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Abstract
We developed a full-field microscope with twin Wolter mirrors for soft X-ray free-electron lasers. The Wolter mirrors for a condenser and an objective were fabricated using an electroforming process with a precisely figured master mandrel. In the imaging system constructed at SACLA BL1, sub-micrometer spatial resolution was achieved at wavelengths of 10.3 and 3.4 nm. Single-shot bright-field images were acquired with a maximum illumination intensity of 7×1014 W/cm2.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
X-ray microscopy allows imaging with high spatial resolution, deep penetration depth, and imaging contrast sensitive to elements and chemical states. X-ray free-electron lasers (XFELs) produce high-intensity X-ray pulses with ultrashort pulse durations [1–5]. Typical pulse energies of XFEL sources are in the range of micro- to millijoules, which enables single-shot acquisition of high-resolution images. X-ray-induced radiation damage may cause image blur, but it can be negligible for an ultrashort pulse with a femtosecond duration [6]. Many interesting targets, such as plasmas [7,8], magnetic domain structures [9], and various biological samples [10,11], have been visualized with microscopic techniques using XFEL pulses.
Lensless imaging methods such as coherent diffraction imaging [6] and holography [7] are utilized for XFELs sources. However, full-field microscopy has not been performed except for extreme ultraviolet microscopy for plasma diagnostics [12]. Full-field microscopy is attractive because of its simple experimental procedure and versatility in terms of sample preparation. Various types of full-field X-ray microscopes for synchrotron and lab-scale X-ray sources have been developed, including those with compound refractive lenses [13], Fresnel zone plates [14,15], multilayer Laue lenses [16], Schwarzschild mirrors [17], and Wolter mirrors [18,19].
A Wolter mirror is an imaging optic that reflects an X-ray beam in the grazing incidence geometry with a wide spectral range, an achromatic property, and a high threshold for X-ray radiation damage [20]. A Wolter mirror for a finite microscope system, which is composed of ellipsoidal and hyperboloidal reflection surfaces, forms an image with a large field of view because the Abbe sine condition is satisfied through the reflections on the two mirror surfaces. The fabrication of a Wolter mirror for soft X-ray microscopy is difficult due to the single-nanometer accuracy required for the inner surface profile of its hollow-shaped body. We recently established a precise fabrication process for such hollow-shaped mirrors based on a replication method [21]. This process was utilized for developing an ellipsoidal mirror, which showed the capability of sub-micrometer focusing at a soft X-ray beam line (BL1) of the SACLA XFEL facility [20] [22].
In the present work, we applied this process to fabricate twin Wolter mirrors to construct a soft X-ray full-field microscope at BL1 of SACLA. Bright-field images with high illumination intensity are formed with the objective Wolter mirror (OWM). The condenser Wolter mirror (CWM) is used to match the divergence angle of the illumination beam to the aperture angle of the OWM. The large divergence angle of the illumination beam of ∼40 mrad enables image formation with collecting wide-angle diffraction from samples and decreases the radiation dose on the OWM to below the damage threshold [20].
2. Mirror fabrication
As schematically illustrated in Fig. 1(a), we fabricated two Wolter mirrors by a replication process using a single master mandrel with their inverse shape [21]. The two mirrors, which had the same shape and whose design parameters are listed in Table 1, were used as a condenser and an objective.
Fig. 1. Fabrication process of the twin Wolter mirrors. (a) Schematic illustration of fabrication process. (b) Residual figure errors of the mandrel in the longitudinal direction.
Table 1. Design parameters of the twin Wolter mirrors. The Wolter mirrors with the same design parameters were used for the condenser and the objective.
The figure error of the Wolter mirrors limits the imaging performance of the microscope. To achieve diffraction-limited imaging with a phase error of below λ/14 at a wavelength of λ = 10 nm, an acceptable peak-to-valley height error of the OWM is estimated to be ∼4 nm by considering its average grazing incidence angle of a few degrees. Additionally, figure accuracy of a few nanometers is required for the CWM and OWM to suppress unwanted speckles under coherent illumination conditions, especially in a high-spatial-frequency region above ∼3 mm−1 [23].
