A SURFACE IR STUDY OF INORGANIC FILM FORMATION GAAS, SILICON AND GERMANIUM BY AQUEOUS NH4F, AND HF

Hydrofluoric acid buffered with ammonium fluoride (BHF) is a common cleaning-stripping solution used in silicon processing. Little is known of the surface chemistry which occurs as a result of BHF treatment. Surface IR spectroscopy in the multiple internal reflection mode was used to study silicon, germanium, and gallium arsenide surfaces treated with BHF. For each of these materials, the BHF not only dissolved the native oxide, but also deposited a film comprising ammonium salts (NH4F and NH4F.HF). These salts subsequently slowly reacted with the semiconductor substrate to form hexafluorometallate compounds ((NH4)2SiF6, (NH4)2GeF6, and (NH4)3GaF6). Although the original ammonium fluoride and bifluoride salts are quite soluble in alcohols as well as aqueous solutions, the hexafluorometallates are completely insoluble in alcohols, and can only be removed by careful water or aqueous acid rinse. The presence of these previously unsuspected films could seriously degrade electronic performance of the materials.