Modeling the Nucleation Kinetics and Aerosol Dynamics of Particle Formation during Cvd of Silicon from Silane

Product contamination by gas-phase nucleation within the processing environment often limits the deposition rate that can be obtained during chemical vapor deposition (CVD) of materials for microelectronics applications. A fundamental understanding of how these particles nucleate and grow may allow us to enlarge the process envelope, providing higher growth rates without particle contamination. Since these particles are formed by a series of chemical reactions involving multiple chemical species with formation and breakage of strong chemical bonds, classical homogeneous nucleation theory fails to adequately describe their generation. The work presented here is part of an ongoing effort to develop a generalized framework for describing these chemical nucleation processes. This study focuses on particle formation during thermal CVD of silicon from silane. We have modeled particle nucleation, growth, and transport in both low-pressure and atmospheric-pressure CVD of silicon from silane, and present results of those modeling efforts here. The nucleation of particles has been modeled by a series of chemical reactions that lead from silane to larger silicon hydrides and finally to hydrogenated silicon clusters that can be considered particles. The thermochemistry and kinetics of these reactions are estimated based on the known reactivity and thermochemistry of the smaller silicon hydrides. A group additivity scheme for estimating the thermodynamic properties of arbitrary silicon hydride clusters was developed by fitting group parameters to the results of the extensive ab initio calculations of Katzer et al (1997). Pressure-dependent rate parameters for reactions of silicon hydrides with up to three silicon atoms were taken from the work of Ho, Coltrin and Breiland (1994). For the larger silicon hydrides, four classes of reversible chemical reactions were considered, as shown in Table 1. The polysilanes produced by these reactions can grow either by eliminating hydrogen to form a silylene (reaction type 1) that then reacts with silane (reverse of reaction type 2), or by reacting directly with silylene (reverse of reaction type 2) produced by thermal decomposition of silane. Silylenes can reversibly isomerize to silenes (reaction type 3), which are thermodynamically more stable than silylenes and are effectively unreactive. Silylenes with the proper geometry can undergo intramolecular insertion to form a ring (reaction type 4). The growth process described above is in competition with polysilane decomposition (reaction type 2). Ring formation plays a key role in the growth process, because once a silicon atom is incorporated into a ring it cannot directly be eliminated by a reaction of type 2. Ring formation also accounts for the dehydrogenation of the particles, since cyclic compounds have lower silicon-to-hydrogen ratios than acyclic compounds. To our knowledge, this cyclization has not been included in any previous models of silicon hydride chemistry.