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 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 period freestanding gratings with support mesh periods of 70, 90, and in a thick membrane on ⟨110⟩ SOI wafers. The size of a single die is divided into four windows. The aspect ratio of a single grating bar achieved is about 150, as required for the CAT grating configuration.
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November 2007
This content was originally published in
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena
Research Article|
December 11 2007
Fabrication of ultrahigh aspect ratio freestanding gratings on silicon-on-insulator wafers
Minseung Ahn;
Minseung Ahn
a)
Massachusetts Institute of Technology
, Cambridge, Massachusetts 02139
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Ralf K. Heilmann;
Ralf K. Heilmann
Massachusetts Institute of Technology
, Cambridge, Massachusetts 02139
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Mark L. Schattenburg
Mark L. Schattenburg
Massachusetts Institute of Technology
, Cambridge, Massachusetts 02139
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a)
Electronic mail: msahn@mit.edu
J. Vac. Sci. Technol. B 25, 2593–2597 (2007)
Article history
Received:
June 06 2007
Accepted:
July 23 2007
Citation
Minseung Ahn, Ralf K. Heilmann, Mark L. Schattenburg; Fabrication of ultrahigh aspect ratio freestanding gratings on silicon-on-insulator wafers. J. Vac. Sci. Technol. B 1 November 2007; 25 (6): 2593–2597. https://doi.org/10.1116/1.2779048
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