Dirk Berger

Performance of a polycapillary halflens as focussing and collecting optic—a comparison

Polycapillary halflenses are widely used to focus X-ray radiation onto a small spot. Additionally they can reduce the field of view of a semiconductor detector when placed in front of one. In 3D micro X-ray fluorescence spectroscopy (3D Micro-XRF) with synchrotron radiation, two polycapillary halflenses are used in a confocal geometry. Up until now, characterization measurements in the focal plane have only been performed in the case of the lens focusing parallel radiation. Assumptions have been made, that in the other case, when isotropic radiation from a spot source is transported to a detector, the acceptance distribution in the focal plane is also Gaussian. We performed measurements with an electron beam as well as a proton beam which confirm this assumption. In addition, a comparison between measurements in collecting and focusing mode show differences in spot size and transmission. These differences exemplify the fact that there is not one global spot size or transmission function of a polycapillary halflens. Illumination and divergence effects can alter both characteristic lens parameters.

Automatable sample fabrication process for pump-probe X-ray holographic imaging

Soft X-ray holography is a recently developed imaging technique with sub-50 nm spatial resolution. Key advantages of this technique are magnetic and elemental sensitivity, compatibility with imaging at free electron laser facilities, and immunity to in-situ sample excitations and sample drift, which enables the reliable detection of relative changes between two images with a precision of a few nanometers. In X-ray holography, the main part of the experimental setup is integrated in the sample, which consequently requires a large number of fabrication steps. Here we present a generic design and an automatable fabrication process for samples suitable, and optimized for, efficient high resolution X-ray holographic dynamic imaging. The high efficiency of the design facilitates the acquisition of magnetic images in a few minutes and makes fully automatic image reconstruction possible.

A general approach to obtain soft x-ray transparency for thin films grown on bulk substrates

We present a general approach to thin bulk samples to transparency for experiments in the soft x-ray and extreme ultraviolet spectral range. The method relies on mechanical grinding followed by focused-ion-beam milling. It results in a uniformly thin area of high surface quality, suitable for nanoscale imaging in transmission. In a proof-of-principle experiment, nanoscale magnetic bits on a commercial hard drive glass disk are imaged with a spatial resolution below 30 nm by soft x-ray spectro-holography. Furthermore, we demonstrate imaging of a lithographically patterned test object via absorption contrast. Our approach is suitable for both amorphous and crystalline substrates and has been tested for a variety of common epitaxy growth substrates. Lateral thinning areas in excess of 100 μm2 and a remaining substrate thickness as thin as 150 nm are easily achievable. Our approach allows preserving a previously grown thin film, and from nanofocus electron diffraction, we find no evidence for morphological changes induced by the process, in agreement with numerical simulations of the ion implantation depth distributon. We expect our method to be widely applicable and especially useful for nanoscale imaging of epitaxial thin films.

Manufacturing and application of a 2 µm dark field aperture in TEM

For an entire TEM characterization of many materials, it is necessary to achieve selected area electron diffraction (SAED) patterns of smallest regions with assigning the reflexions to their origins in the real image. In a previous work we showed that we were able to successfully reduce the field of view by a customized SAED aperture to a 15 nm range [1]. Though it gives us very local information about the samples structure, in daily work it is not always satisfying, since the real image is as important to understand the correlation between certain Bragg spots and the real structure, e.g. given by a series of dark field images. Especially for closely neighboured reflexions, commercial objective apertures are too large and do not allow the separated selection of these spots. Since our conventional TECNAI is equipped with the standard aperture-stripe, we are limited to a smallest size of 10 µm (12 µm in reality) which delivers a field of view of ca. 7.5 mrad inside the back focal plane. The smallest commercially available aperture has a diameter of 5 µm. Figure 1a displays a section of a polycrystalline fcc diffraction pattern. The marked large circle represents the standard 10 µm objective aperture, while the smaller one represents our custom made aperture with a diameter of 2 µm or 1.5 mrad inside the back focal plane. This example shows, with standard apertures it is impossible to select the (311) reflexions without overlap of their neighboured (220) or (222). Therefore, we reworked the present PtIr aperture-stripe by focused ion beam (FIB) in two steps [2]. At first an existing hole of the stripe – there are 2 rows of holes, one provides smaller and the other one larger diameters, which are seldom used – was closed by ion beam-induced Pt-deposition. As a second step, a centred opening was sputtered into that layer by using of circular masks up to 2-µm in diameter. To minimize a conical shape of the opening, at low ion beam current (280 pA) with a high aspect ratio is used and the hole is successively milled from both sides. If the Pt-deposition is too thin, there is a high risk that scattered electrons in the TEM will not be entirely blocked by the new aperture and create artefacts and distortions in the images. Therefore, it has a thickness of around 6.5 µm. First investigations with TEM proved that the deposited layer is not transparent for 200 kV electrons anymore and thermally stable as well. Figure 1b-d shows an application of the new objective aperture on a multi-twinned system of polycrystalline diamonds. Although the twinned areas are in the range of 5–10 nm it becomes possible to correlate the chosen diffraction spots with their origins in the real image. The adjustment of the new objective aperture has to be done very carefully, it can easily outshine the observation screen or the CCD camera, so one can easily lose the designated position, but the selection of certain diffraction spots requires a very accurate positioning. Other than at larger apertures where slight drifts are not critical because of the visibility of the selected area and therefore easier readjustments, slightest drifts must be avoided. In conclusion, the new 2-µm objective aperture can be very helpful for the understanding and structural characterization of samples according their crystallinity, their growth behaviour or even defect studies. Keywords: objective aperture; focused ion beam; dark field

Characterization of the focusing properties of polycapillary X-ray lenses in the scanning electron microscope

Polycapillary X-ray lenses are composed of several thousands of hollow glass capillaries which transport X-rays by total reflection [1]. They have the ability to focus X-rays with spot diameters of only several tens of µm. One recent application is 3D micro X-ray fluorescence spectroscopy (3D Micro-XRF), in which two X-ray lenses are used in a confocal setup, one in the excitation channel and one in the detection channel. The foci of both lenses overlap and form a probing volume which can be moved through a sample. Knowing the exact characteristics of the lenses, quantitative element analysis in this well defined micro-volume is feasible.

Beam quality of self-starting SBS resonators for excimer lasers

The generation of near Gaussian beams with small beam propagation factors M2 is investigated for excimer laser applications requiring small focal spots. A considerable transverse beam quality improvement can be achieved not only with small internal apertures but also be operating the excimer laser with a phase-conjugating SBS-mirror. No additional start resonator was used. SBS-cells filled with a liquid mixture of dimethylbutane/n-pentane exhibit high optical breakdown intensities.