A review of the chemical reaction mechanism and kinetics for hydrofluoric acid etching of silicon dioxide for surface micromachining applications

An etching mechanism for thin silicon dioxide films in hydrofluoric acid solutions has been deduced from experimental results and a review of literature sources. The etching reaction consists of two elementary chemical reactions at the surface of the silicon dioxide thin film. First, acidic components from the etchant break up the siloxane bonds at the SiO2 surface leaving silanol species. This process creates more space at the SiO2 surface for the fluorinated species to react with the silicon nucleophilically. Experiments at low HF concentrations (below 4 M HF) show that the etching reaction is nearly first order. Over a wider range of HF concentration, however, the reaction order approaches 2. Moreover, the addition of strong acids to HF solutions enhances the etching reaction by increasing the rate of the "surface opening" reaction. Literature data support this view of SiO2 etching. Application of these ideas will enhance etching for large area surface micromachining.