An anomalous contrast in scanning electron microscopy of insulators: The pseudo-mirror effect

In a scanning electron microscope, electron-beam irradiation of insulators may induce a strong electric field due to the trapping of charges within th e specimen interaction volume. On one hand, this field modifies the traject ories of the beam of electrons subsequently entering the specimen, resultin g in reduced penetration depth into the bulk specimen. On the other hand, i t leads to the acceleration in the vacuum of the emitted secondary electron s (SE) and also to a strong distortion of their angular distribution. Among others, the consequences concern an anomalous contrast in the SE image. Th is contrast is due to the so-called pseudo-mirror effect. The aim of this w ork is first to report the observation of this anomalous contrast, then to give:an explanation of this effect, and finally to discuss the:factors affe cting it. Practical consequences such as contrast interpretations will be h ighlighted.