J. Cent. South Univ. Technol. (2009) 16: 0730-0737

DOI: 10.1007/s11771-009-0121-4

Chemical vapor deposition of SiC at different molar ratios of

hydrogen to methyltrichlorosilane

YANG Yan(杨 艳)^{1, 2}, ZHANG Wei-gang(张伟刚)¹

(1. State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering,

Chinese Academy of Sciences, Beijing 100190, China;

2. Graduate University of Chinese Academy of Sciences, Beijing 100049, China)

Abstract: Chemical vapor deposition (CVD) of SiC from methyltrichlorosilane (MTS) was studied at two different molar ratios of H₂ to MTS (n(H₂)/n(MTS)). The total pressure was kept as 100 kPa and the temperature was varied from 850 to 1 100 ℃ at a total residence time of 1 s. Steady-state deposition rates as functions of reactor length and of temperature, investigated at different n(H₂)/n(MTS) values, show that hydrogen exhibits strongly influences on the deposition rate. Especially, the deposition of Si co-deposit can be obtained in broader substrate length and at higher temperatures with increasing hydrogen partial pressure. Influence of hydrogen on the deposition process was also studied using gas phase composition and deposit composition analysis at various n(H₂)/n(MTS). SEM micrographs directly show the variation of surface morphologies at various n(H₂)/n(MTS). It can be found that the crystal grain of the deposit at 1 100 ℃ is better developed and the crystallization is also improved with increasing n(H₂)/n(MTS).

Key words: methyltrichlorosilane; silicon carbide; H₂; MTS

1 Introduction

Chemical vapor deposition (CVD) of SiC is one of the most frequently investigated deposition processes. Numerous studies may be distinguished with respect to  (1) the precursor, (2) the objective of the study, and (3) the experimental procedure. Most of the studies were performed using methyltrichlorosilane (MTS) [1-5]. In a few studies, an inert gas like argon was used as diluent gas [6]; in the other studies hydrogen was used not only as a diluent gas [6-10]. Deposition of SiC from methylchlorosilanes was studied with different aspects. Some authors were only interested in the deposition rate [11]. But in Refs.[3, 6, 12-13] the composition of the deposit was determined, additionally. In Ref.[14] this subject was concerned with the gas phase chemistry. Deposition rate and composition of the gas phase were simultaneously determined only in two studies [7]. However, detailed studies related to the deposition rate, gas phase composition, composition and structure of the deposit were not found. Therefore, in spite of numerous studies, CVD of SiC from MTS and hydrogen does not seem to be really understood, especially the influence of hydrogen on the co-deposition of silicon with silicon carbide.

The present study is concerned with CVD of SiC from MTS and hydrogen. Care is taken for the fact that this process includes homogeneous and heterogeneous reactions [15], and the interaction of the two kinds of reactions. Thus the overall deposition process is determined additionally by the molar ratio of hydrogen to MTS.

2 Experimental

2.1 Apparatus of CVD

A vertical hot wall reactor was used to carry out the CVD process as shown in Fig.1. The apparatus consisted of a feed system including mass flow meters, the gas source and the saturator, a deposition system and an exhaust system. The reactant tube was ceramic      (40 mm×700 mm) of Al₂O₃. The flow rate of gaseous MTS could be controlled by the flow rate of the carrier gas, saturator pressure and temperature.

The following equation was used to calculate the equilibrium mass flow of MTS:

                                (1)

where  p_o is the output pressure, F_r is the flow mass of the reactant, F_c is flow mass of the carrier, and p_r is vapor pressure of the reactant.

2.2 Sample preparation

A kind of porous cordierite was used as substrate,

Fig.1 Schematic diagram of chemical vapor deposition reactor: 1—Mass flow meters; 2—Saturator of SiCl₄; 3—Three-way valve; 4—Hot wall furnace; 5—Vacuum pump; 6—Thermo- couple; 7—Temperature controller; 8—Pressure controller

which was polished, cleaned and divided into 5 mm-thick slices. Previous to the SiC deposition, the substrate was coated with pyrolytic carbon using CH₄ as carbon source at 1 100 ℃ for 24 h. The pre-coated carbon exhibited similar thermal expansion coefficient (4.5×10^-6 K^-1, 293-673 K) to SiC, which was beneficial to the formation of a thick SiC coating without any cracks.

