ZEISS
3 Product and Functional Description | 3.3 Optional Components and Accessories
Fig. 45: Display of material contrast with the
3.3.1.2.1 BSE Detector Efficiency
The efficiency of BSE detectors (aBSD1-LH, HDBSD, HDAsB) is determined mainly by three factors:
§
Acceleration voltage
Since BSE detectors only use the energy of the generated backscattered electrons, the effi-
ciency of the detector therefore increases with increasing acceleration voltage.
§
Working distance
Since the BSE detectors are positioned directly below the objective lens, a hole exists in the
center of the detector through which the electron beam scans the specimen. The active layer
(top layer of the semiconductor diodes) is arranged around this hole. If the working distance
is too long, many electrons miss the detector, which also reduces the efficiency. If the se-
lected working distance is too short, only few backscattered electrons hit the detector and
most electrons pass through the hole without contributing to the signal. The optimum solid
angle for detection of the backscattered electrons exists only in a relatively small range of
working distances centered at approximately 9 mm.
Fig. 46: Factors that affect the
detector efficiency: Acceleration voltage (left), working distance (right)
§
Specimen tilt
Because of the viewing angle of the BSE detectors (directly from above), the specimen orien-
tation towards the detector has an effect on the detector efficiency. Whereas the SE detector
responds very well when the specimen is tilted towards it, tilting degrades the detector re-
sponse when using a BSE detector.
Tilting also distributes the generated backscattered electrons in different directions. Larger an-
gles of tilt mean that more electrons are scattered in the forward direction and fewer are
available to contribute to the signal of the BSE detector. Low angles of tilt should therefore be
used with the BSE detector.
Instruction Manual ZEISS SIGMA series | en-US | Rev. 7 | 352102-9344-006
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