Surface Chemistry of Silicon and Germanium Treated with Hydrofluoric Acid Solutions: Slow Reactions of Process Residuals

ABSTRACTSolutions of hydrofluoric acid are used extensively in the processing of semiconductors, primarily in etching and in oxide stripping procedures. Such solutions are often buffered with ammonium fluoride to insure constant oxide stripping rates. Infrared spectroscopy of silicon and germanium surfaces treated with such solutions indicates that a thin film of ammonium salts will deposit on the materials unless they are immediately and very thoroughly rinsed following treatment. This thin film, comprised initially of ammonium bifluoride (NH4FHF), reacts with the semiconductor material to form ammonium fluoride (NH4F) as a reactive intermediate, and ammonium hexafluorometallate ((NH4)2 SiF6 or (NH4)2 GeF6). These reactions occur very slowly, over hours or days. They are apparently driven by favorable thermodynamics. This explanation is supported by the absence of comparable reactions using analogous chlorine-based solutions (HCI/NH4 CI). Aspects of the reaction mechanism for hexafluorometallate production is presented.