3860
Ind. Eng. Chem. Res. 2009, 48, 3860–3866
Reduction of Chemical Reaction Mechanism for Halide-Assisted Silicon Carbide Epitaxial Film Deposition Rong Wang, Ronghui Ma,*,† and Michael Dudley‡ Department of Mechanical Engineering, UniVersity of Maryland, Baltimore County, Baltimore, Maryland 21250, and Department of Materials Science and Engineering, Stony Brook UniVersity, Stony Brook, New York 11794
Simulation of gas-phase and surface chemistry is an essential part in numerical study of chemical vapor deposition for film growth. When integrated with a model for transport processes such as gas flow, heat transfer, and mass transfer, it allows the prediction of gas-phase composition, film deposition rate, and film uniformity. For halide-assisted chemical vapor deposition of silicon carbide film growth, three-dimensional simulation of the deposition process can be time-consuming due to the large number of chemical reactions involved. In this study, a simplified chemical reaction mechanism was developed for silicon carbide growth with silicon tetrachloride and propane as precursors. After model validation, the reduced reaction steps were implemented into a three-dimensional simulation of halide-assisted chemical vapor deposition to predict the distributions of gas velocity, temperature, concentration of the intermediate reactants, and film deposition rate. Specifically, the effects of deposition pressure on the film growth were investigated. The integrated model for chemistry and transport process demonstrated the capability of modeling a deposition process with reasonable computing time. We envision that this model will provide a useful tool for design, test, and optimization of the deposition process for growing silicon carbide films or bulk crystals by use of halideassisted chemical vapor deposition process. 1. Introduction Silicon carbide (SiC) is an attractive wide bandgap semiconductor material with superior properties that enable SiC-based electronic and optoelectronic devices to work under extremely harsh conditions with exceptional functionalities.1 Halideassisted chemical vapor deposition (HCVD) has emerged as a potent technique for growing SiC epitaxial films at high deposition rates.1-12 Several groups 4-12 employed chlorinated silicon-containing precursors or addition of hydrogen chloride (HCl) for SiC film growth and reported a high deposition rate in the range of 100-300 µm/h. It is suggested that the addition of chloride forms silicon dichloride (SiCl2) as a major siliconbearing species in a HCVD reactor. At elevated temperatures, SiCl2 is a chemically stable reactant with high surface activity.4-10 In comparison to the extensively used silane (SiH4), SiCl2 allows the employment of higher Si-containing precursor flow rates and higher deposition temperatures while minimizing the formation of silicon clusters on the film surface. Thereby, the addition of chloride leads to an elevated deposition rate and improved surface morphology.3-5 Although it is a promising technique enabling SiC film or bulk crystal growth with reasonable quality and productivity, HCVD has only been performed on small-area SiC substrates. Further advancement of this technology requires deposition of films on large area substrates with good uniformity, low defect densities, and desired properties. Numerical modeling has been widely exploited as an expedient method for test, design, and optimization of reactor geometry and process conditions for a variety of vapor deposition processes.13-31 An essential part in the numerical study of vapor deposition is the gas-phase and surface chemistry because it has profound impact on film * To whom correspondence should be addressed. E-mail: roma@ umbc.edu. Phone: + 001-410-4551965. Fax: + 001-410-4551052. † University of Maryland. ‡ Stony Brook University.
