Shrinkage Precompensation of Holographic Three‐Dimensional Photonic‐Crystal Templates

Abstract

Holographic lithography is an established technique for fabricating three-dimensional photonic-crystal templates. [1–12] The principle of holographic lithography is to expose a photoresist layer to an interference pattern. After processing, a porous polymer–air structure results. In order to achieve a desirable complete photonic bandgap (PBG), these templates have to be replicated with silicon [13] or other high-index materials. Theory predicts complete PBGs for certain crystal symmetries and crystallographic motifs. [5,8–12] In all these proposals it is tacitly assumed that the surface of the resulting polymer template coincides with an isodose surface of the exposing interference pattern. However, many deviations resulting from various steps in the fabrication process may be present. [14] Based on our studies we find that shrinkage is the most prominent deviation. Photoresist shrinkage can be anisotropic and in some cases reduces the size of the template by up to 50%, which significantly affects the PBG and, therefore, can not be neglected. By quantifying and precompensating the anisotropic shrinkage, we solve this problem for the first time for the most common photoresist system in holographic lithography, namely SU-8 from MicroChem. A lateral shrinkage has previously been discussed [15] for a different resist and