Assessing tolerances in direct write laser grayscale lithography and reactive ion etching pattern transfer for fabrication of 2.5D Si master molds

Abstract

Direct Write Laser (DWL) Grayscale (GS) lithography has gathered attention as a versatile technological solution to fabricate arbitrary and complex 2.5D structures in photoresist (PR). In combination with Reactive Ion Etching (RIE), DWL GS can enable the fabrication of 2.5D micro-structured silicon substrates to be used as molds in high-precision replication processes. To this end, typically, a multilevel pattern is first defined in a low contrast PR and then transferred to the Si substrate via anisotropic RIE. The uniformity and selectivity of the etching process to the PR and substrate materials will define the dimensions of the 2.5D structures in the substrate. However, while there have been numerous examples of etched shapes in Si using this strategy, the wafer scale dimensional variations for etched Si molds fabricated via DWL GS and RIE have not been reported so far. In this work, we present a wafer-scale and wafer-to-wafer analysis of the dimensional variations in the fabrication process of Si molds with 2.5D 100-μm deep cavities using DWL GS lithography and RIE. The dimensional variations and deviations were characterized along each process step over several wafers. The final Si etched shapes consisting of microlens array cavities showed a maximum relative deviation of 10.35% with respect to the intended shape design with a curvature radius variation up to ∼5%, demonstrating this process potential for arbitrary 2.5D Si mold fabrication.

Keywords

• Grayscale lithography
• Direct write laser
• Pattern transfer
• Reactive ion etching
• Microlens arrays