3 Product and Functional Description | 3.2 Main Components
ZEISS
Imaging gun mode is especially useful at low kV or at high magnifications to reduce chromatic
aberration and to achieve a better resolution. Switching from Normal gun mode to Imaging gun
mode is useful for reducing the probe current without any need for beam adjustments.
Analytic Gun Mode
In Analytic gun mode, the temperature of the Schottky emitter and the extraction voltage are in-
creased. This leads to a higher probe current. Overall, the probe current in Analytic gun mode is
about twice the probe current in Normal gun mode.
The Analytic gun mode is especially useful for applications which require high intensities (e.g.
WDX).
3.2.7 Detectors
3.2.7.1 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
Primary Electrons (PEs) are electrons forming the scanning beam before hitting the specimen.
Secondary
Electrons
Secondary electrons are emitted from the topmost layer of the specimen.
§
SE1 Electrons
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.
§
SE2 Electrons
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
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.
Cathodoluminesce
nce
Electrons impacting on luminescent materials cause the emission of photons (Cathodolumines-
cence, CL) which may have wavelengths in the visible spectrum and can be imaged by specialized
detectors.
3.2.7.2 Detectors Overview
The beam scans the specimen and initiates particles to be emitted. A detector collects the emis-
sion and produces an electric signal with an amplitude proportional to the number of particles at
any given time.
40
Instruction Manual ZEISS GeminiSEM series | en-US | Rev. 2 | 349500-8138-000