The master mandrel, made of fused silica, was roughly formed by grinding and polishing, and a part of its surface, which was an area corresponding to the mirror surface that reflects the beam with a numerical aperture (NA) of 0.024 × 0.024, was finished using a numerically controlled figure correction process with organic abrasives. Figure 1(b) shows longitudinal figure error profiles, which were measured with a surface profiler [21], of the hyperboloidal and ellipsoidal sections of the mandrel in the area where the figure correction process was applied. The root-mean-square (RMS) figure errors of the hyperboloidal and ellipsoidal sections along the longitudinal direction were 0.9 and 0.8 nm, respectively. The mirrors were fabricated using a nickel electroforming process with the master mandrel. The typical replication accuracy of the electroforming process is ∼60 nm in terms of peak-to-valley deviation [24]. Therefore, the figure errors of the Wolter mirrors were primarily determined by the replication accuracy. The RMS surface roughness measured using a microscopic phase-shift interferometer in an area of 190 µm × 140 µm on the mandrel was 0.35 nm and the difference between the RMS roughness of the master surface and that of the replicated mirror surface is typically within 0.2 nm [21].
3. Experiment
Figure 2 shows the schematic configuration of the microscope. The illumination system is composed of the Kirkpatrick-Baez (K-B) mirrors, which is a built-in focusing device of SACLA BL1 [22], and the CWM positioned downstream of the K-B mirrors. Bright-field images of samples are formed with the OWM. The magnification factor of the OWM is 240, and the pixel size of a CCD image sensor is 13 µm × 13 µm with a pixel number of 1024 × 1024. The nominal pixel resolution is thus 13/240 × 2 = 0.108 µm. The field of view is 13/240 × 1024 = 55 µm. The distance from a sample, through the OWM, to the detector is 6.25 m, as specified in the optical design of the twin Wolter mirrors, while the distance from the focal point of the K-B mirrors, through the CWM, to a sample is as short as 0.50 m due to the limited space for the optical setup. A quadrant slit is placed upstream of the K-B mirrors to limit the incident beam size. An aperture is placed downstream of the OWM to limit its effective NA.
Fig. 2. Schematic illustration of the soft X-ray transmission microscope. The optical system was constructed at SACLA BL1. Samples are illuminated using an illumination system composed of K-B mirrors and the condenser Wolter mirror. The objective Wolter mirror forms bright-field images on a CCD detector. A quadrant slit is placed upstream of the K-B mirrors to limit the incident beam size. An aperture is placed downstream of the objective Wolter mirror to limit its effective numerical aperture.
Figure 3(a) shows an image of a test target captured with a single-shot exposure at λ = 10.3 nm. The test target was a binary structure figured by a focused ion beam on a membrane composed of a 200-nm-thick silicon nitride layer and a 300-nm-thick nickel deposited layer. A clear image of the letters “SACLA” was acquired.
Fig. 3. (a) Single-shot transmission image of a test target with the letters “SACLA”. (b) Illumination pattern obtained by displacing the condenser focal spot from the sample plane. The quadrant slit upstream the condenser Wolter mirror was opened and the aperture downstream of the objective Wolter mirror was removed when this image was captured. The newton rings in the illumination field were caused by a dust on a mirror surface.
The intensity distribution of the illumination field was evaluated by capturing images without placing a sample. Figure 3(b) shows an image of the illumination field at λ = 10.3 nm. A large field of illumination of 55 µm (vertical) × 49 µm (horizontal) was acquired by displacing the focal spot formed by the CWM from the sample plane (i.e., the focal plane of the OWM) by ∼1 mm. The intensity fluctuation of the illumination field, which was calculated as the standard deviation of the intensity distribution divided by the average intensity in the central 40 µm × 40 µm region, was reduced to 0.10. The vertical and horizontal profiles of the illumination field were obtained from a projection of the surfaces of the Wolter mirrors in their circumferential and longitudinal directions, respectively. The intensity fluctuation is relatively large along the horizontal direction of the image because the figure error in the longitudinal direction of the Wolter mirrors causes a wavefront error with a high spatial frequency, which produces strong interference fringes, due to the grazing incidence reflection.
The spatial resolution of the microscope was evaluated by observation of a test target. The aperture downstream of the OWM was positioned to block beams reflected from a peripheral region of the OWM with a significant figure error to achieve the best spatial resolution. The quadrant slit located upstream of the K-B mirrors was positioned to block the beam that did not pass the aperture downstream of the OWM. The effective NA of the OWM estimated from a diffraction-limited pinhole image at λ = 31 nm was 0.023 (vertical) × 0.013 (horizontal). The area of the OWM reflecting the beam was smaller than the precisely figured area of the mandrel, which corresponds to the mirror surface reflecting the beam with a NA of 0.024 × 0.024. The decrease of the effective area in the longitudinal direction can be attributed to a deformation caused in the electroforming process. The diffraction-limited spatial resolution at the center of the view, calculated as λ/NA, was 450 nm (vertical) × 790 nm (horizontal) and 150 nm (vertical) × 260 nm (horizontal) at λ = 10.3 and 3.4 nm, respectively.
