UV photochemical processes have been developed for rapidly stripping films of LPCYD Si3N4 in a dry reaction environment, free of plasma or plasma effluents. These processes are carried out in a vacuum reactor which allows simultaneous exposure of a substrate wafer to a polyatomic halogen gas and UV radiation. Si3N4 stripping rates approaching 1000 Angstrom/min have been demonstrated for fluorine-based processes, while maintaining the bulk wafer temperature below 250 degrees C; It has been shown that the mechanism for photochemical Si3N4 etching requires both photolytic production of gas-phase F atoms and direct photon exposure of the etching surface. Selectivities between Si3N4 SiO2, and silicon films are controlled through UV lamp exposure, substrate temperature, and with additions of N-2 diluent and various halogen-containing gases. Selectivities for Si3N4-to-SiO2 etching of greater than 30 can be achieved for the stripping of Si3N4 LOGOS mask layers in the presence of field oxide and pad oxide layers.