Fabrication of silicon-blazed phase diffractive gratings using grey scale gallium implantation with KOH anisotropic etch

Abstract

The development of a two-step process for fabricating blazed diffractive gratings, with consistent efficiencies greater than 70%, using a combination of focused ion beams (FIB) for grey scale gallium mask implantation, followed by a potassium hydroxide (KOH) anisotropic wet etch is described. The target wavelength range for the diffractive optics is designed for the infra-red (IR) spectrum from 1 µm to 10 µm. The optical substrate is made from p-type (1015 cm−3) wafer grade silicon (1 0 0). Computer-generated holography is used to determine the implant pattern. This pattern is then translated into FIB script files for spatial implant dose commands. Process optimization is achieved using the implant dose range of 1 pC µm−2 to 25 pC µm−2, using a 1000 pA ion beam current with a focal diameter of 140 nm. The etch process uses a 30% KOH solution at a fixed temperature of 78 °C to achieve an etch rate of approximately 1 µm min for greatest control and repeatability. Measurements confirm that etch times ranging over 1 to 4 min achieve the required phase heights of 1–3.8 µm for the target wavelength range. Therefore, the wet etch process offers the advantages of higher throughput time with greater etch selectivity for larger phase heights when compared with photolithographic techniques. However, experimentation with diffraction gratings and circular lens patterns confirms that repeatability and control are limited to designs that have surfaces with the minimum number of convex corners due to the enhanced etching of the crystalline planes {4 1 1} relative to the {1 0 0} planes. Fabrication of diffraction grating structures on the (1 0 0) silicon wafer for the wavelengths of 1 µm and 10.6 µm confirms optical functionality with binary diffraction structures and a measured diffraction efficiency of 76% for the blazed phase profile.