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The development of Fourier Transform (FT) spectral techniques in the soft X-ray spectral region has been advocated in
the past as a possible route to constructing a bench-top size spectral imager with high spatial and spectral resolution.
The crux of the imager is a soft X-ray interferometer. Auxiliary subsystems include a wide-band soft X-ray source,
focusing optics and detection systems. When tuned over a sufficiently large range of path delays, the interferometer will
sinusoidally modulate the source spectrum centered at the core wavelength of interest, the spectrum illuminates a target,
the reflected signal is imaged onto a CCD, and data acquired for different frames is converted to spectra in software by
using FT methods similar to those used in IR spectrometry producing spectral image per each pixel. The use of shorter
wavelengths results in dramatic increase in imaging resolution, the modulation across the beam width results in highly
efficient use of the beam spectral content, facilitating construction of a bench-top instrument. With the predicted <0.1eV
spectral and <100 nm spatial resolution, the imager would be able to map core-level shift spectra for elements such as
Carbon, which can be used as a chemical compound fingerprint and imaging intracellular structures.
We report on our progress in the development of a Fourier Transform X-ray (FTXR) interferometer. The enabling
technology is X-ray beam splitting mirrors. The mirrors are not available commercially; multi layers of quarter-wave
films (used in IR and visible) are not suitable, and several efforts by other researchers who used parallel slits met only a
very limited success. In contrast, our beam splitters use thin (about 200 nm) SiN membranes perforated with a large
number of very small holes prepared in our micro-fabrication laboratory at JPL. Precise control of surface roughness
and high planarity are needed to achieve the requisite wave coherency. The beam splitters prepared-to-date had surface
RMS and planarity better that <0.3 nm over a 0.45 mm x 1.4 mm area, meeting requirements for spectral imaging at
100eV. Efforts to improve the mirror flatness to a level required for core-level shifts of Carbon are under way.
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