IT-4-P-1852 New possibilities of SEM for two-channel detection of energetically filtered secondary and backscattered electrons

In this paper we present a preliminary study of new potentialities of SEM – microtomograph equipped with a toroidal spectrometer of electrons and two detector systems based on microchannel plates (MCP). A new modification of the instrument is shown in fig.1. Electron probe 1 scans across the surface of the sample under investigation 3. Toroidal spectrometer in a case 4 is mounted under SEM objective lens 2. SE an BSE emitted from specimen 3 pass through annular inlet slit 6 and are energy-separated in toroidal capacitor 5 and pass through outlet annular apertures 7 and 8. The energy filtered SE and BSE are detected by two MCP 9 (A and B) placed opposite each other. The signals A and B from these detectors can be sent either to PC 11 to record the spectra or to the SEM display. Using block 10 one can do the operations of addition (A+B) and subtraction (A-B) of signals. It is known that such operations allow us to obtain contrast from either the chemical composition of the specimen (Z-contrast) or the surface topography. In our case the contrast is enhanced and allows unique interpretation owing to filtration of detected electrons in a narrow energy window. The electrons with small energy losses escape mostly from the subsurface region and are modulated in escape angles, which favors domination of topographic contrast. The electrons, which lost considerable amounts of their energy, are emitted from much deeper regions and therefore mostly produce Z-contrast. Addition and subtraction of signals from the two oppositely oriented detectors enhances this effect considerably.

The examples presented in fig.2 and fig.3 show fragments of the sample having heterogeneous composition, consisting of the alloy of different materials, in particular, Cr, Si, Cu, W.

The images shown in fig.2 are obtained at the primary electron beam energy E₀=10 keV, the current I₀=1nА, the energy of filtered BSE forming the image E_BSE=8 keV ((a) and (b)), and at the SE energy E_SE=4 eV ((c) and (d)). One can see that the image contrast (obtained from signal addition) differs greatly in element composition and in surface topography (obtained from signal subtraction) for both BSE and SE regimes.

Fig.3 presents the images of another region of the sample taken at different energies of E₀. The general image of this region demonstrated in fig.3a is taken in the standard SE mode in SEM at E₀=5 keV. Fig.3b shows the image taken in the BSE added signal at E₀=5 keV, in fig.3c – at E₀=15 keV, and in fig.3d – in the subtracted signal. The fact that, contrast in filtered BSE and SE is higher and more informative than that obtained with standard signals and standard detectors in SEM makes it possible to more accurately visualize and reconstruct the sample 3D surface profile both in BSE and SE.