Study on etch front of piezoelectric ZnO film and new step coverage technique

This paper describes our recent observation on the etch front of a sputter-deposited ZnO film and a novel processing technique for covering a tall step with a thin film. When a polycrystalline ZnO film (sputter-deposited over aluminum in a silicon wafer with c-axis perpendicular to the wafer plane) is etched in various wet etchants, we observe that the etch front is sloped in the opposite direction to that commonly obtained in an isotropic etching of a film. This unexpected etch front presents a major difficulty in covering a step (of a patterned ZnO film) with an electrode. To overcome this step-coverage problem, we have developed a novel technique where we produce an overhanging electrode cantilever (over the patterned ZnO), and intentionally cause it to stick to the substrate (taking advantage of the "stiction" problem encountered in the surface micromachining).