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Tackling complex magnetic interference
The magnetic interference caused by running lots of scientific equipment in close proximity can have a detrimental effect on beam shift and image resolution in electron microscopes and electron beam lithography. Magnetic field cancellation is therefore essential for these high resolution electron beam set-ups. Unfortunately, a ready-made solution is not always available to resolve such complex issues. This proved to be the case at the London Centre for Nanotechnology, when the performance of its £2 million Elionix G100 EBL platform was compromised, so the department turned to Spicer Consulting for a custom solution to magnetic field cancellation
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ince its inception in 2003, the London Centre for Nanotechnology (LCN) has applied nanoscience and nanotechnology to global issues in healthcare, energy, the environment and information processing. The centre itself is a joint venture between University College London, Imperial College London and King’s College London, supporting multidisciplinary research across chemistry, physics, materials science, medicine and engineering at all three universities, as well as other academia and industries both nationally and internationally.
At its core, the facility houses a 255 square metre cleanroom containing an extensive selection of equipment for processes such as photolithography, metallization, and ion milling and characterisation, as well as electron beam lithography (EBL) on a RAITH150 Two. Three Carl Zeiss focused-ion-beam (FIB) platforms are also present in the basement lab, and these EBL, SEM and FIB machines enable highly accurate fabrication and imaging for nano-scale research at the facility.
The need for magnetic field cancellation Top-end systems such as those found at LCN can only withstand minimal interference, which can make finding a suitable environment for the equipment extremely challenging. Ambient magnetic interference can affect the beam shift and accuracy of measurements; the higher the resolution, the more sensitive the technique is to the surrounding magnetic fields, vibrations and sound. It is therefore vital to have magnetic field cancellation systems in place to tackle these issues and, since its creation, the centre has used the standard SC24 Magnetic Field Cancelling System from Spicer Consulting to stabilise any fields. This system can compensate for a maximum interference of 100 milligauss, and was more than sufficient for LCN’s needs.
22 December/January 2021
However, the acquisition of a new platform highlighted previously inconsequential magnetic interference that was affecting beam shift. Suguo Huo, electron beam and ion beam manager at the facility, explains: “We received a grant for quantum computing research that enabled us to buy an Elionix G100 EBL, offering better than 10 nanometre resolution electron beam lithography. Unfortunately, when it came to using it, magnetic interference in the environment far exceeded 100 milligauss, resulting in an electron beam shift of around 1,100 nanometres. This was far too big for our purposes, and we could not get the resolution we needed for our research.”
A number of factors
There are many potential sources of interference, and multiple factors are often involved. The continuous development of new technologies means that laboratories are continually expanding their inventories of electronic equipment, using more space than ever before. On top of this, manufacturers’ environmental specifications are becoming increasingly stringent in the constant drive to improve EM resolution.
In this case, the Elionix was situated on the first floor, just above five superconducting magnets operating on the ground floor at high fields: 14 Tesla, 12 Tesla, 8 Tesla, 3 Tesla and 2 Tesla. In addition, hosting the platform on the first floor required a thick steel brace for support, in which a magnetic field was being induced. Finally, a new AC magnetic field furnace had been installed in close proximity to the Elionix in the cleanroom. The combined magnetic fields and resulting interference led to a considerable loss in performance and resolution if all magnets were in operation.
Suguo continues: “This situation obviously left us in a quandary, and LCN Deputy Director and cleanroom PI Professor Paul Warburton asked me to identify a magnetic field cancellation solution that could accommodate five different magnets in different locations operating at different times and in different directions – it seemed an almost impossible
Components in Electronics
task! We approached Spicer Consulting to ask if they could help develop a custom solution. The Spicer team paid a visit to LCN to take measurements and build up a picture of the magnetic interference, and identified a maximum field strength of 550 milligauss.” Designing a solution proved to be far from simple, as the room layout housing the Elionix couldn’t be altered. In addition, the solution needed to be easy to maintain without requiring support from a third party, and therefore couldn’t have too many coils. The final solution lay in using custom-designed high field cables in conjunction with the SC24. This system was installed in September 2019, and successfully restored the Elionix platform’s resolution to better than 10 nanometres. This not only allowed the machine to get back up
and running, it also ensured that LCN is now equipped to deal with the worst-case scenario, where all the magnets are running concurrently.
Summary
Loss of resolution can negatively impact research data, making costly high-end systems virtually unusable. Magnetic interference can occur as the result of a number of factors, but restrictions in the location of an instrument and its environment can make tackling these effects challenging. The key is always to understand the problem, using a site survey to see the full picture in order to offer the best solution. In this way, resolution can be restored to release the full potential of new technology and help advance research.
spicerconsulting.com
www.cieonline.co.uk
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