Highly selective Si3N4/SiO2 etching using an NF3/N2/O2/H2 remote plasma. II. Surface reaction mechanism

Developing processes for highly selective etching of silicon nitride (Si3N4) with respect to silicon dioxide (SiO2) is a major priority for semiconductor fabrication processing. In this paper and in Paper I [Volynets et al., J. Vac. Sci. Technol. A 38, 023007 (2020)], mechanisms are discussed for highly selective Si3N4 etching in a remote plasma based on experimental and theoretical investigations. The Si3N4/SiO2 etch selectivity of up to 380 was experimentally produced using a remote plasma sustained in NF3/N-2/O-2/H-2 mixtures. A selectivity strongly depends on the flow rate of H-2, an effect attributed to the formation of HF molecules in vibrationally excited states that accelerate etching reactions. Based on experimental measurements and zero-dimensional plasma simulations, an analytical etching model was developed for etch rates as a function of process parameters. Reaction rates and sticking coefficients were provided by quantum chemistry models and also fitted to the experimental results. Etch rates from the analytical model show good agreement with the experimental results and demonstrate why certain etchants accelerate or inhibit the etch process. In particular, the modeling shows the important role of HF molecules in the first vibrationally excited state [HF(v=1)] in achieving high Si3N4/SiO2 selectivity.