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

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°C) metal alloy foils and silicon etched templates. A gold-tin alloy (80wt% Au/20wt% Sn) was used to cast micro-gratings with pitch sizes in the range of 2 to20μm and depth of the structures up to 80μm. 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 (20nm 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. [Display omitted] •Metal microcasting is proposed to realize high aspect ratio X-ray gratings.•The technique relies on hot embossing of AuSn eutectic foil in Si etched templates.•A thin conformal metal layer is used to improve the wettability of the liquid alloy on the Si surface.