High aspect ratio metal microcasting by hot embossing for X-ray optics fabrication

TL;DR

This paper presents a novel, scalable hot embossing method for fabricating high aspect ratio metal microstructures for X-ray optics, using low melting point alloys and silicon templates.

Contribution

It introduces an innovative microcasting approach combining hot embossing with low melting alloy foils and surface treatment to improve filling and scalability.

Findings

Successful fabrication of micro-gratings with pitches of 2-20 micrometers.

Optimized temperature and pressure conditions for complete cavity filling.

Method offers a low-cost, fast, and scalable solution for metal microstructure fabrication.

Abstract

Metal microstructured optical elements for grating-based X-ray phase-contrast interferometry were fabricated by using an innovative approach of microcasting: hot embossing technology with low melting temperature (280{\deg}C) metal alloy foils and silicon etched templates. A gold-tin alloy (80w%Au / 20w%Sn) was used to cast micro-gratings with pitch sizes in the range of 2 to 20 micrometers and depth of the structures up to 80 micrometers. The metal filling of the silicon template strongly depends on the wetting properties of the liquid metal on the groove surface. A thin metal wetting layer (20 nm of Ir or Au) was deposited before the casting in order to turn the template surface into hydrophilic with respect of the melted metal alloy. Temperature and pressure of the hot embossing process were optimized for a complete filling of the cavities in a low viscosity regime of the liquid metal, and for minimizing the shear force that might damage the silicon structures for small pitch grating. The new method has relevant advantages, such as being a low cost technique, fast and easily scalable to large area fabrication.