Humidity effects on hygroscopic particles deposited on HEPA filters and silicon wafer surfaces

Semiconductor wafer manufacturing facilities (fab) must maintain extremely clean air environments to minimize the number of wafers scrapped due to contamination which would result in reduced yields. The fab air is cleaned bypassing it through either HEPA or ULPA filters. A number of airborne fab contaminants may be hygroscopic causing them to exist as a solid or a liquid when in equilibrium with their environment's relative humidity. The effect of relative humidity on such contaminants is poorly documented whether they were to be captured in a filter or deposited on a wafer. The work presented here experimentally characterizes NaCl evolution within HEPA filters when exposed to humidity fluctuations and the effect of humidity on NH4Cl corrosiveness when deposited on cobalt coated wafers with a TiN layer. Successive deliquescence and efflorescence fluctuations were imposed on particles captured on a glass fiber HEPA filter. Scanning Electron Microscopy (SEM) and Environmental SEM (ESEM) studies of the filters showed that the NaCl, under humidity excursions, did not penetrate deep into the filter but deliquesced and effloresced near the top surface of the filter. Pressure drop measurements for filters containing NaCl particles showed differences in pressure drop associated with relative humidity changes. These pressure drop changes suggested some redistribution particle properties. When exposed to a relative humidity of 20%, the NH4Cl particles did not corrode the cobalt wafer beyond the location of the initial deposit. At 61% relative humidity, the surrounding areas of the particles were corroded with a solid artifact left at the original location. At 76% relative humidity the NH4Cl particles were observed to have deliquesced, which is below the expected deliquescence relative humidity. The corrosion of the cobalt wafer was most extensive when the NH4Cl particles had deliquesced.