Magnetic Field Measurements in Multi-Beam Deflector Array

Abstract

The Wien filter is an important part in the multi-beam inspection microscope that is being developed at the Imaging Physics research group at the TU Delft. The multi-beam inspection microscope uses an array of 20x20 parallel beams with a pitch of 1mmthat scan the sample simultaneously. This way the low throughput of current scanning electron microscopes can be increased proportional to the number of parallel beams. The Wien filter uses a magnetic field to separate the array of primary beams from the secondary beams. A design for the Wien filter is described in [6] consisting of electric and magnetic deflection arrays. In this report a first model of the magnetic deflection array is presented and tested to investigate the uniformity of the magnetic field. The magnetic field strength was computed by measuring the deflection distance caused by the field. As predicted by theory, the field has been found to scale linearly with the applied current. Current optimization was done in simulations to find the most uniform distribution of the magnetic field. Experiments showed against expectation that the uniformity of themagnetic field between slits does not increase with increasing current on the auxiliary coil while current on the main coil is kept constant. An possible explanation is provided in this report. For the model presented in this report the current in the auxiliary coil should be
set to zero in order to obtain the most uniform magnetic field. It is proposed to provide every single winding
with its own power supply so that a better uniformity might be achieved by the individual optimization of the
currents applied.