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OVERVIEW
The first scanning electron microscope (SEM) was credited to Max Knoll in 1935. The design was further developed by Manfred von Ardenne over the next few years. Despite these early contributions, Professor Charles Oatley is widely credited as the father of scanning electron microscopy. Professor Oatley developed and refined the technology into a commercially viable instrument at Cambridge University in the 1950s and 1960s.
The SEM incorporates an electron gun at the top of the column which thermionically generates a beam of electrons. As this beam travels down the column it is conditioned by coils which act as lenses that focus the beam and cause it to raster across a sample in the vacuum chamber. As the beam scans the sample, secondary electrons are emitted from the sample. Detectors in the chamber gather these electrons to produce an image of the sample.
Unlike Optical Microscopy, SEM is not limited by the diffraction limit of visible light. This enables researchers to image at several hundred thousand times magnification. This technique also maintains an excellent depth of field, allowing users to clearly resolve surface features of the sample.
In addition to its primary advantages, the scanning electron microscope has proven to be a versatile platform for adding capabilities that go beyond simple imaging. The thermionic gun can be replaced with or complemented by a field emission gun (FEG), which can improve the spatial resolution of the SEM dramatically. SEMs can be outfitted with focused ion beam (FIB) capability which enables ion beam milling and deposition. Energy-dispersive x-ray spectroscopy (EDS or EDX) allows users to analyze the chemical composition of a sample using x-rays generated when the electron beam hits the sample. With the addition of probes inside the chamber, users can analyze the conductivity of a sample. SEM platforms can also be outfitted for lithography and micromachining applications.
There must be a high level of coherence between the electron beam, sample, and detectors to get accurate images, which makes the instrument susceptible to environmental vibrations. Electron microscopes are relatively massive instruments and usually have built-in passive vibration isolation mechanisms. As a result, they are relatively insensitive to higher frequency vibrations. But SEMs are still vulnerable to low frequency vibrations, particularly building vibrations. Thus, when SEMs are placed on higher floors in buildings or in areas with significant seismic vibration levels, they often require supplementary vibration isolation systems. In this case, an Active Vibration Control System is advised.
Electron microscopes are also quite sensitive to EMI because the imaging technique utilizes electromagnetic forces. Care should be taken not to locate an SEM near significant sources of EMI, such as elevators, moving vehicles, or HVAC machinery. When excessive levels of EMI are present, the user will need to construct a large EMI shield around the SEM or install an EMI Cancellation System.
How Stuff Works: Scanning Electron Microscopes |
