Gas-phase chemistry and etching mechanism of SiNx thin films in C4F8 + Ar inductively coupled plasma

In this study, we investigated the etching characteristics of non-stoichiometric SiNx thin films in C4F8 + Ar inductively coupled rf (13.56 MHz) plasma. The SiNx etching rates together with SiNx/Si and SiNx/SiO2 etching selectivities were measured as functions of the gas pressure (4-10 mTorr, or 0.53-1.33 Pa), input power (700-1000 W), bias power (100-400 W), and C4F8/Ar mixing ratio (0%-75% Ar). Data on internal plasma parameters and plasma chemistry as well as steady-state plasma composition were obtained by both Langmuir probe diagnostics and zero-dimensional plasma modeling. The etching mechanism of SiNx was analyzed through the relationship between the measured etching rates and model-predicted fluxes of active species (F atoms, CFx radicals, and positive ions). It was confirmed that within the given range of input process conditions, the SiNx etching process 1) appears in the steady-state etching regime, 2) exhibits the features of ion-assisted chemical reactions in the neutral-flux-limited mode, and 3) is influenced by the thickness of the fluorocarbon polymer film. It was shown that the influence of input process parameters on the effective probability of SiNx + F chemical reaction may be adequately characterized by the fluorocarbon radicals/fluorine atoms and fluorocarbon radicals/ion energy flux ratios.