deposition rate and characteristics of as-grown films.13-16 When integrated with a transport model for gas flow, heat transfer, and transfer of multiple chemical species, it allows one to predict the film deposition rate and uniformity.13-25 Veneroni et al.26-28 investigated the kinetics of gas-phase reactions and surface kinetics with SiHCl3, SiH3Cl2, and SiCl3CH3 as the precursors. They developed a reaction mechanism with 76 surface reactions and modeled the deposition process using a one-dimensional boundary layer model. Wang et al.29 proposed a reaction mechanism for HCVD with silicon tetrachloride and propane (SiCl4/C3H8) as precursors. This mechanism was used in a twodimensional (2D) axisymmetric model for SiC HCVD in a horizontal hot-wall reactor.30,31 The simulation results provided quantitative information on the chemical composition of the gas mixture and the deposition rates for various combinations of the precursor flow rates and deposition temperatures. A parametric study of the deposition process suggests that etching of SiC by HCl is the main reason for the reduced deposition rates at elevated temperatures. Although important insight of SiC HCVD process has been achieved, 2D simulation is not able to address issues that could be critical for process scale-up, such as film uniformity, nonsymmetric reactor geometry, and buoyancy-driven flow in a reactor chamber. Three-dimensional (3D) simulation is imperative but is expensive in terms of computing time and resource, mainly because of the large number of gasphase and surface species and reactions involved. In this study, we developed a simplified reaction mechanism for SiC HCVD using SiCl4 and C3H8 as precursors with considerably reduced number of chemical species and reaction steps. The deposition rates and concentration distributions of the major species predicted by the reduced reaction mechanism were compared with those by the original set of reactions. 3D simulations were performed using the simplified reaction steps. The film uniformity and deposition rates were predicted and the effects of growth pressure were investigated. Also, this
10.1021/ie8017093 CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 3861
Figure 1. The reaction pathway for the reduced gas-phase and surface deposition kinetics for SiC growth from SiCl4/C3H8/H2 mixture.
reduced mechanism was slightly modified and used to study SiC HCVD using SiCl4 and methane (CH4) as the precursors.
are presented in Figure 1. The reduced gas-phase and surface reaction steps are presented in Table 1 and 2, respectively.
2. Reduction of Reaction Mechanism
3. Simulation Configuration
The comprehensive kinetic reaction mechanism we developed in our previous study29 consists of a large number of fundamental chemical reaction steps, including 76 reactions and 33 species in the gas phase and 31 reactions and 9 species on the deposition surface. This mechanism offers a detailed description of the reaction kinetics but is computationally expensive. Our primary study shows that a 3D simulation of SiC growth in a horizontal hot-wall reactor using a grid system of 349 × 133 × 80 takes 168 hours on a customized PC workstation with Intel Core 2 Quad 6600 Processor. Further reduction of the grid density will considerably affect the accuracy of the simulation. Undoubtedly, 3D simulation will benefit from a simplified kinetic reaction mechanism. Previous study of SiC film growth from SiH4 and C3H815 suggested that a large reaction mechanism, although important for identifying the major reacting species and reaction pathway, may not be critical for predicting the deposition rate. For example, simplified reaction steps developed for SiC growth from SiH4/C3H8 has demonstrated the capability of predicting film deposition rate with acceptable accuracy.14,16 For SiC HCVD, some gas species have extremely low concentrations and low surface adsorption rates while others, such as SiCl2 and CH4, contribute to more than 95% of the film growth.29 On the basis of this argument, a selection of the reaction steps was performed. On the basis of 2D simulation results, the species with very low concentrations (lower than 10-6 mol/m3) were identified and the relevant reaction paths were eliminated from the mechanism. It should be noted that some species, although having low concentrations, exist as an important intermediate link between different reactions. The absence of these intermediates will cause substantial difference in the gas-phase composition. With this in consideration, the reduction process was carried out empirically. The elimination of each species was followed by a 2D simulation. The gas composition and deposition rate predicted with the updated reaction steps were then compared with those obtained from the original set of reactions at various deposition temperatures. The selection process leads to a reduced mechanism involving 19 species and 36 reaction in the gas-phase reactions and 6 species and 24 reactions on the deposition surface. The major reaction paths
