Metal-Assisted Chemical Etching and Electroless Deposition for Fabrication of Hard X-ray Pd/Si Zone Plates.

Hanna Ohlin,Rabia Akan,Anne Sakdinawat,Ulrich Vogt,Ulf Johansson,Thomas Frisk,Kenan Li,Fabian Lundberg

doi:10.3390/mi11030301

Abstract

Zone plates are diffractive optics commonly used in X-ray microscopes. Here, we present a wet-chemical approach for fabricating high aspect ratio Pd/Si zone plate optics aimed at the hard X-ray regime. A Si zone plate mold is fabricated via metal-assisted chemical etching (MACE) and further metalized with Pd via electroless deposition (ELD). MACE results in vertical Si zones with high aspect ratios. The observed MACE rate with our zone plate design is 700 nm/min. The ELD metallization yields a Pd density of 10.7 g/cm, a value slightly lower than the theoretical density of 12 g/cm. Fabricated zone plates have a grid design, 1:1 line-to-space-ratio, 30 nm outermost zone width, and an aspect ratio of 30:1. At 9 keV X-ray energy, the zone plate device shows a first order diffraction efficiency of 1.9%, measured at the MAX IV NanoMAX beamline. With this work, the possibility is opened to fabricate X-ray zone plates with low-cost etching and metallization methods.

Highlights

X-ray microscopy is a powerful scientific tool used for the study of different samples in a variety of disciplines [1,2]
A Au zone plate catalyst design with interconnects between the zones plate rings is essential to get a controlled, vertical etching during metal-assisted chemical etching (MACE) at ambient processing conditions (Figure 2a) [20]
We presented a wet-chemical route to fabricate high aspect ratio Pd/Si zone plates aimed for the hard X-ray regime

Summary

Introduction

X-ray microscopy is a powerful scientific tool used for the study of different samples in a variety of disciplines [1,2]. The required high aspect ratios at very small zone widths make hard X-ray zone plates challenging to fabricate [5,6]. Metal-assisted chemical etching (MACE) has recently been used as an alternative fabrication method to transfer a zone plate pattern into a Si substrate with the advantage of vertical etching and the possibility to reach high aspect ratios [17,18,19,20]. Due to its low X-ray diffraction efficiency in the hard X-ray regime, the MACE processed Si zone plates are not suitable to be used directly as optical devices Instead, they can be used as molds for high-Z materials, such as Pt, deposited with atomic layer deposition (ALD) [17,18]. Pd is chosen as a high-Z material owing to an acceptable diffraction efficiency in the hard X-ray regime and the commercial availability of a well-formulated, stable ELD bath

Materials and Methods

MACE Processing of Si Zone Plate Molds

Pd ELD Metallization of Zone Plates

Characterization

Zone Plate Fabrication

Pd Density

Focusing Performance

Conclusions