2.3 Deposition procedure

MTS was used as the source gas and hydrogen was used not only as a carrier gas to transport the reactants to the hot surface, but also as reactant for the chemical reaction to form SiC. Hydrogen was also used in order to remove organic contamination on the substrate surface immediately before the film deposition [16].

The experiment was performed under following conditions: the input gas molar ratios of H₂ to MTS (n(H₂)/n(MTS)) were 4?1 and 8?1; the total pressure was 101.3 kPa; the temperatures varied from 850 to 1 100 ℃ and the total residence time was 1 s.

According to the simulated results of temperature distribution profiles in the reactor using the commercially available Fluent 6.0 (Fluent. Inc) [17-18], the substrates for deposition were placed on the isothermal zone of the graphite holder. The reactor was purged using pure argon during heating up. When the desired temperature reached, the carrier gas was supplied to the saturator and the pressure was controlled by an automatically controlled pump. When the deposition was finished, inputting of the source gas was switched to purging gas of argon. In order to prevent cracks formation in the film, the reactor was cooled down rather slowly.

3 Results and discussion

3.1 Thermodynamic calculations

Calculation of chemical equilibrium composition is always important for the selection of any specific CVD process. Thermodynamic analysis of the equilibrium gas composition not only suggests the degree of variation of the chemical deposition conditions but also changes the thermodynamic yield and the formation of intermediate species [19].

A traded HSC chemistry data bank is used for the thermodynamic calculations. Compositions of the gas phase considered are as follows: CCl₄, CH₃, CH₄, C₂H₂, C₂H₄, C₂H₅, C₂H₆, Cl₂, H, H₂, HCl, Si(CH₃)₄,Si(CH₃)Cl₃, Si(CH₃)₃Cl, Si(C₆H₅)Cl₃, SiCl₂, SiCl₃, SiCl₄, SiH₂, SiH₄, Si₂H₄, Si₂H₆, SiH(CH₃)Cl₂, SiHCl₃, SiH₂Cl₂, SiH₃Cl, C₃H₆, C₃H₈ and C₆H₆. The condensed phases are SiC, C and Si.

The equilibrium compositions of gas phase and solid phase as a function of n(H₂)/n(MTS) at 1 100 ℃ and ambient pressure are shown in Figs.2 and 3, respectively. Species with a concentration of less than 10^-3 (mole fraction) are not given.

Fig.2 shows equilibrium compositions of the gas phase as a function of n(H₂)/n(MTS) without the deposition of Si, SiC and C (Fig.2(a)), with the deposition of Si, SiC and C (Fig.2(b)), and only with the deposition of SiC and Si (Fig.2(c)). It can be seen that H₂ is the predominant equilibrium composition which hardly changes with n(H₂)/n(MTS). The equilibrium compositions of the solids corresponding to the gas phase compositions of Figs.2(b) and 2(c) are presented in Figs.3(a) and 3(b), respectively. Fig.3(a) displays large amounts of SiC and small amounts of Si, and C does not exist according to the experimental results. Fig.3(b) shows large amounts of SiC and little Si, and the amount of Si is lower without the formation of C than that with the formation of carbon. The small amount of Si increases with the increase of n(H₂)/n(MTS), but the ratio of Si to C may be stable at different amounts of hydrogen because of the large progression difference between the amount of SiC and Si, which indicates the pure SiC may be obtained at any n(H₂)/n(MTS).

3.2 Influence of n(H₂)/n(MTS) on deposition rates as function of substrate length

Figs.4(a)-(f) show surface-related, steady-state deposition rates as a function of substrate length in

Fig.2 Equilibrium compositions (x denotes molar fraction) of gas phase as function of n(H₂)/n(MTS): (a) Without formation of SiC, Si and C; (b) With formation of SiC, Si and C; (c) With formation of SiC and Si

temperature range of 850-1 100 ℃ and different n(H₂)/ n(MTS) values (=8, 4).

