1.2.5

Table 1.2-1 Allowable Stray Magnetic Field

 

 

 

AC magnetic Field

DC magnetic Field

 

 

 

Variation

 

Resolution Guaranteeing

(rms value)

 

 

 

(peak to peak)

Conditions

 

 

 

 

 

 

 

 

 

 

 

 

Horizontal

Vertical

Horizontal

Vertical

 

 

 

 

 

 

Direction

Direction

Direction

Direction

 

 

 

 

 

 

 

Accelerating

 

 

 

 

 

 

voltage

:

3 kV

 

 

 

 

Magnification

:

×30,000

140 nT

730 nT

160 nT

820 nT

WD

:

5.0 mm

 

 

 

 

Beam

:

10

 

 

 

 

 

 

 

 

 

 

 

Accelerating

 

 

 

 

 

 

voltage

:

30 kV

 

 

 

 

Magnification

:

×100,000

200 nT

640 nT

230 nT

720 nT

WD

:

5.0 mm

 

 

 

 

Beam

:

0

 

 

 

 

 

 

 

 

 

 

 

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.

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