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ZEISS
3 Product and Functional Description | 3.2 Main Components
When the
Vent
command is received, the column chamber valve closes and gaseous nitrogen
flows into the specimen chamber via the vent valve
9
. 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
Quiet Mode
The automatically controlled
Quiet Mode
is optionally available. This option allows switching off
the pre-vacuum pump after specimen exchange when the vacuum threshold is achieved.
3.2.2 Vacuum System Pressure Modes
Purpose
For operation of the microscope, the gun head, the column, and the specimen chamber have to
be evacuated. The vacuum is essential to operate the gun and to prevent collisions of electrons
with gas molecules.
With the variable pressure mode option, the microscope can be operated in different vacuum
modes:
§
High vacuum (HV) mode
§
Variable pressure (VP) mode
3.2.2.1 HV Mode
Purpose
When changing to HV mode, the acceleration voltage decreases to zero and the column chamber
valve closes. The GFV closes and as soon as the chamber pressure drops below the minimum
chamber pressure (MCP = 15 Pa) the RBV closes and the TIV opens. Then the column chamber
valve opens and the acceleration voltage is turned on again.
The booster voltage is turned on in this mode.
3.2.2.2 Variable Pressure Mode
Purpose
The variable pressure (VP) mode enables you to use the scanning electron microscope to image
specimens that are non-conducting, strongly gassing, or humid, without the need for vapor depo-
sition or other preparation procedures.
A differential pumping system makes it possible to set partial pressures above 10 Pa in the speci-
men chamber while maintaining a high vacuum or ultra-high vacuum in the gun area and in the
beam path.
When changing to VP mode, the acceleration voltage decreases to zero, the column chamber
valve closes, the turbo isolation valve (TIV) closes, the Roughing backing valve (RBV) and the gas
flow valve (GFV) open. As soon as the chamber pressure has reached the user value, acceleration
voltage switched on again and the column chamber valve opens.
The booster voltage is turned off in this mode.
Electron detection is provided via a VPSE detector.
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-
Instruction Manual ZEISS SIGMA series | en-US | Rev. 7 | 352102-9344-006
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