Focused ion beam milling of photonic crystals in silicon on insulator

A photonic crystal slab, consisting of an array of circular sub-micron diameter holes in Silicon on Insulator (SOI), has been fabricated using focused ion beam (FIB) milling. This application requires the sidewalls of the holes to be very smooth and as nearly perpendicular to the slab as possible. The shape of holes is adversely affected by redeposition of milled material. The effects on the sidewall shape due to different FIB parameters, such as beam current, dwell time, and scanning pattern have been researched. It is shown that a spiral-shaped scanning pattern performs much better than the conventional rater scan. A combination of dwell time, beam current and number of scanning loops, optimising the geometry of holes, has been determined.