Etch apparatus

In a process using a hot phosphoric acid etchant to etch silicon nitride on a semiconductor wafer submerged in a tank of the etchant, a recirculating path is established for the etchant. A porous filter is coated with silicon nitride and installed in the recirculating path. As the etchant in the recirculating path flows through the porous filter, the silicon nitride on the porous filter dissolves into the etchant. In the tank, the silicon nitride dissolved in the etchant significantly suppresses the etch of silicon dioxide on the semiconductor wafer, thereby enhancing the etch selectivity of the process. Monitoring and maintaining the concentration of the silicon nitride in the etchant stabilizes the etch selectivity of the process.