A variation in the reactive ion etch (RIE) rate of silicon oxynitride (SiOxNy) films deposited by plasma-enhanced chemical vapor deposition was studied by CHF3 RIE, CHF3+carbon mono-oxide (CO) RIE and CF4 RIE. The source gas flow rate ratio (R=N2O/SiH4) during the SiOxNy film deposition was varied to obtain a film of different atomic composition. Etch rates decreased in the order of CF4 RIE, CHF3 RIE, and CHF3+CO RIE, and the etch selectivity of SiO2 over SiOxNy increased in the same order also. The fluorocarbon (CFx) film deposited during a RIE process was analyzed by x-ray photoelectron spectroscopy measurements. Etch rates are found to be correlated to both thickness and atomic composition of the CFx film. Carbon-rich fluorocarbon films are found to be more resistive against RIE.