Quantum Chemical Investigation for Chemical Dry Etching of SiO2 by Flowing NF3 into H2 Downflow Plasma

A quantum chemical investigation of the chemical dry etching of SiO2 using H2 downflow plasma with flowing NF3 was carried out using the B3LYP/6-31+G(d,p) method. The results provide a reasonable interpretation of how the chemical dry etching of SiO2 takes place in a down flow area. Experimentally, it was found that the etch rates of thermal silicon oxide film range from 1 to 10 nm/min depending on the etching conditions, and white powder was produced on the etched surface. It was deduced that the etchants were HF and NH3 produced by the reaction of H+ NF3, and that the white powder on the etched surface was produced by the decomposition of (NH4)2SiF6 formed on the etched surface. The calculated results support the HF and NH3 production mechanism and clarify the molecular structures of (NH4)2SiF6 and the white powder. Another important point in the chemical dry etching of SiO2 was the realization of a high etching selectivity to Si. As the F atom was deduced to be the main etchant of Si, its generation mechanism in H2 down flow plasma with the addition of NF3 was also studied and a method of suppressing F atom production was proposed in this research.