Toward Hard X-ray Transmission Microscopy at the ANATOMIX Beamline of Synchrotron SOLEIL

Abstract

Among the techniques that are being implemented on the imaging beamline ANATOMIX at Synchrotron SOLEIL [1,2], hard X-ray full-field microscopy is of paramount importance. The transmission X-ray microscope (TXM) on ANATOMIX, based on diffractive optics, is designed for photon energies around three working values of 6.6, 10 and 18 keV. It aims at a spatial resolution down to 100 nm or less, corresponding to pixel sizes down to 30 nm.The diffractive optics for the TXM-beam shapers (condensers), objective zone plates and phase masks for Zernike phase contrast-are manufactured at the Laboratory for Micro-and Nanotechnology of the Paul Scherrer Institut in Villigen, Switzerland. A first set of beam shapers and objectives has been produced, and tests in absorption contrast were recently conducted, using a temporary mechanical setup.These test measurements were carried out with a set of optics designed for 10 keV: a beam shaper [3] illuminating a field of view of 40 × 40 µm², with a physical aperture of 2.5 mm diameter and smallest zones of 50 nm width, and objective zone plates of the same outermost zone width and a diameter of 100 µm, resulting in a focal length of 40 mm at 10 keV. The diffractive X-ray optics were made of iridium using the frequency-doubling method [4], with structure heights of about 1 µm for the beam shaper and 1.6 µm for the objective zone plates, which were patterned on both sides of the membrane [5]. Other elements included a central stop placed a few cm upstream of the beam shaper, an orderselecting aperture (OSA, pinhole with 0.3 mm diameter) 70 mm from the sample and a diffuser (sheets of paper) placed upstream of the OSA. Two indirect, lens-coupled detectors were used: one for alignment, placed just downstream of the objective zone plate and consisting of a 22-µm-thick lutetium aluminum garnet (LuAG) scintillator, a 10× magnifying microscope optics and a CMOS-based digital camera with 6.5-µm pixels (Hamamatsu Orca Flash 4 V2), resulting in an effective pixel size of 0.65 µm. The other detector, used to record the actual TXM micrographs, was placed 4.5 m downstream of the objective zone plate and composed of a 100-µm-thick scintillator, photo-camera optics with an overall magnification of 3.4 and a CCD camera (PCO 4000, 9-µm pixels) using 2×2 pixel binning, resulting in an effective detector pixel size of 5.3 µm. Including the X-ray magnification factor of 110, this resulted in a pixel size of 49 nm at sample level. The alignment detector and sample stage were mounted on standard translation stages (Huber Diffraktionstechnik, Rimsting, Germany), the TXM optics on piezo-based translation stages (SmarAct, Oldenburg, Germany). Figure 1 shows a TXM micrograph obtained with this setup on a resolution test chart (model XRESO-50, NTT-AT, Japan; tantalum structures of 500 nm height). The smallest structures in the center of this reference sample of "Siemens-star" type, resolved in the image (see inset of Figure 1), have a period of 100 nm and are thus just at the Nyquist limit of resolution for the p...

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