Article

Fabrication of diffraction gratings for hard X-ray phase contrast imaging

Paul Scherrer Institut, CH 5232 Villigen-PSI, Switzerland
Microelectronic Engineering (Impact Factor: 1.34). 05/2007; DOI: 10.1016/j.mee.2007.01.151

ABSTRACT We have developed a method for X-ray phase contrast imaging, which is based on a grating interferometer. The technique is capable of recording the phase shift of hard X-rays travelling through a sample, which greatly enhances the contrast of low absorbing specimen compared to conventional amplitude contrast images. Unlike other existing X-ray phase contrast imaging methods, the grating interferometer also works with incoherent radiation from a standard X-ray tube. The key components are three gratings with silicon and gold structures, which have dimensions in the micrometer range and high aspect ratios. The fabrication processes, which involve photolithography, anisotropic wet etching, and electroplating, are described in this article for each of the three gratings. An example of an X-ray phase contrast image acquired with the grating interferometer is given.

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    • "The period g 2 of grating G2 was 2 μm, which is equal to the interference fringe period caused by grating G1. The source grating G0 has a period of g 0 = g 2 × L/d = 14 μm, ensuring that the interference patterns from neighboring source lines will overlap at G2 (David et al 2007a). The sample should be located immediately in front of G1, and the detector should be immediately behind G2. "
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    • "The corresponding values for the second grating (gold absorber grating, G2) were p 2 = 2.00 µm and h 2 = 30 µm. In contrast to the earlier experiments (Weitkamp et al 2005), a novel grating fabrication protocol was used (David et al 2007). This allowed the fabrication of unprecedentedly high aspect ratios (60:1) for the absorbing lines of the analyzer grating (G2), thus increasing the contrast of the intensity modulation recorded during a phase-stepping scan. "
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    • "Then, the gaps of the grating are filled with gold by electro-deposition. Using a shadow evaporation process and selective wet etching, it is possible to let the gold grow from the bottom of the silicon grooves [15], as any deposition on the side walls or the silicon ridges would result in an incomplete filling of the grooves. The lower part of Fig. 3 shows a cross section of a gold-filled silicon grating fabricated by the described process. "
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