Robert Christy, Spicer Consulting, Eden Laboratory, Broadmead Road, Stewartby, Bedfordshire, MK43 9ND, England
Robert Christy studied physical science at university and holds a PhD in chemistry from the University of Kent at Canterbury. His role at Spicer Consulting is primarily customer support, performing environmental surveys and installations globally. Robert provides an independent service surveying magnetic fields, vibrations and acoustics prior to equipment installation, as well as post-installation troubleshooting. He also performs magnetic field surveys as part of the magnetic cancelling system installation process.
The continuous development of new technologies means that laboratories are expanding and investing in an increasing amount of electronic equipment, making space within these labs more precious than ever. This busy setting, combined with the growth – and noise – produced by surrounding cities, causes a significant problem for electron microscopy, which relies heavily on maintaining a stable environment, free from vibration and external magnetic fields. The sensitivity of electron microscopy requires areas to be surveyed meticulously before set-up to ensure that the image quality produced will be unaffected by these external factors.
Customer Support Scientist
The performance of any electron microscope is heavily influenced by the local environment, requiring a location that is free from vibration and external magnetic fields to avoid any movement of the electron beam and achieve optimum image quality.
A crucial part of establishing stable environmental conditions, which are critical for this type of analysis, is a scrupulous pre-installation site survey by either an independent service provider or the equipment supplier, to determine the existence of any external factors likely to exert an adverse effect on the quality of the imaging. Typically, this will include measurement of acoustic levels, magnetic fields and floor vibrations in X, Y and Z directions, and direct comparison with the environmental specifications of the equipment to be installed. Where necessary, action can then be taken to mitigate any unwanted interferences, for example, by installing a magnetic field cancelling system.
WHAT CAUSES INTERFERENCE?
The performance of an electron microscope is affected not only by conditions within the room in which it is installed, but also by the location of the building itself. While there are several possible sources of interference, the most commonly encountered problems will arise as a result of vibrations through the floor or the air – acoustic vibrations – or from magnetic fields.
Anything that moves or rattles – whether regular or random – has the potential to create vibrations, including other electronic equipment, air currents from air conditioning systems, people simply walking around the laboratory, doors opening and closing, even traffic in the street, nearby railways or ocean waves. Similarly, magnetic fields can arise from multiple sources, both within the building and externally. The major source of this kind of interference is from power lines in the walls, with magnetic fields across the world normally occurring at two main frequencies – 50 and 60 hertz. Internal interferences also include magnetic fields generated by other equipment, lights (particularly fluorescent tubes) or nearby lifts. It is important to remember that external factors may also be involved, such as vehicles including trains and the electric trams that are growing in popularity in many urban areas. Magnetic fields generated from these can extend for hundreds of metres, unknowingly affecting electron microscope image quality, unless a site survey is conducted to uncover any problems and take suitable preventative measures.
A stable temperature is vital for performance too, as variations can cause specimen drift in the microscope electronics and changes in the mechanical tolerances of components, which will impact on the instrument’s imaging capabilities.
Space is at a premium in many of today’s facilities. With equipment squeezed into every available inch of lab space, electron microscopists increasingly find themselves working in a crowded environment surrounded by other apparatus that depends on electricity to function, which potentially impacts on image quality. Site surveys have a crucial role to play when investing in microscopy instrumentation, and can help to troubleshoot and resolve issues arising at a later date, especially as cities and towns grow and become busier – with that comes growth in levels of potential interference. In addition, in the drive to continually improve resolution and image quality, manufacturers’ environmental specifications are becoming increasingly stringent, with top end microscopes with spectrometers only able to withstand up to 10 or 20 nanotesla of interference; finding an environment that is as suitable as possible can be extremely challenging.
HOW CAN INTERFERENCES BE ELIMINATED?
In many cases, a new microscopy installation will be within an existing building, either to complement the equipment currently in use or as part of a new laboratory set-up. In this situation, there is generally little that can be done about the prevailing infrastructure and space constraints, and it is often a case of choosing an installation site as far removed from potential interferences as possible and installing a magnetic field cancelling system.
New build laboratories provide the opportunity to design an optimised microscopy facility; all the interested parties can communicate from the outset to ensure the incorporation of the optimal infrastructure, and the laboratory’s location within the building can be selected so that it is as far removed as possible from internal and external influences. Air conditioning units can be installed in a separate plant room away from the laboratory, electrical power cables routed to minimise any potential interference, and consideration given to appropriate siting of other equipment to reduce any likely effects on the microscopy instrumentation. Even the location of the car park in comparison to the microscopy room can be considered!
Several measures can be taken to combat the severity of vibrations and counteract their effects. Concrete blocks are a popular choice, isolating the microscope system from the rest of the building. This reduces the vibrations from building sway that transmit through the floor and into the structures of the microscope, causing them to resonate at different frequencies. Another option is active vibration isolation, which uses a platform of transducers to move the system in the opposite way and is able to respond fast enough to ensure that any phase lag is minimal.
When conducting a site survey, it is important to measure both vibration and magnetic field interference from different locations and heights, to see how signals change amplitude in all three axes. This helps to locate the cause of any problems and it is then far easier to implement measures to reduce or eliminate interference. It can be as simple as relocating the microscope, taking out the cause of the interference, or installing a cancelling system to improve microscopy performance as demonstrated in the following examples.
Power cables run through many areas of a building and can be a common cause of interference for electron microscopes. The live wire in a power cable carries electric current to an appliance, while the neutral cable completes the circuit, to form a path for the current to flow back to the mains. These two wires should effectively cancel each other out but, as the currents in the wires get larger, the current difference between the wires grows and this leads to large magnetic fields, as shown in Figure 1a and Figure 1b demonstrates the impact of such a field on the images from an SEM and how this can be improved if this interference can be managed.
