Fabrication of Ultralow‐Bevel Angle Mesa Structures for Vertical GaN Devices

Abstract

Low‐angle bevel mesa structures are the key technological ideas needed to produce high‐quality vertical GaN devices. Nevertheless, the literature lacks sufficient information regarding the techniques that can yield the best results for obtaining the bevel angles smaller than approximately 5°.[1] Notably, for high p–n junctions NA/ND ratios, such low angles are necessary for the application of negative beveled edge termination and the efficient lowering of electric field at the surface.[1–6] In this work, the optimization of the beveled‐mesa fabrication technique for vertical GaN devices is demonstrated employing a plasma etching of the photoresist mask fabricated using a reflow process. By adjusting the resist reflow process parameters, in particular various BCl3 Cl2−1 ratios and inductively coupled plasma–reactive ion etching plasma power, a range of beveled mask angles as low as 10°–12° is achieved. However, those processes all show etching selectivity close to 1:1 between GaN and the photoresist. Even lower bevel mesa angles are reported for a novel process optimization that resulted in improving GaN to photoresist selectivity by a factor of 5–10. As a result, even smaller bevel mesa angles close to 1°–2° are able to be attained. The developed approach has the advantage of being relatively simple and cost effective.