The reactive ion etching of TiWN, which is used as a gate metal on gallium-arsenide device wafers, was studied in a parallel-plate, single-wafer plasma reactor operating at a frequency of 13.56 MHz. We discuss our experimental program designed to develop a highly uniform TiWN etch process with low linewidth loss for 100 mm GaAs wafers, using a sulfur hexafluoride, trifluoromethane, helium chemistry. The effects of different gas compositions, plasma power, inter-electrode gap, chamber pressure, and electrode temperature on the TiWN etch rate, linewidth loss, and etch uniformity were determined. The effects of adding oxygen and/or nitrogen to the above mixture were also studied. In preliminary experiments on Si wafers, standard design of experiments methods were used to narrow the ranges of parameters for further experiments to develop an optimum process for Si wafers. The results of these experiments guided us to the optimum process for GaAs wafers. The optimum conditions, for both Si and GaAs wafers. are presented.