Fine ZnO patterning with controlled sidewall-etch front slope

This paper describes a wet-etching technique that solves the major difficulty of fine patterning a c-axis oriented polycrystalline ZnO film. The technique uses aqueous NH₄Cl with electrolytically added copper ions and convection flow, and for the first time, allows the ZnO film to be etched 1) with controlled etch rate ratio between the vertical and horizontal etch rates and 2) with controlled etch-front slope. The ratio between the vertical and horizontal etch rates is as high as 20 to 1, while the angle between the sidewall etch-front surface and the substrate surface can be electrically controlled between 73° and 106°. Also, ZnO films can now be patterned to fine features (even sub-μm level) with a wet etchant. The electroless galvanic etching technique described in this paper produces uniform etching over a large area (larger than 3" in diameter).