The process window for the high etching selectivity of silicon nitride to silicon oxide was investigated in CF4/CH4 inductively coupled plasma. This work showed that the etching selectivity could be controlled by modulating the thickness of the fluorocarbon film with the flow rates of CH4 gas. The carbon content in the fluorocarbon by-product layers on the etched films was observed to play a critical role in determining the etching selectivity of silicon nitride to silicon oxide. The increase in the carbon content in the fluorocarbon films with increasing CH4 gas caused the etch rates of both silicon oxide and silicon nitride films to be reduced, eventually leading to the etch stops. The minimum effective thickness of the fluorocarbon films was estimated to be 20 A degrees for the etch stop. The infinite etching selectivity of the silicon nitride to the silicon oxide on the blanket wafers could be achieved for the CH4 flow rate above 30 SCCM (SCCM denotes cubic centimeters per minute at STP) for the CF4 flow rate of 10 SCCM. NF3/CH4 and SF6/CH4 plasma showed etch behavior similar to CF4/CH4 plasma.