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Proceedings Paper

Nanofabrication advances for high efficiency critical-angle transmission gratings
Author(s): Alexander R. Bruccoleri; Dong Guan; Ralf K. Heilmann; Steve Vargo; Frank DiPiazza; Mark L. Schattenburg
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Paper Abstract

We report several break-through nanofabrication developments enabling high efficiency and high resolving power spectrometers in the soft x-ray band. The device is the critical-angle transmission (CAT) grating, which combines the low mass and relaxed alignment tolerances of a transmission grating with the high broad-band efficiency and high diffraction orders of a blazed reflection grating. Past work successfully demonstrated the CAT grating concept; however, the open-area fraction was often less than 20% whilst more than 50% is desired. This presents numerous nanofabrication challenges including a requirement for a freestanding silicon membrane of ultra high-aspect ratio bars at a period of 200 nanometers with minimal cross support blockage. Furthermore, the sidewalls must be smooth to a few nanometers to efficiently reflect soft x-rays. We have developed a complete nanofabrication process for creating freestanding CAT gratings via plasma-etching silicon wafers with a buried layer of SiO2. This removable buried layer enables combining a record-performance plasma etch for the CAT grating with a millimeter-scale honeycomb structural support to create a large-area freestanding membrane. We have also developed a process for polishing sidewalls of plasma-etched ultra-high aspect ratio nanoscale silicon structures via potassium hydroxide (KOH). This process utilizes the anisotropic etch nature of single crystal silicon in KOH. We developed a novel alignment technique to align the CAT grating bars to the {111} planes of silicon within 0.2 degrees, which enables KOH to etch away sidewall roughness without destroying the structure, since the {111} planes etch approximately 100 times slower than the non-{111} planes. Preliminary results of a combined freestanding grating with polishing are presented to enable efficient diffraction of soft x-rays.

Paper Details

Date Published: 26 September 2013
PDF: 18 pages
Proc. SPIE 8861, Optics for EUV, X-Ray, and Gamma-Ray Astronomy VI, 886119 (26 September 2013); doi: 10.1117/12.2024357
Show Author Affiliations
Alexander R. Bruccoleri, MIT Kavli Institute for Astrophysics and Space Research (United States)
Dong Guan, MIT Kavli Institute for Astrophysics and Space Research (United States)
Ralf K. Heilmann, MIT Kavli Institute for Astrophysics and Space Research (United States)
Steve Vargo, SPTS Technologies, Inc. (United States)
Frank DiPiazza, Silicon Resources LLC (United States)
Mark L. Schattenburg, MIT Kavli Institute for Astrophysics and Space Research (United States)

Published in SPIE Proceedings Vol. 8861:
Optics for EUV, X-Ray, and Gamma-Ray Astronomy VI
Stephen L. O'Dell; Giovanni Pareschi, Editor(s)

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