An infrared study of thin-film formation on Si and Ge surfaces treated with aqueous NH4F and HF

The surface chemistry of Si and Ge after treatment with hydrofluoric acid buffered with ammonium fluoride (BHF) was studied using surface infrared spectroscopy. For each of these materials, the BHF not only dissolved the native oxide, but also deposited a thin inorganic film comprised of ammonium salts (NH4F and NH4F.HF). Through one or more complex reactions with the substrate, these salts slowly disappear as the thermodynamically very stable hexafluorometallate compounds [(NH4)2SiF6 and (NH4)2GeF6] form. The NH4F.HF disappearance correlates directly with the hexafluorometallate formation. Though the original fluoride and bifluoride salts are quite soluble in alcohols as well as in aqueous solutions, the hexafluorometallates are completely insoluble in alcohols, and can only be removed by thorough water rinse.