FEI Quanta FEG 250, User'S Operation Manual

Page: 22 / 228

System Operation: Vacuum Modes
3-4
Vacuum Modes
The vacuum mode radio buttons in the Vacuum module / Mode area
are used to select the instrument target operating mode when a
Pump sequence is initiated. The vacuum system recognizes High
Vacuum , Low Vacuum and ESEM modes.
HIGH VACUUM (HiVac) MODE
The high vacuum condition is common throughout the column and
specimen chamber. The typical pressure value is within the order from
10
-2
to 10
-4
Pa.
LOW VACUUM (LoVac) AND ESEM MODES
In these modes, the column section is under the lower pressure than
the specimen chamber where the pressure ranges from 10 to 130 Pa
(LoVac) or from 10 to 4000 Pa (ESEM). These modes uses water
vapour from a built-in water reservoir or a gas from an auxiliary gas
inlet.
The system automatically detects the gaseous detector installed and
offers a relevant vacuum mode in the
Vacuum / Mode module.
When Low Vacuum / ESEM mode is entered from ESEM / Low
Vacuum mode by selecting an appropriate mode radio button,
nothing happens unless there is a different gas type being used for
the two modes. In this case, the appropriate gas type is selected.
When Low Vacuum / ESEM mode is entered from High Vacuum
mode / Vented status by selecting an appropriate mode radio button,
the system prompts the user with the
PLA Configuration dialogue
(this happens only for the first time after a particular Vent procedure).
It is followed by a new mode initializing dialogue and then by the
manual
EBV open dialogue.
Pressure
The Chambre Pressure adjuster is used to set and display the target
chamber pressure. Pascal, Torr or Millibar units are available and can
be selected in
Preferences… / Units (see Chapter 4).
When the system is in Low Vacuum or ESEM mode and the Chamber
Pressure value is changed, the pressure automatically changes to
the new value. When the system is in any other state 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 or
ESEM mode again.
The actual specimen chamber pressure is displayed in the Status
module / Chamber Pressure: field.
Pressure Limiting Aperture (PLA) and Cones
The maximum allowed specimen chamber pressure in LoVac or
ESEM mode is determined by the size of the
PLA and a gas type.
The PLA Configuration dialogue prompts to inform the system about
the used PLA (in case it is not a part of a dedicated gaseous detector)
when pumping or switching to LoVac or ESEM mode. Along with a
gas type, this information sets pressure limits and rates for pressure
changes.