Abstract
Refractive X-ray lenses are in use at a large number of synchrotron experiments. Several materials and fabrication techniques are available for their production, each having their own strengths and drawbacks. We present a grating interferometer for the quantitative analysis of single refractive X-ray lenses and employ it for the study of a beryllium point focus lens and a polymer line focus lens, highlighting the differences in the outcome of the fabrication methods. The residuals of a line fit to the phase gradient are used to quantify local lens defects, while shape aberrations are quantified by the decomposition of the retrieved wavefront phase profile into either Zernike or Legendre polynomials, depending on the focus and aperture shape. While the polymer lens shows better material homogeneity, the beryllium lens shows higher shape accuracy.
Highlights
Compound refractive X-ray lenses (CRLs [1]) are commonly used at synchrotrons as optics to produce micro- and nanobeams, and as objectives in full field microscopy experiments
We report on the implementation of a high resolution, single grating interferometer for the test of refractive lenses performed at the beamline ID06 of the ESRF
Two types of samples were tested in this study: Point focus beryllium lenses fabricated by imprinting [1] mounted in a steel frame, and line focus lenses fabricated by deep X-ray lithography in an SU-8 based polymer on a silicon substrate [10]
Summary
Compound refractive X-ray lenses (CRLs [1]) are commonly used at synchrotrons as optics to produce micro- and nanobeams, and as objectives in full field microscopy experiments. Focusing optics are often characterized in terms of the achieved size and shape of the focal spot While this is an easy way to get an impression of lens quality, it only provides limited information on the lens and offers little feedback for improving the lens production. The resulting interference pattern downstream is altered by the sample, and compared to a reference scan without sample As it can be readily employed for materials with very weak absorption, it is a valuable tool for the characterization of refractive optics and can be used for the quantitative analysis of X-ray lenses [3,4]. Using a high resolution detector in combination with a larger grating period enables us to directly resolve the interference pattern without an analyzer grating This greatly simplifies the alignment such that the set-up and method can be used as a routine analysis tool. In this work the interferometer was employed to test point focus lenses made from beryllium, and line focus lenses from SU-8 based polymer
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