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Moisture Behavior in Ultra-High Purity Gas Distribution Systems
By Curt Fauth
A discussion of the expectations and guidelines for making ultra-trace moisture measurements of UHP gases from the purifier to the tool.
Modern semiconductor fabs employ sophisticated gas distribution systems (GDS) to deliver a variety of ultra high purity (UHP) gases from bulk purifiers to the process tools. These GDSs are controlled and monitored by Continuous Quality Control systems (CQC) that allow fabs to maintain extremely low levels of contaminants in their process gases, leading to high process yields. The task of controlling contaminants is becoming more demanding as device line sizes decrease at an unprecedented rate and the impurity control points of UHP gases are reduced to their limits.
The principal contaminants monitored by CQC systems are oxygen and moisture. Of these, moisture is the most difficult to control due to its adsorption and transport characteristics, particularly at sub-ppb levels. These characteristics not only control the initial dry down of a GDS but also make leak detection and trouble shooting of process excursions difficult.
The purpose of this article is to familiarize users of CQC systems with the difficulty of making ultra-trace level moisture measurements, to set a practical expectation for moisture analyzer performance and to establish guidelines for the proper use of these analyzers in controlling and monitoring moisture in UHP gases.
The following sections of this article will cover leak mechanisms in gas distribution systems, moisture transport phenomena and behavior in GDS, the implications of these on making moisture measurements and guidelines for the proper use of ultra trace moisture analyzers in CQC systems.
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