Author: Jan Berndorff, Journalistenbüro Schnittstelle
PICO sounds like a small likable lad, perhaps even a soccer mascot, but it is approximately four metres tall and is only called this because it focuses on the world’s tiniest parts. The name is based on picometre, a billionth of a millimetre. PICO is the most powerful electron microscope in the world and is located in a building in Jülich and stands on a special vibration-free foundation so that no vibrations, for example, those triggered by the open-cast coal mine only few kilometres away, or passing trucks, can disturb it.
Displaying atoms that step out of line
PICO and about a dozen additional electron microscopes at the “Ernst Ruska – Centre for Microscopy and Spectroscopy” (ER-C), which the Research Centre Jülich and the RWTH Aachen operate, allow scientists to enter a previously hidden nanoworld: It makes the atomic structure of materials visible and can accurately demonstrate why they exhibit certain characteristics and why they change during temperature and/or pressure changes. This helps enormously with improving the storage and processing of computer data. “For example, just a few picometres of certain atoms’ displacement influences the characteristics of modern transistors,” says Rafal Dunin-Borkowski, one of the two directors at ER-C. Electron microscopy also aids the development of solar cells and batteries, as well as new materials for various other industries: For instance, the new “miracle-material” graphene, i.e. carbon in a one-atom-thick layer with many astonishing and promising characteristics for various applications, requires a better understanding. PICO can examine graphene more precisely than any other microscope in the world. According to Joachim Mayer, the second director at ER-C mentions that regardless of whether it is materials science, energy, or computer technology: “We take on global challenges with our equipment.”
PICO has a 50-picometre resolution. This is one twentieth of a nanometre, which is approximately equal to the diameter of a helium atom. In order to bring this into proportion: If the diameter of a strand of hair were as large as a football field, the thickness (not the width!) of a blade of grass would resemble the atom size in the hair strand. Pico and its “comrade microscopes” can display the narrow side of a grass blade and can even detect displacements within a picometre range.
Electron microscopes with optical aberration
Conventional optical microscopes are physically limited: The wavelength of light is a few hundred nanometres too big for such a resolution. In contrast, the wavelength of bundled electron beams, with which the ER-C microscopes illuminate their samples, is a fraction of a nanometre. Nevertheless, conventional electron microscopes only reach a resolution of approximately 200 picometre – sufficient for many applications, but not sufficient to detect the smallest atoms and their changes. “Although precision, electron microscopes, like optical microscopes, suffer from similar aberrations – namely blurring,” says Dunin-Borkowski’s colleague Peng-Han Lu. The two most important aberrations are the “spherical aberration” and the “colour” or “chromatic aberration.” With the electron microscope, the electromagnetic fields perform the function of the optical lenses, namely to focus the electron beam. In this case, the electrons at the edge of the beam are also diffracted more than on the inside. This is spherical aberration. In addition, high-energy electrons are diffracted differently than low-energy electrons. This leads to chromatic aberration.
With optical microscopes, it is relatively simple to correct these errors with dispersing lenses. However, with the electron version, one needs to integrate very complicated devices into the microscope. In the 1990s, engineers, assisted by experts from Jülich, developed a hexa-pole corrector containing six magnetic coils for the spherical aberration, which is currently installed in several electron microscopes worldwide to correct these to an 80-picometre resolution. It has only been recently possible to successfully correct colour aberration with an even more complex system of magnetic and electrostatic multi-pole elements. There are only two microscopes in the world that have both correctors and, thus, a 50-picometre resolution: one at the Lawrence Berkeley National Laboratory in the USA, and PICO in Jülich. More are currently being built – for instance, the one at the University of Ulm should soon be in operation.
Silicon nitride glasses for the electron microscope
Researchers from around the world thus want to cooperate with Jülich to use their ultra-precise microscopes. The Jülich scientists are currently conducting a project with colleagues from Dresden, Ulm, and Tel Aviv. Physicists from the Tel-Aviv University (TAU) have developed nano holograms for electron microscopes. These are tiny mask-like silicon nitride membranes with which one can change the electron beam into almost any form for various purposes: One can specifically diffract it, and split it into grid, groove, or other patterns, as well as modify the phase and amplitude of the wave.
This could, on the one hand, replace the complicated correctors of the PICO electron microscope. According to project leader Arie Ady from the Department of Electrical Engineering and Physical Electronics at the TAU: “Our new methods are more flexible, more compact, and far more cost efficient.” On the other hand, nano holograms could enormously support the monitoring of the semiconductor and computer industry manufacturing: There, the increasingly smaller switching elements on chips are routinely scanned with electron beams to detect possible defects, for example, if an element consisting of only a few atoms is incorrectly positioned.
Arie Ady remarks: “It takes days to extensively scan one chip with a single electron beam. Companies reduce this time to hours by using more scanners. But if we were to split the beam of a single scanner into 100 beams with the help of our masks, it would accelerate the control enormously.”
The Israeli scientists depend on the electron microscopes at Jülich to test and optimize their new method. “In Israel, we do not have such high-performance microscopes. And, beside that, we have the opportunity to learn from our German partners. They are simply the top experts in electron microscopy.”
You can learn more about the intensive cooperation between the Research Centre Jülich and the Israeli partners here.