The Austrian Scanning Transmission Electron Microscope –
The Story Behind
Aberration correction in electron microscopy
Being able to see and analyse materials at atomic resolution, has been a strong motivation for improving the electron microscope. However, the resolution of the conventional transmission electron microscope (TEM) was limited to around 0.2 nanometre, which is slightly larger than the diameter of atoms. This limitation comes from the spherical aberrations of the magnetic lenses, which lead to a blurring of images. Although this deteriorating effect was already recognised 80 years ago, it was only in the late 1990s that the first spherical aberration corrector improved the resolution of the TEM. The following development of commercial aberration-corrected TEMs was a revolution for microscopy research and helped to solve many open questions in physics, chemistry and materials and biological sciences.
The challenge in Austria
Immediately after their commercial introduction in 2005 aberration corrected TEMs became the standard in high resolution electron microscopy. However, due to the high costs there was no realistic chance to establish such an advanced instrument in Austria. Finally, the Association for Electron Microscopy and Fine Structure Research succeeded in 2011 via a complicated project mixture, which was supported by several funding organisations (FFG, ACR & BMWFW, Land Steiermark, WKO-STMK) and the TU Graz. This breakthrough allowed us to build one of the best aberration corrected microscopes in the world – delivered by FEI Company (Eindhoven, The Netherlands) – the Titan cubed 60–300 (Austrian Scanning Transmission Electron Microscope = ASTEM).
The operation of the ASTEM
In order to exploit all the benefits of the new instrument we had to improve the quality of our specimen preparation and to deal with theoretical modelling of the experimental results. Firstly, it was necessary to introduce a new method for low-energy ion milling (Nanomill, Fischione). Secondly, the atomically resolved STEM-images had to be interpreted with the help of advanced simulation tools such as QSTEM (Christian Koch, Berlin, Germany) or µSTEM (Lesley Allen, Melbourne, Australia); collaborations in the field of first principles calculations have been started (R.C. Picu, Troy, USA).
The impact of the ASTEM
With the ASTEM the institute has become the leading research institution in the field of advanced materials microscopy in Austria. The ASTEM plays a major role in the research of the FoE Advanced Materials Science of the TU Graz and increasingly also for the NAWI faculty of the University of Graz and TU Graz. Indeed, it developed into a national resource with intensive collaborations with the Universities of Graz, Leoben, Linz and Innsbruck. Its impact on research collaborations with Austrian companies is manifold, particularly for semiconductor, ceramic and metal industries. Finally, it enabled the access to the leading European research network on advanced electron microscopy (ESTEEM2, FP7).
The future of the ASTEM
The ASTEM is surely a stable platform for the next years to come, nevertheless upgrades and extensions are mandatory. Presently, we are working on projects for funding the introduction of new methods such as magnetic and electric imaging via differential phase contrast and radically improved detectors for imaging and spectroscopy (direct electron detection).
Quantitative analysis at atomic resolution
Elemental mapping using EELS and EDS in a STEM, a well-established method for precision elemental concentration analysis at nanometre level, was firstly demonstrated at atomic resolution in 2010. However, until 2015 these atomically resolved elemental maps have only been interpreted qualitatively, because elastic and thermal scattering of the electron probe confound quantitative analysis. Thanks to the work of Gerald Kothleitner the elemental maps could be quantified. He was one of the first to perform absolute scale comparisons between simultaneous EELS and EDS-experiments and quantum mechanical calculations thus yielding absolute volumetric concentrations at atomic resolution. Werner Grogger and his team study the influence of the four-quadrant X-ray detector (Super-X) of the ASTEM on the quantification of X-ray elemental mapping.
Aluminium EDS map of a Al/Sc structure (simulation), revealing less Al atoms at the defect side than expected.
Collaborations: Lesley Allen and co-workers, University of Melbourne, Australia; Budhika Mendis, Durham University, U.K.
