Fast and Highly Selective GaN-over-AlGaN Etching with Low Surface Damage

Selective etching of GaN over AlGaN is a critical process step in GaN-based high electron mobility transistor (HEMT) fabrication, where precise removal of GaN is required without damaging the thin AlGaN barrier. In this study, we developed a high-selectivity, low-damage etching process using a customized BCl₃/SF₆ inductively coupled plasma reactive ion etching (ICP-RIE) system. Specifically, a 27.12 MHz helical resonance ICP source was utilized to generate high-density radicals and maximize gas dissociation efficiency, while the substrate temperature was precisely controlled using an external oil-circulating bath. The effects of gas composition and substrate temperature on etch rates, selectivity, and surface properties were systematically investigated. By optimizing the gas ratio to 30% SF₆, a record-high GaN-over-AlGaN selectivity of 78:1 was obtained while maintaining a fast GaN etch rate. Temperature-dependent studies revealed that Al-F passivation was stable at low temperatures, whereas it degraded above 40 ℃, resulting in a significant decrease in selectivity. Although low temperatures enhanced passivation, they also promoted AlCl₃ residue oxidation, resulting in unintended surface roughening. At an intermediate temperature of ∼30℃, the etched surface exhibited the best overall characteristics, including smooth morphology, stable chemical states, and minimized leakage current. These results demonstrate that selective, high-throughput, and low-damage GaN etching can be realized by combining optimized BCl₃/SF₆ chemistry with precise substrate temperature control and high-frequency ICP operation.