We have investigated the etching of SiO2 and plasma chemically vapor deposited (CVD) SiNxHy (SiN) thin films in NF3/Ar plasmas as a function of pressure, NF3 concentration in Ar, and rf power. We have also independently measured the complex impedance and real power dissipation of the discharges. By simultaneously varying the pressure and NF3 concentration we have determined that the highest etch rates are obtained at low concentrations of NF3 in Ar (18%-25%) for both SiO2 and SiN at relatively high pressure (1550 mTorr). Etch rates of 14800 Angstrom/min for SiN and 1600 Angstrom/min for SiO2 are achieved under these conditions. The SIN etch rate is found to be a strong function of the applied rf power, which also determines the pressure at which the highest etch rates are achieved. Both the SiN and SiO2 etch rates were found to be closely correlated to the argon and fluorine atom optical emission intensities. For NF3 partial pressures above 200 mTorr the NF3/Ar plasmas had very high impedance magnitudes (> 5000 Omega) and low power coupling efficiencies (< 25%). These results indicate that there is an optimal NF3 partial pressure for obtaining high etch rates which supplies sufficient reactant, but is low enough to maintain a low impedance and high power coupling efficiency. To explain these results we propose that the role of argon in NF3 based plasmas under these conditions is to lower the electronegativity of the discharge through dilution and/or reactions in the discharge. The results of this study provide insight into the optimization of processes which use fluorinated gases in plasmas at high pressures such as plasma enhanced CVD chamber cleaning where fast removal of silicon based materials is required.