Abstract: A method for purifying an etching solution consisting of an aqueous phosphoric acid solution which has been used in etching of a silicon nitride film. In the process, hydrogen fluoride is added to an etching solution consisting of an aqueous phosphoric acid solution which has been used for etching of a silicon nitride film, and the resulting solution is heated to remove fluorides of silicon as reaction products of hydrogen fluoride with silicon compounds which have been contained in the etching solution together with vaporized water.

Abstract: A piezo-resonator comprises a supporting frame having a recess for attaching a piezoelectric resonator element. This recess consists of three portions, that is, a central portion, a pair of connecting portions which are outside of the central portion and a pair of end portions which are outside of the pair of connecting portions. The width W.sub.2 of the connecting portion, the width W.sub.1 of the central portion and the width W.sub.3 of the end portion have the relationship W.sub.2 >W.sub.1 >W.sub.3. Two terminals intersect respective connecting portions to be embedded in the supporting frame.