Imaging of crystals at atomic resolution (aquamarine)
The mineral beryl is a precious gem exhibiting a wide range of colours ranging from the green emerald and the blue aquamarine to the yellow heliodor. These colours are connected with the complex crystal structure of beryl exhibiting crystal channels along the hexagonal axis. Although it is well known that these channels can be filled with various molecules and ions, however, it was only recently possible to image the crystal structure of beryl at atomic resolution. In 2015, Christian Gspan investigated an aquamarine with the ASTEM showing the individual atom columns (Al, Si and O) and even revealing the channel constituents such as water molecules for the first time.
STEM-HAADF image of Beryl viewed in  direction, Al-columns appear as bright dots.
Collaborations: Karl Gatterer, Institute of Physical and Theoretical Chemistry, TU Graz; Helen Chan, Lehigh University, Bethlehem, USA.
Interfaces in semiconductors
Interfaces formed between semiconductors, metals and oxides are of crucial importanceto solid state technology. A general problem is the continuous shrinkage of semiconductor devices, in which critical device dimensions already may approach the size of an atom. Therefore, it is important to understand the precise atom arrangements at interfaces and how they influence device properties. Thanks to its excellent resolution, the ASTEM is the ideal tool for studying semiconductor device interfaces. Evelin Fisslthaler took up this challenge and established a research collaboration with leading Austrian semiconductor companies with a focus on the structural and chemical characterisation of special semiconductor interfaces.
STEM-HAADF image of the interface between Si and SiGe
Collaborations: Peter Hadley, Institute of Solid State Physics, TU Graz; ams, Premstätten; AT&S, Leoben-Hinterberg; EPCOS, Deutschlandsberg; LAM Research, Villach; Infineon Technologies, Villach.
Imaging of nanostructures in three dimensions
TEM or STEM images are only two-dimensional (2D) projections of three-dimensional (3D) objects. In order to reveal the real structure, we have to acquire a tilt series of TEM or STEM images and combine them in a three dimensional reconstruction. This method is called electron tomography and has been introduced at the institute by Gerald Kothleitner and Georg Haberfehlner. Thanks to the excellent stability of the ASTEM and the very advanced spectroscopic methods included, it was possible to record the crystal structure of an individual nanoparticle atom-by-atom. Recently, our tomographic methods were extended to study the photonic environment of plasmonic nanoparticles at sub-nanometre resolution.
3D-reconstruction of core-shell precipitates in an AlMg alloy
Collaborations: Ulrich Hohenester, Joachim Krenn, Institute of Physics, University of Graz; Cecilia Poletti, Institute of Materials Science and Welding, TU Graz; Mathieu Kociak, University of Paris-Sud, France.
In situ studies of nanoparticles and moving atoms
Time-resolved experiments using the aberration-corrected STEM allow direct observations of atomic motions. Recent examples from literature stay with a qualitative description of atom motions induced by the electron beam. However, in a breakthrough experiment, Daniel Knez tracked single Pt atoms on the surface of a very thin silicon crystal. Collaborating with Paul Midgley from Cambridge, we presently analyse the surface dynamics of the ad-atoms in a quantitative way, thus revealing all the influences from absorption, reactions and diffusion. The mechanisms observed may also inspire new ideas for electron beam nanostructuring.
Tracking of a platin atom on the surface of a thin silicon crystal
Collaborations: Paul Midgley, University of Cambridge, U.K.; Wolfgang Ernst, Institute of Experimental Physics, TU Graz; Asuncion Fernandez, CSIC e Univ. Sevilla.
Atomically resolved STEM investigations of alloys and steels
The properties of structural materials strongly depend on the occurrence and distribution of secondary phases, i.e. impurities, precipitates and grain boundary phases on a microscopic scale. With the rise of aberration corrected (S)TEMs it became immediately clear that it will open radically new insights especially for aluminium cast alloys. Mihaela Albu and Angelina Orthacker were successful with atomic resolution studies of precipitates in Al-Si cast alloys and Mg-alloys. They revealed the twinning mechanism in Sr-doped Al-Si cast alloys and explained the self-stabilization of core-shell precipitates in AlMgSc-alloys. However, until recently, high resolution studies of steels have been very rarely reported in literature. Here we have been successful with one of the first atomically resolved STEM investigations of precipitates in a high chromium steel.
