A Novel Polyimide Film Preparation and Its Preferential‐Like Chemical Etching Techniques for GaAs Devices

A novel chemical etching technique using etchants composed of hydrazine hydrate and ethylene diamine to generate V‐shape grooves in thick polyimide films through photoresist masks has been developed. The etching characteristics, in terms of etch rate, side etched width and taper angle, have been found to depend strongly on ( i ) preparation condition of the films, especially baking temperature, and ( ii ) the composition and temperature of the etchants. In order to achieve preferential etching behavior, the polyimide films have been fabricated by repeating 3 times a coating process, which consists of a spin coating followed by two sequential bakings. An optimized baking condition has been found to be 15 min at 150°C in air for each prebaking and 2 min and 30 min at 185°C in for the first two and the last postbaking, respectively. An optimized condition of the etchants has also developed. Under the optimized condition, a side etched width less than 20% of the film thickness and a constant taper angle of about 35° has been achieved. The size of the V‐shape grooves has been controlled by both the opening width in the photoresist masks and the side etch rate. The taper angle of the grooves has been determined by the film preparation condition itself, and is independent of the etching time even when the films are over‐etched.