Ultrahigh selective etching of Si₃ N₄ films over Si O₂ films for silicon nitride gate spacer etching

The process window for the high etching selectivity of silicon nitride to silicon oxide was investigated in C F₄ /C H₄ 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 C H₄ 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 C H₄ 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 Å 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 C H₄ flow rate above 30 SCCM (SCCM denotes cubic centimeters per minute at STP) for the C F₄ flow rate of 10 SCCM. N F₃ /C H₄ and S F₆ /C H ₄ plasma showed etch behavior similar to C F₄ /C H ₄ plasma.