Etching selectivity of SiO₂ to SiN using HF and methanol at higher pressure up to 900 Pa

The isotropic gas-phase etching of SiO₂ was examined using HF and methanol vapor while changing the pressure from 300 Pa to 900 Pa. The temperature dependence of the etching rate of SiO₂ showed a broad maximum around –30 °C, and the rate increased with increasing the pressure. The etching rate of plasma-enhanced chemical vapor deposition (PE-CVD) SiO₂ became more than 60 nm/min at 900 Pa at –30 °C. When the pressure was increased from 300 Pa to 900 Pa, the temperature range that indicates the SiO₂ etching was shifted to a higher temperature. The etching of SiO₂, which did not proceed at 300 Pa, was found to proceed even at 0 °C at 900 Pa. The etching rate of PE-CVD SiN was also found to slightly increase with the pressure. At the higher pressure of 900 Pa, the formation of ammonium hexafluorosilicate, which is byproduct of SiN, was found to increase. As a result, the high selectivity of more than twenty was obtained at the lower pressure of less than 600 Pa and the lower temperature of less than –40 °C.