Very high aspect ratio of photonic crystals holes in InP

We have used a three-level masking technique to achieve high aspect ratio photonic crystal holes in InP-based materials. The masking consists of a ZEP/Cr/SiOx stack where the ZEP layer is used to open the Cr which on its turn is a good mask for opening the 500nm thick SiOx layer. Subsequently InP is etched in an ICP process using Cl2:O2 at a pressure of 1.4 mTorr. High aspect ratio could be achieved in holes ranging from 160 up to 330 nm in diameter with a maximum of 18 obtained with the narrowest holes having a diameter of 210 and 160 nm