ZEISS
3 Product and Functional Description | 3.2 Main Components
3.2.4 Detectors
3.2.4.1 Principle of Signal Detection
The interaction products most frequently used for the generation of images in scanning electron
microscopy are secondary electrons (SEs) and backscattered electrons (BSEs).
Primary electrons are electrons forming the scanning beam before hitting the specimen.
Secondary Electrons (SEs)
Secondary electrons are emitted from the topmost layer of the specimen.
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Electrons emitted at the point of impact between the beam and the specimen are known as
SE1 type electrons. The amount of electrons emitted at the point of impact is related to the
shape of the specimen.
Secondary electron detectors, such as the InLens SE detector, collect SE1 type electrons from
the surface layer of the specimen and are thus ideal for displaying surface structures.
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The emergence of backscattered electrons from the specimen excites further emission of sec-
ondary electrons. These are known as SE2 type electrons.
Detectors that collect SE2 type electrons are especially suitable where the working distance is
large. Surface detail as the effect of “lateral illumination” emphasizes the topography of the
specimen.
Backscattered Electrons (BSEs)
Backscattered electrons (BSEs) emerge from below the surface of the specimen (up to an order
of µm). The number of electrons emitted at the point of impact is highly dependent on the mean
atomic number of the specimen material. This means that a BSE image provides depth informa-
tion and atomic number contrast.
BSE detectors are used to display the materials contrast because the backscatter coefficient is de-
pendent on the mean atomic number of the material under investigation.
Transmitted Electrons
This comprises primary electrons that are transmitted through an ultrathin specimen and weakly
scattered primary electrons with a small range of angles. Depending on the material, primary elec-
trons are scattered under different angles and can be detected by a STEM detector placed below
the specimen. Unscattered electrons are detected in the center of the STEM detector and give a
bright field image. Electrons scattered under higher angles are detected by outer areas of the
STEM detector and produce dark field images.
Electrons impacting on luminescent materials cause the emission of photons which may have
wavelengths in the visible spectrum and can be imaged by specialized detectors.
Instruction Manual ZEISS Crossbeam 550L, Crossbeam 550 | en-US | Rev. 3 | 349500-8122-000
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Summary of Contents for Crossbeam 550
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