The reaction of silicon nitride films on silicon substrates in sub- and supercritical water at temperatures between 100 and 400 degreesC has been studied for the first time. The etching rates for the silicon nitride films were higher than those for silicon dioxide films under the same conditions. A novel selective etching method for silicon nitride films using subcritical. water has been proposed, The etching mechanism for silicon nitride films in subcritical water was studied by means of infrared spectroscopy and ion chromatography. The etching selectivity at 200 degreesC and 10 MPa reached 70. An etching rate of 7.5 nm/min for the silicon nitride films was obtained under the same conditions. The selectivity value was higher than that of the conventional method using phosphoric acid, while the etching rate was comparable. Moreover, water is nontoxic; therefore, the proposed method is environmentally friendly. The proposed selective etching method for silicon nitride films using subcritical water is one of the promising candidates for future etching technologies.