Comparison between SEM and TEM
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SEM
TEM
     
Invented Zworykin et al., 1942 Ruska, 1933
     

Commercially Available

Cambridge Instrument Company marketed the "Stereoscan", 1965 [1] Vickers, 1936
     
Design 2 CRTs, with synchronized e-beams scanning raster patterns. 1 CRT, raster scan not essential.
     
Electron column Electron gun, 2 lenses, 1 aperture, sample and movable stage, various detectors.  See below. Electron gun, 4 lenses, 2 apertures, sample & movable stage.  Half the lenses & apertures are above & half below the sample.
     
Sample Chamber Large.  Allows for sample tilt & rotation.  May also allow electrical connections and mechanical test apparatus Small.  Allows for sample tilt and rotation.
     
Electron scattering in specimen More broadening affects the spactial resolution Less broadening
     
Typical spatial resolution 1 to 50 kV 30 Å or more, depends on sample. Great depth of field.
Modern SEMs with field-emission electron guns resolve better than 1 nm
50 to 300 kV, even a million volts! 10 Å or more. atomic planes visible
Modern TEMs with field-emission electron guns resolve better than 0.2 nm
     
Display CRT #2 provides a TV-like display.  Display brightness is determined by detector output, adjusted for  brightness & contrast.

A fluorescent screen inside the electron column at the bottom.

... or an area detector

     
Photography Take photo of the CRT (cathode ray tube) display or capture image digitally for analysis; Secondary electron (<50 eV) uses scintillator & PMT; Gives good topographical contrast. Load film cartridge inside the e-column under the fluorescent screen; Flip up screen to expose the film; Just the fluorescent screen and photographic film.
     
Optional Detectors Backscattered e- (same energy as incident beam) uses SCD or scintillator & PMT. Gives good compositional contrast. Energy dispersive X-ray uses SCD, detects heavy elements. Wavelength dispersive x-ray uses crystal diffractometer with GFPC.  Detects lower concentrations, lighter elements and avoids peak convolution. Works slow. Photoemission (a.k.a. cathodoluminescence) uses a mirror & PMT. Good for non- or semiconductors. Specimen current to ground
= beam - secondary - backscatter.
Voltage contrast uses a slightly modified secondary e-detector to image regions of varying potential. Ideal for IC chips. Strobe the beam off & on to "freeze" periodic signals. Electron beam induced current, flows between two contacts to the sample, not to ground.
Good for semiconductors.
Thermal wave uses a piezoelectric microphone to detect acoustic noise generated in sample by pulsing (blanking) the e- beam.
Good for imaging features which conduct heat poorly.

Electron energy loss spectrometer detects lighter elements using quadrupole magnetic detector in the transmitted beam.

Energy dispersive X-ray.

Secondary e- detector, plus raster scan capability.

     
Signal collected The SEM collects secondary electrons (SE), backscatter electrons (BE), and X-rays from surfaces of bulk specimens: surface-sensitive The TEM collects X-rays, elastically scattered electrons, and inelastically scattered electrons from thin and ultra-thin specimens. Hybrid TEMs have SEM capabilities: they collect BE that exit the incident-beam side of thin specimens, and they collect SE and X-rays from both sides: probe internal structures directly
     
Contrast Contrast develops in SEM by electrons emitted at or near the surfaces of bulk specimens and therefore topography and composition are examined. Contrast develops in TEM by electrons transmitted through thin specimens and therefore variations in structure and composition are examined.
     
Sample form

Almost any clean solid. Big, thick samples are OK. 

Foil or powders < 100 nm thick. or surface replicas.
     
Sample size

Large area

Very small area
     
Sample preparation Insulators must be coated with a conducting layer ~100 Å thick:  Sputter or evaporate metal or C. Sample prep is usually simple. Use ion mill, focused ion beam, electropolishing, jet polishing, dimpling, etc.  Sample prep is usually a lot of work and may irreversibly change the material.
     
Most useful for: Fracture, wear or corrosion surfaces, powders, polished & etched microstructures, IC chips, chemical segregation. Biology, microbiology, geology, nanotechnology, crystallography Selected area e- diffraction, imaging of dislocations, tiny precipitates, grain boundaries and other defect structures in solids; microbiology, pathology, crystallography
     
Magnification Electrostatic and electromagnetic lenses, as with a TEM. Magnification ranges from 25x to 250,000x. A series of electrostatic and electromagnetic lenses act on an electron beam to produce up to 50 million times magnification
     
Depth of field Large compared to that of a transmission electron microscope Very small
     
Advantages Can view objects' three-dimensional surface. Also, an SEM can provide information about the specimen's elemental composition. Very high resolution
     
Disadvantages As with TEM, requires specialized equipment and a partial vacuum. Cannot be used on living specimens. Requires extensive specimen preparation, including staining and sectioning with an ultramicrotome. Cannot observe the surface of objects.Very specialized equipment that requires a partial vacuum.
Emission of secondary electrons In backward direction only due to thick materials; all incident electrons generate secondary electrons. In forward and backward directions due to thin film; only some incident electrons generate secondary electrons.
     

Definitions:
                e- = electron,
                GFPC = gas filled proportional counter,
                PMT = photomultiplier tube ,
                SCD = semiconductor detector (Si or Ge).

Furthermore, the comparison between the EDS measurements in low-energy SEM and high-energy (S)TEM is listed on a table on page4532.

 

 

 

 

[1] McMullan, D., "SEM-Past, Present and Future," Journal of Microscopy, Vol. 155, No. 3, 1989, pp. 373-392.
[2] Both SEM and TEM are useful in biology and geology, as well as in materials science.
Bibliography:
1) Goldstein, Newbury, Echlin, Joy, Fiori & Lifshin; Scanning Electron Microscopy and X-Ray Microanalysis, Plenum, 1984
2) Hirsch, Howie, Nicholson, Pashley & Whelan, Electron Microscopy of Thin Crystals, Krieger, 1977
3) ASM, Metals Handbook, 9th Edition, vol. 9, p. 89-122, Scanning Electron Microscopy and Transmission Electron Microscopy.
4) ASM, Metals Handbook, 9th Edition, vol. 10, p. 427-546, Electron Optical Methods.
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