A typical hot-wall horizontal HCVD reactor10-12 for SiC deposition is depicted schematically in Figure 2. SiC deposition occurs in a growth chamber that is made of a tubular dense graphite susceptor surrounded by graphite foam. The inner dimensions of the reactor chamber are 55 mm in diameter and 200 mm in length. A 6H-SiC substrate with the dimension of 1 cm × 1 cm is positioned on the inner wall of the susceptor. A mixture of gas precursors and carrier gas is delivered through a graphite channel from the left end and exhausted from the other. The growth chamber is sealed in a double-walled, watercooled quartz tube. The radio frequency induction coils that surround the quartz tube are used to acquire the deposition temperature. The growth experiments were performed in the temperature range of 1400-1700 °C, at the pressure of 200 Torr, with the flow rates of SiCl4, C3H8, and H2 varying in the range of 5-180 sccm, 2.5-30 sccm, and 5-15 slm, respectively.10-12 The gas mixtures were preheated to 50 °C before entering the growth chamber. The deposition temperature refers to the temperature at the center of the substrate hereafter in this paper. 4. Mathematical Model for HCVD Process The prediction of the distributions of the temperature, gas velocity, and species’ concentrations in the reactor chamber requires solving the equations for conservations of mass, energy, momentum, and individual chemical species. The fundamental chemical reaction steps for gas-phase and surface reaction are incorporated into the mass transport equation. Since the reactor is heated to a designated temperature by an inductive heating system, full Maxwell equation needs to be solved for predicting the Joule heat generated in the susceptor, which is subsequently used as input into the heat transfer equation. A detailed description of the transport and electromagnetic dynamic models for HCVD is available in the references.29-31 5. Validation of the Reduced Reaction Mechanism The reduced chemical reaction mechanism presented in Tables 1 and 2 was used in a 2D simulation of HCVD process in the reactor shown in Figure 2. Figure 3 compares the
3862 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 Table 1. Reduced Gas-Phase Reactions for SiCl4/C3H8/H2 Systema reaction
Af
SiCl4 T SiCl3 + Cl SiCl3 T SiCl2 + Cl H + HCl T Cl + H2 HCl T H + Cl H + SiCl2 T HCl + SiCl SiCl2 + H2 T SiH2Cl2 SiCl2 + HCl T SiHCl3 SiCl + H2 T SiHCl + H SiH2Cl2 T SiHCl + HCl H2 + H T 3H 2H2 T 2H + H2 CH4 T CH3 + H C2H4 T C2H2 + H2 C2H5 T C2H4 + H C3H8 T C2H5 + CH3 2CH T C2H2 CH2 + H T CH + H2 CH2 + H2 T CH3 + H CH2 + CH T C2H2 + H 2CH2 T C2H4 2CH2 T C2H2 + 2H 2CH2 T C2H2 + H2 CH3 + H2 T CH4 + H CH3 + CH T C2H3 + H CH3 + CH2 T C2H4 + H 2CH3 T C2H5 + H CH4 + CH T C2H5 CH4 + CH T C2H4 + H CH4 + CH2 T 2CH3 CH4 + CH3 T C2H5 + H2 C2H2 + H T C2H3 C2H2 + H2 T C2H3 + H CH4 + Cl T CH3 + HCl Cl + CH3Cl T HCl + CH2Cl CH3Cl T CH3 + Cl CH3 + Cl T CH2 + HCl
6.9 × 10 4.6 × 1015 1.44 × 1013 9.0 × 1013 2.45 × 1014 4.91 × 1012 1.52 × 1012 4.02 × 1014 6.60 × 1014 2.228 × 1014 9.033 × 1014 8.3 × 1013 1.4 × 1012 1.4 × 108 2.3 × 1022 1.204 × 1014 3.011 × 1013 1.987 × 1013 3.975 × 1013 1.024 × 1012 1.084 × 1014 1.204 × 1013 289.1 3.011 × 1013 1.807 × 1013 7.829 × 1014 1.626 × 1014 3.011 × 1013 1.289 × 1013 1.024 × 1013 5.54 × 1012 2.409 × 1012 106600 3.4 × 1013 1.4 × 1015 2.2 × 1013 16
nf
Ea/R (f)
0 0 0 0 0 0 0 0 0 0 0 0 0.4 1.2 -1.8 0 0 0.5 0 0 0 0 3.1 0 0 0 0 0 0.5 0 0 0 2.6 0 0 0
37760 32815 1731 44010 9490 20364 9285 16391 37054 48350 48350 52246 44670 18722 44637 0 0 0 0 0 400 400 4384 0 0 13275 0 -200 0 11500 1214 32700 502 1425 41373 4152
Ab
2.7 × 1013 6.3 × 10 1.5 × 1014 4.4 × 1014 2.2 × 108 2.3 × 1012 9.792 × 1017 9.792 × 1016 1.204 × 1015 3.011 × 1011 8.431 × 108 4.456 × 1013 13
nb
Ea/R (b)
0
2251
0 0 0 1.74 0 -1 -0.6 -0.4
9800 38068 36152 -608.8 4178 0 0
1.49
19600 499
1.987 × 1013
13240
3
4045
3.674 × 1013
9.635 × 1013 2.65 × 10-8
6.28
1.541 × 1014 1.698 × 1012
0 0
-1580.5 0 436.6
a The rate constants are written in the Arrhenius form, k)ATn exp(-Ea/RT). Unit of A depends on reaction order, but is given in terms of cm, moles, and seconds. f and b designate forward and backward reactions, respectively. Ea is the activation energy in J/mol, R is the ideal gas constant in J/(mol K), and T is the temperature in K. Backward reaction rates are calculated from equilibrium thermodynamics if not explicitly written out. S and B designate surface species and bulk species, respectively. Si($) and C($) represent Si and C surface sites, respectively. Subscripts C and Si indicate a molecule absorbed on C or Si sites.
