Low-aberration ExB deflector optics for scanning electron microscopy.

Abstract

To suppress aberrations in the signal electron optics of a scanning electron microscope, we propose ExB deflector (deflector with superimposed electric and magnetic fields) optics that cancel the aberrations generated during large-angle deflection. This improves the resolution of the angle or position of the signal electrons on the sample surface, allowing them to be discriminately detected. The proposed optics consist of two ExB deflectors and a transfer system with two 4-f systems, or systems that have four times the focal length, placed between them. This configuration maintains the symmetry of the electron beam trajectory throughout the transfer system such that aberrations generated by the first ExB deflector are negated by the second. The effect of the proposed optics was confirmed using a ray-tracing simulation of the electron beam, and the aberration was reduced to at most one-tenth of that in the case with only one ExB deflector. Furthermore, as an example, we examined the implementation of the proposed ExB deflector optics to resolve the signal electron angle and found that the sample emission angle range of 80° can be resolved with an angular resolution of 1°. Therefore, the proposed ExB deflector optics can be applied to the signal electron optics of a scanning electron microscope to improve the resolution of the signal electrons.