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Physics of Thin Films

PES 449 / PHYS 549


Thin Film Characterization - Imaging Techniques

 Ohring Chapter 6 Section 3


Imaging Surfaces

TECHNIQUE
LIMITS
RESOLUTION

eye

retina

700,000 Å

optical microscope

diffraction of light

3000 Å

scanning electron microscope

diffraction of electrons

30 Å

transmission electron microscope

diffraction of electrons

1 Å

field ion microscope

atomic size

3 Å

near-field scanning probe microscopies

"aperture" size

0.1 - 100 Å


Scanning Electron Microscope (SEM)

 
  • Uses
    • topographic images
    • microstructural analysis
    • elemental analysis if equipped with appropriate detector (EDAX, WDX)
    • magnification of 10 - 50,000
  • Samples
    • minimum size: 0.1 mm; maximum size depends on machine
    • samples must be conductive or coated with thin conductive layer
    • compatible with vacuum environment

     

    Schematic diagram of typical SEM:

    SEM schematic


Electron interactions with materials

Electrons do not travel very far, on average, into materials.

graph of electron mean free path vs. electron energy

Electrons incident on a material may


Transmission Electron Microscope

  • uses
    • microstructural analysis
    • interfacial analysis
    • crystal structure
    • magnifications up to 1,000,000 X => atomic resolution
    • small region elemental analysis
  • samples
    • thinned to about 0.1 micron (1000 Å)
    • minimum size: 1 mm; maximum size varies with instrument
    • sample preparation is very time consuming

    Schematic of Transmission electron microscope

    TEM schematic

    Modes of operation:

  • diffraction
    • remove aperture
    • look at diffracted beams from atomic planes oriented to satisfy Bragg's Law
    • TEM diffraction
  • imaging
    • insert aperture
    • look at transmitted beam intensity


Scanning Probe Microscopies

 

  • Scanning Tunneling Microscopy (STM)
  • Atomic Force Microscopy (AFM)
  • Magnetic Force Microscopy (MFM)
  • Scanning Thermal microscope
  • Scanning Near-Field Optical Microscope
  • and many others

    Put "aperture" very close to the sample

    aperture close to sample

    Scanning Tunneling Microscope (STM)

    • STM sketch

      Apply potential and get a current between tip and sample.

      Tunneling Current is a very sensitive function of tunneling gap (space between tip and sample).

      current vs gap size

      Two operating modes

      • Constant current mode
        • keep tunneling current constant
          • move tip in and out to maintain a constant gap size
        • quality of data depends on ability to move sample
          • tends to have poorer resolution
        • often use for initial measurements
      • Constant height mode
        • keep tip position constant
          • measure tunneling current variations as gap changes
          • danger of crashing tip into protrusion on sample
        • quality of data depends on ability to measure current
          • typically higher resolution
        • use for small area, high resolution measurements

      Technology issues

      • positioning tip - use piezoelectric devices
        • within Ångstroms of the surface
        • vibration
        • thermal drift
        • scan horizontally with Ångstrom resolution
      • measuring pA currents
        • not a problem
      • "aperture" size = atomically sharp tip
        • largely black magic

      Link to description of STM and great images: http://www.iap.tuwien.ac.at/www/surface/STM_Gallery/

      Link to IBM Almaden STM gallery: http://www.almaden.ibm.com/vis/stm/stm.html

    Atomic Force Microscope (AFM)


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© Thomas M. Christensen