A scanning electron microscope (SEM), like a transmission electron microscope, consists of an electron optical column, a vacuum system, electronics, and software. The column is considerably shorter because the only lenses needed are those above the specimen used to focus the electrons into a fine spot on the specimen surface. The specimen chamber, however, is larger because the SEM technique does not impose any restriction on specimen size other than chamber size.
The electron gun at the top of the column produces an electron beam that is focused into a fine spot as small as 1 nm in diameter on the specimen surface. This beam is scanned in a rectangular raster over the specimen and the intensities of various signals created by interactions between the beam electrons and the specimen are measured and stored in computer memory. The stored values are then mapped as variations in brightness on the image display. The secondary electron (SE) signal is the most frequently used signal. It varies with the topography of the sample surface much like an aerial photograph: edges are bright, recesses are dark. The ratio of the size of the displayed image to the size of the area scanned on the specimen gives the magnification.
The electron gun and lenses are similar to those of a transmission electron microscope. The most important differences between a transmission electron microscope and a scanning electron microscope are:
- Rather than the broad static beam used in TEM, the SEM beam is focused to a fine point and scans line by line over the sample surface in a rectangular raster pattern.
- The accelerating voltages are much lower than in TEM because it is no longer necessary to penetrate the specimen; in a SEM they range from 50 to 30,000 volts.
- The specimen need not be thin, greatly simplifying specimen preparation.
The interactions between the beam electrons and sample atoms are similar to those described for a transmission electron microscope:
- The specimen itself emits secondary electrons.
- Some of the primary electrons are reflected backscattered electrons (BSE). These backscattered electrons can also cause the emission of secondary electrons as they travel through the sample and exit the sample surface.
- If the sample is thin, the SEM may be operated in STEM mode with a detector located below the sample to collect transmitted electrons.
All these phenomena are interrelated and all of them depend to some extent on the topography, the atomic number, and the chemical state of the specimen.
Next: The Scanning Transmission Electron Microscope (STEM)