Controlled SiO2 etching by HF in an HF/H2O liquid-like layer in a vacuum environment

Controlled SiO2 etching was demonstrated using hydrogen fluoride (HF) in an HF/H2O liquid-like layer in a vacuum environment. Under vacuum conditions in the absence of H2O, there was no etching of SiO2 by HF. In the presence of H2O, the SiO2 etch rates were dependent on the HF pressure (1.5–6 Torr), H2O pressure (1.5–30 Torr), and substrate temperature (18.1–30.4 °C). These parameters defined the ultrathin HF/H2O liquid-like layer on the SiO2 substrate. The reduction of the SiO2 thickness versus HF exposures was linear. Using 5 s HF exposure times, SiO2 etch rates from 1 to 93 Å/exposure were obtained at lower HF and H2O pressures at 30.4 °C. Much larger SiO2 etch rates were measured at higher HF and H2O pressures where the SiO2 etch rates varied from 83 to 1255 Å/exposure at 30.4 °C. The large increase in the SiO2 etch rates suggested that a thicker HF/H2O layer may be condensing above threshold HF and H2O pressures. Additional studies confirmed that the high SiO2 etch rates correlated with condensation at higher HF and H2O pressures. The SiO2 etch rates in this condensation regime were also similar to SiO2 etch rates in bulk HF/H2O solutions at comparable temperatures. The SiO2 etch rates may be sensitive to the thickness of the liquid-layer needed to hydrate the F−, HF2−, SiF62−, and H+ ion species involved in the etching. H2O may be clustering around the ions because the ultrathin liquid-like layer is often less than the size of the hydrated ions.