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Physics of Thin Films

PES 449 / PHYS 549

Thin Film Characterization - Imaging Techniques

 Ohring Chapter 6 Section 3

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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

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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:

  

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Electron interactions with materials

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

Electrons incident on a material may

• scatter back
• knock out other (secondary) electrons

  

  The number of secondary electrons produced is relatively insensitive to atomic number of the atoms in the material. The number of backscattered electrons, however, is sensitive to atomic number of the material.

  

  The number of electrons leaving the surface varies with the incident electron energy.

  This can influence the charging of the sample.

  

  other links:

• GOOD: Scanning Electron Microscopy: http://www.mse.iastate.edu/microscopy/
• GOOD: description of electron microscopies (SEM, TEM): http://www.unl.edu/CMRAcfem/em.htm
• environmental SEM - for samples that do not handle vacuum well
• history of the SEM: http://www2.eng.cam.ac.uk/~bcb/history.htm
• SEM information: http://www.lifelong.com/carnivalWorld/SEM/
• Institute of Physics: on-line Science Museum: http://www.nmsi.ac.uk/on-line/Electron/section4/sem.html
• SEM for art: http://www.clinch.edu/cvc/r_baird/arts.htm

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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

  

  Modes of operation:

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

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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

  

  Scanning Tunneling Microscope (STM)

  • 

    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).

    

    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)

  •  

    tip is deflected by atomic forces

    tip radius of curvature typically < 400 Å

    forces can be attractive or repulsive

    

    Link to Digital Instruments AFM/STM/MFM application notes: http://www.di.com/AppNotes/ANMain.html

    Information on scanning near-field optical microscopy: http://hamers.chem.wisc.edu/techniques/NSOM/NSOM.html

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© Thomas M. Christensen