STEM-HAADF image of a precipitate in Al-Si alloy; bright spots are Ag- and Cu rich atom columns.
Collaborations: Gerhard Schindelbacher, Austrian Foundry Institute (ÖGI), Leoben; Christof Sommitsch, Institute of Materials Science and Welding, TU Graz; Elisabetta Gariboldi, Politecnico Milano, Italy, Jiehau Li, Institute of Casting Research, MU Leoben.
Surface plasmon polaritons
Electron energy-loss spectroscopy (EELS) has recently emerged as the ideal method for the study of plasmonic nanoparticles and nanostructures. When high energy electrons pass or penetrate through a noble metal nanoparticle, surface plasmons are excited. They can be observed only in TEM/STEM systems, which are equipped with a monochromator for the electron source and a high resolution imaging filter/spectrometer. This is the case for the ASTEM and in combination with its high brightness gun we have excellent conditions for studying surface plasmons at high spatial and high energy resolution. In the last two years we have been using the ASTEM for studies of plasmon modes of a silver thin film taper and a detailed investigation of edge mode coupling within rectangular nanoparticles.
STEM-EELS investigation of a silver nanostar with surface plasmon polaritons.
Collaborations: Ulrich Hohenester, Joachim Krenn, Institute of Physics, University of Graz; Mathieu Kociak, University of Paris-Sud, France.
The ASTEM Project:
Austrian Scanning Transmission Electron Microscope
The ASTEM is built by FEI Company, Eindhoven, The Netherlands.
It is a Titan3 TM 60-300 which consists of the following components:
- The ASTEM can be operated from 60 to 300 kV with a high-brightness electron gun (X-FEG) and an electron monochromator.
- The ASTEM provides a wide range of operating conditions and can be used for the study of all kind of materials and devices.
- The microscope is equipped with a Cs probe corrector for high resolution STEM-imaging (correction of spherical aberrations).
- A new energy-dispersive x-ray spectrometer (ChemiSTEM) for fast elemental mapping.
- A special energy-filter (Quantum filter, Gatan Inc, USA) for high energy-resolution EELS spectroscopy and EFTEM mapping.
- Electron tomography holders and software for atomic resolution 3D analysis.
Scientific and Technical Goals
The ASTEM is the only ultra-high performance in Austria. It provides a common platform for advanced material characterisation and is widely available to the materials and nanoscience community in Austria and Europe.
The microscope allows to address important scientific challenges such as:
- Direct atomic resolution of materials and devices
- Elemental analysis at atomic resolution
- Local electronic structure and bonding in interfaces and nanostructures
- Nanotomography of functional materials
- Nanometrology of semiconductor devices (inorganic and organic)
To meet these scientific challenges we have defined a number of technical issues:
- Spatial resolution of 0.07 nm for real-time structure imaging in the focused probe mode of operation (STEM)
- Engergy resolution for EELS-spectroscopy better 0.2 eV.
- High brightness electron source with 0.6 nA in a 0.1 nm probe.
Silicon-dumbbells in  STEM HAADF image
The ASTEM is jointly run by two institutions, the Graz Centre for Electron Microscopy of the Association for the Promotion of Electron Microscopy and Fine Structure Research and the Institute of Electron Microscopy and Nanoanalysis at the Graz TU.
The microscope is also embedded in local activities:
- Field of Expertise of the Graz TU: Advanced Materials Science
- Research field “Processes, Products and Materials”
- Members of the Association for the Promotion of Electron Microscopy and Fine Structure Research
- NAWI faculty of the Graz TU and the University of Graz
The FELMI-ZFE is a national research and user facility open to universities, research institutions and industry.