Analysis of X-ray multilayer Laue lenses made by masked deposition

Abstract

Multilayer Laue lenses are diffractive optics for hard X-rays. To achieve high numerical aperture and resolution, diffracting structures of nanometer periods are required in such lenses, and a thickness (in the direction of propagation) of several micrometers is needed for high diffracting efficiency. Such structures must be oriented to satisfy Bragg’s law, which can only be achieved consistently over the entire lens if the layers vary in their tilt relative to the incident beam. The correct tilt, for a particular wavelength, can be achieved with a very simple technique of using a straight-edge mask to give the necessary gradient of the layers. An analysis of the properties of lenses cut from such a shaded profile is presented and it is shown how to design, prepare, and characterize matched pairs of lenses that operate at a particular wavelength and focal length. It is also shown how to manufacture lenses with ideal curved layers for optimal efficiency.

Precise wavefront characterization of x-ray optical elements using a laboratory source

“…We use a simple method to achieve such wedging using a straight mask [35]. The mask, which is attached above the substrate and moves with it, casts a shadow, which automatically results in a thickness gradient of the layers [38]. After the deposition, the thickness profile is measured with the help of a focused ion beam (FIB, FEI Helios NanoLab, Thermo Fisher Scientific, USA).…”

“…However, from a single multilayer deposition one can also prepare many MLLs optimized for different energies or extract MLLs that work at the same energy but have different focal lengths. The latter can be achieved by using two masks at different mask-tosubstrate distances [38].…”

“…The two lenses were cut from a single multilayer, which was deposited using two masks with different distances to the substrate. Hence, lenses of the same energy but slightly different focal lengths and numerical apertures [38] could be cut from the same multilayerhere, the focal lengths were 1.15 mm and 1.25 mm.…”

On the use of multilayer Laue lenses with X-ray free electron lasers

“…To obtain high efficiency over the entire lens, it is critical that each layer is also correctly tilted to satisfy Bragg's law in each position along the lens. We developed a simple method to achieve this wedging of the layers by affixing a straight-edged mask at a pre-selected height above the substrate and spinning it together with the substrate during the deposition process 24 . A desired thickness profile is then formed in the penumbra of the mask.…”

“…Consequently, we could prepare lenses optimized for the same energy but of two different focal lengths during the same deposition run. After the deposition was finished the multilayer thickness profiles were determined by making "depth soundings" of the structure at a number of positions using a focused ion beam (FIB) 24 . This was used to identify the positions where layer spacing and layer tilt curvature matched the design 25 .…”