We studied the etching mechanisms of thermal oxide and borophosphosilicate glass (BPSG) under various HF/H2O etching conditions. It was found that a high etch selectivity of BPSG over thermal oxide can be obtained by suppressing the adsorption of H2O on thermal oxide. Adsorption of H2O at the thermal oxide surface can be limited by increasing the substrate temperature or by reducing the H2O vapor partial pressure. On the other hand, adsorption of H2O at the BPSG surface is not significantly influenced by the substrate temperature and the H2O partial pressure, because an H3PO4(H2O) liquid layer is formed on the BPSG surface, even at a substrate temperature of 65 degrees C. The etch rate of oxides in HF vapor strongly depends on the amount of adsorbed H2O at the oxide surface. The BPSG etch selectivity over thermal oxide can therefore significantly be improved by reducing H2O adsorption on the thermal oxide surface via an increased substrate temperature or a reduced H2O vapor partial pressure. In addition, the influence of the substrate temperature and the H2O vapor partial pressure on the particle generation during the etch process was studied. It was found that particles are generated in the case of etching at elevated substrate temperatures in combination with high H2O pressure under the influence of reaction by-products.