Modeling chemical vapor deposition of thin solid films

Citation

Jabbour, Michel E. (2000) Modeling chemical vapor deposition of thin solid films. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pa2d-3n29. https://resolver.caltech.edu/CaltechTHESIS:10042010-153504863

Abstract

Chemical vapor deposition (CVD) is a process by which thin solid films are deposited on solid substrates for various technological applications. Roughly speaking, a multispecies chemically reacting gas flows past a heated substrate on top of which the deposition and subsequent formation of the alloy or compound of interest take place via a series of heterogeneous chemical reactions. The growth rate of the thin film is determined by the competition between the diffusive and convective transports of species in the gas phase, the homogeneous and heterogeneous chemical kinetics, and the morphology of the gas-film interface. In chapter 2, a thermomechanical macroscopic model is proposed that couples the multicomponent chemically reactive gaseous flow to the bulk of the growing thin solid film via the equations that govern the morphological evolution of the film-gas interface. The surface is modeled as a separate anisotropic elastic phase, and such phenomena as surface species diffusion, heat conduction and chemistry are accounted for. In particular, the driving force at the surface is identified, and a thermodynamically consistent kinetic relation linking it to the growth velocity is proposed. A specialization of this general framework to the case of a multicomponent ideal gas and a linearly elastic solid film separated by an isotropic surface is considered. In chapter 3, we examine a multicomponent gas flow in a vertical axisymmetric MOCVD reactor whose geometry is characterized by a small aspect ratio (defined as the ratio of the height of the reactor channel to the radius of the substrate) and operating under conditions insuring a small Mach number. A two-parameter asymptotic analysis yields, in the limit of vanishingly small aspect ratio and Mach number, a set of approximate equations governing the gas phase, combined with approximate boundary conditions at the showerhead and the gas-film interface. A specialization to the steady-state approximation is then proposed, and an analytical solution to the approximate problem is derived. It is found that this solution is of the similarity type, thus insuring a uniform temperature and chemical composition profiles along the film surface. Finally, in chapter 4, the growth of a generic thin film via a ledge-and-terrace mechanism is examined. Of particular interest is the interaction between the microstructure of the surface and the chemical kinetics by which the adsorption/desorption of species along the terraces and the formation of the compound at the steps occur. A simple step-flow model is proposed and its specialization to the case of a binary compound is used to illustrate the complex dependence of the averaged growth rate on the chemical composition of the gas phase as well as on the morphology of the evolving surface.

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