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Encyclopedia > Scanning electron microscope
SEM Cambridge S150 at Geological Institute, University Kiel, 1980
SEM Cambridge S150 at Geological Institute, University Kiel, 1980
SEM opened sample chamber

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. The SEM was pioneered by Manfred von Ardenne in the 1930s [1] [2]. The instrument was further developed by Charles Oatley and first commercialized by Cambridge Instruments Image File history File linksMetadata Download high-resolution version (1346x878, 112 KB) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Scanning electron microscope Metadata This file contains additional information, probably added from the digital camera or scanner used... Image File history File linksMetadata Download high-resolution version (1346x878, 112 KB) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Scanning electron microscope Metadata This file contains additional information, probably added from the digital camera or scanner used... Image File history File linksMetadata SEM_chamber1. ... Image File history File linksMetadata SEM_chamber1. ... This article does not cite any references or sources. ... Manfred von Ardenne (January 20, 1907 - May 26, 1997) was a German inventor. ... Sir Charles Oatley (1904–1996), Professor of Electrical Engineering, University of Cambridge, 1960–1971, and developer of one of the first scanning electron microscopes. ...

Contents

Scanning process

Low-temperature SEM magnification series for a snow crystal.
Low-temperature SEM magnification series for a snow crystal.
An insect coated in gold, having been prepared for viewing with a scanning electron microscope.
An insect coated in gold, having been prepared for viewing with a scanning electron microscope.

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 100 keV, is focused by one or two condenser lenses into a beam with a very fine focal spot sized 0.4 nm to 5 nm. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron optical column, typically in the objective lens, which deflect the beam horizontally and vertically so that it scans in a raster fashion over a rectangular area of the sample surface. When the primary electron beam interacts with the sample, the electrons lose energy by repeated scattering and absorption within a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than 100 nm to around 5 µm into the surface. The size of the interaction volume depends on the electrons' landing energy, the atomic number of the specimen and the specimen's density. The energy exchange between the electron beam and the sample results in the emission of electrons and electromagnetic radiation, which can be detected to produce an image, as described below. Download high resolution version (900x3994, 1532 KB)Low temperature scanning electron microscope [1] magnification series, from 93x to 36,000x magnification series, of a snow crystal. ... Download high resolution version (900x3994, 1532 KB)Low temperature scanning electron microscope [1] magnification series, from 93x to 36,000x magnification series, of a snow crystal. ... Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. ... Image File history File links Metadata Size of this preview: 496 × 600 pixelsFull resolution (1859 × 2248 pixel, file size: 698 KB, MIME type: image/jpeg) Photo credit: Peter Halasz. ... Image File history File links Metadata Size of this preview: 496 × 600 pixelsFull resolution (1859 × 2248 pixel, file size: 698 KB, MIME type: image/jpeg) Photo credit: Peter Halasz. ... Thermionic emission (archaically known as the Edison effect) is the flow of electrons from a metal or metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. ... General Name, Symbol, Number tungsten, W, 74 Chemical series transition metals Group, Period, Block 6, 6, d Appearance grayish white, lustrous Standard atomic weight 183. ... Lanthanum hexaboride (LaB6, also called lanthanum boride and (incorrectly) LaB) is an inorganic chemical, a boride of lanthanum. ... Diagram of a copper cathode in a Daniells cell. ... Diagram of a zinc anode in a galvanic cell. ... Also known as Fowler-Nordheim tunneling, field emission is a form of quantum tunneling in which electrons pass through a barrier in the presence of a high electric field. ... The electronvolt (symbol eV) is a unit of energy. ... A lens. ... An objective lens is the lens in a microscope, telescope, camera or other optical instrument, that receives the first light rays from the object being observed. ... Suppose the smiley face in the top left corner is an RGB bitmap image. ...


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 an Everhart-Thornley detector which is a type of 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. The Everhart-Thornley Detector is a detector used in scanning electron microscopes (SEMs). ... A scintillator is a device or substance that absorbs high energy (ionizing) electromagnetic or charged particle radiation then, in response, fluoresces photons at a characteristic Stokes-shifted (longer) wavelength, releasing the previously absorbed energy. ... Photomultipliers, or photomultiplier tubes (PMT) are extremely sensitive detectors of light in the ultraviolet, visible and near infrared. ... A digital image is a representation of a two-dimensional image as a finite set of digital values, called picture elements or pixels. ...


