 enlarged version
enlarged version
enlarged version | | At the impact point of the primary electron beam secondary and backscattered electrons are generated. The secondary electrons (SE), having an energy of less than 50 eV, are emerging from the very surface of the specimen. Backscattered electrons (BSE) are generated below the surface in a larger volume than the SEs.
For high resolution imaging, elastically scattered BSEs have to be detected. These high angle BSEs typically in a cone with a 15° angle to the primary beam are attracted by the electrical field of the GEMINI® column and rejected into the column.
For the separation and detection of the SEs and BSEs one has to consider two parameters: energy and angle distribution. The secondary electrons emerging from the top surface of the specimen contain surface information, as their angle distribution is virtually perpendicular to the surface and orientation dependent. Due to their relatively low energy, SEs are attracted by the electrical field of the GEMINI® column and are all deflected by the excited objective lens to the plane of the annular In-lens SE detector.
The SEs are detected through a wide angle range depending on the surface of the specimen. The high angle backscattered electrons, carrying an energy close to the landing energy of the primary beam, are projected into the GEMINI® column as well. If the angle is too low they will not enter the column but will land on the end cap of the pole-piece.
The BSEs inside the GEMINI® column are deflected by the objective lens, but due to the higher energy they are deflected to a different plane than the secondary electrons.
The method of separating and detecting the backscattered electrons is called: Energy and angle selective Backscattered detection, hence the name EsB detector.
The distribution of the secondary and backscattered electrons in the plane of the In-lens SE detector is such that the detection efficiency from both the In-lens SE detector as well as the EsB detector are optimised. The detection efficiency of the EsB is around 85% of the high angle backscattered electrons. Only a fraction of the secondary electrons are deflected into the hole of the In-lens SE detector giving a detection efficiency of over 90%. The few secondary electrons passing through the hole in the In-lens SE detector are suppressed by the filtering grid in front of the EsB detector. The filtering grid does not interfere with the primary beam and does not effect the beam booster of the GEMINI® column.
Benefits:
SE imaging with the In-Lens detector
- Ultra-high spatial resolution
- High contrast surface information
- Voltage contrast imaging
BSE imaging with the EsB detector
- Nano-scale compositional information
- Sub-surface information visible
- Not sensitive to charging
- Less sensitive to edge contrast
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