concentrations of the major gas species (HCl, SiCl2, and CH4) predicted with the reduced and the original set of reactions along the axial centerline of the reactor chamber at the deposition temperature of 1400 °C. The film deposition rates at different deposition temperatures and carrier gas flow rates for the pressure of 200 Torr are presented in Figure 4. The differences in the predicted deposition rate between the reduced and original set is less than 2%, indicating that the reduced reaction mechanism retains the major features of the original reaction mechanism and is able to predict the chemical process with reasonable accuracy. 6. 3D Simulation of HCVD Process Three-dimensional simulations of the SiC HCVD process in the horizontal hot-wall reactor were performed with a grid system of 349 × 133 × 80 using the commercially available software package ESI-CFD. The grid dependence has been examined by use of a refined grid system of 700 × 266 × 160. Consistent results have been produced with a difference less than 2%. With the customized PC workstation (Intel Core 2 Quad 6600 Processor and 4GB RAM), it took about 40 h to reach the designated convergence. In comparison to the time used (168 h) for 3D simulation of the same process with the original set of reactions, the reduced reaction mechanism can significantly save the computatinal time.
7. Results and Discussions Film Uniformity and Average Deposition Rate. The simulation of SiC HCVD is performed at temperatures ranging from 1400 to 1700 °C and the pressure of 200 Torr. The flow rates of SiCl4, C3H8, and H2 are 120 sccm, 10 sccm, and 5000 sccm, respectively. Figure 5a shows the calculated film deposition rate distribution on the substrate at the deposition temperature of 1400 °C. The predicted film deposition rate on the substrate surface varies in the range of (15%, with the high film deposition rates observed near the edge of the substrate, as the low ones at the center. We attribute the spatial variation of the deposition rate to the gas velocity on the substrate, temperature variation of the substrate, as well as the complex temperaturedependent surface reactions. The simulation of HCVD showed that a high concentration of HCl present in the reactor chamber causes film etching.29-31 The rate of film etching is dependent on the deposition temperature and HCl concentration. According to the analysis of surface chemical reaction kinetics,29-31 etching effect is more pronounced at elevated temperatures and a reduced deposition rate is observed with increased deposition temperatures.29-31 In 3D simulation, the substrate was modeled as a flat surface positioned on the bottom of a tubular susceptor as shown in Figure 5b. The gas velocity over the substrate surface is not uniform, with the center exposed to a gas flow of higher velocity than the edge. The higher gas velocity leads to a thinner boundary layer which facilitates the transport of
Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 3863 a
Table 2. Reduced Surface Reactions for SiCl4/C3H8/H2 System reaction
Af
CH4 + Si($) f C(S) + 2H2 CH3 + Si($) T CH(S) + H2 CH2 + Si($) f C(S) + H2 CH + Si($) f CH(S) C2H5 + 2Si($) f C(S) + CH(S) + 2H2 C2H4 + 2Si($) f 2C(S) + 2H2 C2H3 + 2Si($) f C(S) + CH(S) + H2 C2H2 + 2Si($) f 2C(S) + H2 SiHCl3 + 2Si($) + 2C($) f SiCl(S) + H(S) + 2ClSi(S) SiH2Cl2 + Si($) + 3C($) f SiCl(S) + 2H(S) + ClSi(S) SiCl4 + 2Si($) + 2C($) f SiCl(S) + ClC(S) + 2ClSi(S) SiCl2 + 2C($) T SiCl(S) + ClC(S) SiCl2 + Si($) + C($) T SiCl(S) + ClSi(S) SiCl + C($) f SiCl(S) SiHCl + 2C($) f SiCl(S) + H(S) SiHCl + C($) f Si(S) + HCl ClC(S) f Cl + C($) ClSi(S) f Cl + Si($) 2H(S) f H2 + 2C($) SiCl(S) + C(S) f SiC(B) + Si($) + C($) + Cl SiCl(S) + CH(S) f SiC(B) + HCl + C($) + Si($) Si(S) + C(S) f SiC(B) + C($) + Si($) Si(S) + CH(S) f SiC(B) + C($) + Si($) + H HCl + SiC(B) f SiCl + CH