At 850 ℃, the deposition rate increases with the substrate length at the front part of the substrates, followed by the decrease at the end of the substrate (Fig.4(a)) at two n(H₂)/n(MTS) values. The deposition rate at higher n(H₂)/n(MTS) reaches another unconspi- cuous peak at an even substrate length, which indicates that high concentration of hydrogen can promote the

Fig.3 Equilibrium composition of solid phase as function of n(H₂)/n(MTS): (a) With formation of SiC, Si and C; (b) With formation of SiC and Si

deposition process. The two maximum deposition rates are caused by the high deposition rate of Si at lower temperature [17-18], which is also testified by the Raman spectrum analysis (see Fig.5).

The deposition rates decrease at the two n(H₂)/ n(MTS) values, 900 ℃ (Fig.4(b)) and 1 000 ℃ (Fig.4(c)) except that at 900 ℃ the rate increases in the front part of the substrate and is limited by a rate maximum (Fig.4(b)) for n(H₂)/n(MTS)=8. The rate maximum is induced by the high deposition rate of Si in the lower temperature range and the rate maximum is higher than that at 850 ℃. These results show that the deposition temperature is promoted to be higher in the lower temperature range and the residence time is promoted to be longer at the same temperature with the increase of hydrogen concentration.

At 1 050 ℃, the deposition rates dramatically

decrease at the front part of the substrate, followed by an increase as a function of the substrate length at the two n(H₂)/n(MTS) values (Fig.4(d)), which should be caused by the overlap of the deposition peaks at two different substrate lengths. The weak peak approaching the end of the substrate is caused by the maximum deposition rate

Fig.4 Deposition rate as function of substrate length at different n(H₂)/n(MTS) values and different temperatures: (a) 850 ℃; (b) 900 ℃; (c) 1 000 ℃; (d) 1 050 ℃; (e) 1 100 ℃

of SiC at higher temperatures. This result shows that the CVD process is divided by two different regimes: lower and higher temperature deposition processes. Co-deposition of Si and SiC occurs at these medium temperatures.

At 1 100 ℃ (Fig.4(e)), the rate maxima are caused by the very high deposition rate of SiC in high temperature range at the two n(H₂)/n(MTS) values. The maxima are higher than those at other temperatures in Figs.4(a)-(d). And it is also obvious that the deposition can be obtained in broader range of the substrate length at higher n(H₂)/n(MTS).

In general, it can be concluded that there is co-deposition of Si and SiC at lower temperatures but pure SiC can only be deposited at higher temperature. Deposition rates are always promoted with broader substrate length and broader temperature ranges by increasing hydrogen. Additionally, the deposition rates at n(H₂)/n(MTS)=8 are smaller than those at n(H₂)/n(MTS)=4, which is caused by a lower concentration of MTS.

3.3 Influence of H₂ on deposition rates as function of temperature

The tendency of the overall deposition rates as a function of temperature at different n(H₂)/n(MTS)   values is nearly the same (see Fig.6), which shows         the deposition rate peaks at about 900 ℃ and the rates

Fig.5 Raman spectrum of deposits obtained at 850 ℃ and n(H₂)/n(MTS)=8

Fig.6 Overall deposition rates as function of temperature at different n(H₂)/n(MTS) values

dramatically increase at high temperatures. This clearly suggests that deposition of SiC from MTS is based on two different processes, low- and high-temperature regimes. The overall rate maxima are induced by the rate maxima at the front part of the substrate [13]. The overall rate is smaller at higher n(H₂)/n(MTS) because the original concentration of gas phase composites reduces. This can be supported by the following analysis of the gas phase composition. The deposition rate at low temperature can be the same to that at higher temperature by increasing n(H₂)/n(MTS).

3.4 Gas phase composition

The gas phase composition as a function of temperature is studied at different n(H₂)/n(MTS) values under an ambient pressure of about 100 kPa, which is obtained with a GC analysis. The results are presented in Fig.7.

Fig.7(a) shows the concentration of MTS decreases with the temperature and almost decomposes completely as the temperature approaches 900 ℃. The hydrogen exhibits a little effect on the concentration of MTS during the decomposition, which may suppress the decomposition of MTS at lower temperature to a certain extent.

