On the wet etching of Pyrex glass

This paper addresses a commonly raised question regarding wet etching of Pyrex glass: “How are the defects on the glass generated during etching process while most of the masking materials are chemically inert in the etching solution?” The response to this question relies in controlling the residual stress in the masking layer (its value, gradient and nature: compressive or tensile) and controlling the hydrophobicity of the mask surface. With this response, the solution for achieving a suitable process can be easily set up: a low stress masking layer (preferably performed by successive depositions) and a hydrophobic surface of the masking layer (easily achieved, for example by hard baking of the photoresist masking layer). Nevertheless, these factors must be correlated with a correct selection of the glass material (low content of oxides that gives insoluble products after the reaction with the etching solution) and a fast etch rate (achieved using highly concentrated hydrofluoric acid). The best reported results in the literature are analyzed for this perspective.