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Electron Channelling Contrast Imaging - An Effective Technique in Scanning Electron Microscopy

In this application note, Zeiss demonstrates the use of the ZEISS GeminiSEM 500 for electron channelling contrast imaging (ECCI).

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Electron channeling contrast imaging (ECCI) is a technique for auantitative characterization of deformation structures in scanning electron microscopy (SEM). Relative orientation of crystalline lattice and incident electron beam influence back scattered electrons (BSE). ECCI makes use of this. ECCI in SEM is especially useful for imaging crystallographic defects such as dislocations, stacking faults and twin boundaries and thus starts to have an impact on materials science.

Introduction
The BSE yield of crystalline samples depends on the relative orientation of the electron beam to the crystalline lattice. In particular when the incident electron beam fulfils the Bragg condition with respect to the lattice, the backscattered electron yield will have an abrupt change. This variation of signal can be used to qualitatively distinguish materials with different crystalline orientations. 

A particularly interesting feature of channelling contrast is that the BSE yield has a very large change around the Bragg condition. Even small changes in crystallographic direction can be detected when the electron beam and the sample are
aligned to fulfil the Bragg condition. The distorted lattice around the defects in crystalline materials can thus be imaged. An example for a dislocation is shown where an extra half-plane of atoms is introduced.

This extra atomic plan causes local distortion of the crystalline lattice, which can be detected due to the large variation of BSE yield around the Bragg condition. 

Electron Channelling Contrast Imaging of Crystalline Defects
In order to effectively image defects in material using ECCI, it is important to adjust the orientation of the sample such that the Bragg condition is fulfilled for the incident electron beam. This condition can be achieved when imaging a single crystalline sample where the orientation of the sample can be readily determined.

Summary

  • Strongest channelling contrast is observed when orientation of the incident electron and the crystalline lattice fulfill the Bragg condition.
  • Lattice defects such as dislocations can be observed by channelling contrast.
  • Higher electron beam energy and lower working distance are suitable for getting good results in channeling contrast.
  • Ideally the sample should have a polished surface.

Full details can be viewed by downloading the application note using the link below.

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