Analysis of moisture purge in high purity gas distribution systems

Moisture can easily adsorb on the inner surface of high purity gas distribution systems for semiconductor manufacturing processes, when the inner surface is exposed to the ambient air during regular or troubleshooting services. Before restarting the equipment, the adsorbed moisture has to be removed, typically by purging the gas distribution system with a high purity gas. An important system design issue therefore is to minimize the required moisture drydown time, so as to increase the productivity of the equipment. Here, the moisture purge/drydown process is analyzed using a simple phenomenological model, in which the moisture desorption kinetics is extracted from experimental data for moisture drydown in a single straight pipe. Based on that model, we also propose to minimize the overall moisture drydown time of a gas distribution system by properly allocating the purging gas flowrates in all branches of the system. It is demonstrated by a case study that, without altering the piping network design and total flowrate, our flowrate allocation scheme substantially reduces the overall moisture drydown time of the gas distribution system.