Thermophoretic Deposition of Small Particles in the Modified Chemical Vapor Deposition (MCVD) Process

Abstract

Thermophoresis is conclusively established as the particulate deposition mechanism in the MCVD process by comparing experimental measurements and quantitative theoretical predictions. The deposition efficiency, E , is defined as the fraction of the silica in the gas stream (initially as SiCl 4 ) that is deposited. For normal MCVD operating conditions, the deposition efficiency is only a function of the equilibrium temperature, T e , at which the gas and walls equilibrate downstream of the torch and the temperature, T r , at which reaction occurs. The deposition efficiency is ∼0.8[1‐( T e / T r )]. It is determined that T e is a strong function of the torch traverse velocity, the traverse length, the temperature of the ambient environment, and the tube wall thickness but only a weak function of the gas flow rate. At high gas flow rates, the efficiency is limited by incomplete reaction.