Piezoelectric ZnO chemical etching for zero-power MEMS applications

This paper presents wet chemical etching processes of piezoelectric ZnO (wurtzite atomic structure) films. ZnO films can be etched by most of acid and basic solutions. However, in MEMS processes, many etchants also attack sublayers (electrodes or insulating layers) and are therefore not compatible with MEMS processes. HCl, HF and H3PO4 were selected among the possibilities and carried out etching on Ti/Pt/ZnO sputtered silicon wafers at room temperature. Vertical etching rates are found to be in the 0.2 to 5 mu m/min and lateral etching rates are in between 0.5 to 17 mu m/min. The ratio between lateral to vertical etching speed was found to reach a minimum value for low etchant concentrations of approximately 2%. The morphology of the semi-etched borders of the samples were observed by SEM and practical conclusions are extracted on the adequation of the wet etching process on the top electrode subsequent fabrication step.