| Table 4997a. Comparison between SEM and TEM (simplified version of this table: link).
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SEM |
TEM |
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|
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| Underlying physics |
Electron beam–matter
interaction |
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|
|
| 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: 0.5 Å or better. Atomic planes visible.
Modern TEMs with field-emission electron guns resolve better than 0.2 nm |
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|
|
| Depth of field |
Large |
Very small |
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|
|
| Field of view |
50 nm to 10 mm |
tens of
nm to tens of µm |
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|
|
| Invented |
Zworykin et al., 1942 |
Ruska, 1933 |
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|
|
Commercially Available |
Cambridge Instrument Company marketed the "Stereoscan", 1965 [1] |
Vickers, 1936 |
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|
|
| 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. |
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|
|
| Sample Chamber |
Large. Allows for sample tilt & rotation. May also allow electrical connections and mechanical test apparatus |
Small. Allows for sample tilt and rotation. |
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|
|
| Electron scattering in specimen |
More broadening affects the spactial resolution |
Less broadening |
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|
|
| 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 |
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|
|
| 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. |
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|
|
| 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. |
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|
|
| Sample form |
Almost any clean solid. Big, thick samples are OK. |
Foil or powders < 100 nm thick. or surface replicas. |
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|
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| Sample size |
Large area |
Very small area |
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|
|
| 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. |
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|
|
| 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 |
| |
|
|
Current instrument
limitations |
Drift; vibration;
contamination; beam damage; lack of sub-nanometre beam placement accuracy; information
volume must be folded into
size or shape determination |
Sample needs to
be cross-sectioned
(therefore
destructive); beam
projection artefacts
and noise; relatively
small high-resolution
field of view |
| |
|
|
| Advantages |
Can view objects' three-dimensional surface. Also, an SEM can provide information about the specimen's elemental composition. Local and global information |
Very high resolution |
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|
|
| Disadvantages |
As with TEM, requires specialized equipment and a partial vacuum. Cannot be used on living specimens, electromagnetic fields |
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, beam
steering errors, electromagnetic fields, Lens aberration |
| 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. |
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|
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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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