Scanning electron microscope

The scanning electron microscope (SEM) is a type of electron microscope capable of producing high resolution images of a sample surface. Due to the manner in which the image is created, SEM images have a characteristic three-dimensional appearance and are useful for judging the surface structure of the sample.

Scanning process

In a typical SEM electrons are thermionically emitted from a tungsten or lanthanum hexaboride (LaB₆) cathode and are accelerated towards an anode; alternatively electrons can be emitted via field emission (FE). Tungsten is used because it has the highest melting point and lowest vapour pressure of all metals, thereby allowing it to be heated for electron emission. The electron beam, which typically has an energy ranging from a few hundred eV to 50 keV, is focused by one or two condenser lenses into a beam with a very fine focal spot sized 1 nm to 5 nm. The beam passes through pairs of scanning coils in the objective lens, which deflect the beam in a raster fashion over a rectangular area of the sample surface. As the primary electrons strike the surface they are inelastically scattered by atoms in the sample. Through these scattering events, the primary electron beam effectively spreads and fills a teardrop-shaped volume, known as the interaction volume, extending from less than 100 nm to around 5 µm into the surface. Interactions in this region lead to the subsequent emission of electrons which are then detected to produce an image. X-rays, which are also produced by the interaction of electrons with the sample, may also be detected in an SEM equipped for energy dispersive X-ray spectroscopy or wavelength dispersive X-ray spectroscopy.

Detection of secondary electrons

The most common imaging mode monitors low energy (<50 eV) secondary electrons. Due to their low energy, these electrons originate within a few nanometers from the surface. The electrons are detected by a scintillator-photomultiplier device and the resulting signal is rendered into a two-dimensional intensity distribution that can be viewed and saved as a Digital image. This process relies on a raster-scanned primary beam. The brightness of the signal depends on the number of secondary electrons reaching the detector. If the beam enters the sample perpendicular to the surface, then the activated region is uniform about the axis of the beam and a certain number of electrons "escape" from within the sample. As the angle of incidence increases, the "escape" distance of one side of the beam will decrease, and more secondary electrons will be emitted. Thus steep surfaces and edges tend to be brighter than flat surfaces, which results in images with a well-defined, three-dimensional appearance. Using this technique, resolutions less than 1 nm are possible.

Detection of backscattered electrons

In addition to the secondary electrons, backscattered electrons can also be detected. Backscattered electrons may be used to detect contrast between areas with different chemical compositions. These can be observed especially when the average atomic number of the various regions is different.

Backscattered electrons can also be used to form an electron backscatter diffraction (EBSD) image. This image can be used to determine the crystallographic structure of the specimen.

There are fewer backscattered electrons emitted from a sample than secondary electrons. The number of backscattered electrons leaving the sample surface upward might be significantly lower than those that follow trajectories toward the sides. Additionally, in contrast with the case with secondary electrons, the collection efficiency of backscattered electrons cannot be significantly improved by a positive bias common on Everhart-Thornley detectors. This detector positioned on one side of the sample has low collection efficiency for backscattered electrons due to small acceptance angles. The use of a dedicated backscattered electron detector above the sample in a "doughnut" type arrangement, with the electron beam passing through the hole of the doughnut, greatly increases the solid angle of collection and allows for the detection of more backscattered electrons.

Beam-injection analysis of semiconductors

The nature of the SEM's probe, energetic electrons, makes it uniquely suited to examining the optical and electronic properties of semiconuctor materials. The high-energy electrons from the SEM beam will inject charge carriers into the semiconductor; that is, the beam electrons will loose energy by promoting electrons from the valence band into the conduction band, leaving behind holes.

In a direct bandgap material, recombination of these electron-hole pairs will result in cathodoluminescence; if the sample contains an internal electric field, such as is present at a p-n junction, the SEM beam injection of carriers will cause electron beam induced current (EBIC) to flow.

Cathodoluminescence and EBIC are referred to as "beam-injection" techniques, and are very powerful probes of the optoelectronic behavior of semiconductors, particularly for studying nanoscale features and defects.

Resolution of the SEM

The spatial resolution of the SEM depends on the size of the electron spot which in turn depends on the magnetic electron-optical system which produces the scanning beam. The resolution is also limited by the size of the interaction volume, or the extent of material which interacts with the electron beam. The spot size and the interaction volume are both very large compared to the distances between atoms, so the resolution of the SEM is not high enough to image down to the atomic scale, as is possible in the transmission electron microscope (TEM). The SEM has compensating advantages, though, including the ability to image a comparatively large area of the specimen; the ability to image bulk materials (not just thin films or foils); and the variety of analytical modes available for measuring the composition and nature of the specimen. Depending on the instrument, the resolution can fall somewhere between less than 1 nm and 20 nm. In general, SEM images are much easier to interpret than TEM images, and many SEM images, beyond their scientific value, are actually beautiful.

External links

Notes on the SEM Notes Covering All Aspects Of The SEM
JEOL SEM Scanning Electron Microscope Manufacturer SEM/STEM/TEM/FIB/DUAL-BEAM/EBEAM)
Hitachi High-Technologies Europe GmbH, Krefeld/Germany Scanning Electron Microscope Manufacturer SEM/CD-SEM/STEM/TEM/FIB)
Environmental Scanning Electron Microscope (ESEM)

History

Microscopy History links from the The University of Alabama Department of Biological Sciences
The Early History and Development of SEM from Cambridge University Engineering Department
Milestones in the History of Electron Microscopy from the Swiss Federal Institute of Zurich

Images

Tescan Image Gallery Some great SEM images of various specimens, as well as analytical results
Dennis Kunkel Microscopy, Inc. Large collection of SEM images - mostly false colour
Jeol SEM Images Twelve SEM images of various specimens
SEM Lab at the Smithsonian National Museum of Natural History; includes a gallery of images

Gallery of SEM images

The following are images taken using a scanning electron microscope.

SEM picture of a bend in a seven-lobed polyester fiber. Note dotted line equals 20 micrometers.
An SEM image of various types of pollen.
SEM Picture of a Diatom at magnification of 5000X.
False coloured SEM image of soybean cyst nematode and egg.
SEM image of an ant head.
SEM image of asbestos fibers.
Compound eye of Antarctic krill Euphausia superba.
Ommatidia of Antarctic krill eye.

Ommatidia of Antarctic krill eye.
SEM image of a discharged nematocyst.

SEM image of a discharged nematocyst.
SEM image of the upper body of a Drosophila.
SEM images of the compound eye of a Drosophila.
SEM image of the grasshopper's spiricale valve.