A HIGH-CONTRAST DIRECTIONAL DETECTOR FOR SCANNING ELECTRON MICROSCOPE

Directionally sensitive detectors have been used in the scanning electron microscope to reveal additional detail due to deflection or non-uniformity of the spatial emission distribution of the secondaries which form the electron picture of the surface under examination. Deflection may occur as a result of electric or magnetic fields above the surface; also the emission characteristic may be anisotropic, or modified in shape and direction by local surface structure. These effects all occur in addition to the principal source of contrast in the scanning electron microscope, i.e. changes of secondary emission coefficient. The directional content of contrast is normally small, but the system discussed causes considerable enhancement of these effects. Results have the appearance of 'shading' of surface detail, revealing more clearly the nature of shallow undulations, with a three-dimensional effect similar to that obtainable by optical microscopy with side illumination. Comparative results are presented which indicate the advantages to be gained for examination of small surface detail. By appropriate adjustment of electrostatic bias on electrodes, the detector system can provide magnetic contrast, potential contrast due to changes in surface electric potential, electric contrast from horizontal electric fields and improved sensitivity to surface topography. To a considerable extent these can be separately identified by observation under several different detector field configurations.