Acid-Free Electrochemical Chromium Etch and Release of Nanoscale Gold Films

We describe a method for releasing coated and uncoated 100-nm gold films from a 5-nm chromium-coated glass substrate. The method is rapid at room temperature within a nontoxic solution of sodium phosphate or sodium hydroxide. Etching processes using hydroxide successfully etch the chromium layer without affecting a ZnO coating layer deposited on the gold film. Etching using sodium phosphate at neutral pH causes some surface damage to the ZnO surface while leaving the bulk of the film intact. Other anions tested include nitrates and chlorides, both demonstrating poor etching selectivity, damaging the gold as well as the chromium layers. A copper counter electrode is used to collect the chromium from the solution, minimizing chromium persistence in the etching solution and any contamination of the ZnO surface. This single step acid-free process can be applied to a variety of materials and structures formed on gold, where insulating materials and many oxides do not require any special protection from the etching solution. An uncoated or polyimide coated 2 cm × 1.5 cm film can be released in under 1 h, while ZnO-coated films require up to 7 h to to release films 1 cm × 1 cm in size.