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Journal of Vacuum Science & Technology B / Volume 25 / Issue 6 / PAPERS FROM THE 51st INTERNATIONAL CONFERENCE ON ELECTRON, ION, AND PHOTON BEAM TECHNOLOGY AND NANOFABRICATION / Nanodevices

J. Vac. Sci. Technol. B 25, 2593 (2007); http://dx.doi.org/10.1116/1.2779048 (5 pages)

Fabrication of ultrahigh aspect ratio freestanding gratings on silicon-on-insulator wafers

Minseung Ahn, Ralf K. Heilmann, and Mark L. Schattenburg

Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

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(Published online 11 December 2007)

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The authors report a silicon-on-insulator (SOI) process for the fabrication of ultrahigh aspect ratio freestanding gratings for high efficiency x-ray and extreme ultraviolet spectroscopy. This new grating design will lead to blazed transmission gratings via total external reflection on the grating sidewalls for x rays incident at graze angles below their critical angle (about 1°–2°). This critical-angle transmission (CAT) grating combines the alignment and figure insensitivity of transmission gratings with high broadband diffraction efficiency, which traditionally has been the domain of blazed reflection gratings. The required straight and ultrahigh aspect ratio freestanding structures are achieved by anisotropic etching of ⟨110⟩ SOI wafers in potassium hydroxide (KOH) solution. To overcome structural weakness, chromium is patterned as a reactive ion etch mask to form a support mesh. The grating with period of 574 nm is written by scanning-beam interference lithography (SBIL) which is based on the interference of phase-locked laser beams. Freestanding structures are accomplished by etching the handle and device layers in tetramethylammonium hydroxide and KOH solution, respectively, followed by hydrofluoric acid etching of the buried oxide. To prevent collapse of the high aspect ratio structures caused by water surface tension during drying, the authors use a supercritical point dryer after dehydration of the sample in pure ethanol. The authors have successfully fabricated 574 nm period freestanding gratings with support mesh periods of 70, 90, and 120 μm in a 10 μm thick membrane on ⟨110⟩ SOI wafers. The size of a single die is 10×12 mm2 divided into four 3×3.25 mm2 windows. The aspect ratio of a single grating bar achieved is about 150, as required for the CAT grating configuration.

© 2007 American Vacuum Society

ACKNOWLEDGMENTS

The authors gratefully acknowledge the assistance of Robert Fleming of the MIT Space Nanotechnology Laboratory and James Daley of the MIT NanoStructures Laboratory. They thank Nicki Watson and Erika Batchelder for help with the supercritical point dryer at the MIT Whitehead Institute and the MIT Microsystems Technology Laboratories for facility support. This work was supported by NASA Grant No. NNG05WC13G and a Samsung Scholarship.

Article Outline

  1. INTRODUCTION
  2. FABRICATION PROCESS
    1. Patterning process
    2. Anisotropic wet etching
  3. RESULTS AND DISCUSSION
  4. CONCLUSION AND FUTURE WORK

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KEYWORDS and PACS

Keywords

diffraction gratings, optical fabrication, silicon-on-insulator, ⟨110⟩ Silicon, Silicon-on-Insulator, Potassium hydroxide (KOH)

PACS

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History
Received 6 June 2007
Accepted 23 July 2007
Published online 11 December 2007
Digital Object Identifier
http://dx.doi.org/10.1116/1.2779048

PUBLICATION DATA

ISSN

1071-1023 (print)  
1520-8567 (online)

Publisher

American Vacuum Society

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