System Options:
Optional Detectors
9-10
Loading samples
Materials or hard samples should be prepared as for the TEM by
appropriate thinning technique. The STEM I holder (which is a part of
the detector assembly) can either be loaded with samples while
outside the microscope chamber (which is more convenient) or when
it is mounted and fixed to the stage.
1.
Remove the holder top (by loosening the central screw). This
exposes the 8 grid positions (round holes with tweezers slots).
2.
Load the TEM grids with samples face-up into the grid holes.
The holder top has raised rings to press down in the grid holes to
hold the TEM grids firmly in place. The holder top numbers should
overlay the same ones on the base plate.
3.
Replace the holder top carefully and tighten down the central
screw.
To remove sample grids proceed in the reverse order.
Obtaining Imaging
1.
Switch on the accelerating voltage at 20 kV, set the spot size to 3.
2.
Using a fast scan focus the top of the STEM I holder surface with
SE detector.
3.
Link Z to FWD and bring the focused surface to a 5 mm WD.
4.
Move to the appropriate sample position and focus the TEM grid
bars. The WD and Z position has now lengthened and re-setting of
the Z-axis value to 5 mm is necessary.
This procedure is necessary to prevent inadvertently bringing the
detector in contact with the final lens. The minimum safe distance to
the sample surface is 1 mm. Be aware that the STEM I holder surface
is now closer to the lens than the sample.
By moving off the grid bars and fine focusing most imaging corrections
(image rotation, astigmatism) can be performed in the SE Mode.
5.
Choose the
Detectors
menu /
STEM I
detector and select the
segment mode, depending on the sample position in the holder. A
transmission sample image should be visible at a low magnification.
•
The
BF image:
change the accelerating voltage to suit the
contrast necessary through the sample. For example light element
materials (such as silicon or silicon oxide) may work better with 5 -
10 kV to create contrast, whereas dense materials (such as
metals) may require 10 - 20 kV or higher. Finally set the
magnification, fine focus and correct the astigmatism.
If the aperture adjustment is needed, it may be achieved more
easily by momentarily switching to ETD, because a faster scan
speed can be used.
•
The
DF image:
the samples that reside in the 1D and 5D positions
can be observed in the dark field mode. The separator line of the
two diodes crosses vertically the positions of 1D and 5D. An area
of interest on the left / right side of the line can be observed with
the right-hand / left -hand diode for DF / BF observation.
DF observation may require higher HV to create a suitable image
as the angle subtended to the detection diode can be wide.
Choosing 2× the value used for BF is a good guide level.
Detector home position
While the detector is not used, it could be placed into a holder which is
mounted inside the specimen chamber, saving it from a mechanical
damage and pollutions.
Содержание Quanta FEG 250
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