Microfabrication of 3D Free-Forms with Textured Surface Morphology in Monocrystalline Silicon using Grayscale Technology

Abstract

Grayscale lithography involving a controlled polymerization of the photoresist, by a gradual modification of the exposure intensity is combined with reactive ion etching process having an etching selectivity of 1:1 (i.e. for the used photoresist and monocrystalline silicon wafer), in order to microfabricate 3D free-forms with complex geometry having a textured surface into monocrystalline silicon wafer. The microfabrication of complex 3D free-forms is exemplified by a geometry of convex micro-lens having concentric rings on its surface representing the surface texture and a concave depression at its tip. The 3D geometry is exposed in positive AZ4562 photoresist having a thickness of 3.8 µm by grayscale lithography using direct writing laser. The exposed 3D geometry in the photoresist is successfully transferred into monocrystalline silicon wafer using the optimized reactive ion etching process with a selectivity of 1:1 and an anisotropic factor close to 1 in SF6+CHF3 etching environment. The results show that the complex geometry exposed in the photoresist was successfully transferred into the silicon substrate with a pattern transferability with < 0.2 µm deviation between the convex micro-lens diameter in the exposed photoresist and the transferred geometry into silicon wafer. Therefore, the results show the efficacy of the proposed technique even for the pattern transferring of 3D microstructures having a textured surface using the proposed grayscale technology.