Measurement and control of impurity distribution in ultra pure gas delivery systems.

Abstract

As the integrated circuit technology evolves to a greater complexity, the purity of process gases becomes a critical issue requiring extreme diligence in gas handling. There are three major challenges in gas contamination control: Gas purification, gas transport, and metrology of contaminants. In the area of gas purification, a novel integrated purification/filtration technique is developed--the reactive filter. The fundamental concept of a reactive filter involves impregnating the surface of an efficient particle filter with chemically reactive sites. The technique is capable of purifying inert as well as reactive gases to sub parts-per-billion levels of atmospheric contaminants at ambient temperature. Development and evaluation results for the reactive filter are presented. In the area of gas transport, this work has experimentally and theoretically studied the fundamental mechanisms of various sources of impurity intrusion and the fate of impurities in an ultra-pure gas transport system. The contamination sources investigated include impurity adsorption and desorption from transport lines and gas filters, impurity back-diffusion, and impurity permeation through polymeric materials. Fundamental kinetic parameters for impurity adsorption/desorption in stainless steel tubes and ceramic and metallic filters are reported. The desorption kinetics expression developed assumes two energy levels for the desorbing species. A systematic filter outgassing data analysis and normalization technique is presented along with a computer code to simulate a simple gas delivery system. The back-diffusion study developed a mathematical model to predict the extent of back-diffusion for various contaminants in high purity gas lines. Back-diffusion is very sensitive to system pressure. Impurity permeation study was conducted on Kel-F and PFA polymeric tubes. The effect of both temperature and pressure on the permeation coefficients was investigated. In the area of metrology, state-of-the-art analytical tools and techniques like Atmospheric Pressure Ionization Mass Spectroscopy (APIMS) were employed to detect and measure trace impurity levels. APIMS has the advantage of short response times and low parts-per-trillion level detection limit for most impurities of interest.