Study and formation of 2D microstructures of sapphire by focused ion beam milling

Dry etching of C-plane sapphire wafer has been studied using Ga+ focused ion beam milling (FIBM). Due to a much lower milling rate of sapphire compared to GaN, it has been proven that gas-assisted FIBM (GAFIBM) is a necessity. Furthermore, it needs to be determined whether XeF2- or I2-GAFIBM can improve the technique. We found that XeF2-GAFIBM gave the highest milling rate. The obtained enhancement factor of the XeF2-GAFIBM milling rate compared to FIBM rate varied from 2.3 to 1.2 for the ion beam current in the range 20pA-1nA. A favorable milling rate selectivity of sapphire to nickel film of about 1.5 was obtained by XeF2-GAFIBM at 350pA. We have successfully fabricated a variety of 2D sapphire-based submicron pillar or hole arrays in regular crystals, quasicrystals and aperiodic symmetries. The nearest neighbor distance was down to 230nm. The depth achieved was deeper than 400nm. The air filling factor ranged from 12% to 60%. These sapphire-based 2D microstructures were applied on flip-chip GaN-based light emitting diodes (LEDs) and more than 40% improvement in light extraction was obtained.