Multilevel interference lithography--fabricating sub-wavelength periodic nanostructures

Abstract

Periodic nanostructures have many exciting applications, including high-energy spectroscopy, patterned magnetic media, photonic crystals, and templates for self-assembly. Interference lithography (IL) is an attractive method for fabricating such structures, as it offers several advantages including large exposure area and high spatial-phase coherence. However, the spatial resolution of IL is limited, and the smallest attainable period is roughly half the wavelength of the light used. To overcome this wavelength-limited resolution, we have developed a multilevel interference lithography process that is capable of fabricating sub-wavelength periodic nanostructures over large areas. In this process, multiple grating levels with different phase-offsets are overlaid and spatial-phase aligned to a common reference grating. Each grating level is pattern-transferred into a single hard mask layer, resulting in spatial-frequency multiplication. To ensure high grating overlay accuracy, each grating level is aligned to the reference grating with various interferometric techniques. In addition, an image-reversal process with plasma etch trimming was developed to control the linewidth of each grating level to nanometer-repeatability. Extensive optical simulations using rigorous coupled-wave analysis were used to examine the intensity distribution of exposures over multilayer periodic structures. The immediate goal of this work is to extend the wavelength-limited resolution of interference lithography with high precision metrology and well-controlled fabrication processes. Using this multilevel process, we have successfully fabricated 50 nm-period gratings using light with 351.1 nm wavelength. This process presents a general scheme for overlaying periodic nanostructures, and can be used to fabricate more complex 2D and 3D geometries.