Metrology challenges for fabrication of high aspect ratio, nano scale x-ray diffraction gratings

Next-generation space telescopes under development by NASA will require high efficiency and resolving power soft x-ray spectrometers which can provide an order of magnitude increased performance compared to present space instrumentation. For the past decade the MIT Space Nanotechnology Lab has developed new x-ray spectrometer concepts based on so-called critical-angle transmission (CAT) gratings which exploit the principle of efficient x-ray reflection below the critical angle for total external reflection. These devices require challenging high-aspect ratio (HAR) "nano mirror" grating patterns with well controlled bar tilt angle and sub-nm surface smoothness. Gratings are patterned with precision (110) SOI wafers using 193 nm lithography and etched using a Bosch process. In this paper we present the first results of applying Mueller Matrix OCD mapping to our silicon HAR device geometry and compare with small angle x-ray scattering results.