas flow rate of 150 sccm. The Ra and RMS microroughness were 0.6 nm and 0.7 nm, respectively. The measured area was 0.2 × 0.2 μm.
increased due to the formation of short hillocks, as shown in Fig. 4 (b), which was formed following a grain nature. However, its surface appearance was still specular by visual inspection. The Ra and RMS microroughness were evaluated to be 0.6 nm and 0.7 nm, respectively. Some of the SiC films obtained from MMS gas at 1073 K showed a small grain-like surface [15,16], but the other films showed a very smooth surface. Because the specular surface is expected to have a higher coating quality than that of a grain-like surface, the condition for obtaining the smooth surface with a high reproducibility should be studied in future. 3.3. Low-temperature deposition In this section, the low-temperature SiC film formation is studied. For maintaining the gas condition over this study, hydrogen chloride gas was introduced with MMS gas, even at room temperature, at which hydrogen chloride gas hardly reacted with silicon. In the SiC film formation for 60 s at various temperatures between 1073 K and room temperature following Steps (A) and (B) in Fig. 2, the obtained film thickness was very small, around 0.1 μm, and their surface often had a grain-like morphology, as shown in Fig. 5 and a yellowish
Average film thickness (μm) 0.47 0.72 4.38 6.56 0.34 0.24 0.74 0.84 0.67 0.24 0.36 0.50 0.10 – b 0.1 –
Fig. 5. Surface morphology of the film formed at room temperature for 60 s using MMS gas (92 sccm) and hydrogen chloride gas (150 sccm), immediately after the surface cleaning in ambient hydrogen at 1373 K for 10 min.
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In order to verify the SiC film formation, Fig. 6 shows the XPS spectra of C 1s of the 0.1-μm thick deposited film which was obtained from MMS gas at room temperature, following the condition listed in Table 1, and further etched by hydrogen chloride gas at 1073 K for 10 min, following Steps (A), (B) and (C) shown in Fig. 2. Fig. 6 clearly shows the existence of the Si–C bond at 283 eV. Because silicon atom bonding with the carbon atom is consistently detected at 101 eV, the obtained film is concluded to contain the Si–C bond. In order to show the influence of hydrogen chloride gas at room temperature, the SiC film formation was performed only with MMS gas. This resulted in the same film formation when using MMS and hydrogen chloride gases. Thus, hydrogen chloride gas reasonably has less significant influence at room temperature. 3.4. Film formed at room temperature Fig. 6. XPS spectra of C 1s of the SiC film. The film was obtained from MMS gas and hydrogen chloride gas on silicon surface at room temperature after annealing in hydrogen ambient. This film was further exposed to hydrogen chloride gas at 1073 K for 10 min before the XPS measurement.
appearance indicating the existence of the SiC film. Thus, the film formation at the lowest temperature, that is, at room temperature, was further studied. The average film thickness obtained at room temperature, following Steps (A) and (B) in Fig. 2, was 0.1 μm, which was comparable to the thickness obtained at 1073 K. In order to quickly evaluate the coating quality of the SiC film, the film surface was further exposed to hydrogen chloride gas at 1073 K, following Step (C) in Fig. 2. Because the film surface showed no decrease in weight and because the surface appearance was maintained, the film formed at room temperature was expected to be SiC. In order to show the necessary condition for the film formation at room temperature, MMS gas was supplied to silicon substrate skipping the silicon surface cleaning (Step (A)) of the process shown in Fig. 2. This resulted in no weight increase to indicate no film formation; its surface was significantly etched by hydrogen chloride gas at 1073 K, by Step (C) in Fig. 2. Thus, the surface cleaning in ambient hydrogen (Step (A)) is shown to take an important role for the SiC film formation from MMS gas at low temperatures.
In order to evaluate the robustness of the SiC film formed at room temperature, the deposited film was exposed to hydrogen chloride gas at 1073 K, following Fig. 2; its surface was compared with that of a silicon substrate after exposed to hydrogen chloride gas. Fig. 7 shows SEM photograph of the silicon substrate surface and SiC film. Fig. 7 (a) is the silicon substrate surface after etching using hydrogen chloride gas at the flow rate of 100 sccm diluted by hydrogen gas of 2 slm, at the substrate temperature of 1073 K for 10 min, without SiC film formation. This figure shows the existence of many pits indicating the occurrence of etching by hydrogen chloride gas. Fig. 7 (b) shows the SiC film surface formed using MMS gas of 69 sccm at room temperature for 1 min. This figure shows that there was no large pit at the deposited film surface. Next, this surface was exposed to hydrogen chloride gas at the flow rate of 100 sccm diluted in hydrogen gas of 2 slm at 1073 K for 10 min. This condition was exactly the same as that performed for the silicon surface, shown in Fig. 7 (a). As shown in Fig. 7 (c), a considerable morphology change was not observed at the deposited film surface, except of particles intentionally taken in order to clearly focus the surface for SEM observation. Fig. 8 is the TEM micrograph of the cross section of the SiC film obtained in this study. The film, shown in Fig. 8, was obtained from MMS gas and hydrogen chloride gas on silicon surface at room
Fig. 7. Surface of (a) silicon substrate after etching using hydrogen chloride gas at 1073 K for 10 min, (b) deposited film using MMS gas of 69 sccm at room temperature, and (c) the film of (b) further etched using hydrogen chloride gas at 1073 K for 10 min.
