High-aspect-ratio gratings by deep reactive ion etching

In X-ray grating interferometry (XGI) setups, silicon gratings are usually adapted as interferometry pattern generators, and could serve as beam splitters and fringe analyzers. The trenches of the gratings are filled with high absorption materials, such as gold. To improve the sensitivity of phase contrast and dark field imaging, XGI system designers ask for gratings with periods in the micrometer range. Large grating height is indispensable for high X-ray energy applications. In a word, the fabrication of gratings with high aspect ratios has become one of the key factors affecting the development of XGI. The grating fabrication team has developed a mature process flow and is now pushing the limits of the fabrication technology with new designs and processing for improved gratings performance [1]. We also bring the most advanced nano- and micro- processing technologies into the lab to overcome obstacles on the road to high quality gratings. For example, displacement Talbot lithography was introduced to have uniformly patterned grating lines over wafer scale [2]. Advanced Bosch process is used in plasma etching [3] with oxide hard mask patterns to achieve aspect ratio as high as 75:1 at 4-8 inch wafer scale. An in-house method to generate a gradual tilt in the etching direction is used to fabricate fan-shaped gratings [4]. Gold absorption gratings without voids and delamination defects are prepared in collaboration with GratXray (PSI spin off), EPFL and NIST.