Figure 4(a) shows an image of a test target captured with a scanning ion microscope. The test target had line and space patterns with different intervals. In the observation with the soft X-ray microscope, the NA of the CWM was set to 0.022 × 0.022, and the finest part of the test pattern, with which the spatial resolution was evaluated, was place at the center of the view, where the diffraction limited spatial resolution is estimated to be λ/NA. Figures 4(b) and 4(c) show the soft X-ray images acquired at λ = 10.3 and 3.4 nm, respectively. These images were captured with the same optical geometry, confirming the achromatic property of the microscope. The image acquired at λ = 10.3 nm was captured with a single-shot exposure, and the image acquired with its third harmonics at λ = 3.4 nm was captured by accumulating images over 40-shot exposures. Intensity fluctuations in the bright part of the images originates from the highly coherent nature of the illumination by the XFEL. Figure 4(d) shows the vertical and horizontal intensity profiles of the soft X-ray images. We were able to resolve an interval of the line and space patterns of as small as 500 nm (vertical) × 900 nm (horizontal) and 250 nm (vertical) × 500 nm (horizontal) at λ = 10.3 and 3.4 nm, respectively, which correspond to the diffraction-limited resolutions in the two cases.
Fig. 4. Images of test pattern. The intervals of line and space pattern are 3300 nm, 3000 nm, 2500 nm, 2100 nm, 1700nm, 1300 nm, 900 nm, 500 nm, and 250 nm in the vertical direction, and 3000 nm, 2500 nm, 2100 nm, 1700 nm, 1300 nm, 900 nm, 500 nm, and 250 nm in the horizontal direction. (a) Image captured with scanning ion microscope. (b) Soft X-ray image at λ = 10.3 nm. (c) Soft X-ray image at λ = 3.4 nm. (d) Cross-sectional intensity profiles of soft X-ray images. Profiles between the lines A - A’ and B - B’ shown in (b) and (c) are plotted.
The illumination intensity of the microscope increases when a sample is illuminated with a tighter focal spot. We captured an image of the focal spot formed by the CWM to achieve a high illumination intensity. The incident pulse energy of 40 µJ was reduced to 1 µJ by the quadrant slit, which blocked the beam that did not pass the aperture downstream of the OWM, and then reduced to 0.6 µJ by the reflections on the CWM, assuming that the reflectivity of the CWM is 76% (i.e., a throughput of 0.76 × 0.76 = 0.58) [25]. To avoid saturation of the CCD detector, the signal intensity on the CCD detector was attenuated by a zirconium filter downstream of the OWM. Figure 5 shows an image of the focal spot formed by the CWM. The widths of the focal spot, defined as the full width at half maximum, were 420 nm (vertical) × 2060 nm (horizontal), which correspond to a peak illumination intensity of 7×1014 W/cm2 with a pulse duration of 100 fs [26] and a pulse energy on the focal spot of 0.6 µJ. The spot size was large in the horizontal direction because of an astigmatic aberration of the CWM. This was caused by a discrepancy of the distance of 0.50 m between the focus of the K-B mirrors and the focus of the CWM from the designed distance of 6.25 m.
4. Conclusion
We demonstrated soft X-ray full-field microscopy with twin Wolter mirrors at BL1 of SACLA. The precisely fabricated Wolter mirrors enabled us to acquire single-shot bright-field images with sub-micrometer spatial resolution. The maximum illumination intensity was 7×1014 W/cm2 at λ = 10.3 nm (photon energy, hν = 120 eV), which is high enough to induce saturable absorption of Al and Si [20,27]. The microscope will be available for imaging-before-destruction and direct visualization of non-linear optical effects such as saturable absorption [20,27–29] and harmonic generation [30,31].
Funding
Japan Society for the Promotion of Science (15H02041, 16J10126, 19H00736).
Acknowledgements
The authors thank the SACLA engineering team for their help during the beam time. S. E. thanks the SACLA Research Support Program for Graduate Students.
Disclosures
The authors declare no conflicts of interest.
References
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