Formation of hexafluorosilicate on Si surface treated in NH₄F investigated by photoemission and surface infrared spectroscopy

The chemical nature of Si surfaces treated with ammonium fluoride (NH ₄F) has been investigated using photoemission and surface infrared spectroscopy. On the surface after treatment in NH₄F solution, there remain ammonium compounds such as NH₄F and NH₄F.HF. Photoemission data demonstrate that under the atmospheric environment, the ammonium compounds remaining on the NH₄F-treated Si surface react with the Si substrate to generate the hexafluorosilicate salt, (NH ₄)₂SiF₆. We propose that the formation of (NH₄)₂SiF₆ or SiF₆^2- ions is the dominant reaction pathway in the NH₄F etching of Si crystals.