Accurate Etch Depth Control to Optimize Critical Process Step for p‐GaN High Electron Mobility Transistors Fabrication

This work reports the results of an advanced end point (EP) technique that performs in‐situ thickness measurements of thin (<80 nm) p‐GaN layers during plasma etch processing. Utilizing the calculated etch rates allows automated switchover to a higher selectivity process in the final part of the etching, when less than 20 nm of the p‐GaN layer remains. Controlling the process in this way builds on previously reported in‐situ monitoring for recessed gate AlGaN devices and allows more accurate determination of over etching to minimize AlGaN loss and reduce exposure to additional ion‐bombardment compared with timed etching. Process optimization tests performed using the EP technique to measure in‐situ etch rates and tune the selectivity to AlGaN using a Cl 2 ‐O 2 ‐based plasma chemistry highlights the need for careful balancing of process conditions in agreement with previous reports. Finally, the results of adding an additional processing step utilizing the smoothing benefits of atomic layer etching (ALE) to give an improved final surface quality are reported. Surface roughness (Ra) measured by atomic force microscopy is found to improve from 0.8 nm with just the high selectivity inductively coupled plasma (ICP) compared to 0.4 nm after the high selectivity ICP followed by ALE.