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4.4.5
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4.4.5 When Image is Very Noisy
There may be three types of noise on scanning images.
The first is a snow noise that appears uniformly on the image. It is mainly due to statistical
fluctuation of number of primary or secondary electrons. It appears when probe current is too
small, secondary electron emission efficiency of the specimen is too low, or efficiency of signal
detection is too low.
If images show such a noise, check the following.
(1) Is the emission current too low?
Reset the accelerating voltage, the filament, and the gun bias.
See <3.3.2 Setting the Accelerating Voltage and Filament Current>.
(2) Is the probe current setting too low?
If the probe current level is less than 30, it should be set to a level greater than or equal to
30.
See <3.10.2 Probe Current Setting and Image Quality>.
(3) If you are using TV or FAST scanning speed, open the Image tab of the Setup dialog
window and check if the Frame Averaging for TV1, TV2, Fast1 and Fast2 is set at 1 or 2.
If so, try setting at 4 and 8.
(4) Decrease contrast and increase brightness.
(5) If the BSE is selected, re-check the BSE settings in the DETECTOR tab on the Operation
Panel. If contrast is too high, reduce the BSE Gain; if contrast is too low, increase the
BSE Gain. The detector should normally set to the COMP mode.
In the case of a BSE detector, a long WD reduces the detection efficiency. If that is the
case, set a WD less than 15 mm. If the BSE is selected in the high-vacuum mode, try to
select the SE detector.
See <3.5.1.2 Backscattered Electron Detector (BSE)>.
(6) On Operate menu, set the Filament Image mode. If a round image appears off center on
the screen or the circle is chipped, re-adjustments should be performed.