Efficient focusing of 8 keV X-rays with multilayer Fresnel zone plates fabricated by atomic layer deposition and focused ion beam milling

Abstract

Fresnel zone plates (FZPs) recently showed significant improvement by focusing soft X-rays down to ∼10 nm. In contrast to soft X-rays, generally a very high aspect ratio FZP is needed for efficient focusing of hard X-rays. Therefore, FZPs had limited success in the hard X-ray range owing to difficulties of manufacturing high-aspect-ratio zone plates using conventional techniques. Here, employing a method of fabrication based on atomic layer deposition (ALD) and focused ion beam (FIB) milling, FZPs with very high aspect ratios were prepared. Such multilayer FZPs with outermost zone widths of 10 and 35 nm and aspect ratios of up to 243 were tested for their focusing properties at 8 keV and shown to focus hard X-rays efficiently. This success was enabled by the outstanding layer quality thanks to ALD.Viathe use of FIB for slicing the multilayer structures, desired aspect ratios could be obtained by precisely controlling the thickness. Experimental diffraction efficiencies of multilayer FZPs fabricatedviathis combination reached up to 15.58% at 8 keV. In addition, scanning transmission X-ray microscopy experiments at 1.5 keV were carried out using one of the multilayer FZPs and resolved a 60 nm feature size. Finally, the prospective of different material combinations with various outermost zone widths at 8 and 17 keV is discussed in the light of the coupled wave theory and the thin-grating approximation. Al2O3/Ir is outlined as a promising future material candidate for extremely high resolution with a theoretical efficiency of more than 20% for as small an outermost zone width as 10 nm at 17 keV.

Multilayer Fresnel zone plates for high energy radiation resolve 21 nm features at 12 keV

“…The alignment was carried out by tilting the SXR-FZP in pitch and yaw until a full ring was observed in the diffraction pattern recorded on the CCD. This alignment process is similar to that described earlier [40]. To determine the resolution, a Siemens star (X30-30-2 Xradia, USA) with specified smallest features of 30 nm, as well as two 500 and 200 nm thick FIB lamellae sliced from a GaAs/Al 0.7 Ga 0.3 As multilayer sample (L200, BAM, Germany) [41] with certified layer thicknesses, were positioned at the 1st order focal plane and raster scanned over the focus.…”

“…6(a), demonstrates that the innermost features with smallest line widths of 30 nm (on average, 60 nm period) were clearly resolved regardless of the direction. Previous reports of similar ML-FZPs at SXR range demonstrated lower resolving powers [24,40] and apparently astigmatic images [15]. These were attributed to one or a combination of the following facts: the absence of a high quality test object [24], a sub-optimal FZP thickness and the lack of a beamstop [40] and most importantly the absence of any means of tilt correction [15,24,40].…”

“…This places the technique among the serious potential candidates for ultrahigh resolution (<10 nm) FZP fabrication methods for both soft and hard X-rays. Future perspectives include the optimization of the material selection [40], increased active region and the reduction of the Δr. Demonstration of the best HXR and SXR resolutions, ideally by direct imaging in order to unleash the full potential of these optics remains the ultimate goal.…”

“…In the soft X-ray range tests at the MAXYMUS STXM, the alignment of the SXR-FZP with the optical axis was carried out using a 2-axis tilt stage, improving the procedure described earlier [40] and now allowing for the highest reported resolution demonstrated via direct imaging, for a ML-FZP. In order to align the SXR-FZP, the far-field diffraction pattern was observed on a CCD camera, while tilting the SXR-FZP about x-and y-axes (pitch and yaw) until the diffraction pattern registered at the CCD was circularly symmetric.…”

“…ML-FZPs can be fabricated with very high aspect ratios (A r ) that are required for efficient (up to >15% [40]) focusing of HXR. Despite the proposed improvements in the EBL for increased A r [14,43], aspect ratios in the order of 10 2 are achievable, so far, only for ML-FZPs [15].…”