Fabrication of High-Aspect Ratio Nanogratings for Phase-based X-ray Imaging

TL;DR

This paper presents a cost-effective fabrication method for high-aspect-ratio nanogratings filled with gold, enabling improved phase-based X-ray imaging with relaxed coherence requirements and demonstrated diffraction performance at a synchrotron.

Contribution

It introduces an innovative fabrication process combining laser interference, nanoimprint lithography, and a post-MACE drying step to produce nanogratings with aspect ratios over 40.

Findings

Achieved nanogratings with aspect ratios >40.

Demonstrated effective diffraction at 12.2 keV X-ray energy.

Proposed a scalable, cost-efficient fabrication approach.

Abstract

Diffractive optical elements such as periodic gratings are fundamental devices in X-ray imaging - a technique that medical, material science and security scans rely upon. Fabrication of such structures with high aspect ratios at the nanoscale creates opportunities to further advance such applications, especially in terms of relaxing X-ray source coherence requirements. This is because typical grating-based X-ray phase imaging techniques (e.g., Talbot self-imaging) require a coherence length of at least one grating period and ideally longer. In this paper, the fabrication challenges in achieving high aspect-ratio nanogratings filled with gold are addressed by a combination of laser interference and nanoimprint lithography, physical vapor deposition, metal assisted chemical etching (MACE), and electroplating. This relatively simple and cost-efficient approach is unlocked by an innovative post-MACE drying step with hexamethyldisilazane, which effectively minimizes the stiction of the nanostructures. The theoretical limits of the approach are discussed and, experimentally, X-ray nanogratings with aspect ratios >40 demonstrated. Finally, their excellent diffractive abilities are shown when exposed to a hard (12.2 keV) monochromatic x-ray beam at a synchrotron facility, and thus potential applicability in phase-based X-ray imaging.