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SiC film deposited at room temperature. For further development, the way to control the coating film thickness should be developed. 3.5. Film formation mechanism
Fig. 8. TEM micrograph of the cross section of the SiC film. The film was obtained from MMS gas and hydrogen chloride gas on silicon surface at room temperature after annealing in hydrogen ambient. This film was further exposed to hydrogen chloride gas at 1073 K for 10 min before the TEM measurement.
temperature after annealing at 1373 K in hydrogen ambient. This film was further exposed to hydrogen chloride gas at 1073 K for 10 min, before the TEM measurement. Fig. 8 shows that the entire silicon substrate surface was sufficiently covered with the SiC film consisted of arranged many grains, diameter of which was about 0.2–0.3 μm. The average film thickness in the observed area was about 0.3 μm Additionally, any etch pit and pin-hole caused due to etching by hydrogen chloride gas could not be found at the SiC–silicon interface in this figure. Thus, the SiC film deposited at room temperature in this study is recognized to be tough to a hazardous ambient including hydrogen chloride gas. Although the film thickness in Fig. 8 was larger than 0.1 μm, the area covered with quite thin film also existed, with no etched shape or pin-hole at SiC–silicon interface. Thus, the entire silicon substrate surface was protected from hydrogen chloride gas, by means of the
Based on the result that the surface cleaning in ambient hydrogen was necessary for producing the SiC film, the surface chemical process for the low temperature SiC formation using MMS gas is pictorially shown in Fig. 9 and discussed below. The SiC film formation is considered to be initiated by Step (a), as shown in Fig. 9. At Step (a), MMS molecule approaches to silicon dimer present at hydrogen-terminated silicon surface. The silicon dimer is assumed to be broken in order to accept the MMS molecule. Here, Step (a) is for an initiation of the surface chemical reaction; Steps (b) and (c) are for a repetition of the surface chemical reaction to produce multilayer film. After Step (a), Processes 1, 2 and 3, are expected to occur. At Step (b1) in Process 1, silicon atom in MMS is chemisorbed with silicon atom of the substrate. Here, hydrogen radicals are produced. The hydrogen radicals bond to the neighboring silicon atoms. At Step (c1) in Process 1, two hydrogen atoms are produced; dangling bonds remain at the neighboring silicon atoms. At Step (b2) in Process 2, silicon atom in MMS is chemisorbed with silicon atom of the substrate, similar to Process 1. Next, one of the 1hydrogen radicals produced can approach the hydrogen atom bonding with the carbon atoms in the chemisorbed MMS molecule. At Step (c2) in Process 2, two hydrogen atoms are produced; dangling bonds remain at the neighboring silicon atom and at the carbon atom in the MMS. At Step (b3) in Process 3, silicon atom in MMS is chemisorbed with silicon atom of the substrate, similar to Processes 1 and 2; one of the hydrogen radical produced can approach the hydrogen atom bonding
Fig. 9. Surface processes for low temperature SiC film growth. (a) approach of MMS to silicon dimer at hydrogen-terminated silicon surface. Process 1: (b1) chemisorption of MMS and production of hydrogen radicals, and (c1) production of hydrogen molecules, and silicon dangling bonds at the neighboring silicon. Process 2: (b2) chemisorption of MMS and production of hydrogen radicals, and (c2) production of hydrogen molecules, silicon dangling bond at the neighboring silicon, and carbon dangling bond in the chemisorbed MMS. Process 3: (b3) chemisorption of MMS and production of hydrogen radicals, and (c2) production of hydrogen molecules, and silicon dangling bonds at the neighboring silicon and in the chemisorbed MMS.
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with the silicon atom in the chemisorbed MMS molecule. At Step (c3) in Process 3, two hydrogen atoms are produced; dangling bonds remain at the neighboring silicon atom and at the silicon atom in the MMS. Because the dangling bonds formed after Steps (c1), (c2) and (c3) can accept more MMS molecules, chemisorption of MMS is expected to be spread and repeated over the substrate surface. When Process 2 is slower than Process 3, a larger amount of C–H bond can remain at the film surface. Because this induces the C–H termination over the entire surface, the SiC film formation finally saturates. Here, silicon dimer was reported to be very weak [25]; many research groups [6,26] reported the occurrence of the dissociative adsorption of organosilane on silicon dimer at room temperature. Additionally, Silvestrelli et al. [27] reported that SiH2CH3 can bond to silicon dimer, when the MMS molecule vertically approached the surface. Taking into account these previous studies, the surface process, shown in Fig. 9, is considered to be consistent with the results of the low temperature SiC film formation and its saturation, using MMS gas. 4. Conclusions A silicon carbide film is formed at a substrate temperature lower than 1073 K on a silicon surface by chemical vapor deposition using monomethylsilane gas along with hydrogen chloride gas in ambient hydrogen at atmospheric pressure. Some of the films, obtained at 1073 K and at the hydrogen chloride gas concentration greater than that of monomethylsilane gas, showed a specular surface having the root-mean-square microroughness of 0.7 nm, which was measured by AFM. At low temperatures, particularly, at room temperature, the 0.1–0.3-μm thick silicon carbide film could be formed on the silicon surface immediately after the hydrogen annealing at 1373 K. Following the SEM and TEM observation, the silicon carbide film formed at room temperature consisted of many small grains; hydrogen chloride gas at 1073 K caused no etch pit and pin-hole at its surface and at the interface between the silicon carbide film and the silicon substrate. Silicon dimers produced by the hydrogen annealing are considered to accept monomethylsilane molecules in order to produce covalent bonds. Because the obtained film suffered no etching by hydrogen chloride gas, the SiC film produced by the process
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developed in this study is expected to be a tough coating film applicable to a hazardous environment.
Acknowledgment The authors thank Dr. Masahiko Aihara of Yokohama National University for his helpful suggestions for this study.
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