Ultrahigh selective etching of Si_3N_4 films over SiO_2 films for silicon nitride gate spacer etching

摘要

The process window for the high etching selectivity of silicon nitride to silicon oxide was investigated in CF_4/CH_4 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 CH_4 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 CH_4 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 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 CH_4 flow rate above 30 SCCM (SCCM denotes cubic centimeters per minute at STP) for the CF_4 flow rate of 10 SCCM. NF_3/CH_4 and SF_6/CH_4 plasma showed etch behavior similar to CF_4/CH_4 plasma.