Stereo Scan 440 - DCU

ChalKer 1. Opentioo 2.-)7
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magnification win result in changes of probe current, so it is advisable to enable
auto brightness.
Further guidance on selecting the correct probe current is given in the Chapter 3
Advanced Operation.
Optibeam
The Optibeam program takes the require~nts for probe current. working
distance, etc and determines the optimum lens settings to achieve the best
performance from the column. Usually the best performance is defined as the
highest resolution, but Optibeam can be told to optimise for the greatest depth of
focus instead. This is done by selecting Depth Mode on the Beam menu.
2.3.3. Adjusting the Aperture
Oprimum imaging can sometimes depend on the correct choice of aperture size.
The internal software model. Optibeam. continually calculates the best setup for
the selected aperture. however it will sometimes recommend using a different
aperture. This message is displayed in the Beam->Apertures menu. as
Recommended Aperture =. If preferred. this infonnation can be displayed
automatically on screen by switching off Suppress Aperture Warning.
The four apertures installed (sizes as displayed in the Apertures menu) are all
located on the Y axis of the aperture changer mechanism. To reach another
aperture, simply rotate the Y micrometer either clockwise or counter-clockwise
according to the direction of the required aperture - refer to the X and Y positions
displayed in the Apertures menu.
As the micrometer is rotated. the signal will disappear as the cunent aperture
moves out of position. The signal returns as the next aperture moves into
position. however there will either be much less or much more signal according
to the change in aperture size. For this reason. it is sometiux:s easier if auto
brightness is enabled during the change.
Having located the aperture in position. click on the "radio button" next to the
new current aperture selection and OK the menu. This ensures that Optibeam
will continue to achieve the best column performance.
Finally, it is necessary to make a fine adjustment to the aperture position to
ensure that it lies exactly on the electron optical axis. Select a focused field of
view at about lOOOx and switch on focus Wobble under the Beam menu. The
speed and amplitude of the wobble can be adjusted from the operating panel that
Stereoscan 440 Operator Manual