Understanding how the SEM works   In the SEM a beam of electrons is generated in the electron gun (cathode, located at the top of the column), this beam is attracted through the anode, condensed by a condenser lens, and focused as a very fine point on the sample by the objective lens. The electron beam hits the sample, producing, among others, secondary and backscattered electrons from the sample. These electrons are collected by a secondary electron or a backscattered electron detector, converted to a voltage, and amplified. The beam scanning the sample surface and the display (CRT) beam are synchronised. In the figure on the right, the liquid nitrogen Dewar of the EDS detector is shown on the right, with the snout containing the Si(Li) crystal entering the specimen chamber in the low left. The electron gun and the ion getter pump are on the top of the specimen chamber.   The electron column   The electron column consists of an electron gun and two or more electron lenses operating in a vacuum. When the SEM is used, the column must be at a vacuum. Vacuum must be used because:   Gases could react with the electron source, causing it to burn out, or cause electrons in the beam to ionize the surrounding gas, which produces random discharges and leads to instability. In gas-filled environment, an electron bean cannot be generated or maintained because of a high instability in the beam. The transmission of the beam through the electron optic column would also be hindered by the presence of other molecules, scattering the beam and counteracting the electromagnetic focusing. The other molecules (from the sample or the gas) could form compounds and condense on the sample. This would lower the contrast and make details in the image unclear. Schematic drawing showing the electron column, the deflection system and the electron detectors1       The electron gun     The electrons are ejected from a filament, made of tungsten (our SEM). This filament functions as the cathode. During operation the filament and its heating supply are maintained at a high negative potential by the high voltage supply. At the operating temperature, electrons are emitted from the V- shaped filament tip towards the anode. The anode is positive with respect to the filament - this forms powerful attractive forces for the electrons. A hole in the anode allows a fraction of these electrons to continue down the column towards lenses, and onto the sample. Schematic diagram of the electron gun1       Electron lenses     Electron lenses beyond the electron gun are used to demagnify the image of the crossover in the electron gun to the final spot size on the specimen (1 nm-1 um). This represents a demagnification of as much as 10,000x for a thermionic source. In a field emission system, since the source size is already small, a 1-2 nm probe size in this case requires only a demagnification of 10-1000x. Electrons can be focused by either electrostatic or magnetic fields.1   1 Scanning Electron Microscopy and X-Ray Microanalysis, Second Edition, Joseph Goldstein, Dale Newbury, Patrick Echlin, David Joy, A. Romig, Charles Lyman, Charles Fiori, and Eric Lifshin, Plenum Press, New York and London, 1992.   Back

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