We investigated dry etching of GaAs and AiGaAs in a high density planar inductively coupled plasma system with BCl 3 and BCl 3 /Ar gas chemistry. A detailed etch process study of GaAs and ALGaAs was peformed as functions of ICP source power, RIE chuck power and mixing ratio of BCl 3 and Ar. Chamber process pressure was fixed at 7.5 mTorr in this study. The ICP source power and RIE chuck power were varied from 0 to 500 W and from 0 to 150 W, respectively. GaAs etch rate increased with the increase of ICP source power and RIE chuck power. It was also found that etch rates of GaAs in 15BCi 3 /5Ar plasmas were relatively high with applied RIE chuck power compared to pure 20 sccm BCl 3 plasmas. The result was the same as AlGaAs. We expect that high ion-assisted effect in BCl 3 /Ar plasma increased etch rates of both materials. The GaAs and AlGaAs features etched at 20 sccm BCl 3 and 15BCl 3 /5Ar with 300 W ICP source power, 100 W RIE chuck power and 7.5 mTorr showed very smooth surfaces(RMS roughness