Posted On: Wednesday, January 24, 2018
Changes in DC magnetic field levels near an electron microscope or e-beam lithography tool degrade image quality and inhibit production. A change in DC fields often results in image distortion, a blurred image, or image drift. Common sources of change in DC fields are variation in the Earth’s magnetic field and the movement of ferromagnetic objects. Ferromagnetic objects including elevators, trains, trucks, and metal doors which are common in and around many urban laboratories. The approach to resolving these issues varies based on each laboratory’s unique design and setup.
Frequently, lab owners can remove the source, such as replacing metal doors with a non-ferromagnetic alternative. However, if the lab owner cannot remove the source, the laboratory requires DC magnetic field shielding or a DC magnetic field cancelling system. Vibration Engineering Consultants can measure the magnitude of change in DC field in your laboratory, identify the source, and provide expert advice to optimize the performance of your microscope or e-beam lithography tool.
What are changes in DC fields?
DC magnetic fields can be confusing. The Earth has a magnetic field ranging from 0.25 to 0.65 milligauss that fluctuates over time and throughout the day. Scientists believe that electric currents in the conductive material of the Earth’s molten lava core generate this changing field. Therefore, all electron microscopes are located in the Earth’s changing DC magnetic field.
In addition, the movement of ferromagnetic mass bends the Earth’s field lines resulting in a change in DC field levels. In the most simplistic form, the magnitude of change in DC field levels is correlated to the mass of the moving ferromagnetic source and the distance of that source to the point of measurement (i.e. the electron microscope or e-beam lithography tool). Therefore, the passing of a train several hundred meters away can result in the same change in DC magnetic field levels as the movement of a metal swivel chair less than a meter away.
Why are changes in DC magnetic fields problems for electron microscopes and e-beam lithography tools?
Changes in DC magnetic fields cause the electron beam to move or shift resulting in image distortion, blurring, and drift. This is particularly problematic in urban laboratories where trains, trucks, elevators and other massive ferromagnetic sources are common. As a result, electron microscope operators can waste hours trying to acquire an image because of excessive drift, or e-beam lithography tools can fail to meet production targets.
How to mitigate changes in DC fields?
Several options exist to mitigate changes of DC magnetic fields. Eliminating the source directly is the best option. Lab owners can replace metal doors with wooden core doors, restrict the use of cell phones (they contain magnets), and replace all metallic chairs and trash cans with plastic ones. For sources such as trains, elevators, and truck traffic, the laboratory requires either magnetic field shielding or a DC magnetic field cancelling system.
DC magnetic field shielding requires the use of a material with very high permeability, such as mu-metal. This is a viable option but can be prohibitive because of the high cost of materials. In addition, effective DC magnetic field shielding requires the installation of mu-metal on all walls, the floor, and the ceiling.
The other alternative is to install a DC magnetic field cancelling system, such as the Spicer SC-24. Field cancelling systems are relatively easy to install, less expensive than magnetic field shielding, do not require extensive on-site construction, and can be installed after the microscope is in place. Field cancelling systems are a cost-effective and unintrusive way to eliminate effects of changes in DC magnetic field.
How VEC can help?
If you think that your laboratory space might have change in DC field problems, VEC can help. We conduct detailed measurements to quantify the size of the changes in DC field in your space. In addition, we pinpoint the sources of change in DC field levels and provide detailed remediation recommendations. If a DC magnetic field cancelling system is required, VEC will design and install a system based off your unique tool and laboratory requirements.