We investigate etching characteristics of WSiN gates using electron cyclotron resonance ion stream etching with a SF6-CF4-SiF4-O-2 gas mixture, and we fabricate 0.1 mu m WSiN gates for ultrahigh speed GaAs metal-semiconductor field effect transistors (MESFETs). A vertical WSIN gate etched pattern is obtained as a result of CF4 addition. Moreover, uniform etching over the entire wafer can be attained with high selectivity between the WSiN and the GaAs and with accurate control of the gate length. A current-gain cutoff frequency (f(T)) of 131.4 GHz with a 3 sigma value of 5.0 GHz in whole wafer has been obtained for 0.1 mu m gate GaAs MESFETs.