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Physics of Thin Films

PES 449 / PHYS 549


Thin Film Characterization - Structural Techniques

 Ohring Chapter 6 Section 2 and 3.4


Crystalline Structure: Diffraction Techniques


X-ray Diffraction

wavelength of X-rays is around 1 Å => can diffract from crystal lattices
  • Uses
    • determine crystal structure and lattice spacings
    • identify phases present
    • measure crystal defects
  • Samples
    • must by crystalline
    • need a minimum of about 0.001 gram
    • sampling depth is 10 - 100 µm

    Diffraction (reflection): constructive interference only occurs at some incident angles

    • l = 2d sin q

      We know the wavelength, we measure the angle at which we see diffraction and find the lattice spacing.

      diffraction of reflected beam

    Diffraction of transmitted beam

    • X-ray diffraction schematic

      note: get diffraction from many sets of planes as you rotate

      • => complicated sets of peaks

    X-rays penetrate many microns into sample. => hard to see thin films (mainly see substrate)

    • Use glancing angle primary beam without sample rotation for thin films.


Low Energy Electron Diffraction (LEED)

see handouts


Reflection High Energy Electron Diffraction (RHEED)

see handouts

 
Film thickness and roughness


Stylus profilometer (Dektak, Alphastep, Talystep)

profilometer schematic

electro-magnetic sensors detect the vertical motion of the stylus as it is moved horizontally across the sample.

measure

  • film thickness (step height)
    • changes of 200 Å to 65 µm
    • vertical resolution of about 10 Å
  • roughness
    • horizontal resolution depends on tip radius

    problem: stylus penetrates soft films


Quartz Crystal Monitor (QCM)

use piezoelectric properties of quartz crystal
  • put electrodes on thin (0.3 mm) disk of quartz
  • alternating voltage deforms crystal periodically (vibrates)
  • resonant vibration frequency between 5 and 6 MHz
  • deposition of film on crystal shifts frequency to lower values
  • relate film thickness to frequency shift IF film density is known
  • crystal can be used for up to about 50,000 Å of material

    caution: stress and heat can also change vibration frequency


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© Thomas M. Christensen

© Thomas M. Christensen