Si3N4 acts as both passivation isolation layer and infrared absorber layer in infrared thermopiles, and its thin film etching is crucial in the MEMS process. The RIE-10NR reactive ion etcher was used to etch the silicon nitride film by varying the oxygen flow rate, RF power, and chamber pressure using SF6 as the main etching gas. The etching rate and etching uniformity were characterized by step meter and confocal microscope. The experiments showed that the etching rate of silicon nitride reached 509 nm/min with a non-uniformity of 2.4 % under the conditions of 50 sccm SF6 flow rate, 10 sccm O2 flow rate, 11 Pa chamber pressure and 250 W RF power, and the etching rate of polysilicon reached 94.5 nm/min with a selection ratio of 5.39:1 under the same conditions.