cathode wehnelt cap anode condenser lens system aperture scan coils objektive lens detectors sample

scanning electron microscopy (SEM) Similar to a tem the source of illumination is a thermo ionic gun consisting of a cathode (tungsten, LaB6), a whenelt cap and an anode. Electrons produced by heating the filament are accelerated towards the anode. The electrical negatively charged whenelt cap causes that the emitted electrons are directed towards the optical axis. The accelerating voltage between cathode and anode controls the speed of the electrons. The condenser lens system controls the size of the beam. Increasing the size of the beam achieves a better signal to noise ratio, but because the beam diameter is larger, it gives a lower resolution. Depending on the magnification, a compromise between signal to noise ratio and resolution achieves the best image quality. The beam width and therefore the depth of field is adjusted by the aperture. By varying the voltage between the scan coils, the electron beam can be deflected. If a scan generator is used to trigger this, the beam can be made to scan lines across the surface of the sample. The size of the scanned area determines the degree of magnification. The objective lens focuses the beam into a spot on the sample. This is necessary to have an image in proper focus. Due to the different interactions of the electron beam with the sample, different signals are induced and recorded by the various detectors. Secondary electrons are knocked out by the inelastic interaction of the electron beam with the atomic shell from a thin layer of the sample surface (0.5-2 nm). They have a weak energy (2-5 eV) and are collected to form a SEM image. Backscattered electrons have high energies (> 50 eV) and depend on the sample composition. They are detected to get information about the chemistry and/or about the shape of the sample. The sample is placed on a stage and can be moved, rotated and tilted. Images can be observed on a TV monitor and can be recorded.