An analysis of flow, temperature, and chemical composition distortion in gas sampling through an orifice during chemical vapor deposition

Measurement of the chemical composition of gases sampled through a small ho le in the substrate can be a useful diagnostic for investigations of the ch emistry of chemical vapor deposition (CVD) processes. Ideally, one would me asure the composition of the gas at the growth surface. However, the flow d isturbance due to sampling causes the conditions at the mouth of the orific e to be different from those at the growth surface. Unless the orifice diam eter is sufficiently small, relative to the thickness of chemical and therm al boundary layers above the growth surface, the sampled composition will d iffer from the composition at the growth surface. In this work, we present results of two-dimensional simulations of the flow, heat transfer, and chem ical reactions in an axisymmetric stagnation point how with gas sampling th rough a small orifice in the substrate on the symmetry axis of the how fiel d, Detailed results are given for atmospheric-pressure radio-frequency plas ma CVD of diamond, corresponding to experiments performed in our laboratory . We also present more general results, approximate analytical representati ons of the flow field, and scaling rules for the size of the disturbance du e to the sampling orifice. (C) 1999 American Institute of Physics. [S1070-6 631(99)01204-0].