Characterization of the gray-scale photolithography with high-resolution gray steps for the precise fabrication of diffractive optics

Abstract

To fabricate a diffractive optical element (DOE) with optimum surface profile, we characterize the relation between the developed height profile on the photoresist and the optical density (OD) value of a high-energy-beam-sensitive (HEBS) gray-scale photomask with high-resolution gray steps. The lithography characteristics of a gray step less than 10 µm are inspected in detail. The gray patterns with various gray-step widths are fabricated for the investigation of the relation between the OD values and the height profiles on the specific photoresist. We demonstrate the dependence of the photoresist height profiles on the width of the gray step, and this dependence agrees well with the simulation results from our model. We provide a proper method to design the OD value of a gray-scale photomask for the fabrication of DOEs with precise surface profiles.