UNINTENTIONAL DOPING OF WAFERS DUE TO ORGANOPHOSPHATES IN THE CLEAN ROOM AMBIENT

We report here the unintentional doping of silicon device wafers durin g furnace operations caused by a volatile organophosphate contaminant in the clean room ambient, The unintentional doping is characterized e lectrically using spreading resistance and sheet resistance measuremen ts. The onset of the unintentional doping was found to coincide with t he installation of new high efficiency particulate air (HEPA) filters. Using static secondary ion mass spectroscopy and gas chromatography t echniques on monitor wafers exposed to the clean room ambient the cont aminant was identified as phosphorus in the form of Fyrol PCF, an orga nophosphate flame retardant. Fyrol was traced to the polyurethane pott ing material used in the HEPA filters to secure and seal the filter me dia in place. The unintentional doping was caused by exposure of the f urnace dummy wafers to the clean room ambient air during the period be tween oxidations. These wafers then acted as phosphorus dopant sources during device wafer oxidation.