2.4 × 10 8.5114 × 1011 8.9125 × 1011 9.12 × 1011 5.7544 × 1020 9.3325 × 1017 5.8884 × 1020 1.2023 × 1019 2.63 × 1016 3.8 × 108 2.34 × 1016 3.09 × 1019 3.09 × 1019 4.17 × 1011 3.31 × 1020 4.17 × 1012 1 × 1017 1 × 1017 1 × 1017 1 × 1017 1 × 1017 1 × 1017 1 × 1017 3.55 × 1010 9
nf
Ea/R (f)
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 0 0 0 0 0 0 0.5
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Ab
nb
Ea/R (b)
2.291 × 1011
0.5
44432
0 0
10151 45367
1 × 1019 1 × 1019
a The rate constants are written in the Arrhenius form, k)ATn exp(-Ea/RT). Unit of A depends on reaction order, but is given in terms of cm, moles, and seconds. f and b designate forward and backward reactions, respectively. Ea is the activation energy in J/mol, R is the ideal gas constant in J/(mol K), and T is the temperature in K. Backward reaction rates are calculated from equilibrium thermodynamics if not explicitly written out. S and B designate surface species and bulk species, respectively. Si($) and C($) represent Si and C surface sites, respectively. Subscripts C and Si indicate a molecule absorbed on C or Si sites.
Figure 2. The schematic of a horizontal hot-wall HCVD reactor.10-12
reactants to the center of the substrate. On the other hand, the temperature distribution on the substrate is neither uniform nor axisymetric. The highest temperature locates at the center of the substrate, and decreases with distance away from the center in the longitudinal direction. Under the combined effects of reactant transport and surface etching, a lower deposition rate is observed at the center where the temperature is higher and HCl transport to the surface is facilitated. The simulation result implies that a uniform deposition places a high requirement on the uniformity of the temperature and velocity on the substrate surface. Figure 6 compares the predicted average film deposition rates with those measured in SiC growth experiment under identical deposition conditions. The prediction agrees reasonably well with the experimental measurements except for the large deviation observed at the temperature of 1500 °C. We consider this deviation caused by both the uncertainty of the measurements and the model. The reference 11 from which the experimental data was taken indicated that the deposition rate measured at the same conditions varied by 40 µm/h. On the other hand, the verification of the chemical reactions and the transport model is very limited due to the unavailability of the measurement of the gas composition, temperature distribu-
Figure 3. Comparison of the major species’ concentrations along the axial centerline of the reactor predicted by the reduced and the original reaction mechanisms at 1400 °C. The flow rates of SiCl4, C3H8, and H2 are 120 sccm, 10 sccm, and 5 slm, respectively.
tion, and the velocity field inside of a reactor. In-situ measurement is a challenging task because of the high temperature, low pressure, and erosive environment in the reactor chamber. Detailed discussion of the film growth and film growth kinetics can be found in the references.29-31 Simulations of SiC HCVD were performed in the temperature range of 1400-1700 °C as employed in the experimental studies.10-12 The analysis of surface reaction kinetics suggests that elevated deposition temperatures favor surface etching by HCl and result in reduced deposition rates for the reactor configuration and process conditions used in this study.29-31 SiC HCVD conducted at temperatures lower than 1300 °C is rarely reported partly because the low deposition temperature
3864 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009
Figure 4. Comparison of the film deposition rates at different deposition temperatures and carrier gas (H2) flow rates predicted by the reduced and the original reaction mechanism.
