History

Widespread use of SEM followed many years after the TEM. The cause of this delay was likely due to the fact that scientists had been using optical microscopes without scanning for hundreds of years and the TEM was based on more familiar methods. The concept of producing an image by scanning was perhaps first concieved in 1842 when Alexander Bain invented the first fax machine. The proposal of using charged particle beams (including electron beams) for scanning was later proposed in a German patent by Stintzing in 1929 [Stintzing H: Verfahren und Einrichtung zum automatischen Nachweiss, Messung und Zählung von Einzellteilchen beliebiger Art, Form und Grösse. German patent No 485155 (1929a)]. The first scanned electron image was produced in 1935 by Max Knoll, coinventor of the TEM which received more attention at the time. It wasn't until 1952 that Sir Charles Oatley invented the SEM in its current form [D. McMullan, "Scanning Electron Microscopy 1928 - 1965" Presentation, 51st Annual Meeting of the Microscopy Society of America, Cincinnati, August 1993.].

Principles of Operation

SEM operates by scanning an electron beam over the sample and measuring the electrical interactions with the surface. When the electrons hit the surface, weakly bound electrons will be ejected to produce secondary electrons. These secondary electrons can then be measured by a detector, and used to calculate the color for each pixel of an SEM image. Since these secondary electrons are of low energy their trajectories can be easily influenced by electromagnetic fields. In order to avoid a charge build-up on the surface of the sample which would alter the path of the secondary electrons, the surface must be conducting. However, shadowing methods have been developed for coating non-conductive samples with a thin layer of metal so that SEM measurements become possible.

Advantages

• Provides images of surface features
• Much higher resolution than optical microscopy, due to the very small wavelength of the electron
• Faster than Scanning Probe Microscopy as the beam can be scanned with electromagnetic fields rather than mechanical actuators

Disadvantages

• Requires a conductive sample (or that the surface of a non-conductive sample be metallized)
• Must be carried out in a vacuum, which is not only inconvenient but may also result in artifacts
• Unlike SPM or interferometry, the height and chemical properties of the surface can not be quantitatively determined by SEM.

SEM Company

• JEOL SEM instrument vendors - Conventional, Research Grade, Low Vacuum, Analytical Field Emission, Semi-in-Lens Field Emission (Also sells TEM, EBX, MS & NMR).