The mechanism of HF/H2O chemical etching of SiO2

Quantum chemistry is used to investigate the HF/H2O chemical etching mechanism of silicon dioxide. Etching proceeds through four sequential steps to remove silicon as SiF4(g) for silicon dioxide, eventually leading to a fluorine-terminated silicon surface which HF attacks, resulting in a hydrogen-passivated silicon surface. Our predicted activation barriers show that the concerted attack by HF and H2O enhances the etch rate over etching by HF alone by reducing the barrier for each etching step. This is consistent with experimental observations that HF etching is enhanced by the presence of water. The QCISD barrier for the rate-limiting step of this catalytic HF/H2O etching mechanism is 22.1 kcal/mol, while the QCISD etching barrier for HF etching is 35.1 kcal/mol. In addition, we find that the F-terminated silica surface is not readily hydrolyzed by exposure to water since the forward barrier of 32.1 kcal/mol for hydrolysis of F-terminated silicon dioxide is larger than the reverse barrier of 26.2 kcal/mol. (C) 2002 American Institute of Physics.