Comparison of TEM and SEM

                                        PJL 11/16/98 rev. 11/22/05

     definitions:   e- = electron

                  GFPC = gas filled proportional counter

                   PMT = photomultiplier tube                        

                   SCD = semiconductor detector (Si or Ge)

          SEM                         TEM                 

            ===============================  =============================== 

Invented:   Zworykin et al., 1942            Ruska, 1933

Commercially

Available:  Cambridge Instr., 1965           Vickers, 1936           

Design:     2 CRTs, with synchronized e-     1 CRT, raster scan not essential.

            beams scanning raster patterns.

Electron    Electron gun, 2 lenses           Electron gun, 4 lenses, 2 apertures,

column:     1 aperture, sample and           sample & movable stage.  Half the

(CRT #1)    movable stage, various           lenses & apertures are above & half

            detectors.  See below.           below the sample.

Sample      Large.  Allows for sample        Small.  Allows for sample tilt and

Chamber:    tilt & rotation.  May also       rotation.

            allow electrical connections

            and mechanical test apparatus.

Typical     1 to 50 kV                       50 to 300 kV, even a million volts!

Voltage &   30 Ang. or more, depends on      10 Ang. or more. atomic planes visible

Resolution:   sample. Great depth of field. 

Display:    CRT #2 provides a TV-like        A fluorescent screen inside the

            display.  Display brightness     electron column at the bottom.

            is determined by detector

            output, adjusted for                 ... or an area detector

            brightness & contrast.

Photog-     Take photo of the CRT display    Load film cartridge inside the e-

 raphy:     or capture image digitally       column under the fluorescent screen.

            for analysis.                    Flip up screen to expose the film.

Standard    Secondary e- (<50 eV) uses       Just the fluorescent screen and

Detector:     scintillator & PMT.  Gives       photographic film.

              good topographical contrast.

Optional    Backscattered e- (same energy    Electron energy loss spectrometer

Detectors:    as incident beam) uses SCD       detects lighter elements using

              or scintillator & PMT. Gives     quadrupole magnetic detector in

              good compositional contrast.     the transmitted beam.

            Energy dispersive X-ray uses     

              SCD, detects heavy elements.   Energy dispersive X-ray. 

            Wavelength dispersive x-ray     

              uses crystal diffractometer    Secondary e- detector, plus raster  

              with GFPC.  Detects lower        scan capability.

              concentrations, lighter

              elements and avoids peak

              convolution.  Works slow.

            Photoemission (a.k.a. cathodolumin-

              escence) uses a mirror & PMT. 

              Good for non- or semi-conductors.

            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.

Sample      Almost any clean solid.          Foil or powders < 1000 Ang. thick.

form:       Big, thick samples are OK.       or surface replicas. 

Sample      First clean off dirt & grease.   Use ion mill, focused ion beam,

prep:       Insulators must be coated        electropolishing, jet polishing,

            with a conducting layer          dimpling, etc.  Sample prep is

            ~100 Ang thick.  Sputter or      usually a lot of work and may

            evaporate metal or C.            irreversibly change the material.

            Sample prep is usually simple.                                                                

Most        Fracture, wear or corrosion      Selected area e- diffraction,

useful      surfaces, powders, polished      imaging of dislocations, tiny

for:        & etched microstructures,        precipitates, grain boundaries

            IC chips, chemical segrega-      and other defect structures in

            tion.                            solids.

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.