Precise fabrication of 3D microstructures via optimized diffuser grayscale lithography

Abstract

Grayscale lithography enables the fabrication of three-dimensional microstructures in photoresist, which can subsequently be transferred into substrate materials by dry etching. Among the available techniques, diffuser-based grayscale lithography offers a cost-effective and mask-efficient route to continuous-profile micro-optics without the need for expensive graytone masks or slow direct-write tools. In this work, we present an optimized diffuser-based grayscale lithography process using Shipley S1818 photoresist and a structured photomask consisting of dots with diameters ranging from 0.5 to 4.2 μm at a 5 μm pitch. We demonstrate that iterative correction of the mask dot diameters linearizes the dose-depth response and enables highly precise ramp profiles. Surface roughness arising from the periodic mask structure is characterized using both optical (Zygo NewView 7300) and stylus (KLA Tencor D-120) profilometry, and is mitigated using UV-assisted Thermally Activated Selective Topography Equilibration (U-TASTE), a 172 nm flood-exposure process that selectively reduces the glass transition temperature of the resist surface, enabling localized reflow without compromising the overall shape. After optimization, the fabricated ramp profiles achieve a linearity of R2 = 0.9996 with a surface roughness of 7.81 nm RMS. The versatility of the method is demonstrated through the fabrication of inverted pyramid arrays and an Alvarez lens. The method is suitable for fabricating elements such as an Airy beam phase mask, and an optical vortex element. These results establish diffuser-based grayscale lithography as a practical platform for rapid prototyping of complex micro-optical elements.