An etching method. The method includes etching a first plurality of silicon wafers in a first enchant, each silicon wafer having SiO2 and Si3N4 deposited thereon, where the etching includes dissolving a quantity of the SiO2 and a quantity of the Si3N4 in the first etchant. A quantity of insoluble SiO2 precipitates. A ratio of a first etch rate of Si3N4 to a first etch rate of SiO2 is determined to be less than a predetermined threshold. A portion of the first etchant is combined with a second etchant to form a conditioned etchant. A second plurality of silicon wafers is etched in the conditioned etchant. A ratio of a second etch rate of Si3N4 to a second etch rate of SiO2 in the conditioned etchant is greater than the threshold. A method for exchanging an etching bath solution and a method for forming a selective etchant are also disclosed.