3 Product and Functional Description | 3.3 Optional Components and Accessories
ZEISS
Life sciences:
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Mineral deposits in plant structures
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Biological sections
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Bone structures
The detector has one video output channel. The 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 influ-
ence 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.
Function
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.
3.3.1.2.4 HDAsB Detector
Purpose
The HDAsB is an improved version of the angular-selective backscattered electron (AsB) detector.
It is integrated into the objective lens and therefore also referred to as cap-mounted BSD.
Function
In comparison to the retractable BSD, the HDAsB detector allows operation at shorter working
distances. The HDAsB detector is equipped with four diode quadrants that can be controlled inde-
pendently using a dedicated menu. The HDAsB detector offers improved low kV capability be-
cause it has a thinner oxide layer than older versions of the AsB.
Info
The HDAsB detector enables BSE imaging at acceleration voltages above 1 kV.
The HDAsB detector has two operating modes:
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Compositional mode:
produces images showing the atomic contrast of the specimen
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Topography mode:
produces images showing the surface details of the specimen
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Instruction Manual ZEISS SIGMA series | en-US | Rev. 7 | 352102-9344-006
