Symmetry breaking in a stagnation-flow CVD reactor

Flow-symmetry breaking owing to buoyancy effects in cold-wall stagnation flow chemical vapor deposition reactors has been studied through numerical solution of the 3-D laminar Navier-Stokes equations. The mechanisms behind symmetry breaking have been outlined, and the process windows in which it may occur have been determined. For reactor height-to-diameter aspect ratios larger than one, and Rayleigh numbers (based on the reactor diameter) between Ra = 2×10³ and Ra = 10⁵, steady, non-axi-symmetric flows may occur in axi-symmetric geometries. These non-axi-symmetric flows coexist with a solution family of axi-symmetric flows. Symmetry-breaking is found to be due to buoyancy effects alone, and does not result from an interaction between forced and free convection. On the other hand, symmetry-breaking can be suppressed through a relatively low inlet flow and/or rotation rate of the wafer, corresponding to Ra/(Re² Pr)≤50.