We investigate etching conditions for photonic crystals (PhCs) in InGaAsP/InP and AlGaAs/GaAs using a new regime of CAME operation. We show that the beam voltage-current ratio is critical in obtaining high material/mask selectivity. For one-dimensional PhCs, i.e., air slots, selectivities of 22:1 and 50:1 were achieved in InP and GaAs, respectively, using a very high beam voltage (about 1500 V) and a low beam current (about 10 mA). Etched features were observed to be very smooth, i.e., edge roughness was low. Two-dimensional PhCs were etched in InGaAsP/InP under similar conditions achieving selectivities up to 27:1 and 34:1 for hole diameters of 170 and 270 nm, respectively. (C) 2004 American Vacuum Society.