Electron beam lithography on uneven resist for uniform Bragg gratings in GaSb based distributed-feedback lasers

Nanofabrication of metallic Bragg gratings vertically connected to the AlGaAsSb ridges for distributed-feedback (DFB) lasers based on AlGaAsSb/InGaAsSb was successfully conducted in this work. Owing to the densely distributed 4-μm wide ridges on the surface, electron beam lithography on the spin-coated PMMA suffered from severely uneven thickness of the resist, significantly deteriorating the grating quality. To overcome this difficulty, a special e-beam exposure strategy was developed by optimizing the dose distribution on the non-flat PMMA to achieve uniform Bragg gratings throughout the whole wafer surface. Single mode operation at room temperature of the fabricated DFB lasers was achieved at the designed lasing wavelength around 1973 nm, indicating the high quality of the gratings generated on extremely uneven PMMA. This exposure method provided us with a novel approach to pattern uniform nanostructures on the resist with uneven thickness as a whole.