An electrochemical and ellipsometric study of oxide growth on silicon during anodic etching in fluoride solutions

The electrochemical etching of (111) and (100) p-Si in buffered ammonium fluoride solutions (0.01–1.0 mol dm −3 fluoride, pH 1.0–4.5) has been studied by voltammetry, electrochemical impedance spectroscopy and in situ ellipsometry. Measurements were performed in a flow cell to eliminate mass transport limitations. The presence of an oxide film in the electropolishing region was detected by impedance spectroscopy and ellipsometry, but ellipsometry revealed thicker films (up to 30 nm) than those detected by impedance spectroscopy (up to 4 nm) or in work by other authors using ir spectroscopy. This is consistent with the existence of an inner layer of “dry” oxide and a thicker outer layer of hydrated oxide. The ellipsometric response is influenced by surface roughening at higher potentials: the effect is more pronounced for the (100) surface. The surface morphology of etched samples has been examined by scanning electronmicroscopy and atomic force microscopy, and the differing ellipsometric responses of the (100) and (111) surfaces have been related to morphological changes.