How to ... improve electron microscopy resolution

Materials World magazine
,
7 Aug 2019

Discover how magnetic field cancelling can improve electron microscopy resolution.

Electron microscopy (EM) is a 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 shorter wavelengths 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 acoustic vibrations or surrounding magnetic fields, generated by day-to-day electrical equipment, can cause this electron beam to deflect, which affects the resolution of the microscope decreasing the quality of the images. As such, users often turn to cancelling systems to reduce both alternating current and direct current magnetic fields and overcome these problems.

External influences

There are many potential sources of interference that need to be considered when installing an electron microscope. 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 can potentially create vibrations, including other electronic equipment, air conditioning systems, people walking around the laboratory, doors opening and closing, traffic in the street, nearby railways and ocean waves. External factors such as magnetic fields generated by trains and electric trams can extend for hundreds of metres, and unexpected factors like the location of the car park in comparison to the microscope can make a difference.

While there are challenges in setting up and maintaining a stable microscopy environment, 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 measure vibration and magnetic field interference from different locations and heights, identify common and unusual causes of interference, and understand the magnitude of their effects.

The sensitivity of electron microscopy requires areas to be surveyed to ensure the image quality produced will be unaffected by any external factors. An analysis system allows electron microscopes to be installed in the most stable environment possible. Some systems measure and analyse magnetic fields, vibrations and acoustics along the X, Y and Z axes, allowing the user to perform a direct comparison to the microscope specifications. The results can be displayed graphically in a user-friendly form.

Overcoming interference

There is a constant drive to improve EM resolution and image quality, and manufacturers’ environmental specifications are becoming increasingly stringent. Top-end microscopes with electron energy loss spectrometers can only withstand up to 20 nanotesla of interference, so finding a suitable environment for the equipment can be challenging. It is becoming increasingly common for EM users to seek advice from experts to mitigate unwanted interferences, with one solution being to install a magnetic field cancelling system.

Some systems consist of a magnetic field control unit, magnetic field sensors and three multicore cables installed around the electron microscope. There are two different types of sensors that can be used, depending on the interference present. The first only measures AC fields, while other sensors can detect both high frequency AC and low-frequency DC fields. There are three power amplifiers in the control unit which, when interference is present, drive currents through the cables to create a nearly equal and opposite field. This dynamic response – which automatically responds to field changes within a microsecond – stabilises the ambient field, restoring resolution.

One system available is the SC24 which does not cancel the field everywhere in the room, but instead creates a region around the magnetic field sensors where the field is much reduced. The volume of this region depends on the uniformity of the ambient field and the layout of the cables, highlighting the importance of an initial survey, as well as the need for expert assistance at installation.

Peking’s success

At Peking University, China, College of Chemistry and Molecular Engineering, the core SEM facility assists many departments and research groups with a range of projects, from studying bacteria and materials to the analysis of nanoparticle crystal structures. For SEM, the researchers needed a high-resolution image – typically a 200,000 times magnification of the sample – with minimal interference.

The laboratory is in an old building. There is a large electricity power cabinet emitting AC interference nearby, and a subway 100m away contributes to the high levels of DC magnetic fields and acoustic noise from vibrations. This level of interference deflected the electron beam of the microscope, distorting the images that they acquired.

The SEM system ideally needs to operate with an external magnetic field of less than one milligauss, but DC fields measured on the Z-axis were 19 milligauss, and AC fields on the X-axis were more than 21 milligauss. Both AC and DC magnetic fields were regularly out of range according to the SEM requirements, so the team explored using a cancelling system to stabilise the ambient field.

As the lab is a busy environment and handles many samples, a system needed to be put in place to detect and stabilise the interference. An SC24 system was installed in 2018 and the team has seen a significant improvement to the microscope’s resolution – reaching a magnification of 300,000 times – and an increase in the quality of images that they can produce. The lab is planning on moving the SEM to a new site.