ZEISS
3 Product and Functional Description | 3.1 Vacuum System
1
Water vapor kit (de-ionized), can be ni-
trogen or air
2
Gun at high vacuum
3
Chamber at variable pressure
4
Single pressure-limiting aperture
mounted under the objective lens
Either 100 μm VP aperture or 400 μm
EasyVP aperture
5
Turbo Isolation Valve closed
VP Mode
Configuration
The 100 μm VP aperture is mounted under the objective lens. The 750 μm mid-column aperture is
used for imaging. The beam first goes through the mid-column aperture and then passes through
the VP aperture before landing on the specimen surface. In VP mode configuration, only the
Nor-
mal
and
Analysis
Optibeam modes can be used for imaging. It is possible to switch between HV
mode and VP mode when using this configuration. The maximum pressure that can be achieved
in the VP mode configuration is 400 Pa for EVO W systems, 273 Pa for EVO LaB₆ and EVO HD mi-
croscopes.
In VP mode configuration, focus wobble is not available.
EasyVP Mode
Configuration
The 400 μm EasyVP aperture is mounted under the objective lens. The 20 μm mid-column aper-
ture is used for beam control. In EasyVP mode configuration, the beam first goes through mid-col-
umn aperture and then passes through the EasyVP aperture before landing on the specimen sur-
face. The maximum pressure that can be achieved in the EasyVP mode configuration is 133 Pa for
all EVO series microscopes.
In the EasyVP mode configuration, Optibeam modes such as
Resolution
,
Depth
, and
Analysis
modes can be used for imaging. It is possible to switch between HV mode and EasyVP mode and
the beam alignment is optimum in both conditions without adjusting any gun or column parame-
ters.
In this EasyVP mode configuration, focus wobble is available.
Function
The residual gas atmosphere in the specimen chamber creates an interaction region of electrons
and residual gas molecules between the objective lens and the specimen. In this region, high-en-
ergy electrons in the primary electron beam hit the residual gas molecules and ionize them. The
ions generated in these collisions contribute to the compensation of negative charge on the speci-
men.
However, another effect of these collisions is to scatter the electron beam. This is called the “skirt
effect”. The electrons that are lost from the primary beam as a result of this effect provide only a
resolution-limited background signal for imaging purposes. Although it is possible to tolerate
these leakage losses at chamber pressures up to a few hundred Pa, it is necessary to carefully se-
lect and control the important factors such as acceleration voltage, chamber pressure, and beam
path. The signal-to-noise ratio in variable pressure mode can also be improved via the noise re-
duction features of SmartSEM. For details refer to the Software Manual SmartSEM.
Fig. 8: Non-conducting specimen imaged with an acceleration voltage of 20 kV and a 30 μm aperture. Left: High vac-
uum (
) mode, showing strong charging effects. The electron beam is distorted and high-quality imaging is not pos-
sible. Right:
mode at 21 Pa. Charges are completely compensated, allowing easy imaging of the specimen.
Instruction Manual ZEISS EVO | en-US | Rev. 10 | 354706-0780-006
25
Summary of Contents for EVO
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