CRITERIA FOR MAKING UNIFORM FILMS BY CHEMICAL VAPOR-DEPOSITION

The mechanisms which determine the stability of films made by chemical vapor deposition, CVD, are fundamentally different from those which govern physical vapor deposition, PVD, or solidification. Therefore different criteria govern the growth of uniform films when one uses these processes. We analyze CVD with a first order decomposition reaction at the surface. The competition among the mechanisms of gas phase diffusion, adsorption, capillarity, chemical reaction and heat transfer determine the stability of the film. Consequently this is a conjugated boundary value problem which involves gas phase, interfacial and solid phase processes that occur simultaneously. The diffusion of a reactant from the gas phase to a protuberance, or peak of the perturbation, occurs at a faster rate than to the plane portion of the film because the diffusion path is smaller. Therefore the concentration gradient in the gas phase enhances the growth of the perturbation. On the other hand if heating is from below the film the protuberance is at a lower temperature and as a result undergoes a lower rate of reaction and growth than the plane surface. Thus, heating the substrate, from below, tends to stabilize the film. Furthermore, when the wavelength of the perturbation is small, capillarity reduces the extent of adsorption at the peak of the perturbation and inhibits its growth. Surface diffusion also favors stable growth by reducing the concentration gradients on the surface. A linear stability analysis of this conjugated boundary value problem yields a criterion for stable growth. It is shown that lower reactor pressure, low temperature of the substrate, high activation energy, and heating from below the film with a large temperature gradient from the bottom to the top of the substrate favor morphological stability and therefore uniform films.