A two-step process using CHF3 /CO2 and CHF3 /O2 gas mixtures for the reactive ion etching (RIE) of silicon nitride, which allows stopping on thin oxides, is described and analyzed. Taking into account the etch rate of silicon nitride, oxide, and photoresist, the etch rate nonuniformity, and the nonuniformity of the silicon nitride and oxide layers, the etching times can be chosen to insure three conditions: (i) complete removal of all silicon nitride, (ii) a finite fraction of the oxide remaining, and (iii) small linewidth losses. The process has been implemented in an AME 8110 ‘‘HEX’’ plasma etching machine. By controlling the uniformity of the HEX etch rates, it has been possible to etch 1200 Å of silicon nitride on 175 Å of pad oxide with an average of 108-Å oxide remaining (σ=10 Å). The number of stacking faults generated after B implantation and reoxidation is significantly reduced for samples etched with the two-step process as compared with RIE etches in which the pad oxide is removed completely and the Si therefore exposed to the reactive ions during the overetch.