Zeiss EVO - Variable Pressure Modes

ZEISS 3 Product and Functional Description | 3.1 Vacuum System
Gun Vacuum
In the gun head, a high vacuum is maintained by an ion getter pump. The vacuum in the gun
head is displayed as Gun vacuum in the SmartSEM user interface. It should be below
5×10⁻⁷mbar.
Venting
The specimen is located in the evacuated specimen chamber. To open the specimen chamber for
specimen exchange, you have to break the vacuum in a controlled manner. This is done by the
Vent command via the SmartSEM user interface or by pressing the Exchange push button on the
optional control panel.
When the Vent command is received, gaseous nitrogen flows into the specimen chamber via the
vent valve
8
. As soon as the pressure equilibrium is obtained, the chamber door can be
opened to change the specimen.
Evacuating
In order to continue operation, the Pump command makes the pre-vacuum pump and the turbo
pump evacuate the specimen chamber.
As soon as the vacuum in the specimen chamber is ready for operation, the column chamber
valve opens and the EHT Vac ready message is displayed in the SmartSEM user interface. Gun
and EHT can be switched on.
Vacuum Modes
The microscope can be operated in different vacuum modes:
§ High vacuum (HV) mode
§ Variable pressure (VP) mode (optional)
§ EasyVP mode (optional)
§ Extended pressure (EP) mode (optional)
System Vacuum in
HV Mode
In HV mode, both gun and chamber are at high vacuum. The 20 μm or 30 μm mid-column aper-
tures are used for the beam profile control. No pressure limiting aperture is mounted under the
objective lens.
HV mode provides the means to obtain higher resolution and enables using the SEdetector.
HV mode is the regular operation mode for standard applications and is recommended for imag-
ing conductive specimens.
System Vacuum in
Variable Pressure
Modes
The configuration of the microscope needs to be changed to enable variable pressure modes (VP,
EasyVP and EP Mode), refer to Variable Pressure Modes [
} 23].
3.1.1 Variable Pressure Modes
Variable pressure (VP) modes enable imaging specimens that are non-conducting, strongly
gassing, or hydrated, without the need for vapor deposition or other preparation procedures.
This is made possible by using a differential pumping system. A pressure-limiting aperture (differ-
ential pumping aperture DPA) is mounted under the objective lens, which allows partial pressures
above 10Pa to be set in the specimen chamber while maintaining high vacuum in the gun area.
Three different pressure-limiting aperture configurations are available:
§ VP mode with 100μm VP aperture
§ EasyVP mode with 400 μm EasyVP aperture
§ EP mode with 100μm EP aperture and 500μm or 1000μm beamsleeve aperture.
The following table summarizes the hardware, applications and pressure ranges of the different
vacuum modes:
Mode Application
Pressure-lim-
iting aper-
ture
Mid-col-
umn aper-
ture
Filament Min/Max
Pressure in
Pa
VP mode Charge com-
pensation
100μm VP 750μm W 10–400
LaB₆ 10–273*
Instruction Manual ZEISS EVO | en-US | Rev. 10 | 354706-0780-006 23