Methods and apparatus for etching materials using tetramethylammonium hydroxide (TMAH) are described. The methods may involve including an additive when applying the TMAH to the material to be etched. The additive may be a gas, and in some situations may be clean dry air. The clean dry air may be provided with the TMAH to minimize or prevent the formation of hillocks in the etched structure. Apparatus for performing the methods are also described.