1.2.5

1.2.5Stray Magnetic Field

Image disturbance will be negligible when the stray magnetic field conditions shown in Tables

1.2-1 are satisfied at the instrument installation place. Before proceeding to installation, check the stray magnetic field conditions as required. Make sure that the conditions shown in the tables are satisfied.

If there is a large-sized magnet clutch or power cable for other equipment in the vicinity of this instrument, abrupt variation in current or magnetic field may take place. Avoid installing the instrument in such a location.

If the instrument is installed in an environment having intense stray magnetic fields, the scanning electron beam is forced to tremble due to the magnetic field, causing abnormal expansion, shrinkage, distortion, unintentional shift, or whisker noise during imaging. These adverse effects occur differently depending upon whether the frequency component of stray magnetic field is synchronous with the power current frequency or not. Therefore, the allowable value of stray magnetic field component synchronous with power current frequency is different from that of stray magnetic field component asynchronous with power current frequency (DC magnetic field variation).

•The stray magnetic field component synchronous with power frequency appears as image distortion or deformation on the CRT display (Fig. 1.2-1). This is because the electron beam is scanned in synchronization with the power frequency.

The maximum allowable value of stray magnetic field component synchronous with power frequency is specified so that no adverse effect will be given to CD measurement reproducibility.

•The stray magnetic field component asynchronous with power frequency affects imaging directly. That is, movement of the electron beam due to variation in external magnetic field appears as unintentional shift or whisker noise in imaging (Fig. 1.2-2).

Even a slight variation in external magnetic field may result in conspicuous irregularity of the image. The maximum allowable value of stray magnetic field component asynchronous

with power frequency is specified so that CD measurement reproducibility will not be affected by image shift or whisker noise.

The effect on the scanning electron beam varies also according to whether the stray magnetic field component is in the horizontal or vertical direction, and the allowable value differs for each of these components. Given below are the allowable values stipulated under the resolution guaranteeing conditions (high resolution observation conditions), and under conditions different from these. Image disturbance may occur even if the stray magnetic field at the site satisfies the allowable values given in Tables 1.2-1.

In general, the effect of a stray magnetic field (wavering of scanning electron beam) is inversely proportional to the square root of the accelerating voltage, provided the other parameters are kept constant. The effect increases nearly proportionally to the working distance.

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