Water window ptychographic imaging with characterized coherent X-rays

Abstract

A ptychographical coherent diffractive imaging experiment in the water window with focused soft X-rays at 500 eV is reported. An X-ray beam with high degree of coherence was selected for ptychography at the P04 beamline of PETRA III synchrotron radiation source. The beam coherence was measured with the newly developed non-redundant array method, and a coherence length of 4.1 µm and global degree of coherence of 35% at 100 µm exit slit opening in the vertical direction were determined. A pinhole, 2.6 µm in size, selected the coherent part of the beam that was used to obtain ptychographic reconstruction results of a lithographically manufactured test sample and a fossil diatom. The achieved resolution was 53 nm for the test sample and was only limited by the size of the detector. The diatom was imaged at a resolution better than 90 nm.

Keywords: diatom; non-redundant array; ptychography; water window imaging.

Figure 1

(a) Soft X-ray beamline layout. The X-ray scattering vacuum chamber HORST in the coherence measurement setup (b) and the ptychography setup (c).

Figure 2

(a) Diffraction pattern from a vertically oriented NRA measured at 500 eV and = 100 µm. The white rectangle indicates the area used for the analysis. (b) The Fourier transform of the NRA diffraction pattern. Both images are displayed on a logarithmic scale. (Inset) Optical microscope image of the NRA and its aperture separations shown in micrometers.

Figure 3

Results of coherence measurement in the vertical direction for three exit slit openings at 500 eV. Black dots indicate measured data and dashed lines represent Gaussian fits. (a)–(c) Intensity profiles measured with scans of a 1.5 µm pinhole. (d)–(f) Modulus of the CCF . The gray shaded areas in (b) and (e) indicate the coherent part of the beam selected by the beam-defining 2.6 µm pinhole. The r.m.s. values σ of the beam size obtained from Gaussian fits, the coherence length , as well as the values of the global degree of coherence ζ determined from equation (4) are also shown. For the Gaussian fits we used the data points up to 9 µm in (e) and up to 7 µm in (f).

Figure 4

Diffraction images averaged over all positions i for the Siemens star (a, b) and for the diatom (c, d). Measured (a, c) and reconstructed diffraction patterns (b, d) are separated by the white line. The diffraction patterns are displayed on a logarithmic scale.

Figure 5

Ptychographic probe function reconstruction from the Siemens star (a–c) and the diatom (d–f). A plane wave simulation for propagation of the wavefield from a 2.6 µm pinhole aperture is shown in (g, h, i). Amplitude distribution at the position of the pinhole (a, d, g); distribution of the wavefield amplitude downstream of the pinhole (b, e, h); amplitude distribution of the probe amplitude at the position of the sample (c, f, i) [Siemens star (c), and diatom (f)]. The simulated amplitude distribution (i) is shown at the same z position as for (f). Dashed white lines in (b, e) indicate the position of the sample relative to the pinhole at z = 0. White circles in (c, f, i) indicate the pinhole size.

Figure 6

Ptychographic reconstruction of the Siemens star test pattern. (a) Amplitude and (b) phase image. (c) Contrast C between high and low transmission as a function of spatial frequency from the angular scans denoted by red lines in the amplitude image.

Figure 7

Ptychographic reconstruction of the amplitude (a) and the phase (b) of the fossil diatom. (c) Integrated SiO₂ mass along the depth of the diatom (see text for further details) and (d) FWHM values of two error function fits along the black lines indicated in the phase reconstruction in (b).