Electron optical property of a charged foil, observed with a transmitted electron detector device in an SEM

Abstract

AbstractExperimental evidence is presented for the electron optical behaviour of a charged foil area, using the transmitted electron detection device of the scanning electron microscope JSM 50 A (JEOL). The primary electron beam scanning a thin pioloform foil on the one hand produces a charged foil region which on the other hand acts as an electron lens to the primary and scattered electrons.Scanning electron microscopical investigations of air particulates in the submicron size range can be eased by using a transmitted electron detection device both of the bright and dark field operation mode. The image contrast thus may be improved by orders of magnitude, also allowing on line operation of an image analysis system. Using a special preparation technique, depositing the particles on a thin supporting foil which is also used for LAMMA analysis – Wieser et al. 1980, the x‐ray spectra of single particles provided by an energy dispersive x‐ray spectrometer may be quantitatively interpreted on the basis of the peak‐to‐background method (Statham and Pawley 1978, Small et al. 1979).Figure 1 shows a schematic of the transmission detector device of the JSM 50 A when operated in the dark field mode. Geometrical dimensions and apertures also are given in Fig. 1. The dark field diaphragm (DFD) on the optical axis of the microscope blocks all electrons (primary electrons and scattered electrons) within an angle of about 10−2 rad from contributing to the video signal. As long as magnifications above about 350 × are used the primary electron beam hits the DFD thus yielding a transmission scanning electron micrograph in dark field mode. Below this limit or above the corresponding maximal scanning angle (about 7 × 10−3 rad) of the primary electron beam the rim of the DFD becomes visible in the displayed image as shown in Fig. 2a. At the same magnification Figure 2b shows the sharpened contours of the DFD as obtained by focussing the primary electron beam to the plane of the DFD by lowering the objective lens excitation. By means of the thin bar attached to the DFD (left hand upper corner of Fig. 2b) the DFD may be centered to the optical axis or exchanged to the bright field aperture.Looking to the circular center of Fig. 2a, we recognize the black grid bars and a few black particles whereas the supporting foil looks bright. No video signal can be obtained, because both the grid bars, and to a lesser extent the particles, are not transparent to the primary electrons of 15 keV energy. On the other hand all electrons scattered by the thin foil to an angle of more than 10−2 rad are seen by the scintillator and hence accumulate a measurable video signal: This is also favoured by the large solid angle outside the DFD, which is about 30 times the solid angle of the DFD itself.