System control: Vacuum system
2-11
C O N F I D E N T I A L
– limited rights
User Manual
Revision A
Feb 2018
Vacuum modes
The Vacuum module /
High Vacuum
or
Low Vacuum
radio buttons
are used to select the instrument target operating mode when a
Pump sequence is initiated.
When changing the vacuum mode from one mode to another, a
confirmation dialog appears.
High Vacuum (HiVac) mode
For
High Vacuum
, the system achieves the lowest pressure
possible throughout the column and specimen chamber. The
typical pressure value is within the order of 10
-2
to 10
-4
Pa.
Low Vacuum (LoVac) mode
In this mode, the column section is under lower pressure than the
specimen chamber, where the pressure ranges from 10 to 500 Pa
(0.08 to 4.0 Torr) depending on the Use case and PLA used:
•
Standard Use case without PLA:
10 – 50 Pa
•
OptiPlan Use case with PLA:
10 – 150 Pa
•
Standard Use case with PLA:
10 – 500 Pa
This mode uses water vapor from a built-in water reservoir, or a gas from an auxiliary gas inlet (the drop-down list).
When entering the LoVac mode from the vented state or when changing the gas type, chamber pressure goes to
the HiVac value and subsequently to the desired LoVac value, so the old gas mixture in the chamber is first
exhausted and then replaced by gaseous atmosphere. Entering the HiVac mode can by unwanted for highly
outgassing samples (see the
Preferences…
dialogue /
Low Vacuum
section /
Fast Low Vacuum Pumping Through High
Vacuum
check box functionality).
The system automatically detects any Low Vacuum detector installed (LVD and GAD) and offers to switch to the
LoVac vacuum mode. If a relevant detector is not in place, a warning appears informing the user that the detector
should be installed for the mode selected. However, the system will not abort the pump sequence.
Pressure
The
Chamber Pressure
adjuster is used to set and show the target chamber pressure. Pascal, Torr or Millibar units
are available and can be selected in the
Preferences…
dialogue /
Units
tab.
When the system is in LoVac mode and the
Chamber Pressure
value is changed, the pressure automatically changes
to the new value. When the system is in any other status and the chamber pressure value is changed, the new
value is used as the target pressure when the system starts pumping to a Low Vacuum mode again.
The actual specimen chamber pressure is shown in the
Status
bar
/
Chamber Pressure
field.
PLA (Pressure limiting aperture) Accessories
When the
LoVac
mode is entered from the
HiVac
mode or from the
Vented
status, the system prompts the user with the
PLA Accessories
dialogue.
This only happens the first time after a particular vent procedure.
Note
The number and types of accessories within the PLA Accessories dialog vary
according to the actual system configuration.
The maximum allowed specimen chamber pressure in LoVac mode is
determined by the size of the PLA cone and the gas type. The
PLA
Accessories
dialogue prompts to inform the system about the PLA cone
size (part of a dedicated gaseous detector) according to the figure when
pumping or switching to LoVac mode. Along with gas type, this
information sets pressure limits and rates for pressure changes.
Clicking the appropriate accessory button informs the system that it is
mounted on the final lens. The system starts pumping to the LoVac mode.
From that point on, this information is automatically used until the system
is vented again, when the PLA is set to “unknown”. Clicking the
Cancel
button leaves the system in its actual status mode (Pumped or Vented).
The
X-Ray Cone
(500
µ
m aperture) is used for EDX analysis (option) and
for the LoVac and low voltage imaging (i.e. below 5 kV) to reduce beam
