In this work, we investigated the etching characteristics of SiOxNy thin films in CF4+CHF3+O-2 inductively coupled radiofrequency (13.56MHz) plasma. SiOxNy etching rates were measured as functions of the CF4/CHF3 mixing ratio at constant O-2 fraction, gas pressure (10 mTorr), input power (500W) and bias power (100W). The conditions of the etched surfaces were examined by X-ray photoelectron spectroscopy, atomic force microscopy and contact angle measurements. Data on internal plasma parameters and steady-state plasma composition were obtained by Langmuir probe diagnostics and zero-dimensional plasma modeling. It was found that the substitution of CF4 for CHF3 suppresses the SiOxNy etching rate as well as results in increasing both amount of residual fluorocarbon polymer and SiOxNy/Si etching selectivity. The SiOxNy etching mechanism was analyzed by considering the relationships between measured etching rates and model-predicted fluxes of active species (F atoms, CFx radicals and positive ions). It was proposed that the SiOxNy etching process: (1) exhibits features of ion-assisted chemical reactions in the neutral-flux-limited mode, and (2) involves the contributions of by HF molecules. The effective probability of the SiOxNy+F reaction is correlated with the amount of deposited fluorocarbon polymer while the hydrophobic nature of the plasma-treated SiOxNy surface confirms the presence of a continuous fluorocarbon polymer film.