OPTO-EDU A63.7069, Instruction Manual

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3.3.3. Bias control
❖
Gun bias is the voltage between filament and wehnelt cap, which is used to confine the
emission current of electron gun and stabilize the emitted electron numbers.
❖
Drag the scroll bar of [Bias] on panel with mouse can control the value of bias. The adjustment
range of Bias is 0-30KV, recommended less than 300V; if more than 300V, it is recommended
to increase the relative position of filament and wehnelt cap properly (adjustable by adding
adjustment shims) or reduce the distance between wehnelt and anode.
❖
The vacuum and Bias voltage are interlocked, so the Bias controls can start only when the
vacuum satisfies requirements and V1 valve is opened. The Bias voltage will be cut off
automatically when the vacuum is low.
3.3.4. Filament control
❖
Drag the scroll bar of [Filament] on panel with mouse can control the value of filament. The
adjustment range of filament is 0-3A.
The recommended value of saturation value of
filament current is among 2.4-2.6A. When finish the work, the filament current should be
adjust to 0A.
❖
The value of electron beam current depends on many factors. When operating SEM, users
can adjust bias voltage and filament current to get stable emission current of filament.
❖
Filament of electron gun of SEM is a V-shaped tip with very small diameter made from
tungsten filament, and can be heated by its power supply. When the filament reaches working
temperature, the emitted electrons will leave the tip. Due to the action of high voltage,
these electrons will be accelerated to the specimen and finally form electron beam current.
❖
After selected the HV, drag the scroll bar with mouse can adjust the Filament, the Beam
current value will display on the control panel. Filament should be worked at saturation
point. At the same time, the lightness and stability of image is the best. If the filament
current is not saturated, the brightness will be unstable, the image is also not stable. When
finished the work, filament should be turned off.
❖
The larger the filament current, the larger the beam current. Because the electron gun is self-
biased, when the filament current exceeds a certain value, the beam current will not change
any more, and the point is so-called saturation point. Generally, after reaching saturation, the