Cascaded Ni hard mask to create chlorine-based ICP dry etched deep mesas for high-power devices

Abstract

A highly selective cascaded Ni hard mask without stress and film damage issues suitable for the formation of deep vertical mesas for high-power devices is presented. A Cl2/BCl3/Ar inductively coupled plasma dry etch was used to investigate the effect of photoresist, and patterned Ni hard masks applied via four different methods on the etch rate of GaN, and mask selectivity. Strain in the e-beam only deposited Ni films produced low yield due to poor adhesion. Electroplated Ni showed rough mask morphology. Sputtered Ni left damage/chemically reacted layers on the GaN under the mask. A cascaded 50 nm e-beam evaporated Ni followed by 2 µm of sputtered Ni mask method improved the yield to 100%, achieved high selectivity and retained an undamaged GaN surface morphology. The cascaded Ni hard mask successfully achieved GaN mesas up to 13 µm. The dry etching rates of the photoresist, GaN and Ni masks were 250, 275 and 13 nm min−1, respectively giving an etch selectivity of the GaN/Ni hard mask of ∼20, much higher than the etch selectivity of 1.1 for the photoresist. Furthermore, yield of the cascaded Ni hard mask was 100% making it suitable for commercial high-power electronics.