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-conti nuous profile Diffractive Optical Elements (DOEs) to be used as beam shaper s. The process employs Electron Beam Lithography as a tool to obtain comple x 3D profiles by direct writing onto PMMA resist. We also present a charact erization of the resist for different types of developer and different deve loping 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 che ck the accuracy of the dose calibration and the sensitivity to process erro rs. Preliminary optical tests on a DOE realized with a 16-level profile wer e also performed, and gave a good response.