Scanning Electron Microscopes
Secondary Electron Microscope or SEM was developed in 1938 to image the surface of samples with high resolution. Both a SEM and a light microscope apply the same principle; however, instead of using visible light, the SEM use electrons for imaging. Nonetheless, the wavelength of visible light limits resolution of the images from the optical microscope, while accelerated electrons in the SEM, which has much shorter wavelength, make it possible to investigate features in microscale to nanoscsale with high resolution. Therefore, this instrument has opened doors in numerous fields such as, physics, materials science, biology, chemistry etc.
To create an image, a beam of incident electrons(or primary electrons) is generated at the top of the microscope by a thermal emission source, for example, a heated tungsten filament, or a field emission cathode. The energy of the incident electrons can be varied from 100 eV to 30 keV depending on the evaluation objectives. The electron beam follows a vertical path through the vacuum chamber. The beam also passes through electromagnetic lenses which focus the beam down toward the specimen. When the beam hit the specimen, secondary electrons and X- rays are emitted from the specimen to chamber. Secondary electrons, backscattered electrons and X-rays are detected and converted to into a signal by detectors in order to produce images.
However, the primary limitation of the SEM is that a sample has to be clean, dry and electrically conductive. For non-conductive specimens, they must be coated with a conductive film. Moreover, a high vacuum chamber is required during operating to reach high resolution because the incident beam may interact with atoms in air before hitting the sample.