Fabrication of nanoscale, high throughput, high aspect ratio freestanding gratings

Abstract

A nanofabrication process has been developed for a novel critical-angle transmission grating for astronomical x-ray spectroscopy. The pitch of the gratings is 200 nm and the depth is 4 μm, which exceeds the state-of-the-art in aspect ratio by over a factor of 2 for ultrahigh aspect ratio freestanding nanoscale gratings with open areas on the order of 50% and spanning several square centimeters. They have a broad array of other applications, including neutral mass spectroscopy, ultraviolet filtration, and phase contrast imaging x-ray spectroscopy. The gratings are fabricated as a monolithic structure in silicon via two lithographic and pattern transfer processes, integrated together on a silicon-on-insulator wafer. The grating is patterned via interference lithography and transferred into the 4 μm device layer via a Bosch deep reactive-ion etch (DRIE). The grating channels are then filled without voids by spinning photoresist on them, which wicks into the channels. The sample is then bonded under vacuum via Crystal Bond™ to a carrier wafer, and a honeycomb pattern is etched via DRIE through the handle layer until it stops cleanly on the buried SiO2. The buried SiO2 is etched away, and the sample is separated from the carrier. The resist filling is cleaned from the channels and the grating is critical-point dried to create a freestanding structure. The freestanding gratings can now be mounted to external frames and structurally analyzed and tested for launch and deployment in space.