PREDICTING INTRAWAFER FILM THICKNESS UNIFORMITY IN AN ULTRALOW PRESSURE CHEMICAL-VAPOR-DEPOSITION REACTOR

We present a reaction engineering analysis of a multiple wafer-in-tube ultrahigh vacuum chemical vapor deposition reactor which allows an es timate of wafer throughput for a reactor of fixed geometry and a given deposition chemistry with specified film thickness uniformity constra ints. The model employs a description of ballistic transport and react ion based on the pseudosteady approximation to the Boltzmann equation in the limit of pure molecular flow. The model representation takes th e form of an integral equation for the flux of each reactant or interm ediate species to the wafer surfaces. Expressions for the reactive sti cking coefficients (RSC) for each species must be incorporated in the term which represents reemission from a wafer surface. In our model we use a published expression for the RSC of silane as a function of flu x and wafer temperature developed from molecular beam measurements. Nu merical solution of the resulting integral equation using Gauss-Legend re quadrature yields quantitative estimates of intrawafer film thickne ss uniformities for epitaxial silicon deposition from silane for speci fied process conditions and wafer radius:wafer separation. For given r eactor dimensions and specified uniformity, throughputs can then be es timated.