Probe Size and Current

The resolution of SEM imaging is determined by the cross-sectional diameter of the scanning probe. Thus, the size of the probe limits the size of features on the specimen surface to be resolved. To obtain high resolution, we should know how to minimize probe size. The probe diameter is approximately expressed as dp. 

Even though reducing the probe size is necessary, the reduction of probe size alone is not sufficient to obtain a high resolution image in an SEM. For obtaining a high resolution image, 

The relationship between the probe current and image visibility can be understood by analyzing the basic requirements that an image will not be obscured by background noise. Background noise in an SEM system is generated by fluctuation of the electron beam current and signal amplification in the detector, and cannot be totally eliminated. 

The noise level can be expressed as the standard deviation of signals. Suppose that a signal S consists of n counts at the detector its noise level is then considered to be the square root of its mean n value. Thus, we can rewrite the Rose visibility criterion. 

Recalling the definition of contrast (C) (Equation 1.6), we can express the criterion for signal 

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