Application of rigorously optimized phase masks for the fabrication of binary and blazed gratings with diffractive proximity lithography

Abstract

Gratings with binary and blazed profiles and periods in the low micron and sub-micron range define a class of microstructures with a huge application potential. We present a mask based photolithographic fabrication method for these demanding grating geometries. It combines the advantages of electron beam lithography and holographic exposure, which are superior homogeneity, high resolution and pattern flexibility on one hand, and a fast, large aerial exposure with the option for smooth profiles on the other hand. This is accomplished by the use of an electron beam written phase mask which contains a very homogeneous pattern of diffractive features and is used for a full-field exposure in a proximity mask aligner. The key for the beneficial use of the technology is the proper design of the phase mask surface profile which can have a binary or multilevel geometry. Since the patterns to be exposed are periodic, this is also the case for the phase mask which allows calculating their physical light transmission with exact methods like rigorous coupled wave analysis. An optimization algorithm has been developed which can find mask geometries that synthesize a desired complex aerial image in the proximity distance of choice. Aerial images offering e.g. high resolution features, phase shifts, and tilted propagation directions can be realized that way. This technology has been successfully used to fabricate e.g. binary gratings of very high quality with a period of 800 nm as well as blazed gratings with a period of 3 μm.