Transport and distribution of gaseous impurities in UHP gas delivery systems and process equipment.

Abstract

Removal of gaseous contaminants from process environment is essential for improving the yield and lowering the cost during the integrated device processing. Due to major advances in purification and delivery of high purity gases, the bulk and the process gases are no longer major sources of impurities. The burden is now shifted to the process tools which in most cases are the primary sources of impurities. This study focuses on the characterization of various sources of impurities in a manufacturing unit. An experimental methodology using different metrology tools is developed to extract the fundamental parameters of impurity transport modes. A simulation is also designed to predict the extent of contamination in a process tool, with an emphasis on ascertaining impurity distribution in a vertical diffusion furnace. The major sources of impurity in a manufacturing unit are permeation through polymeric seals, adsorption/desorption from the various surfaces and back-diffusion against the convection. The extent of leakage by permeation through polymeric materials depends primarily on permeation coefficient (product of solubility and diffusivity) of the impurity in the material and the concentration gradient across the surface. Rate of adsorption/desorption has strong dependence on type of impurity and the outgassing surface. In general, the adsorption process is not very activated. However, desorption is highly activated process and activation energy varies with the surface concentration. Back-diffusion involves both bulk and surface diffusion and is higher for lower pressures, lower flow rates, and for small nonadsorbing molecules. Back-diffusion is particularly significant in the gas boundary layer where the convective flow is small. The overall model for a typical purge process in the vertical reactor consists of modules representing individual impurity introduction and transport steps. Results show a stagnant gaseous volume in the wafer spacing, causing the pronounced concentration gradient of impurity over the wafer surface. The purge schedule of the wafers and furnace is significantly affected by gasket leakage, permeation through polymers and quartz lining as well as the feed gas impurities. A parametric study has also been done to determine the effects of wafer size, reactor size and purge flow rates on impurity distribution.