Selective etching bath methods

An etching method, comprising:

• etching a first plurality of silicon wafers in a first etchant, each silicon wafer of said first plurality of silicon wafers having a first silica layer of SiO2 and a first silicon nitride layer of Si3N4 deposited thereon, wherein said etching comprises dissolving a quantity of said SiO2 from the first silica layer at a first etch rate of SiO2 and a quantity of said Si3N4 from the first silicon nitride layer at a first etch rate of Si3N4 in said first etchant and a quantity of insoluble SiO2 precipitates in said first echant in response to said dissolving;
• after said etching:
• determining the first etch rate of SiO2, determining the first etch rate of Si3N4, determining a ratio of the determined first etch rate of Si3N4 to the determined first etch rate of SiO2 from said first plurality of silicon wafers in said first etchant, and ascertaining that said determined ratio is less than a predetermined threshold;
• in response to said ascertaining, combining a portion of said first etchant with a second etchant to form a conditioned etchant, wherein said portion is essentially free of insoluble SiO2;
• and
• after said combining, etching in said conditioned etchant a second plurality of silicon wafers having a second silica layer comprising SiO2 and a second silicon nitride layer comprising Si3N4 disposed thereon, wherein a ratio of a second etch rate of Si3N4 to a second etch rate of SiO2 from said second plurality of silicon wafers in said conditioned etchant is greater than said predetermined threshold.