A Numerical Model for Hot-Wire Chemical Vapor Deposition of Amorphous Silicon

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79 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society

MODEL DESCRIPTION For simplicity, only the simplest possible hot-wire reactor is considered here: a long, straight wire enclosed by a concentric cylinder that serves as the substrate. End effects are neglected, and therefore spatial variations in mean gas properties only occur in the radial direction. The gas is simulated with the Direct Simulation Monte Carlo (DSMC) method [4]. The DSMC method is a direct, physical simulation technique, in which simulation particles representing groups of physical molecules are allowed to move, collide, react, and interact with surfaces according to rules designed to produce statistically correct collision and reaction rates. Mean flow properties are determined by partitioning the domain into cells and sampling the number, identity, momentum, and energy of particles in each cell, averaging over many samples once steady state has been achieved. The technique has been tested arid validated in many different low-density gas flows with good results [4]. CHEMISTRY Si and H Production on the Wire The primary chemistry on the wire is conversion of incident silicon hydride species and molecular hydrogen into atomic Si and H [5]. The wire is assumed to atomize incident SinH, species with probability 0.7 [1]. For H2 , the dissociation is modeled as an activated process with an activation energy of 50 kcal/mol, resulting in a probability of reaction of 0.14 at 2000 °C. Film Growth Chemistry Film growth is modeled approximately, using measured or estimated reactive sticking probabilities [1, 6]. Since the total simulation time required for the gas to come to steady state is well below one monolayer coverage time, no attempt is made to dynamically compute the state of the surface. Instead, surface chemistry is modeled with global reactions. Based on the measurements of [6], atomic H has unity sticking probability on the surface, and SiH 3 can either abstract a surface hydrogen with probability 0.16, or stick to the surface with probability 0.10. Radicals that can insert into silicon-hydrogen bonds (Si, SiH, SiH 2 , Si 2, Si 3, H3 SiSiH) are expected to be highly reactive on the surface, and are assigned a sticking probability of 0.7 [1]. At each timestep, the molecules that stick are removed from the simulation, and reaction products, if any, are emitted from the surface. Reactions involving a surface hydrogen are balanced by simply adjusting the H2 flux back to the gas to insure overall H balance. No attempt is made to simulate hydrogen incorporation into the film. Gas-Phase Reactions The gas-phase mechanism consists of 18 reversible reactions among the species H, H2, Si, Si 2 , SiH, SiH 2 , SiH3, SiH 4 , Si 2 H2 , H2 SiSiH 2, H3 SiSiH, Si 2 H6 , and Si3H 8 . The reaction rates are taken from literature sources. The silane pyrolysis mechanism of Ho, Coltrin, and Breiland [7] is used, with additional radical chemistry taken primarily from Woike, Catoire, and Roth [8]. In practice, bimolecular radical chemistry is the mos

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