Selective anisotropic etching of GaN over AlGaN for very thin films

Selective etching of gallium nitride (GaN) over aluminum gallium nitride (AlxGa1-xN) with inductively coupled plasma and reactive ion etching (RIE) was examined using only chlorine and oxygen gasses. Etch selectivity was heavily influenced by the amount of oxygen present during etching and was slightly influenced by RIE power. Surface roughness was also influenced heavily by the oxygen flow and RIE power which is important for local and across-wafer uniformity. Etch rates were intentionally minimized for use for highly controlled etching of very thin GaN and Al0.25Ga0.75N epitaxial layers. Maximum tested etch rates for GaN and Al0.25Ga0.75N were 200 and 15 Å/min, respectively, and maximum selectivity between GaN and Al0.25Ga0.75N achieved was at least 68.5 to 1. Above a certain oxygen flow, the etch rate of both GaN and Al0.25Ga0.75N drop so drastically that it was impractical to obtain the etch rate and selectivity in a timely manner. Optimum selectivity was obtained with a low oxygen flow to inhibit Al0.25Ga0.75N etching while steadily etching GaN. Although Al0.25Ga0.75N acts as an etch stop with excellent selectivity, significant over-etching can still cause damage to the underlying layers through ion bombardment. This damage can be predicted through an extrapolation of collected experimental data points for a target a specific epitaxial sheet resistance. This allows sufficient over etch to maximize process margin while minimizing epitaxial damage.