Detection of backscattered electrons

Backscattered electrons consist of high-energy electrons originating in the electron beam, that are reflected or back-scattered out of the specimen interaction volume. Backscattered electrons may be used to detect contrast between areas with different chemical compositions, especially when the average atomic number of the various regions is different, since the brightness of the BSE image tends to increase with the atomic number.


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. Electron Backscatter Diffraction Image - Taken from National Institute of Standards and Technology Materials Reliability Division Electron backscatter diffraction (EBSD), also known as backscatter Kikuchi diffraction (BKD) is a microstructural-crystallographic technique used to elucidate the crystallographic texture or preferred orientation of any crystalline or polycrystalline materials. ...


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. The Everhart-Thornley Detector is a detector used in scanning electron microscopes (SEMs). ...


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 semiconductor materials. The high-energy electrons from the SEM beam will inject charge carriers into the semiconductor. Thus, beam electrons lose energy by promoting electrons from the valence band into the conduction band, leaving behind holes. Charge carrier denotes in physics a free (mobile, unbound) particle carrying an electric charge. ... In solids, the valence band is the highest range of electron energies where electrons are normally present at zero temperature. ... In semiconductors and insulators, the conduction band is the range of electron energy, higher than that of the valence band, sufficient to make the electrons free to accelerate under the influence of an applied electric field and thus constitute an electric current. ... For the following two reasons the electron hole was introduced into calculations: If an electron is excited into higher state it leaves a hole in its old state. ...


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. In semiconductor physics, a direct bandgap means that the minimum of the conduction band lies directly above the maximum of the valence band in momentum space. ... Cathodoluminescence is an optical and electrical phenomenon whereby a beam of electrons is generated by an electron gun (e. ... A p-n junction is formed by combining N-type and P-type semiconductors together in very close contact. ... Electron beam induced current (EBIC) is a semiconductor analysis technique performed in a scanning electron microscope (SEM) or scanning transmission electron microscope (STEM). ...


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.


Cathodoluminescence

Cathodoluminescence, the emission of light when atoms excited by high-energy electrons return to their ground state, is analogous to UV-induced fluorescence, and some materials such as zinc sulphide and some fluorescent dyes, exhibit both phenomena. Cathodoluminescence is most commonly experienced in everyday life as the light emission from the inner surface of the cathode ray tube in television sets and computer CRT monitors. In the SEM, CL detectors either collect all light emitted by the specimen, or can analyse the wavelengths emitted by the specimen and display a spectrum or an image of the cathodoluminescence in real colour. Cathodoluminescence is an optical and electrical phenomenon whereby a beam of electrons is generated by an electron gun (e. ...


X-ray microanalysis

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. Energy dispersive X-ray spectroscopy (EDS or EDX) is an analytical tool predominantly used for chemical characterization. ... The Wavelength dispersive X-ray spectroscopy is a method used to determine the energy spectrum of an X-ray radiation. ...


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 to which the material interacts with the electron beam. The spot size and the interaction volume both might be large compared to the distances between atoms, so the resolution of the SEM is not high enough to image individual atoms, 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 easier to interpret than TEM images. Transmission electron microscopy (TEM) is an imaging technique whereby a beam of electrons is focused onto a specimen causing an enlarged version to appear on a fluorescent screen or layer of photographic film (see electron microscope), or can be detected by a CCD camera. ...


Environmental SEM

Conventional SEM requires samples to be imaged under vacuum, which means that samples that would produce a significant amount of vapour, e.g. biological samples, need to be either dried or cryogenically frozen. This means that processes involving transitions to or from liquids or gases, such as the drying of adhesives or melting of alloys, liquid transport, chemical reactions and solid-air-gas systems in general could not be observed.


The first commercial development of the Environmental SEM (ESEM) in the late 1980s [3] [4] allowed samples to be observed in low-pressure gaseous environments (e.g. 1-50 Torr) and high relative humidity (up to 100%). This was made possible by the development of a secondary-electron detector [5] [6] capable of operating in the presence of water vapour and by the use of pressure-limiting apertures with differential pumping in the path of the electron beam to separate the vacuum regions around the gun and lenses from the sample chamber. The torr (symbol: Torr) or millimeter of mercury (mmHg) is a non-SI unit of pressure. ... This article or section is not written in the formal tone expected of an encyclopedia article. ...


