Robust, efficient grating couplers for planar optical waveguides using no-photoacid generator SU-8 electron beam lithography

摘要

450 nm pitch coupling gratings with greater than 10% input coupling efficiency have been fabricated on thin film planar optical waveguides using electron beam lithography on a novel SU-8 resist formulation that contains no photoacid generator (PAG). Even without the PAG component, we found the sensitivity of this material to be unexpectedly high, with a dose to clear of less than 10 μC/cm~2 at 50kV for our substrates. Our optical waveguides consisted of 350 nm Ta_2O_5 ion beam sputtered onto superpolished fused silica substrates. The grating fabrication flow was designed to minimize process steps on the Ta_2O_5 surface and ensure that the original low surface roughness was sufficiently maintained after processing.