Comparison of Micro Chrome Patterns in Gray Scale Lithography

Gray scale lithography is an attracting technology to fabricate 3D structures such as a microlens, prism etc. In this study, a gray scale chrome mask was fabricated by using image-processing software and E-beam lithography system that can use bitmap as drawing data. Random dither pattern and half-tone pattern by circles were generated as a fundamental micro pattern. UV exposure was performed on 10μm thick positive resist (AZ-p4620). The mask was distanced 80μm from the resist surface to defocus the micro pattern. As a result, half-tone pattern provides smooth surface while the surface condition of dither pattern had a horizontally large roughness. The resist depth was changed in the range of 20%-80% gray-scale value for the dither pattern, and in the little wider range of 20%-90% for the half-tone pattern. Then, Fresnel lens profile was formed by using half-tone pattern, which profile was calculated by C-code program and polar-coordinates conversion by image-processing software.