ZEISS
3 Product and Functional Description | 3.2 Main Components
Apertures
The electron beam passes through the anode aperture
3
first, afterwards through the multi-
hole aperture
5
.
The anode aperture defines the maximum possible probe current.
The multihole aperture is the final beam limiting aperture. It is decisive for the probe current. The
standard is the 20 μm aperture hole that is the central aperture. Other aperture sizes are se-
lectable to meet the requirements of a wide range of applications.
For the Gemini 3 column, two types of multihole aperture are available:
§
20 nA high resolution configuration
Multihole aperture type
Probe current
Typical application
7 hole aperture
3 pA to 20 nA
High resolution
§
100 nA high current configuration
Multihole aperture type
Probe current
Typical application
6 hole aperture
6 pA to 100 nA
High current
Specific to the Gemini 3 column with high resolution configuration is, that aperture matching is
also optimized for the non standard apertures 15 µm and 30 µm. For low EHT settings the 15 µm
aperture offers the best imaging performance.
Condenser
The condenser
4
is used for aperture matching of the objective lens in order to guarantee op-
timum resolution at each probe current and EHT setting.
Together with the multihole aperture, the condenser allows to regulate the probe current.
Stigmator
The stigmator is located inside the condenser and compensates for astigmatism so that the elec-
tron beam becomes rotationally symmetrical.
Deflection System
The electron beam is focused by the objective lens
7
onto the specimen
9
while being de-
flected in a point-by-point scan over the specimen surface by the scanning coils
8
.
The Nano Twin lens in the Gemini 3 column has a double deflection system, which offers different
scan modes to offer navigation modes (Overview, Low Mag Mode) and third order optimized scan
distortions.
Before the electron beam exits the objective lens, the electrostatic lens creates an opposing field
which reduces the potential of the electrons by +8 kV. The energy of the electrons reaching the
specimen surface therefore corresponds to the set acceleration voltage (EHT).
Signal Detection
When the primary electron beam hits the specimen, certain interaction products are released,
which can be recorded by specific detectors, e.g. the InLens SE detector
6
. For more informa-
tion refer to
Principle of Signal Detection [
.
3.2.6 Gun Modes
The microscope can operate in different gun modes:
§
Normal
§
Imaging
§
Analytic
Normal Gun Mode
In Normal gun mode, the temperature of the Schottky emitter (gun / filament) and the extraction
voltage are optimized for a long lifetime of the Schottky emitter. Normal gun mode is suitable for
most applications.
Imaging Gun Mode
In Imaging gun mode, the temperature of the Schottky emitter and the extraction voltage are re-
duced in comparison to the Normal gun mode. This leads to a reduction of the energy spread of
the primary electrons. Overall, the probe current in Imaging gun mode is about half the probe cur-
rent in Normal gun mode.
Instruction Manual ZEISS GeminiSEM series | en-US | Rev. 2 | 349500-8138-000
39
