Fabrication of semi-continuous profile diffractive optical elements for beam shaping by electron beam lithography

We report a description of the design and fabrication process of semi-continuous profile Diffractive Optical Elements (DOEs) to be used as beam shapers. The process employs Electron Beam Lithography as a tool to obtain complex 3D profiles by direct writing onto PMMA resist. We also present a characterization of the resist for different types of developer and different developing times. This study shows how important is to fix such parameters for the assignment of the electron dosage values required to obtain the correct resist heights, through the Normalized Resist Thickness (NRT) vs. absorbed dosage curve. Simple patterns were fabricated and examined in order to check the accuracy of the dose calibration and the sensitivity to process errors. Preliminary optical tests on a DOE realized with a 16-level profile were also performed, and gave a good response.