Breath figures: Self-organizing masks for the fabrication of photonic crystals and dichroic filters

Abstract

Controlled condensation of water vapor on a nonpolar liquid polymer film leads to the formation of submicrometer arrays of water droplets. Dissolution of a small amount of a polymer surfactant in the volatile liquid prevents the drops from coalescing and stabilizes their hexagonal arrangement. When subsequently the solvent and the water evaporate a solid polymer film patterned with hexagonally ordered holes remains—similar in shape to a honeycomb pattern. The hole size and the polydispersity are controlled by the solvent type, the flow rate of the condensing vapor, and the polymer concentration. The pattern is used as a lithographic mask to structure semiconductor surfaces. To this end, metal is deposited on the prestructured surface and subsequently lifted off to create two complementary structures: an array of metal disks on the semiconductor and a metal network. The lateral periodicity of the patterns can be varied between 300nm and several micrometers covering macroscopic areas (∼1cm2). Employing the metallic disk pattern as a mask in a plasma dry-etching process we have fabricated two-dimensional photonic crystals with small lattice constants in silicon. We have also used the complementary metal net for the production of metallic dichroic filters. The thin metal net acts as an optical short-pass in the infrared region of the spectrum due to its small lateral structure dimensions. The optical transmission of the dichroic filters is measured and simulated by a model calculation.