Controlled Chemical Etching of ZnO Film for Step Coverage in MEMS Acoustic Sensor

In this letter, we report a novel wet etching technique of a c -axis-oriented ZnO film that solves the step coverage problem during formation of electrodes on this film. The negative profile or hanging structure of ZnO film deposited by RF magnetron sputtering was obtained during wet etching in HCl and NH₄Cl solutions. The developed technique uses aqueous NH₄Cl with electrolytically added copper ions. By suspending the wafer in the horizontal direction in a 20% NH₄Cl solution, positive slope (more than 90 ^°) was obtained at the edge of the ZnO film. In this process, p-type 〈100〉 silicon wafers of 10-20-Ω·cm resistivity have been used. Al deposition was done to confirm the step coverage on ZnO film after getting the positive slope. The thickness of ZnO film was varied from 1.3 to 3.4 μm to observe the coverage of sidewall of ZnO film. The structure was also electrically tested and was found to function satisfactorily.