Nanofocusing of X-ray free-electron laser using wavefront-corrected multilayer focusing mirrors

Abstract

A method of fabricating multilayer focusing mirrors that can focus X-rays down to 10 nm or less was established in this study. The wavefront aberration induced by multilayer Kirkpatrick-Baez mirror optics was measured using a single grating interferometer at a photon energy of 9.1 keV at SPring-8 Angstrom Compact Free Electron Laser (SACLA), and the mirror shape was then directly corrected by employing a differential deposition method. The accuracies of these processes were carefully investigated, considering the accuracy required for diffraction-limited focusing. The wavefront produced by the corrected multilayer focusing mirrors was characterized again in the same manner, revealing that the root mean square of the wavefront aberration was improved from 2.7 (3.3) rad to 0.52 (0.82) rad in the vertical (horizontal) direction. A wave-optical simulator indicated that these wavefront-corrected multilayer focusing mirrors are capable of achieving sub-10-nm X-ray focusing.

Figure 1

Schematic of the proposed scheme consisting of wavefront measurement and direct shape correction of the focusing mirrors.

Figure 2

Geometrical arrangement of the two-stage focusing system. Length unit: mm.

Figure 3

Wavefront measurements obtained using a single-grating interferometer. (a) Typical recorded self-image and (b) its cross-sectional profile. The white arrows in (a) indicated the uncoated area on the mirrors, used as position references in order to convert the wavefront data to shape error data. (c) Systematic errors induced by the camera. (d) Wavefront aberrations after introducing known comatic aberration by inclining the vertical focusing mirror by 10 μrad. The red and blue lines are the experimentally obtained results and those calculated using the Fresnel–Kirchhoff integral. The black line is the difference between the experimental and calculated results. (e,f) Wavefront aberrations obtained before and after direct shape correction. The term “normalized position on camera” in the graphs means the normalized position on the camera with respect to the effective bright field width, where the dimensions of the bright field at a camera length of 0.82 m were 16.4 × 19.4 mm² (H × V).

Figure 4

Differential deposition method. (a) Magnetron sputtering deposition apparatus, which was also employed for multilayer film deposition on the mirror. (b) Schematic of the inside of the chamber. A test substrate is exposed to sputtered Pt atoms through the slit with 2 mm clearance while scanning the substrate. (c) Measured stationary Pt deposition spot (exposure: 20 min). (d) Deposition thickness distribution for the horizontal focusing mirror. The red line is the target deposition thickness distribution calculated based on the wavefront aberration. The black line is the deposition thickness expected based on the actual dwell time distribution and the stationary deposition spot. (e) Comparison of the measured and target deposition thicknesses on test pieces. The target is the same as that used to obtain (d), and the horizontal coordinate corresponds to the horizontal coordinate in (d).

Figure 5

Focused beam intensity before and after correction, calculated using Fresnel–Kirchhoff integrals based on the corresponding wavefront aberrations. In the calculation, reflectivity distribution on the mirrors, calculated from the multilayer design and its performance evaluated in advance, was considered. (a) Beam intensity at the focus. Calculated area = 500 nm (x) × 500 nm (z). (b) Beam caustics in the vertical direction along the optical axis (y). Calculated area = 20 μm (y) × 500 nm (z). (c) Cross-section of the beam at the focus. The vertical axes were normalized by the maximum values of the ideal profiles for clear comparison of the relative peak heights.

Figure 6

Measured beam intensity profiles at the focus. The profiling was performed twice (Exp1 and Exp2 in the graphs).