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3.10.2
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Table 3.10-3 Objective Lens Movable Aperture and Image Quality
Stop No. 1 2 3 4
Aperture diameter
（µm）
150 80 50 30
Focal depth Shallow Deep
Probe current High Low
Resolution Low High
Application
X-ray analysis on
low accelerating
voltage
Observation and normal X-ray
analysis
Observations
requiring a great
focal depth
See 3.4.2 Axial Alignment.
3.10.4 Influence of WD in Low-vacuum Mode
When observation is conducted in the low-vacuum mode, in which the specimen chamber is
maintained at a low vacuum level, some of the primary electrons and the backscattered
electrons that provide signals are subject to scattering by residual gas molecules, and this may
result in a less than clear image. To ensure a clear image, the following settings may be required.
(1) Accelerating voltage
The accelerating voltage should be set as high as possible (see Table 3.10-1).
(2) Probe current
If image roughness is noticeable, the probe current should be increased (see Table 3.10-2).
(3) Working distance
To increase the sensitivity of the detector, the working distance should be minimized as
much as possible (see Table 3.10-4).
Table 3.10-4 Impact of WD in Low-vacuum Mode
Working Distance (mm) 10 50
Scattering of irradiation electrons Low High
Scattering of backscattered electrons Low High