By John Sackett, Director of Spicer Consulting, Stewartby, Bedfordshire, U.K.
The performance of an electron microscope relies on maintaining a stable environment, free from vibration and external magnetic fields. Pre-installation site surveys are vital for uncovering any potential sources of interference, resulting in a need for purpose-designed equipment for the measurement and analysis of acoustics, and magnetic fields and vibrations in X, Y and Z directions. This article discusses the importance of comprehensive site surveys for identifying and eliminating potential sources of interference of electron microscopes (EMs) and similar sensitive equipment, and describes how one company has addressed this through the continual evolution of measurement instrumentation
Electron microscopy is a powerful, sensitive technique used to investigate the intricate structures of cells, materials and nanoparticles for many technical disciplines, including metallurgy, chemistry and biology. All EM techniques – including the two most common, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) – use a beam of accelerated electrons as a source of illumination for the sample. As electrons have a shorter wavelength than visible light protons, this allows electron microscopes to have a significantly higher resolving power than light microscopy, revealing the detailed structure of smaller objects.
However, interference from acoustics, vibrations or surrounding magnetic fields – generated by day-to-day equipment – can cause this electron beam to deflect, which decreases the quality of the images obtained, and therefore affects the resolution.
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. Electron microscopists often find themselves working in a crowded environment surrounded by other apparatus that create magnetic fields, vibrations or acoustic interference, which potentially adversely affects image quality. This busy setting, combined with the growth – and noise – of towns and cities, causes a significant problem for electron microscopy. In addition, in the drive to continually improve resolution and image quality, manufacturers’ environmental specifications are becoming increasingly stringent, with top end microscope spectrometers only able to withstand up to 10 or 20 nanotesla of interference; unsurprisingly, finding a suitable environment can be extremely challenging.
Site surveys have a crucial role to play both when initially investing in microscopy instrumentation, and for helping to troubleshoot and resolve issues arising at a later date as a result of environmental changes that introduce sources of interference. The performance of the instrument is affected not only by conditions within the room in which it is installed, but also by the location of the building itself. Anything that moves or rattles – whether regular or random – can potentially create vibrations, including other electronic equipment, air conditioning systems, people simply walking around the laboratory, doors opening and closing, traffic in the street, nearby railways and even ocean waves. External factors, such as magnetic fields generated by trains and electric trams that are hundreds of miles away, and unexpected influences like the proximity of the parking lot to the microscope, can make a tremendous difference.
While there are undoubtedly challenges in setting up and maintaining a stable microscopy environment, painstakingly surveying the site before set-up allows measures to be put in place to ensure these are mitigated. 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. Measuring and understanding the magnitude of such effects will enable action to be taken to alleviate unwanted interferences, for example, by installing a magnetic field cancelling system, to ensure that the image quality produced is unaffected by external factors.
Keeping up with technology
As technology has advanced over the years, microscopes have become more sensitive to interference, and sensing equipment has had to keep pace to ensure that the environment meets the manufacturer’s specifications for optimal instrument performance. Today, vendors and consultants have access to purpose-designed site survey equipment for examining new installations or to troubleshoot technical issues with an existing microscope by measuring and analyzing any interference. But how has this evolved over the years?
Advances in hardware
In the early days of EM, labs relied on some quite crude magnetic field sensors to monitor the environment, with limited options for measuring fluctuations in sound levels. There was a clear need for a single system that could monitor the entire lab situation around a microscope and, in 1996, Spicer Consulting launched an instrument based on an AC magnetic field sensor, with added inputs for an accelerometer and a sound level meter, that would do just that. This system could make all the measurements required, although the user still had to physically turn the vibration sensor in each direction to measure interference in the X, Y and Z axes.
Subsequently, the system was upgraded so that more bandwidth of data could be collected, and higher frequencies could be evaluated on the spectrum analyzer. Further upgrades, including a move to USB connection, enabled site surveyors to perform more comprehensive measurements with extra sensors. A plug-in for DC sensors was added – so that AC and DC fields could be measured – along with three accelerometer inputs, allowing measurement in all three directions at the same time, instead of having to turn the sensor around sequentially. While the original instrument only had one magnetic field sensor input, later versions had two, enabling the simultaneous measurement of fields at two heights. This feature was important for TEM environments, which typically have tight field specifications over a length of more than two meters.
What about software?
Of course, the software of newer systems has also advanced in parallel to changes in the hardware. On first release, three separate programs were required – an oscilloscope, a chart recorder and a spectrum analyzer – to make the necessary measurements, but the user needed a good understanding to be able to set them up correctly. This was made far easier by the creation of a software wizard to guide the user through the process of measuring for different types of microscope. Users were able to simply turn the machine on, select the instrument they wanted to measure for, and the wizard would bring up the correct program to make that specific measurement. Graph plotting software was also developed, which allowed results to be viewed more easily than with the original method, which was based on Microsoft Excel macros. While the wizard software was capable of running a single measurement, further iterations saw the launch of an automation program capable of running a whole sequence of measurements to simplify the survey workflow even more. Today, users have access to a completely automated system capable of repeat surveying without human intervention. This enables long-term measurements, expanding the surveying snapshot to study the environment at different times of the day.
Acting on the results
Magnetic field interference identified during a site survey can be eliminated by implementing a magnetic field cancellation system. This presents further challenges, requiring a three-axis magnetic field sensor of the necessary bandwidth with low noise levels and low drift. It should include a control unit that can drive the cables to form a stable negative feedback loop, and be easy to set up without complex adjustments. Finally, effective placement of room-sized cancelling cables that make uniform orthogonal magnetic fields and are practical for use in microscope labs or clean rooms of all shapes and sizes must be determined. Alternatively, suitable frames can be constructed to support cables and there are also techniques for installing them inside existing enclosures.
Control units providing readings in three axes, plus total magnetic field readings – including AC and DC simultaneously for some models – are available from Spicer Consulting, with performance tailored to the application. These are convenient to use and enable fields to be cancelled to the demanding levels required by today’s high-resolution electron microscopes. Automatic set-up can provide helpful error and warning messages, and some cancelling systems support a dual-sensor option that creates a virtual sensor where a physical sensor cannot be placed, such as ‘inside’ the EM column. A wide range of cables for different types of microscope in various types of room are also available.
Magnetic field cancellation technology has come a long way and will continue to advance in the future, with demand likely to increase as electron microscopes become higher in resolution and more sensitive to magnetic fields. While the technology is now quite mature, the expectation is that users will seek even better magnetic field sensors, incremental improvements to control units and easier ways to install cancelling cables.
For more information, visit spicerconsulting.com