A proposed x-ray instrument - a Fourier-transform x-ray microscope - would enable high-resolution imaging of objects with sizes of the order of 20 nm. (This size range is typical of cell organelles.) The instrument would also offer a spectroscopic capability for the sensitive detection and identification of selected chemical elements and chemical bonds. The instrument could be used, for example, to examine microscopic biological samples, to study surface chemistries, or to identify chemical contaminants on the surfaces of microelectronic devices during fabrication.

Heretofore, information of the sort that the proposed instrument would provide has been obtained through (1) imaging of photoelectrons excited by illumination with a monochromatic beam of soft x-rays from a synchrotron x-ray source or (2) transmission microscopy with monochromatic radiation, also from a synchrotron source. The disadvantage of both techniques is that a large apparatus (a synchrotron) is needed to produce sufficient fluxes at the wavelengths of interest. The Fourier-transform function of the proposed instrument would exert an effect equivalent to multiplying the flux density of x-rays by a factor of the order of 103; this effect would make it possible to use a smaller, electron-beam-based x-ray source. Consequently, unlike x-ray imaging systems based on synchrotron sources, the proposed system would be amenable to miniaturization.

The proposed instrument (see figure) would include an electron-beam-based x-ray source with an output spectrum spanning a wavelength band (1) approximately centered at an absorption edge of a chemical element of interest and (2) broad enough to include possible chemical shifts (± about 10 eV). Two examples of such a wavelength band are 284.3±10 eV for C or 346.2±10 eV for Ca.

A Mach-Zender interferometer would modulate the spectrum with a period of about 2ΠDx/c in frequency (where Dx is the difference between the lengths of the two optical paths in the interferometer and cis the speed of light). By use of a microactuator, the mirrors in the interferometer would be moved to vary Dx by small increments. A specimen would be illuminated by the x-ray beam coming out of the interferometer. X rays reflected from the specimen would be imaged on a scintillator, which would convert the x-ray image to a visible one that, in turn, would be acquired by a charge-coupled-device (CCD) camera.

An image would be acquired at each Dx. The image data acquired over all of the many small increments of Dx would be Fourier-transformed and otherwise processed to obtain x-ray spectra for the pixels in the image. From the magnitudes and shifts of absorption edges in these spectra, high-resolution atomic-concentration and chemical-bonding maps of the chemical element of interest could be generated.

This work was done by Kirill Shcheglov and Victor White of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category.


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Fourier-Transform X-Ray Microscope

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This article first appeared in the November, 2001 issue of Photonics Tech Briefs Magazine.

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