Once obsessed with metallic impurity levels, reducing variations in the delivered gas purity has become the primary focus of contamination control engineers managing gas distributions systems. When used correctly at bulk sources or at the point-of-use, gas purifiers can remove harmful impurities down to the ppt range.

Metal contaminant levels are now only one of several challenges when it comes to gas purity for semiconductor processing. Even when a gas supplier delivers the highest grade of ultra-high purity (UHP) process gas available to meet process specification, impurities from process lines and container valving within a fab can still contaminate the system before it enters the tool or point-of-use (POU).

While filters are a necessary part of any gas delivery system, filtration alone cannot remove all impurities in a gas stream. Many filters are particle size specific and can only remove individual groups of particles. Filters also cannot remove moisture, and some moisture can even be adsorbed on PTFE filters. In fact, some filters can be a substantial source of moisture contamination if not properly introduced to a dry gas line system.

To address these issues, gas purifiers are incorporated into the gas flow lines — removing, in some cases, 99.9999999% of specific impurities.

WHERE GAS PURIFIERS ARE USED AND WHY
Local purifiers can maintain and ensure ultimate purity of gases at the point-of-use, even though as-delivered bulk gas purity levels appear to be sufficient for multiple generations of devices. The main focus will be reducing variations in purity rather than continuing to improve gas specifications. Figure 1 shows where purifiers could be located in a gas delivery system.