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

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/cm2 at 50kV for our substrates. Our optical waveguides consisted of 350 nm Ta2O5 ion beam sputtered onto superpolished fused silica substrates. The grating fabrication flow was designed to minimize process steps on the Ta2O5 surface and ensure that the original low surface roughness was sufficiently maintained after processing.