Scanning Electron Microscopy

The scanning electron microscope (SEM) is, next to the optical microscope, one of the most important and versatile tools available to investigators not only in forensic science but in many other disciplines in the biological and physical sciences. There are some fundamental differences in the image formation process in the two instruments but the SEM, like the optical microscope, also shows an image of the sample under investigation.

The SEM has two major advantages over a conventional microscope: higher magnification and greater depth of field. At higher magnification more details may be evident, which can certainly help in establishing a specimen's origin. The great depth of field of the SEM is also extremely important. In an optical microscope, objects are only in focus only in a shallow plane at higher magnification, which can make the examination of specimens difficult.

SEMs may be classified into three generations according to the type of source used to produce the electron beam. The oldest instruments with the poorest resolution (beam diameters of perhaps 20 nanometers) used tungsten filaments to emit electrons. The next generation used lanthanum hexaboride emitters (beam diameters of 5 to 6 nanometers). The most modern instruments use a field emission (FE) source and have a beam diameter of about 1.5 nanometers. The FE SEMs can also operate at much lower beam voltages while yet achieving adequate spatial resolution. The Aerospace Corporation has a new field emission instrument in addition to several lanthanum hexaboride and tungsten emitter instruments.

SEMs are very often equipped with an energy-dispersive x-ray (EDX) analysis system for the elemental analysis of specimens.

No discussion of electron microscopy would be complete without showing at least one electron micrograph. To the left is a photo of a human hair. The scales that you see are quite typical of hairs. A micrograph of a hair is not unusual, but note the Aerospace logo on the hair. In fact, you can see a still smaller logo about 5 micrometers in diameter carved within the outer logo. These features were made using a focused ion beam (FIB) milling machine. FIB systems are now virtually indispensible for the analysis of microelectronic components and some day may well find applications in forensic science.

For more information on scanning electron microscopy contact Dr. Gary Stupian at stupian@law-west.org.

National Law Enforcement &
Corrections Technology Center - West
c/o The Aerospace Corporation
2350 East El Segundo Boulevard
El Segundo, California 90245-4691
Phone: 888-548-1618
Fax: 310-336-2227
E-mail: nlectc@law-west.org