The concentrations of CH₄, SiHCl₃ and SiCl₄ increase rapidly with the increase of temperature (Figs.7(b), 7(c) and 7(d)) while MTS decomposes below 900 ℃, which shows the decomposition of MTS results in the formation of CH₄, SiHCl₃ and SiCl₄ in the gas phase. Fig.7(b) shows the concentration of CH₄ decreases when hydrogen increases because of the suppression of hydrogen on MTS below 900 ℃. During the process of reaction, the following reaction is promoted:

CH₃?+H₂→CH₄+H?                          (2)

Therefore, CH₃? decomposed from MTS reacts with more hydrogen and more CH₄ is obtained, which results in the less concentration difference below 900 ℃ and the higher concentration above 900 ℃ between different amount of hydrogen. The concentration of CH₄ almost keeps constant within a range of temperature above  900 ℃ because of the complete decomposition of MTS and then the concentration of CH₄ decreases at higher temperature because of the participation in the reaction of formation of SiC at high temperatures.

Fig.7(c) shows the hydrogen has little effect on the concentration of SiHCl₃ below 900 ℃ because below 900 ℃, the main dissociation reaction of MTS is as follows:

CH₃SiCl₃→CH₃?+SiCl₃?                      (3)

HSiCl₃ is produced mainly due to the reaction of SiCl₃? with MTS:

SiCl₃?+CH₃SiCl₃→SiHCl₃+Cl₃SiCH₂?           (4)

Therefore, from reaction (4), it can be seen that hydrogen shows less relevance to the concentration of HSiCl₃ at lower temperatures.

But obviously hydrogen exhibits dramatic effect on the concentration on SiHCl₃ above 900 ℃. SiHCl₃ can be decomposed into SiCl₂?? that is important reactive specie during the CVD process. SiCl₂?? can form the higher chlorosilanes, leading to formation of Si. It can also react with CH₄, which forms the higher carbochlorosilane, leading to the formation of SiC at higher temperature. Therefore, the concentration of SiHCl₃ decreases rapidly with increasing temperature.

At lower temperatures, SiCl₃? from the decomposition of MTS can form SiCl₄ as follows:

2SiCl₃?→SiCl₄+SiCl₂??                       (5)

With the increase of temperature and after the dissociation of MTS, SiCl₄ can react with H₂ to form SiHCl₃ and HCl in the gas phase in the higher temperature range.

Fig.7 Concentrations of MTS and volatile products as function of temperature at different n(H₂)/n(MTS) values: (a) MTS;  (b) CH₄; (c) SiHCl₃; (d) SiCl₄; (e) HCl

SiCl₄+H₂→SiHCl₃+HCl                       (6)

Reaction (6) can therefore be promoted when the amount of hydrogen increases. So the concentration of HSiCl₃ at higher n(H₂)/n(MTS) is higher than that at lower one.

Fig.7(d) shows the concentration of SiCl₄ increases rapidly below 950 ℃ because of the pyrolysis of MTS in the gas phase. Reaction (5) is the formation reaction of SiCl₄. The concentration of SiCl₄ is low when the hydrogen increases because of the suppressing influence of hydrogen on the MTS. The concentration of SiCl₄ keeps stable above 950 ℃ in a certain temperature range, which shows that SiCl₄ is more stable for providing the origin of Si at higher temperatures compared to SiHCl₃. Concentration of SiCl₄ slightly decreases between 1 050 and 1 100 ℃ because of reaction (6), which is promoted by increasing the molar ratio of hydrogen to MTS. This result is accordant with the thermodynamic calculations.

Fig.7(e) shows that there is a little of HCl in the gas phase during the pyrolysis of MTS below 950 ℃. During the consecutive reaction process, the increase of the concentration of HCl with the temperature trends to be more rapid above 950 ℃ because of the reactions of SiHCl₃, SiCl₄ and CH₄ produced in the gas phase during the dissociation of MTS, as the following reactions (7) and (8):

SiHCl₃→SiCl₂??+HCl                         (7)

SiCl₂??+CH₄→ClSiCH­₃??+HCl                 (8)

It is obvious that the concentration of HCl at n(H₂)/n(MTS)=8 is higher than that at n(H₂)/n(MTS)=4, which can be explained by reaction (6). When the amount of H₂ is increased, reaction (6) is promoted, which results in large amount of HCl in the gas phase.

