Infrared identification of ammonium hexafluorosilicate on silicon nitride.

During the processing of STIC wafers, the tantalum nitride is etched in a CF4 plasma. The underlying silicon nitride is briefly exposed to the plasma at the end of the tantalum nitride etch. Sometimes a blue film is left on the silicon nitride which after metalization leads to a poorly defined Pt pattern called ratty platinum. The blue film was identified as ammonium hexafluorosilicate from its infrared spectrum. We believe a similarly produced blue film reported in the literature was misidentified as ammonium fluoride. Also, ammonium fluoride that is evaporated from solution onto a Si wafer leaves a stained surface that is ammonium hexafluorosilicate.