Scanning Electron Microscope (SEM)
SEMs are patterned after Reflecting Light Microscopes and yield similar information:
The surface features of an object or "how it looks", its texture; detectable features limited to a few manometers
The shape, size and arrangement of the particles making up the object that are lying on the surface of the sample or have been exposed by grinding or chemical etching; detectable features limited to a few manometers
The elements and compounds the sample is composed of and their relative ratios, in areas ~ 1 micrometer in diameter
The arrangement of atoms in the specimen and their degree of order; only useful on single-crystal particles >20 micrometers
A detailed explanation of how a typical SEM functions follows (refer to the diagram below):
- The "Virtual Source" at the top represents the electron gun, producing a stream of monochromatic electrons.
- The stream is condensed by the first condenser lens (usually controlled by the "coarse probe current knob"). This lens is used to both form the beam and limit the amount of current in the beam. It works in conjunction with the condenser aperture to eliminate the high-angle electrons from the beam.
- The beam is then constricted by the condenser aperture (usually not user selectable), eliminating some high-angle electrons.
- The second condenser lens forms the electrons into a thin, tight, coherent beam and is usually controlled by the "fine probe current knob".
- A user selectable objective aperture further eliminates high-angle electrons from the beam.
- A set of coils then "scan" or "sweep" the beam in a grid fashion (like a television), dwelling on points for a period of time determined by the scan speed (usually in the microsecond range).
- The final lens, the Objective, focuses the scanning beam onto the part of the specimen desired.
- When the beam strikes the sample (and dwells for a few microseconds) interactions occur inside the sample and are detected with various instruments.
- Before the beam moves to its next dwell point these instruments count the number of interactions and display a pixel on a CRT whose intensity is determined by this number (the more reactions the brighter the pixel).
- This process is repeated until the grid scan is finished and then repeated, the entire pattern can be scanned 30 times per second.