<i>In situ</i> analysis of SiO2 films etched under cryogenic conditions using H2/F2/Ar gas mixture plasma

Abstract

High aspect ratio hole etching processes require high-speed etching of SiO2 and Si3N4 films. Cryogenic etching significantly increases the etch rates of these two films by lowering the substrate temperature. However, the etching behavior and mechanisms in the temperature range below −70 °C remain unclear. In this work, we investigate the etching behavior of blanket SiO2 films, from 25 to −200 °C, and examine the mechanisms through in situ analyses. Our results show that the etch rate at −100 °C is approximately 3.2 times higher than that at 25 °C, and is associated with the highest etching efficiency in our experiments. This enhancement in the etch rate is attributed to the co-adsorption of H2O and HF, which increases the number of etchants on the SiO2 surface. At temperatures lower than −100 °C, the solidification of H2O reduces the co-adsorption of HF, decreasing the etch rate. At temperatures below −150 °C, the etch rate declines further, owing to the reduced volatility of the reaction product SiF4. These findings provide valuable insights for optimizing etching processes under cryogenic conditions.