Airborne Contamination on Semiconductor Wafers Traced to Humidification Plant Additives

Airborne contamination was observed on wafers being processed in a Class 100 semiconductor processing facility during a period of extremely cold weather. The contamination appeared as 1 to 2-μm-thick microdroplets, soluble in water and methanol. An analysis suggested that a reaction between additives (morpholine and sodium sulfite) in the humidification plant had formed a volatile product. A mechanism for formation and transport of this compound is proposed. Because of the chemical affinity of the contaminant for gold surfaces, a simple method for monitoring proved useful. Gold-coated, AT-cut quartz crystal monitors, normally used for thickness monitoring during vacuum deposition, were placed in the contaminated airstream and maintained at 10°C. The rate of change of resonant frequency gave an objective, qualitative measure of the concentration of the contaminant in the air. A test for the presence of the compound on surfaces and in aqucous samples was also devised. Warm, humid nitrogen is capable of carrying the contaminant selectively from such samples to a clean, cooled, gold mirror where the presence or absence of microdroplets can be observed. Such a source of contamination, due to standard cleanroom procedures, can have a significant effect on process yield.