Figure 1a. Magnetic fields from power cables. The circle represents the field produced by current flowing in a wire. The two arrows show the tangential field produced at the two heights. At the height of the power cable the field is vertical, while at lower heights, the horizontal component dominates
Figure 1b. The effect of a field cancelling system on interference caused by an AC field
Distribution boards are central hubs; power comes directly into a building or lab and is distributed from the boards around various circuits. They are a source of large magnetic fields and every effort should be taken to place an electron microscope as far away as possible because, even at a small distance away – as shown in Figure 2 – the interference levels are much higher than the tolerance level of the microscope.
Figure 2. Magnetic fields from distribution boards
In some instances, conducting a site survey can uncover problems in the power supply as shown in Figure 3. Large leaks down the earth wire produce huge magnetic fields, which can often be identified by turning off different areas of power supply.
Figure 3. Magnetic fields from earth current leakage
Although lights are frequently turned off in microscopy rooms during imaging, lighting can be a cause of interference, as demonstrated in Figure 4. In general, LEDs tend to be the best to use, whereas fluorescent lights should be avoided. Changing the lights is often the simplest solution to overcoming the problem.
Figure 4. Magnetic fields from lighting. The microscope image on the left was taken with the lights on. The image demonstrates a high level of interference, shown by the jagged edge. This is reduced in the image on the right which was taken with the lights turned off
Trains produce interference in multiple orders of magnitude higher than can be tolerated by electron microscopes, depending on how far away the tracks are. The runtimes of trains often also cause problems as there is regular activity causing disruption, not only from vibrations as trains pass by, but also from magnetic fields running along the power lines, which affect instruments hundreds of metres away.
Trains in different countries – and even regions within countries – run at varying frequencies and voltages, producing an array of magnetic fields. Figure 5a shows the results of a survey conducted outside a lab on the embankment next to a 16.67 Hz railway line in Germany to determine how much interference was coming from the lines supplying power to passing trains. In this case, the survey did not measure vibration levels, although they might also have an impact on image quality.
Figure 5a. Magnetic fields from trains. X and Y are the two horizontal axes while Z is the vertical axis. X is parallel to the track in this image
Similarly, Figure 5b shows readings taken 50 metres from a tram line. The tram line itself is set on a hill and this influences the level of interference, based on whether a tram is going up or down the slope. However, in this situation, a microscope was still installed but with the addition of a cancelling system.
Figure 5b. Magnetic fields from trams
UNDERGROUND TRAIN SYSTEMS
Both vibrations and magnetic fields should be measured when studying the effects of an underground system. In the following example, a survey was conducted quite close to the tracks and there is a level of constant base interference which peaks when trains go past, as seen by the spikes at three, six, 19 and 28 minutes in Figure 6. As the frequency of public transport systems increases due to demand, so does the level of interference. Cancelling systems are frequently needed to overcome the effects of the world becoming busier.
Figure 6. Magnetic fields from trams and trains. The three magnetic axes show how interference from the power cables is constantly changing. Vibration bursts can be seen when trains are closest, and demonstrates how magnetic interference can be high when vibration level is low
Quite often, people simply don’t realise how close they are to a lift. It might seem as if you’ve walked along two or three corridors to get to a microscopy room, but, in fact, you may have just turned back on yourself and walked sides of a square, ending up only a few metres away from a lift. As lifts move up and down, they cause interference gradients that constantly change over different heights, as shown in Figure 7, causing images to wander and jump around the screen as there are large changes in magnetic fields. It is even possible for site survey readings to show you what floor a lift is on. Lifts are of course ubiquitous, especially in larger facilities, and, in these instances, a cancelling system is the only option.
Figure 7. Magnetic fields from lifts
Microscopy rooms situated close to roads or car parks can display varied levels of interference from vehicle movements. Larger peaks in interference come from heavier vehicles, such as lorries, or the regular passing of buses, as seen by the large spikes in Figure 8. A magnetic field cancelling system is often required to overcome these disturbances, as it is increasingly difficult to find areas that are not in close proximity to roads.
Figure 8. Magnetic fields from vehicles
Although it may seem obvious that magnets have magnetic fields, large systems built around magnets – such as NMR platforms – are designed to keep as much field inside as possible, and therefore the extent of their effect should, in theory, be minimal. In reality, it is unknown without conducting a site survey. In the situation shown in Figure 9, even at a distance of 25 metres the field is multiple orders of magnitude above what can be tolerated, and it is advisable to move the microscope and magnet further apart.
Figure 9. Magnetic fields from magnets
Site surveys commonly uncover unexpected causes of interference. The readings shown in Figure 10 were taken at three different heights in a microscopy room that had been converted from a lecture theatre. There was significant interference from an unknown source, and a survey was able to locate the source of interference as coming from a hearing loop that had been left behind ceiling tiles. In this case, the problem was easily rectified by renovation work.
Figure 10. Magnetic fields from a hearing loop
While there are undoubtedly challenges in setting up and maintaining a stable microscopy environment – especially as the world becomes a busier place – measures can be put in place to ensure these are mitigated. The key is to think ahead and engage in discussions with microscopy suppliers, site surveyors and, in the case of a new build, architects at the earliest possible opportunity. Having an experienced site surveyor with the right equipment is vital to identify common and unusual causes of interference and understand the magnitude of their effects. They can then advise on the best way to counteract these problems. This will allow electron microscopy to be installed in the most stable environment possible, whether it is in an existing facility or a newly set-up laboratory, ensuring the highest quality imaging is maintained.