Abstract
Reactive-ion etching (RIE) based lithography has been the prevailing technology for the fabrication of diffractive optical elements (DOEs). However, the inherent physical effects, such as RIE lag and aspect ratio dependent etching, make it challenging to produce spatially-varying features with uniform depths in large areas. In this work, we propose an additive lithographic fabrication process for a reflective Tilt-Gaussian-Vortex (TGV) mask for focal plane wavefront sensing. The unique design of the TGV mask requires simultaneous presence of feature sizes ranging from micrometers to millimeters with low surface roughness. The proposed additive fabrication eliminates the error-prone RIE step after lithography, but instead, a sputter deposition and bi-layer liftoff step is fine-tuned to form the microstructures. Compared to removing materials from the substrate in RIE, our method grows materials up onto the substrate. The combination of sputter deposition and liftoff is uniform across the whole wafer area, resulting in high surface quality and depth accuracy. We demonstrate the effectiveness of the fabrication method by a reflective TGV mask with aluminum deposition on a fused silica substrate. The central Gaussian pattern has a diameter of 130 mu m with minimum spacing of 2 mu m, and the background vortex pattern is 3 mm x 3 mm, with the largest flat region spanning 1.5 mm. A preliminary 4-level prototype has been tested in the Gemini Planet Imaging calibration unit upgrading project, and an improved 16-level sample has been measured. The results show uniform depth and surface roughness control for both the Gaussian and Vortex patterns.