Magnetic Field Testing
As magnetic field consultants, we have been working with magnetic field issues for sensitive tools for many decades from cutting edge development of scanning electron microscopes (SEMs) to active cancellation systems for MRI tools, to site surveys for specification compliance, debugging, and tool Magnetic Field Sensitivity Testing.
We have worked for many years on the development of new SEMs. A SEM image disturbance will often be caused by a combination of environmental vibration, acoustic noise, and stray magnetic fields, as well as on board sources, such as fan noise, fan imbalance, stage transients, stage control related disturbances (servo instability) and field from stray grounding currents.
These mechanical disturbances are filtered by the structural dynamics of the system and combine with magnetically induced disturbances of the electron beam and result in very confounding image problems.
We have made the task of untangling these issues our work for many decades. Our extensive structural dynamics and acoustics experience is indispensable to our ability to sort out cause and effect from these very different disturbance source types. A very firm grasp of physics allow us a broad range of problem solving ability on real systems (see Diagnostic Testing for more details).
We provide site analysis to insure that magnetically sensitive tools will perform well prior to installation. We typically look at floor and pedestal vibration, acoustics noise, and magnetic fields when performing a site survey for a SEM, or electron beam based tool (see Site Evaluation for more details).
We measure and consider magnetic field disturbances from DC. This varries from very low frequency fluctuations in magnetic field from DC currents that drive elevators or trains and fluctuations in the earths field due to moving ferris materials, all the way up to very high frequency fluctuations that may alias with the scan rate and appear as a low frequency disturbance. Most often the disturbances are at the line frequency, but just as often, in a process or manufacturing facility, there are strong sources of magnetic field disturbance that are not at the line frequency.
In the last 10 years MRI installations have become more common place and we have performed numerous site surveys for these sensitive imaging tools.
In some instances we have created custom active cancellation systems to correct a magnetic field problem that could not be resolved by other means. Many spaces where a SEM or MRI tool can be placed have unique source and/or steel framing geometries that necessitate a custom cancellation geometry to effectively cancel a disturbing field.
We consider the concepts of magnetic flux path analysis and source geometry to debug and troubleshoot magnetic field issues. Mu metal shielding, flux containment, electrical current routing, and active cancellation are tools we use to mitigate magnetic disturbances.