The first commercial ESEMs were produced by the ElectroScan Corporation in USA in 1988 [7]. ElectroScan were later taken over by Philips (now FEI Company) in 1996 [8]. FEI Company (NASDAQ: FEIC), founded in 1971, is a supplier of electron microscopy tools to researchers, developers and manufacturers working on the nanoscale. ...


ESEM is especially useful for non-metallic and biological materials because coating with carbon or gold is unnecessary. Plastics and Elastomers can now be routinely examined, as can biological samples. Coating is irreversible, and may reduce the value of the results obtained. Thus very small details on the surface of the sample may be concealed by the coating, let alone that coating is done under vacuum, which drastically alters hydrated specimens. The term plastics covers a range of synthetic or semi-synthetic organic condensation or polymerization products that can be molded or extruded into objects or films or fibers. ... The term elastomer is often used interchangeably with the term rubber, and is preferred when referring to vulcanisates. ...


See also

Wikibooks
Wikibooks has more about this subject:

Image File history File links Wikibooks-logo-en. ... List of surface analysis methods LIBS - Laser induced breakdown spectroscopy EBSD - Electron backscatter diffraction XRF - X-ray fluorescence analysis LOES - Laser optical emission spectroscopy LS - Light (Raman) scattering IRS - Infra Red spectroscopy SEIRA -Surface enhanced infrared absorption spectroscopy FTIR - Fourier transform infrared absorption spectroscopy; e. ... Transmission electron microscopy (TEM) is an imaging technique whereby a beam of electrons is focused onto a specimen causing an enlarged version to appear on a fluorescent screen or layer of photographic film (see electron microscope), or can be detected by a CCD camera. ...

References

  1. ^ von Ardenne, Manfred (1938). "Das Elektronen-Rastermikroskop. Theoretishce Grundlagen" (in German). Zeitschrift fur Physik 108: 553-572. 
  2. ^ von Ardenne, Manfred (1938). "Das Elektronen-Rastermikroskop. Praktische Ausfurung" (in German). Z. Techn. Phys. 108: 407-416. 
  3. ^ Danilatos, G,D (1988). "Foundations of environmental scanning electron microscopy". Advances in Electronics and Electron Physics 71: 109-250. 
  4. ^ US4,823,006 (PDF version) (1989-4-18) Gerasimos D Danilatos, George C Lewis Integrated electron optical/differential pumping/imaging signal detection system for an environmental scanning electron microscope 
  5. ^ Danilatos, G,D (1990). "Theory of the Gaseous Detector Device in the ESEM". Advances in Electronics and Electron Physics 78: 1-102. 
  6. ^ US4,785,182 (PDF version) (1988-11-15) James F Mancuso, William B Maxwell, Gerasimos D Danilatos Secondary Electron Detector for Use in a Gaseous Atmosphere 
  7. ^ History of Electron Microscopy, 1980s
  8. ^ History of Electron Microscopy 1990s

Year 1988 (MCMLXXXVIII) was a leap year starting on Friday (link displays 1988 Gregorian calendar). ... is the 319th day of the year (320th in leap years) in the Gregorian calendar. ...

External links

General

History

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
  • Rippel Electron Microscope Facility Many dozens of (mostly biological) SEM images from Dartmouth College.

Gallery of SEM images

The following are examples of images taken using a scanning electron microscope.


  Results from FactBites:
 
Electron microscope - Wikipedia, the free encyclopedia (1789 words)
The electron microscope is a microscope that can magnify very small details with high resolving power due to the use of electrons as the source of illumination, magnifying at levels up to 2,000,000 times.
The first practical electron microscope was built at the University of Toronto in 1938, by Eli Franklin Burton and students Cecil Hall, James Hillier and Albert Prebus.
Unlike the TEM, where electrons are detected by beam transmission, the Scanning Electron Microscope (SEM) produces images by detecting secondary electrons which are emitted from the surface due to excitation by the primary electron beam.
Scanning electron microscope - Wikipedia, the free encyclopedia (1180 words)
The scanning electron microscope (SEM) is a type of electron microscope capable of producing high resolution images of a sample surface.
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 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.
  More results at FactBites »

 
 

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