Highly selective Si3N4/SiO2 etching using an NF3/N2/O2/H2 remote plasma. I. Plasma source and critical fluxes

Highly selective plasma etching of silicon nitride (Si3N4), while not etching silicon dioxide (SiO2), is a critical step in the fabrication of microelectronics devices. In many applications, this etching must be damage-free and isotropic, which then motivates the use of remote plasmas where the reactants interacting with the substrate are dominantly neutral species. In this paper and Paper II, mechanisms for highly selective Si3N4 etching in remote plasmas are discussed based on results from experiments and simulations. It has been shown experimentally that high Si3N4/SiO2 etch selectivity (≈380) can be achieved in the downstream effluent of an NF3/N2/O2/H2 plasma. The authors found that H2 plays a principal role in the reaction mechanism as Si3N4/SiO2 selectivity shows a sharp maximum as a function of the H2 flow rate. Based on this observation, and measured densities of F-atoms and H2 in the process chamber, a mechanism of selective Si3N4/SiO2 etching is proposed in which HF molecules in vibrationally excited states accelerate etching reactions. A reaction mechanism for NF3/N2/O2/H2 plasmas and its afterglow was developed to computationally determine the species densities and fluxes on the wafer level, validated by comparing with experimentally measured F-atom and H2 densities. The calculated species densities and fluxes were used as input to an analytical model of Si3N4 and SiO2 etching based on the results of quantum chemistry simulations. This paper presents experimental results (etching data and species densities), the reaction mechanism for NF3/N2/O2/H2 plasmas, and the results of simulations of gas phase chemistry. Quantum chemistry simulations of elementary etching reactions, description of the analytical model of Si3N4 and SiO2 etching, calculations of the etch rates, and Si3N4/SiO2 selectivity with this model are presented in Paper II.