Reactive ion etching using BCl₃-based plasma chemistries is a promising technique to pattern high-k metal gate stacks. High-k materials for non-volatile memory and CMOS applications, in particular Al₂O₃, possess high chemical resistance. Accordingly, a steep sidewall angle at the device edges is difficult to achieve by reactive ion etching. Advanced etch conditions at elevated temperatures (above 250 °C) is an alternative to solve this challenge but generate various other technological difficulties. In particular the patterning of TANOS devices reveals severe etch damage effects at the metal gate layer. A study of damage protection has been carried out and in particular the chemical stability of different metal gate options during plasma treatments was investigated in detail. Advanced process approaches to prevent the metal gate deterioration are proposed.