Front- and backside structuring of gratings for phase contrast imaging with x-ray tubes
2010
Phase contrast imaging with conventional X-ray tubes as e.g. in computer tomography scanners (CTscanners)
requires a setup of three different types of optical gratings. One grating is used to obtain
a spatially coherent radiation, the second grating defines a periodic phase shift and the third is used
as a periodic absorption grating. In order to absorb high energy radiation, absorption gratings with
periods of a few microns only and extreme aspect ratios (>80) are fabricated, employing a modified
LIGA process. However, above a critical structural height, structures collapse due to e.g. capillary
effects. To overcome this limitation a new variant of the LIGA process has been developed. It is
characterized by structuring of a resist on both sides of a membrane, resulting in a moderate aspect
ratio on both sides of the membrane instead of an extreme aspect ratio on one side. To get a perfect
overlay of both structures the grating structure on the front side of a membrane patterned by the
standard LIGA-process is used as the mask for structuring the second resist layer on the backside of
the membrane. A second electroforming step fills the gaps on the backside.
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