From Fig.7 it can be seen that the correlation between gas phase compositions and temperature shows two different reaction processes: lower temperature and higher temperature processes, i.e. below 950 ℃ and above 950 ℃, respectively. This fact is not changed when n(H₂)/n(MTS) increases. The influence of hydrogen on the gas phase compositions can be perfectly explained by the model of gas phase reaction suggested by reactions (2)-(8).

3.5 Composition of deposits

From Fig.8 it can be seen that the molar ratio of Si to C increases when hydrogen increases at 900 and       1 000 ℃, especially at the front part and the end of the substrate, corresponding to the shortest and longest substrate length, respectively. From the above analysis of gas phase composition, reaction (6) is promoted by increasing hydrogen content. Thus the contents of HSiCl₃, and further SiCl₂?? are increased. It is well known that SiCl₂?? is a very important radical for forming larger species in the gas phase, such as chlorosilanes and carbochlorosilanes.

Fig.8 Molar ratio of Si to C in deposits as function of substrate length at various temperatures and n(H₂)/n(MTS)

At lower temperatures, free Si can be formed from these chlorosilanes. Therefore, the formation of free Si is promoted, which can explain the molar ratio of Si to C at higher n(H₂)/n(MTS).

At 1 100 ℃, reaction (8) occurs, which leads to a typical high-temperature reaction and forms larger carbochlorosilane in the gas phase and produces SiC further. The reaction can be improved by hydrogen because of the increasing SiCl₂?? and as a result, the formation of SiC is promoted. It is well known that the content of Si is the same as that of C for a SiC molecule, so the molar ratio of Si to C is not changed with increasing n(H₂)/n(MTS).

3.6 Surface morphologies of deposits

SEM micrographs of chemical vapor deposited films at 1 100 ℃ and different n(H₂)/n(MTS) are shown in Fig.9, and a pure hexangular crystalline β-SiC is detected using XRD (not shown). It can be seen from Fig.9 that, the hexangular facet structure of SiC at the two n(H₂)/n(MTS) values is compact with typical morphologies of high degree crystallization. The size of each hexangular facet structure is about 200 nm at n(H₂)/n(MTS)=4 and about 400 nm at n(H₂)/n(MTS)=8, that is to say, the size becomes larger when the content of hydrogen in the gas phase increases. Furthermore, the crystallization boundary is better developed with the increase of hydrogen.

200 nm

Fig.9 SEM micrographs at different n(H₂)/n(MTS) values and       1 100 ℃: (a) n(H₂)/n(MTS)=4; (b) n(H₂)/n(MTS)=8

4 Conclusions

(1) Chemical vapor deposition of SiC from MTS/H₂ system is a complex homogeneous-heterogeneous process. The gas phase reactions and compositions strongly depend on the temperature, substrate length and especially hydrogen concentration.

(2) Co-deposition of Si in low temperature regime (less than 950 ℃) is caused by the formation of chlorosilanes and polychlorosilanes in the gas phase. Increasing hydrogen benefits the formation of chlorosilanes, which leads to deposition of Si in an even broader substrate length range.

(3) Increasing the concentration of hydrogen in high temperature regime (above 950 ℃) enhances the decomposition of SiCl₄ that is a stable component formed during the dissociation of MTS at low temperature. Chemical reactions of SiCl₄ with CH₄ at high temperature (above 950 ℃) result in the formation of carbochlorosilane in the gas phase, which leads to the deposition of pure hexangular SiC at 1 100 ℃.

(4) These experimental results are in good accordance with the thermodynamic and kinetic analysis.

References

[1] PAPASOULIOTIS G D, SOTIRCHOS S V. On the homogeneous chemistry of the thermal decomposition of methyltrichlorosilane: Thermodynamic analysis and kinetic modeling [J]. J Electrochem Soc, 1994, 141(6): 1599-1611.

[2] BESMANN T M, SHELDON B W, LOWDEN R A, STINTON D P. Vapor-phase fabrication and properties of continuous-filament ceramic composites [J]. Science, 1991, 253: 1104-1109.

[3] SOTIRCHOS S V, PAPASOULIOTIS G D. Experimental study of the atmospheric pressure chemical vapor deposition of silicon carbide from methyltrichlorosilane [J]. J Mater Res, 1999, 14: 3397-3409.

