Patterned photoresist, SiO2, SiNX, and W were compared as masking materials for GaAs and other III-V semiconductors during CH4/H-2/Ar plasma etching. The use of electron cyclotron resonance conditions is a much more severe test of the mask integrity in comparison to reactive ion etching. Ar flow rate, microwave power and rf power have significant effects on mask erosion (or polymer deposition on the mask), while process pressure is found to have little effect over the range 1.5-10 mTorr. SiNX retains high geometrical integrity and displays little erosion under electron cyclotron resonance conditions, while SiO2 is quickly coated with a polymer that can degrade critical dimensions. Morphological changes in both W and photoresist films during exposure to electron cyclotron resonance CH4/H-2/Ar discharges indicate that these are less desirable choices than SINX.