ZEISS
3 Product and Functional Description | 3.3 Optional Components and Accessories
The BSD detector has a relatively large central hole and therefore has the advantage that it does
not limit the field of view of the SEM and does not influence the electron optical properties of the
objective lens. A disadvantage is that especially at low kV a lot of backscattered electrons are lost
in the central hole and cannot be detected. A solution for that is the aBSD detector.
Operating
Principle
On the surface of the specimen, some of the primary electrons are backscattered. The backscat-
tered electrons then move towards the silicon segments of the BSD detector. If the energy of the
backscattered electrons is high enough, then the electrons pass through the very thin dead layer
of the diode and create electron-hole pairs in the silicon segments.
In each individual segment, the charge separation due to the electron-hole pairs is measured as a
current, which is used as a signal for image generation. Only electrons that have a high enough
energy can create electron-hole pairs and can contribute to image generation. Electrons that have
a lower energy (e.g. secondary electrons) are not detected by the BSD detector.
The emission of backscattered electrons from a specimen is related to the atomic number of the
involved material: Elements with high atomic numbers generate more backscattered electrons (i.e.
the backscatter coefficient is higher). When imaging, regions that contain elements with higher
atomic numbers appear brighter. Regions that contain elements with lower atomic numbers ap-
pear darker.
Since the detector has a limited speed, it is recommended to use scan speed 6 or higher (slower),
especially at small magnifications. The lower the gain is, the faster is the detector.
Fig. 22: Quad mode showing different contrast on different segments of the
detector: Seg-
ment S1: Pure material contrast (top left), Segment S2: Material contrast and topography (top
right), Segments S3 to S6: Pure topography (bottom left). For comparison: Surface contrast seen
with the InLens
At small magnifications, the field of view is limited by the hole inside the aBSD detector:
Instruction Manual ZEISS Crossbeam 350 | en-US | Rev. 3 | 349500-8111-000
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