[4] LOUMAGNE F, LANGLAIS F, NASLAIN R. Reactional mechanisms of the chemical vapour deposition of SiC-based ceramics from CH₃SiCl₃/H₂ gas precursor [J]. J Cryst Growth, 1995, 155: 205-213.

[5] LOUMAGNE F, LANGLAIS F, NASLAIN R. Experimental kinetic study of the chemical vapour deposition of SiC-based ceramics from CH₃SiCl₃/H₂ gas precursor [J]. J Cryst Growth, 1995, 155: 198-204.

[6] KIM H S, CHOI D J. Effect of diluent gases on growth behavior and characteristics of chemically vapor deposited silicon carbide films [J]. J Am Ceram Soc, 1999, 82: 331-337.

[7] CAGLIOSTRO D E, RICCITIELLO S R. Model for the formation of silicon carbide from the pyrolysis of dichlorodimethylsilane in hydrogen (I): Silicon formation from chlorosilane [J]. J Am Ceram Soc, 1993, 76(1): 39-48.

[8] CAGLIOSTRO D E, RICCITIELLO S R. Model for the formation of silicon carbide from the pyrolysis of dichlorodimethylsilane in hydrogen (II): Silicon carbide formation from silicon and methane [J]. J Am Ceram Soc, 1993, 76(1): 49-53.

[9] CAGLIOSTRO D E, RICCITIELLO S R. Comparison of the pyrolysis products of dichlorodimethylsilane in the chemical vapor deposition of silicon carbide on silica in hydrogen or argon [J]. J Am Ceram Soc, 1994, 77(10): 2721-2726.

[10] TAKEUCHI T, EGASHIRA Y. A kinetic study of the chemical vapor deposition of silicon carbide from dichlorodimethylsilane precursors [J]. J Electrochem Soc, 1998, 145(4): 1277-1284.

[11] SONE H, KANEKO T, MIYAKAWA N. In situ measurements and growth kinetics of silicon carbide chemical vapor deposition from methyltrichlorosilane [J]. J Cryst Growth, 2000, 219: 245-252.

[12] KANEKO T, OKUNO T, YUMOTO H. Growth kinetics of silicon carbide CVD [J]. J Crystal Growth, 1988, 91: 599-604.

[13] LU Y M, LEU I C. Microstructural study of residual stress in chemically vapor deposited B-SiC [J]. Surface and Coatings Technology, 2000, 124: 262-265.

[14] VOROBEV A N, KARPOV S Y, ZHMAKIN A I, LOVTSUS A A, MAKAPOV Y N, KRISHNAN A. Effect of gas-phase nucleation on chemical vapor deposition of silicon carbide [J]. J Cryst Growth, 2000, 211: 343-346.

[15] HUTTINGER K J. CVD in hot wall reactors: The interaction between homogeneous gas-phase and heterogeneous surface reactions [J]. Chem Vap Deposition, 1998, 4(4): 151-158.

[16] HABUKA H, WATANABE M, MIURA Y, NISHIDA M, SEKIGUCHI T. Polycrystalline silicon carbide film deposition using monomethylsilane and hydrogen chloride gases [J]. J Cryst Growth, 2007, 300: 374-381.

[17] ZHANG W G, HUTTINGER K J. CVD of SiC from methyltrichlorosilane deposition rates [J]. Chem Vap Deposition, 2001, 7(4): 167-172.

[18] ZHANG W G, HUTTINGER K J. CVD of SiC from methyltrichlorosilane [J]. Chem Vap Deposition, 2001, 7(4): 173-181.

[19] KLEPS I, ANGELESCU A. Correlations between properties and applications for the CVD amorphous silicon carbide films [J]. Appl Surf Sci, 2001, 184: 107-112.

(Edited by YANG You-ping)

Foundation item: Project supported by the One Hundred Talents Program of Chinese Academy of Sciences

Received date: 2008-11-10; Accepted date: 2009-04-07

Corresponding author: ZHANG Wei-gang, Professor, PhD; Tel: +86-10-62520135; E-mail: wgzhang@home.ipe.ac.cn