Explanation of the high resolution backscattered electron image in the scanning electron microscope by the twin-population theory historical review

Abstract

Several different “twin-population” theories have been proposed to explain the high resolution backscattered electron (BSE) image in the scanning electron microscope (SEM). In all of these, the BSE are divided into local BSE that give an image having a resolution that is significantly better than the electron penetration depth R and diffused BSE that give a less sharp image. Other theories attach more importance to the diffused BSE.The BSE image was originally proposed by von Ardenne in 1940 with the specimen at right angles to the incident electron beam (Fig. 1). This has been summarised by McMullan. Von Ardenne predicted high resolution secondary electron (SE) imaging by local SE (now called SE-I) and pointed out the advantages of low voltage operation, but failed to realise that there was a high resolution BSE signal also. McMullan distinguished between the BSE image as defined above (Fig. 2(b)) where the principal contrast mechanism is compositional and the reflected electron image in which the sample is tilted by typically 45° and forward-scattered BSE are collected (Fig. 2(a)).