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Proceedings Article

Nanofabrication advances for high efficiency critical-angle transmission gratings

[+] Author Affiliations
Alexander R. Bruccoleri, Dong Guan, Ralf K. Heilmann, Mark L. Schattenburg

MIT Kavli Institute for Astrophysics and Space Research (United States)

Steve Vargo

SPTS Technologies, Inc. (United States)

Frank DiPiazza

Silicon Resources LLC (United States)

Proc. SPIE 8861, Optics for EUV, X-Ray, and Gamma-Ray Astronomy VI, 886119 (September 26, 2013); doi:10.1117/12.2024357
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From Conference Volume 8861

  • Optics for EUV, X-Ray, and Gamma-Ray Astronomy VI
  • Stephen L. O'Dell; Giovanni Pareschi
  • San Diego, California, United States | August 25, 2013

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. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Citation

Alexander R. Bruccoleri ; Dong Guan ; Ralf K. Heilmann ; Steve Vargo ; Frank DiPiazza, et al.
" Nanofabrication advances for high efficiency critical-angle transmission gratings ", Proc. SPIE 8861, Optics for EUV, X-Ray, and Gamma-Ray Astronomy VI, 886119 (September 26, 2013); doi:10.1117/12.2024357; http://dx.doi.org/10.1117/12.2024357


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