Investigation of surface reactions of etched SiN films using HF/PF3 gas mixture plasma under cryogenic conditions via in-line XPS: The role of ammonium salts in etch rate enhancement

Enhanced etch rates of SiO2 and SiN films are increasingly required for high aspect ratio hole etching in 3D nand flash memory. Cryogenic etching using HF/PF3 gas mixtures has emerged as a promising approach; however, the etching mechanism for SiN, particularly, the role of PF3, remains unclear. To elucidate the factors controlling SiN etch rates during cryogenic etching, we investigated the relationship between surface reactions and etch rates using a plasma beam irradiation apparatus equipped for low temperature in-line XPS analysis. In this study, each experiment was conducted under lower pressure conditions compared to the capacitive coupled plasma reactive ion etching process. Our results for HF-based etching indicate that the ammonium salt NH4F forms on the SiN surface under cryogenic conditions and acts as a source of etchant, substantially increasing the SiN etch rate. Addition of PF3 promotes the formation of the lower volatility ammonium salt NH4PF6 on SiN films at temperatures above those where NH4F forms. Efficient decomposition of NH4PF6 by irradiation with sufficiently energetic ions can further enhance the SiN etch rate.