FEI Quanta FEG 250, Руководство пользователя, Страница: 178 / 228

System Options: Cooling Stage / Waterless Cooling Stage
9-30
COOLING STAGES BASIC OPERATIONS
Condensation Point
When determining the Cooling stage condensation point, use the flat
sample holder. This positions the sample closer to the thermistor, thus
giving a more accurate reading. Use carbon paint or carbon tape to
hold samples onto the holder. Better contact between the sample and
the holder yields better heat transfer.
Pressure and Temperature control
It is better to control the condensation by a pressure control, as
opposed to temperature control. Firstly, the thermoelectric module
heat pumping capability is very small; therefore a temperature is hard
to control accurately. Secondly, it is easier to keep the pressure below
the condensation point, which prevents water condensate from raising
the sample temperature (cooling water takes longer time to reach the
setpoint). As a general rule, condensation is achieved by the following
procedure:
1. Set the pressure to 540 Pa (4.0 Torr).
2. Bring the sample to 5 °C.
3. Raise the pressure until water condenses on the sample. Keep the
pressure below the condensation point 860 Pa (6.5 Torr).
Keeping the sample wet
Water in the sample tends to evaporate during the pump-down cycle.
The simplest way to keep the sample wet is to accurately control
sample temperature and pressure conditions. Another method is to
cool mounted sample to its operating temperature before it is put into
the chamber. Then add several drops of water to the stage base; this
displaces air faster during pump-down. There is an indentation on
each corner of the base for this purpose.
FIGURE 9-25 ADDING WATER TO COOLING STAGE BASE
Once the system enters Wet mode and the chamber pressure has
reached the setpoint value, the chamber automatically
Purges . It
repeats this process 5 times. Wait until the pressure returns to the
setpoint.
Condensation and the Detector
When imaging with the GSED, water droplets on the sample appear
darker than the surrounding features. When imaging with the BSED,
however, water appears lighter than the surroundings. One exception