Discussion of Ways to Energy-Filter the Electron Backscattering Pattern (EBSP) in the Scanning Electron Microscope (SEM).

Abstract

An electron backscattering pattern (EBSP) is formed by the electrons that are scattered with the highest energies from a crystalline target that is illuminated with an electron beam (EB) of kiloelectron-volt (keV) energy. In appearance it closely resembles the electron channeling pattern (ECP) that is formed in the scanning electron microscope (SEM) when the incident EB is rocked about a point. According to one possible explanation, it is assumed that in either case the pattern is formed by the electrons that leave the specimen with a minimum loss of energy in the reflected electron peak following a single wide-angle Rutherford-type scattering event (typically through about a right angle) close to the surface. Now if ECP and EBSP are reciprocal, they can be related by reversing the direction of the (zero-loss) electrons in the specimen. It can then be argued that these patterns must be formed because the probability of such a Rutherford-type wide-angle scattering event is modulated by both the incoming (ECP) and outgoing (EBSP) channeling conditions [1-6]. The slower scattered electrons undergo scattering, channeling and diffraction events that are in addition to this basic process. The related reflection high-energy electron diffraction (RHEED) pattern is formed with grazing incident and exit directions with electrons that are diffracted by Bragg planes parallel to the surface. Generally the EBSP is formed over a limited solid angle by placing a detecting screen in line-ofsight from the specimen or (in principle) with a retarding-field energy filter [6]. An alternative approach that is suggested here is to magnetically filter the scattered electrons by mounting the sample between the polepieces of a magnetic immersion lens, with the limitations: (a) The sample must be totally non-magnetic. (b) It will be irradiated more heavily than with the standard EBSP. Fig. 1 shows a solid sample mounted between the polepieces of a magnetic immersion lens. The magnetic field is adjusted so as to focus the incident EB onto the specimen. This same magnetic field also deflects the scattered electrons to follow spiral paths that periodically return to the lens axis. The fastest scattered electrons (reflected electron peak) can reach a limiting surface at the end of the first half-turn that cannot be reached by the slower scattered electrons. The slower scattered electrons can, however, go beyond this limiting surface on subsequent turns of the spiral path if not prevented from doing so [7]. It has been demonstrated that a detector that is placed at a short distance inside this limiting surface will collect only the fastest scattered electrons and can be used to give a magnetically filtered low-loss electron (LLE) image in the SEM [8-10].