The influence of cathode coverplate material on inductively coupled plasma etching of GaN and AlGaN with 1% and 10% of Al was investigated. It was revealed that coverplate material has a great impact on the etching of At-containing layers. Results obtained with a graphite coverplate and a Si wafer on top of a quartz coverplate indicate that etch products of coverplate material such as SiClx, CClx, reactive species and SiClx+, CClx+ ions play a significant role in fast and smooth etching of Al-containing layers. They act as getters to remove oxygen in the process chamber and as effective etchers for oxide layers formed by background oxygen in the process chamber. Experiments where SiCl4 gas was added to Cl-2/Ar plasma confirmed the role of SiClx reactive species and SiClx+ ions for fast and smooth etching of AlGaN layers. (C) 2004 American Vacuum Society.