Analysis of the scanning electron microscope mirror image based on the dielectric surface microstructure

Abstract

This paper addresses hitherto unexplained phenomena associated with the electrostatic mirror technique, such as (i) the influence of the surface microstructural parameters of a polycrystalline dielectric (e.g., alumina) on mirror images, and (ii) the distortion of mirror images. The formation of a mirror image on an as-machined alumina surface was found to be nearly impossible, but on a polished specimen, possessing low surface damages, mirror images were easily formed. A smooth and low-damage surface seems to be a prerequisite for the formation of a mirror image. The size of mirror images, observed at different locations on a polished alumina surface, was found to vary significantly, indicating that an inhomogeneous surface charging occurs in polycrystalline dielectrics. It was shown that the variation in the grain and grain boundary volumes constituting the charged region is responsible for the variation in the size of the mirror image. The distortion of the mirror images was studied using single crystal quartz specimens. The asymmetry of the equipotential boundary—caused by (i) the tilt of the crystal axis (θ≠0°) with respect to the probing electron beam, or by (ii) the asymmetrical electron interaction volume, i.e., deviation from the standard pear shape—was demonstrated to be responsible for the distortion of the mirror images.