Until recently, electron microscopy’s main engineering constraint has been spherical aberration of the objective lens - a feature of all round lenses that causes image distortion. Not only has resolving the aberration issue kept physicists busy since the 1940s, but it has also restricted the development of new materials. Now, using aberration correction technology pioneered by Professor Ondrej Krivanek in 1995 and funding from the EPSRC, a new SuperSTEM facility has been set up in the UK at CLRC Daresbury. The success of the SuperSTEM project, an initiative to set up a world class spherical aberration correction microscope facility in the UK promises significant progress in a number of fields, including alloys, drug delivery, nuclear fusion for power, glassy materials and semiconductor lasers by allowing analysis to be carried out with sub-Angstrom resolution.
The Uniqueness of the Daresbury SuperSTEM Facility
Two other aberration corrected microscopes exist in Europe, but the Daresbury set-up is unique in its formation as a user facility that has been built from scratch. The laboratory is intentionally set in a neutral location, unattached to a university so that as many people as possible can access it. Also, for successful correction, high mechanical and electrical stability is vital, and a new building was constructed to exacting standards around the needs of the SuperSTEM microscopes.
The Purpose Built Facility
The main structure is a bow-shaped building containing two concrete boxes. Separate electrical supplies, computer controlled air handling, close temperature control and air conditioning with long term oscillation characteristics ensure reliable microscopy Daresbury was chosen as the site of the facility partly on the merit of its geological characteristics. The site is surrounded by a sandstone bedrock with stratified rocks and has excellent geological stability - critical for an instrument as sensitive as the SuperSTEM.
Central to the exceptional resolution of the SuperSTEM microscope is the successful aberration correction (the Super denotes the addition of an aberration corrector to the STEM microscope), which improves image resolution by a factor of three. The spatial resolution is currently 0.103nm. Dr Andrew Bleloch, Technical Director of the SuperSTEM facility, describes aberration correction as ‘like giving a microscope glasses, or the difference between analogue and digital.’
How the Aberration Correction Works
The aberration corrector retrofitted to the SuperSTEM uses quadrupole and octupole aberration correction. The image aberration measured in terms of shifts in beam offsets, tilts and changing of defocus is recorded and translated into computer-controlled adjustment of all the quadrupole and octupole currents - in effect introducing the negative of the microscope’s aberration. A quadrupole squeezes the beam in one direction and stretches it in the other for the octupole to perform correction, changing the beam from round to square.
This breakthrough in aberration correction is timely. Modern computer power was needed so that the aberration could be measured and fed back quickly. Also, advances in materials were needed - the pole pieces in the quadrupoles and octuploes are made from a new alloy with very low hysteresis, which provides the predictable behaviour necessary for such sensitive equipment.
The new SuperSTEM is capable of ultra high resolution imaging with single atom sensitivity The analysis takes the form of high angle annular dark field (HAADF) imaging with the formation of a subAngstrom probe and atomic column electron energy loss spectroscopy (EELS) analysis of thin specimens.
Resolution and Positioning
The microscope’s strength lies in its ability to focus on a single atom and move to an exact target. The probe is moved in a known pattern to work out how the different parts of the image are moving, which is fed back to the computer for aberration correction. Each sub micron thick sample is painstakingly prepared, and with a good sample, results can be achieved within a day. Incoherent images are obtained that are easier to interpret than images from other forms of microscopy. There is also a clear differentiation between columns, and nanocrystals are sharply visible. In addition, a better signal to background ratio means improved sensitivity
Current and Future SuperSTEM’s
The current SuperSTEM, known as STEM1, is an existing STEM model made by VG Microscopes, which was retrofitted with an aberration corrector made by Nion who are supplying STEM2, the first dedicated SuperSTEM microscope. STEM2 will be easier to use and among other benefits will have the capacity for six samples to be left overnight, giving shorter lead times.
SuperSTEM’s Envisaged Effect on Materials
Bleloch’s favourite maxim is, ‘If you can see what atoms there are, where they are and how they bond that’s all you can hope to know.’ The new microscope takes the guesswork out of engineering materials. For example, if an interface in a material is not conducting, a visibly rough interface surface could be the reason. As for the future, ‘Changes will be evolutionary rather than revolutionary,’ says Bleloch. ‘This (the successful implementation of aberration correction) is almost a once in a lifetime improvement.’
Areas of Application
Areas of research that could potentially benefit from the SuperSTEM are diverse. ‘We don't really know what the full capabilities of the STEM are, or what people will use it for. When you build a railway fine you have no idea who’s going to travel on it,’ says Bleloch. However, possibilities include:
• Catalysis - identification of catalytically active atoms, showing which side of an interface the atom lies
• Pollution in biological samples - higher amounts of ferritin in oxidation states associated with disease in liver samples can point to pollution. STEM2 will allow better viewing of biological samples with less damage
• Biological molecules - by identifying the positions of a few atoms in the overall shape of a molecule, its structure can be deciphered - like finding the corners and edge pieces of a jigsaw puzzle
• Nanotechnology - this is a major area in which the SuperSTEM can aid research for ever shrinking components. For example viewing silicon and nickel silicide interfaces in gate oxide transistors, which are now 1nm thin, can show the presence of the sharp interfaces required.
Other areas in which Peter Goodhew, the SuperSTEM Project Director, is expecting substantial advances include alloys, drug delivery, nuclear fusion for power, glassy materials, such as smart glass, and semiconductor lasers. ‘Nearly all the work we will do will be in collaboration with a person who wants to solve a problem, it will be teamwork,’ says Goodhew, who sees the next 10-15 years as a time to make the facility financially viable.
Outlook for The Future
The microscope will potentially operate on an ‘e-grid’, a new protocol similar to the internet that allows high security sharing of resources. A client would send their samples up the day before, then an operator would load the samples and the client could see the images online and guide the testing procedure. For the time being, appointments for time on the microscope will be decided by a peer review panel. Interest has arisen as far a field as Singapore and China for remote microscopy. The group sees themselves as a last port of call for people who have tried other forms of microscopy, and at the centre of the development of new materials.