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THEORETICAL STUDY OF GAS-PHASE THERMODYNAMICS RELEVANT TO SILICON CARBIDE CHEMICAL VAPOR DEIPOSITION*

MARK D. ALLENDORF AND CARL F. MELIUS Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551-0969 ABSTRACT Equilibrium calculations are reported for conditions typical of silicon carbide (SiC) deposition from mixtures of silane and hydrocarbons. Included are 34 molecules containing both silicon and carbon, allowing an assessment to be made of the importance of organosilicon species (and organosilicon radicals in particular) to the deposition process. The results are used to suggest strategies for improved operation of SiC CVD processes. I. INTRODUCTION The excellent resistance of silicon carbide (SiC) to corrosive, oxidizing or hightemperature environments, in addition to its outstanding hardness and semiconductor properties, make it of considerable industrial interest. Comprehensive models including detailed chemical reactions that occur in the gas-phase and on the surface, coupled with reactor fluid mechanics are required to optimize new deposition processes. Recent work provides several indications that gas-phase chemistry is an important element in the chemical vapor deposition (CVD) of SiC. First, a comparison of measured deposit compositions with thermodyrqamic predictions for mixtures containing silicon, carbon, and hydrogen showed that deposits typically contain excess silicon in regions where pure SiC is predicted at equilibrium[l]. This is attributed to the suspected higher surface reactivity of gas-phase silicon-containing species relative to that of stable hydrocarbons (such as CH 4 and C2 H 4 ). Several experimental and theoretical studies have confirmed this difference in reactivity [2,3]. Second, the reactivity of gas-phase hydrocarbons themselves with silicon substrates varies widely, depending on the degree of unsaturation in the molecule. Saturated species, such as C3 H 8, which is commonly used to deposit epitaxial SiC, have reactive sticking coefficients on Si(1 11) (whose crystal structure is the same as that of P-SiC) on the order of 10-5 to 10-4 [3], while unsaturated molecules such as C2H2 react with a higher probability [4] on the order of 0.001 - 0.01. Radical species are even more reactive, with sticking coefficients near unity [2,5]. This low reactivity of the initial precursors relative to their decomposition products implies that some decomposition by gas-phase pyrolysis may be required to achieve efficient deposition with these reactants. Indeed, at the high gas-phase temperatures typical of SiC-CVD (> 1500 K), hydrocarbons such as C 3 H 8 are known to decompose, yielding large amounts of CH 4 , C2 H 4 , and C2 H 2 [6]. This conversion could be the rate-limiting step in SiC deposition under some conditions. Clearly, with such large variations (a factor of 104) in surface reactivity, the exact composition of the gas phase can make a large difference in the deposition rate. Finally, surface temperatures lower than 1500-1600 K are desirable in order to deposit Si