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Scanning Electron Microscopy

Abstract

The scintillator-photomultiplier combination (Everhart-Thornley detector) for detecting secondary and backscattered electrons (SE and BSE) has the best properties concerning signal-to-noise ratio and bandwidth as compared to other detectors (semiconductor detectors or channel plates).

Two opposite Everhart-Thornley detectors A and B are proposed for a better and reproducible angular selection of the SE. The field strength at the specimen is reduced either by a grid or ring electrode to separate the SE with regard to their exit momenta. This offers the possibility to record the signals A, B, A+B, and A-B. The signal A+B shows material and channelling contrast and the signal A-B topography with a clear distinction of elevations and indentations. Furthermore, this signal A-B is proportional to dz/dx for tilt angles ϕ=0- 60° and the surface profile can be recorded by analogue or digital on-line integration.

Backscattered electrons can be recorded optimally by using scintillators or a conversion of BSE to SE at plates covered with MgO. Multi-detector systems offer a determination of the specimen tilt ϕ and azimuth 𝜘 which can be used for a surface reconstruction or in x-ray microanalysis for a ZAF-program of tilted surfaces. The signal A+B shows predominately material and channelling contrast and A-B the topography, however, with a worse resolution than the SE mode and with image artifacts like steps at flat interfaces between materials of different Z.

The electron-backscattering pattern (EBSP) shows advantages compared to an electron channelling pattern (ECP), and methods have been developed to record EBSPs and to use the shadow of the 'skyline' in an EBSP for a three-dimensional reconstruction of the specimen surface.

Energy-selection of the BSE and digital image processing will be further improvements for these imaging and recording techniques.

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