Buoyancy, Soret, Dufour, and Variable Property Effects in Silicon Epitaxy

In most of the previous numerical and semi-analytical studies of silicon epitaxial deposition, a common practice has been to neglect the buoyancy flow, Dufour, Soret, and property variation effects. In this paper, we take a critical look at the validity of that approach and point out some fallacies. The geometric configuration studied is a horizontal reactor for the susceptor tilt angles of 0 and 2.9 deg. The full Navier-Stokes equations coupled with those for the energy and species transfer are solved numerically for a range of parameters typical of commercial silicon epitaxial deposition systems. The effects of ignoring terms due to buoyancy, Dufour, Soret, and variable properties on the mass transfer rate are systematically evaluated. The results indicate that for typical horizontal epitaxial deposition parameters, the buoyancy and Dufour effects have negligible effect on the mass transfer rate, while the Soret and property variation have a large impact. In light of this information, it is shown that the agreement reported in the past between the experimental and numerical/analytical studies is coincidental. The implication is that these assumptions must be critically examined for a given CVD system and not ignored a priori. Finally, the effects of important parameters—reactor height, inlet velocity, inlet concentration, and susceptor temperature—on the deposition characteristics are included to provide guidelines for controlling the epitaxial layer thickness and uniformity.