Figure 7. Distributions of temperature and gas velocity on different crosssections of the reactor chamber. The deposition temperature is 1400 °C and the pressure is 600 Torr.
Figure 5. (a) Predicted film deposition rate distribution on a 1 cm × 1 cm substrate situated on the susceptor at the deposition temperature of 1400 °C and pressure of 200 Torr; and (b) side view of the position of a substrate in the susceptor.
Figure 6. Comparison of the predicted and measured average film deposition rates at various deposition temperatures.
will cause various problems such as incomplete decomposition of C3H8, inclusions of 3C-SiC, and imperfect crystalline structure. Effects of Deposition Pressure and Buoyancy Effect. In the previous 2D simulation of SiC HCVD in a horizontal hotwall reactor,29-31 the non-symmetric gravitational force and the associated buoyancy flow is not considered based on an order-
of-magnitude analysis that indicates negligible buoyancy flow due to the low pressure (P ) 200 Torr), small size of the reactor (D ) 55 mm), and strong forced convection. In this study, we performed 3D modeling for deposition pressure ranging from 200 to 600 Torr with the gravitational body force taken into consideration in the model of gas flow and heat and mass transfer. Displayed in Figure 7 are the distributions of temperature and gas velocity vector for the pressure of 600 Torr on different cross-sections perpendicular to the axial centerline of the growth chamber. It is observed that both the temperature and velocity distributions deviate from a perfect axisymmetric shape due to the gravitational effect. However, the gas flow inside the reactor remains to be dominated by forced convection. This observation of negligible buoyancy effect agrees with the order-of-magnitude analysis. The ratio of Grashof to Reynolds number square, Gr/Re2, which presents the strength of buoyancy force to the inertial force, is evaluated in the range of 0.01-0.07 for a carrier gas flow rate of 5 slm. Although the buoyancy effect is insignificant for the process conditions used in this study, it might have appreciable influence on film growth for enlarged reactor size and reduced carrier gas flow rate, leading to the formation of vertex over the substrate. Detailed discussion of buoyancy flow in CVD processes can be found in the work of Jaluria, Y., 1998.17 Figure 8 presents the predicted film deposition rates at three different deposition pressures for the deposition temperature of 1400 °C with SiCl4, C3H8, and H2 flow rates of 120 sccm, 10 sccm, and 5 slm, respectively. The film deposition rate is observed to increase with pressure. At the deposition pressure of 600 Torr, a deposition rate as high as 500 µm/h is achieved. However, the simulations of SiC growth from SiH4 by Danielsson and Jaze´n15 indicated that a high deposition rate is achieved at low pressure. The contradiction can be explained by the different growth mechanism. For SiC CVD with SiH4, a low flow rate of SiH4 is used to avoid gas-phase nucleation of silicon. Thereby, the growth is limited by the transport of the reactants to the deposition surface. A lower pressure leads to a thinner boundary layer that facilitates the transport of reactants to the deposition surface, resulting in an elevated film deposition rate.
Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 3865
Figure 8. Variations of the deposition rates with deposition pressure at H2 flow rate of 5 slm and the deposition temperature of 1400 °C. Table 3. The Precursor and Carrier Gas Flow Rates (sccm) for SiC HCVD
case 1 case 2
SiCl4
CH4
120 120
30
C3H8
H2
10
5000 5000
On the other hand, HCVD is featured by high flow rate of silicon-bearing precursors and high deposition rates. As implied by the Damko¨hler number of 2.5-9.0,30 the deposition is controlled by the rate of surface reaction as well as that of transport. At elevated pressures, the high concentrations of the reactants on the substrate favor fast surface reactions and lead to enhanced deposition rates. Experimental studies6-9 suggested that a high deposition rate will compromise the SiC crystalline perfection by creating defects and voids on the film surface. Further, such a high deposition rate can move film growth from epitaxial growth to polycrystalline growth. Deposition pressure is an important parameter that substantially affects the SiC growth and should be selected carefully for a HCVD process. CH4 as Carbon-Containing Precursor. In SiC HCVD, it has been reported that under certain growth conditions, premature decomposition of C3H8 into gaseous carbon leads to carbon deposition on the gas delivery channel and the reactor wall. In the worst case, it may cause carbon inclusion in the film.6 One solution to this problem is to employ methane (CH4) as the carbon source. Compared with C3H8, the decomposition of CH4 into gaseous C proceeds at a much slower rate, which greatly reduces the carbon deposition and improves the carbon delivery efficiency.6 Besides, CH4 has higher mass diffusivity in the gas mixture and simpler decomposition mechanism. Our previous reaction kinetics study indicated that CH4 is the most chemically stable C-containing species in the deposition temperature range of HCVD. In an effort to investigate the effect of CH4 as the carboncontaining precursor, we performed simulations of HCVD using CH4 in the deposition temperature range of 1400-1700 °C. The precursor and carrier gas flow rates applied are presented in Table 3. The CH4 flow rate triples that of C3H8 so that the total amount of carbon atoms delivered remains the same. Increasing the flow rate of carbon-bearing precursor from 10 to 30 sccm will not make appreciable difference in the temperature distribution because it will only increase the total gas flow rate by 0.4%. The thermal and flow conditions are dominated by the flow rate
Figure 9. Variations of the deposition rates with deposition temperatures for different carbon-bearing precursors (deposition pressure: 200 Torr).
of the carrier gas H2 (5000 sccm). The predicted film deposition rates for the four simulation cases are presented in Figure 9. Slightly higher deposition rates are observed when CH4 is used. Conclusion Reduction of chemical reactions was performed and led to a smaller reaction set for SiC HCVD from SiCl4 and C3H8. Compared with the original reaction set, the reduced reaction mechanism has less number of reactants and reactions, which enables 3D simulation of SiC HCVD with acceptable simulation time and computational resource. Numerical simulation of SiC with the large reaction mechanism performed in our previous publications has demonstrated the capability of predicting the film deposition rate as well as the thermal, flow, and chemical conditions in the reactor, enabling advanced understanding and improved design to control the etching effect. The reduced reaction set further allows 3D simulations to be performed to address a variety of issues including, but not limited to, film uniformity, buoyancy effect, and deposition pressure. The simulation results suggested that the reactor configuration and process conditions need to be designed carefully in order to grow epitaxial SiC film with desired deposition rate, uniformity, and quality. We envision that the simplified reaction model provides an expedient means of studying the HCVD system and is the first step toward the development of a useful tool for reactor design and optimization of the process conditions for large area SiC film growth using different types of reactor configurations. Literature Cited (1) Powell, J. A.; Will, H. A. Epitaxial Growth of 6H Silicon Carbide in the Temperature Range 1320°C to 1390°C. J. Appl. Phys. 1973, 44, 5177. (2) Myers, R. L.; Kordina, O.; Rao, S.; Everly, R.; Saddow, S. E. Increased Growth Rate in a SiC Reactor Using HCl as an Additive. Mater. Sci. Forum 2005, 483, 73. (3) Crippa, D.; Valente, G. L.; Ruggiero, A.; Neri, L.; Reitano, R.; Calcagno, L.; Foti, G.; Mauceri, M.; Leone, S.; Pistone, G.; Abbondanza, G.; Abbagnale, G.; Veneroni, A.; Omarini, F.; Zamolo, L.; Masi, M.; Roccaforte, F.; Giannazzo, F.; Di Franco, S.; La Via, F. New Achievements on CVD Based Methods for SiC Epitaxial Growth. Silicon Carbon Relat. Mater. 2005, 1, 67. (4) La Via, F.; Galvagno, G.; Foti, G.; Mauceri, M.; Leone, S.; Pistone, G.; Abbondanza, G.; Veneroni, A.; Masi, M.; Valente, G. L.; Crippa, D. 4H SiC Epitaxial Growth with Chlorine Addition. Chem. Vap. Dep. 2006, 12, 509.
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ReceiVed for reView November 10, 2008 ReVised manuscript receiVed February 17, 2009 Accepted February 25, 2009 IE8017093
