Wavefront Sensing for Evaluation of Extreme Ultraviolet Microscopy

Abstract

Wavefront analysis is a fast and reliable technique for the alignment and characterization of optics in the visible, but also in the extreme ultraviolet (EUV) and X-ray regions. However, the technique poses a number of challenges when used for optical systems with numerical apertures (NA) > 0.1. A high-numerical-aperture Hartmann wavefront sensor was employed at the free electron laser FLASH for the characterization of a Schwarzschild objective. These are widely used in EUV to achieve very small foci, particularly for photolithography. For this purpose, Schwarzschild objectives require highly precise alignment. The phase measurements acquired with the wavefront sensor were analyzed employing two different methods, namely, the classical calculation of centroid positions and Fourier demodulation. Results from both approaches agree in terms of wavefront maps with negligible degree of discrepancy.

Soft X-ray wavefront sensing at an ellipsoidal mirror shell

“…In consequence of a good adjustment, the figure error of the optic can be isolated, and the wavefront is expected to be determined at an enhanced accuracy well below the Mare ´chal criterion (Probst et al, 2020a). To simplify the image pre-processing, a clear, unique criterion for definition of the optical axis and for centring of the CCD frames to the exit pupil must be specified (Ruiz-Lopez et al, 2020). To shorten the integration time of 40 s per image, the source flux may be enhanced and stray light should be lowered, preserving a high signal-to-noise ratio in near photon-limited detection.…”

Special Issue “EUV and X-ray Wavefront Sensing”

“…In the seventh contribution [ 7 ], the wavefront is analyzed to perform otherwise very challenging, if not impossible, metrology on capillary optics. The Special Issue closes with an article dedicated to small-spot optimization, in the presence of a relatively large numerical aperture [ 8 ]. The work is performed on a Free-Electron Laser and is aimed to the optimization of the focal spot for microscopy application.…”

EUV and Hard X-ray Hartmann Wavefront Sensing for Optical Metrology, Alignment and Phase Imaging

“…Wavefront aberrations can be described by Zernike or Legendre polynomials. More details about the data analysis and the wavefront reconstruction are given in [ 13 ].…”

“…It operates in vacuum, and therefore, can be positioned close to the focal spot, and covers an energy range from 28 to 124 eV. It has been tested on harmonics in solid with preliminary results showing the strong link between the interaction regime and the WF [ 13 , 21 ]. In the hard X-ray range, a WFS based on indirect detection was tested, providing WF measurement over a broad 5–25 keV energy range (HASO HXR–Imagine Optic).…”

“…Such a wavefront monitoring alignment was performed at other wavelengths, for example at 32 nm at FERMI@Elettra [ 22 , 23 , 24 ] or at 10 keV at APS using the hard X-ray wavefront sensor (HASO HXR) [ 22 ], as well as with other kind of focalization optics: elliptic mirrors [ 25 ], Schwarzschild [ 13 ] or Wolter-like telescopes [ 26 , 27 ].…”