Measurement and compensation of misalignment in double-sided hard X-ray Fresnel zone plates.

Viktoria Yurgens,Christian David,Frieder Koch,Mario Scheel,Timm Weitkamp

doi:10.1107/s1600577520001757

Viktoria Yurgens, Christian David + Show 3 more

Open Access

https://doi.org/10.1107/s1600577520001757

Copy DOI

Abstract

Double-sided Fresnel zone plates are diffractive lenses used for high-resolution hard X-ray microscopy. The double-sided structures have significantly higher aspect ratios compared with single-sided components and hence enable more efficient imaging. The zone plates discussed in this paper are fabricated on each side of a thin support membrane, and the alignment of the zone plates with respect to each other is critical. Here, a simple and reliable way of quantifying misalignments by recording efficiency maps and measuring the absolute diffraction efficiency of the zone plates as a function of tilting angle in two directions is presented. The measurements are performed in a setup based on a tungsten-anode microfocus X-ray tube, providing an X-ray energy of 8.4 keV through differential measurements with a Cu and an Ni filter. This study investigates the sources of the misalignments and concludes that they can be avoided by decreasing the structure heights on both sides of the membrane andby pre-programming size differences between the front- and back-side zoneplates.

Highlights

Fresnel zone plates (FZPs) are diffractive lenses used as focusing elements in a range of soft and hard X-ray imaging applications in synchrotrons all over the world
We present an X-ray tube-based setup that yields absolute measurements of first-order diffraction efficiency of Fresnel zone plates, as well as maps of the zone plate diffraction efficiency
Since the central part of the zone plate mainly consists of low-Z hydrogen silsesquioxane (HSQ), the zeroth-order counts at that position can be well approximated by the number of photons passing through the silicon nitride membrane outside of the zone plate

Summary

Introduction

Fresnel zone plates (FZPs) are diffractive lenses used as focusing elements in a range of soft and hard X-ray imaging applications in synchrotrons all over the world. An additional approach to achieving small linewidths at high aspect ratios is provided by a technique called line-doubling (Jefimovs et al, 2007; VilaComamala et al, 2011) In this method, template structures are fabricated from a low-atomic-number material, e.g. hydrogen silsesquioxane (HSQ), which has a very small effect on the transmission of hard X-rays. If strong misalignments are present, the efficiency distribution over the zone plate area – here referred to as the efficiency map – will contain moirefringes Such fringes are visual artefacts arising from the interference in the overlay of ordered patterns, in our case the two zone plates. The step to a laboratory-based setup is crucial to provide quick feedback to the fabrication process that would be impossible to obtain on a regular basis at a synchrotron

Zone plate fabrication

Results

Conclusions