Ultra-high-aspect multilayer zone plates for even higher x-ray energies

Abstract

Penetration lengths in the millimetre range make hard x-rays above 60 keV a well-suited tool for non-invasive probing of small specimens buried deep inside their surroundings, and enable studying individual components inside assembled, complex devices (solar cells, batteries etc.). The real-space resolution of typical imaging modalities like fluorescence mapping, scanning SAXS and WAXS depend on the available beam size. Although routine in the 5–25keV regime [1-4], spot sizes below 50nm are very challenging at x-ray energies above 50 keV: Compound refractive lenses lack in refractive power, the multilayer thickness of coated mirrors is bounded by interfacial diffusion, and lithographic Fresnel Zone Plates loose their efficiency in the two-digit keV regime. Multilayer Laue Lenses and Multilayer Zone Plates (MZP) are promising candidates for high-keV focusing to small spot sizes; compared to Fresnel Zone Plates, the aspect ratio comparing outermost layer width (~focal spot size) to optical thickness (efficiency) is virtually unlimited by the fabrication. Using Pulsed Laser Deposition on a rotating wire (several millimetre long), we have fabricated an MZP with 10nm outermost zone widths and optical thickness of 30 μm(optimum phase shift at 60 keV), yielding an unprecedented ultra-high aspect ratio of 1:3000 (outermost zone width compared to optical thickness). We present experimental results obtained at ESRF’s high energy beamline ID31, where for the first time scanning experiments with real-space resolutions below 50nm even at x-ray energies ranging from 60 keV to above 100 keV have been achieved.