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

Isotropic gas-phase etching of SiO2 was examined using HF and methanol vapor while changing the pressure from 300 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 pressure. The etching rate of plasma-enhanced CVD (PE-CVD) SiO2 was more than 60 nm min−1 at 900 Pa and –30 °C. When the pressure was increased from 300 to 900 Pa, the temperature range that indicates 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 increase slightly with pressure. At the higher pressure of 900 Pa, the formation of ammonium hexafluorosilicate, which is a by-product of SiN, was found to increase. As a result, a high selectivity of more than 20 was obtained at a lower pressure of less than 600 Pa and a lower temperature of less than –40 °C.