Tunable Antireflection Coating to Remove Index-Matching Requirement for Interference Lithography

Interference lithography is a flexible technique for creating 3D periodic nano- and microstructures that can be used to make a wide variety of crystal lattices, but as it is found, some lattices require index-matched substrates to eliminate reflections at the photoresist-substrate interface. In this study a tunable-refractive index quarter wavelength-thickness polystyrene/poly(vinyl methyl ether) homopolymer blend backside antireflection coating, which alleviates this issue, is presented. The coating's refractive index can be tuned from 1.47 to 1.6, drastically reducing reflections at the photoresist-substrate interface for substrates with refractive indices as low as 1.35 for normal incidence and even lower for angled illumination. By injecting the light through the substrate and applying the antireflection layer to the top of the photoresist, interference lithography can even be performed on high refractive index substrates, such as indium tin oxide (ITO)-coated glass. Fabrication of hexagonal, face-centered cubic, and simple cubic lattices in SU-8 photoresist (refractive index of 1.59) is demonstrated using 532 nm laser light on nonindex matched substrates including ITO-coated glass and borosilicate glass, and the effects of reflection interference on the photonic bandstructure are investigated.