Tech Notes Basic design of the scanning electron microscope

The main components of gun to the specimen the scanning electron surface. Electron gun microscope include the electron gun, probe-forming Lens system Filament column (consisting of Magnetic electron lenses magnetic electron lenses, focus the image of the apertures, and scanning electron source to the final Wehnelt cylinder coils), electron detectors, spot size on the sample and vacuum system. surface. In most cases, three lenses are needed. The electron gun produces Apertures are placed Anode a narrowly divergent beam between the lenses to limit of electrons directed down the beam diameter. the centerline of the First condenser column. The electron source Two sets of scanning coils lens is a tungsten filament that are placed in the bore of the acts as a cathode. It is objective lens to scan the 0.25-mm in diameter and is electron beam across the heated to approximately sample. The electron beam Spray aperture 2500°C (4530°F). The scans the sample in much electrons essentially boil off the same way as a cathode (thermionic emission) the Second condenser ray tube for image formation lens sharply bent tip of the on a television screen, and filament and are attracted to the output of the electron the anode, which consists detectors is displayed on of a grounded plate with a the screen of a CRT. In Final aperture hole to let the electrons modern instruments, the pass. The anode is analog scanning systems maintained at a positive are replaced by digital Scanning coils voltage relative to the systems in which the filament, ranging from 5 to movements of the 30 kV. This voltage, con- beam are controlled Objective lens trolled by the operator, is with a computer. generally held at 20 kV, but it The analog signal from can be varied for X-ray and the electron detectors structure analysis. is digitized and stored Secondary as a number for High-vacuum pump electron detector A third electrode (the each pixel. alkaline-oxide-coated Wehnelt cylinder) has a negative bias of a few Specimen hundred volts relative to the Vacuum pump cathode. It is introduced to limit the emitting area to the tip of the tungsten filament Backscattered electron detector The electron gun and the Typical design of a scanning electron column must be evacuated This information is from the microscope for secondary and to avoid damage to the ASM Handbooks Online: backscattered electron imaging. electron source and high- Volume 9, Metallography voltage breakdown in the and Microstructures —> gun. High vacuum is also Scanning Electron necessary to minimize Microscopy —> Basic Design scattering of the electrons of the Scanning Electron during their travel from the Microscope ADVANCED MATERIALS & PROCESSES/FEBRUARY 2008 89