Basic principles of the SEM
The SEM uses a beam of high energy electrons generated by an electron gun, processed by magnetic lenses, focused at the specimen surface and systematically scanned (rastered) across the surface of a specimen. Unlike the light in a light microscope (LM), the electrons in a scanning electron microscope (SEM) never form a real image of the sample. The SEM image is in the form of a serial data stream i.e. it is an electronic image. It is a result of the beam probe illuminating the sample one point at a time in a rectangular scanning pattern (raster), with the strength of the signal generated from each point being a reflection of differences (e.g. topographical or compositional) in the sample.
The screen is scanned in synchrony with the beam on the specimen in a one-to-one relationship between points on the specimen and points on the image viewing screen i.e. a point-by-point translation. Increased magnification is produced by decreasing the size of the area scanned.
In more detail
The formation of an image requires a scanning system to construct the image point-by-point and line-by-line. The scanning system uses two pairs of electromagnetic deflection coils (scan coils) that scan the beam along a line then displace the line position to the next scan so that a rectangular raster is generated both on the specimen and on the viewing screen. The first pair of scan coils bends the beam off the optical axis of the microscope and the second pair bends the beam back onto the axis at the pivot point of the scan. In order to produce contrast in the image the signal intensity from the beam-specimen interaction must be measured from point-to-point across the specimen surface. Signals generated from the specimen are collected by an electron detector, converted to photons via a scintillator, amplified in a photomultiplier, and converted to electrical signals and used to modulate the intensity of the image on the viewing screen.