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Physics of Thin Films

PES 449 / PHYS 549

Thin Film Characterization - Optical Techniques

 Ohring Chapter 6 Section 2.2


  • Use
    • dielectric/optical properties of materials
    • thickness of thin films
    • porosity and roughness information
    • can be used during deposition or after
  • Samples
    • solids
      • metals, semiconducors, insulators, films
    • liquids
    • samples must be flat and reasonably specularly reflecting
  • Limitations
    • usually visible wavelengths (350 - 700 nm)
      • some IR and near-UV ellipsometry is done
    • sampling region diameter: 30 microns - 10 mm
    • depth of sampling depends on absorption of light in sample
      • typically 100 Å or more


    • Ellipsometry schematic

      Use change of polarization of light reflected from a sample

    Polarization of Light

    • polarization is described by the relative amplitude and phase of the Electric field components in the direction perpendicular (s) and parallel (p) to the plane of incidence

      equation of electric field components

      We often use these to define reflection coefficients:

      rp= Erp / Eip and rs = Ers / Eis

      In ellipsometry we define two other parameters (psi and delta):

      equations defining psi and delta

    Experimental parameters

    • instrument parameters
      • angle of incidence
        • variable angle ellipsometry
      • wavelength of light
        • spectroscopic ellipsometry
    • material parameters
      • film thickness
      • film structure
      • temperature

    Data analysis

    • ellipsometry layers

      ellipsometry data depends on index of refraction of ambient medium (air), film(s) and substrate and thickness of film(s).

      index is generally complex (real index and absorption) N = n - ik

      single measurement of one film on substrate:

      • assume index of ambient and substrate are known

        three unknown parameters (n, k, d of film)

        two measurable parameters (psi and delta)

      need to vary experimental parameters to obtain enough data to solve

    Ellipsometry Models

    • Most analysis uses two types of models:
  • layer model (as described above)
    • assume homogeneous layers can be used to describe sample
  • effective medium approximations (EMA)
    • describes how physical properties change the optical properties
    • use for porosity, roughness

    Use measured data and layer model to determine optical properties and thickness of layers.

    Then use EMA to determine porosity and roughness of layers.


use interference of beams of light with different optical paths to determine film thickness


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© Thomas M. Christensen