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1.2.5
1 - 9
Table 1.2-1 Allowable Stray Magnetic Field
AC magnetic Field
(rms value)
DC magnetic Field
Variation
(peak to peak)
Resolution Guaranteeing
Conditions
Horizontal
Direction
Vertical
Direction
Horizontal
Direction
Vertical
Direction
Accelerating
voltage : 3 kV
Magnification : ×30,000
WD : 5.0 mm
Beam : 10
140 nT 730 nT 160 nT 820 nT
Accelerating
voltage : 30 kV
Magnification : ×100,000
WD : 5.0 mm
Beam : 0
200 nT 640 nT 230 nT 720 nT
nT: nano-tesla
NOTICE: 1. The power frequency synchronous component (AC magnetic field) is in rms value.
To evaluate in peak-to-peak value, the allowable value will be 2.8 times the rms
value.
2. The DC magnetic field variation is an abrupt variation of stray magnetic field with
respect to time. A slow, gently sloping variation with time is handled as a field
drift.
3. The allowable AC magnetic field is the value at the prescribed image disturbance
(expansion/contraction or bending of image) of 0.5 mm and at the specified
magnification.
4. The allowable DC magnetic field variation is the value at the prescribed image
disturbance (image shift or whisker noise) of 0.2 mm and at the specified
magnification.
5. The DC component of stray magnetic field without fluctuation does not cause
swaying of the electron beam, and will not affect the image if the amount is
around several ten µT.
6. The allowable values in Table 1.2-1 are the ones under the resolution
guaranteeing conditions in the table. In general, the longer the WD or the lower
the accelerating voltage, the more the electron beam is susceptible to stray
magnetic field effect.
7. If the allowable values in Table 1.2-1 are exceeded, the stray magnetic field must
be reduced by one of the countermeasures below:
(1) Try a different installation location.
(2) Provide a magnetic shielding box.
(3) Provide an active magnetic field canceller.