Trapezoid-kinoform zone plate lens - a solution for efficient focusing in hard X-ray optics.

Xujie Tong,Chengwen Mao,Zhenjiang Xing,Jun Zhao,Xiangjun Zhen,Chengyang Mu,Yifang Chen,Yijie Li,Zijian Xu,Renzhong Tai

doi:10.1107/s1600577522000893

Abstract

X-ray microscopes are powerful tools in the nano-inspection of materials owing to their ultra-high resolution at the molecular level. However, the focusing efficiency of binary zone plate lenses as key components in such probes is merely 5% in practice, hindering their application in advanced scientific research. Although kinoform zone plate lenses are in principle supposed to possess high efficiency beyond binary ones, little progress has been reported so far due to the shortage of both a theoretical calculation approach and greyscale lithography for generating fine three-dimensional (3D) kinoform zones of the lenses. This paper reports our theoretical work for a modified beam propagation method to compute the focusing performance and state-of-the-art 3D greyscale electron beam lithography for kinoform zone plate lenses. Three different zone shapes - binary, kinoform and top-flat kinoform (nicknamed the trapezoid-kinoform) - were compared both theoretically and experimentally. Theoretical calculations suggest, for the first time, that the trapezoid-kinoform zone plate gives rise to the highest focusing efficiency among the three lenses, which was proved by optical characterization of the fabricated lens with hard X-rays. As high as 40% of the focusing efficiency by Au trapezoid-kinoform lenses with resolution of 250 nm at 8 keV has been achieved, which is two times higher than that of binary zone plate lenses. The origin of the enhanced efficiency in the trapezoid-kinoform zone plate lens was explained by the joint contributions from both the refraction through the kinoform slope and the diffraction through the top flat part of the trapezoid-kinoform zone plate. Such a breakthrough in focusing efficiency sheds light on the further development of X-ray lenses with both high resolution and high efficiency.

Highlights

Fresnel zone plate (FZP) lenses as diffractive X-ray optics are key components in X-ray microscopes such as the full-field transmission X-ray microscope (Chu et al, 2008) and the scanning transmission X-ray microscope (Kirz, 1974; Spector et al, 1997) for nanoscale focusing and imaging (Sakdinawat & Attwood, 2010; Chen et al, 2021; Oktem et al, 2018; Fan et al, 2018)
Since the urgently necessary to conduct systematical research two-dimensional X-ray lenses in this work have rotational on both a theoretical approach and reliable 3D greyscale symmetry, we propose to replace the fast Fourier transform lithography for a new generation of zone plate lens with high in the BPM by the QDHT in the calculation of the near-field resolution as well as efficient focusing efficiency for X-ray wavefield
Based on the actual zone profiles of the fabricated TKZPs, which were obtained from the scanning electron microscope (SEM) images in Fig. 4(c), the calculated efficiency (Au-TKZP+) is added to Fig. 5(b) for comparison

Summary

Introduction

Fresnel zone plate (FZP) lenses as diffractive X-ray optics are key components in X-ray microscopes such as the full-field transmission X-ray microscope (Chu et al, 2008) and the scanning transmission X-ray microscope (Kirz, 1974; Spector et al, 1997) for nanoscale focusing and imaging (Sakdinawat & Attwood, 2010; Chen et al, 2021; Oktem et al, 2018; Fan et al, 2018). Since the urgently necessary to conduct systematical research two-dimensional X-ray lenses in this work have rotational on both a theoretical approach and reliable 3D greyscale symmetry, we propose to replace the fast Fourier transform lithography for a new generation of zone plate lens with high in the BPM by the QDHT in the calculation of the near-field resolution as well as efficient focusing efficiency for X-ray wavefield.

Results

Conclusion