Secondary Electron Counting Images in SEM

Abstract

The conventional method of image recording in scanning electron microscopy is photographic recording of the images displayed on a monitor screen of CRT installed with the SEM. An increasing number of works related to the development of computerized systems for digital recording and processing of the image data have been reported But a serious problem exists that the digiatal image data are acquired by discretizing the analogue output of the conventional detector system. In actuality, proportionality between the analogue output and the number of detected electrons is not ensured because of various kinds of noises and nonlinearity which are parasitic with the electronic circuitry. As a solution, we proposed a detection system to measure directly the number of signal electrons. In this paper, we describe electron counting for secondary electron detection in SEM, and show that the electron counting can provide a far better image quality than the analogue output.I. Images by Counting Method and by Analogue MethodThe signal obtained by the counting method is the electron number reaching the detector. This means that the method can eliminate the thermal noise and the fluctuation of the amplification which are parasitically generated during amplification of any type of secondary electron detectors including Everhart-Thornley detector and semiconductor detectors. In addition, noises caused by electronic circuits connected to the detector can be reduced. As shown in Fig.1, even when a primary electron current is lowered to the level less than 1 pA, we could obtain clear images by the counting method as compared with those by the analogue method. It is clear that the electron counting possesses remarkable advantages, as the secondary electron signal becomes weak.