ZEISS
3 Product and Functional Description | 3.2 Main Components
Operating
Principle
Gun
A Schottky field emitter serves as gun
1
. The filament is heated by applying the filament cur-
rent. Electrons are emitted from the heated filament while an electrical field, called extractor volt-
age (U
Ext
), is applied. To suppress unwanted thermionic emission from the shank of the Schottky
field emitter, a suppressor voltage (U
Sup
) is applied as well.
EHT
The emitted electrons are accelerated by the acceleration voltage (U
EHT
). The beam booster (U
,
booster voltage), which is always at a high potential if the acceleration voltage is 20 kV or less, is
integrated directly after the anode. This guarantees that the energy of the electrons in the entire
beam path is always much higher than the set acceleration voltage. This considerably reduces the
sensitivity of the electron beam to magnetic stray fields and minimizes the beam broadening.
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.
For the Gemini I column, two different column configurations are available:
§
20 nA high resolution configuration
Anode aperture diameter
Probe current
Typical application
40 μm
5 pA to 20 nA
High resolution
§
100 nA high current configuration
Anode aperture diameter
Probe current
Typical application
90 μm
12 pA to 100 nA
Combined high resolution
and analytical investigations
The multihole aperture is the final beam limiting aperture. It is decisive for the probe current. The
standard is the 30 μm aperture hole that is the central aperture. Other aperture sizes are se-
lectable to meet the requirements of a wide range of applications.
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
.
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).
Instruction Manual ZEISS Crossbeam 350 | en-US | Rev. 3 | 349500-8111-000
31
