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Physics of Thin Films

PES 449 / PHYS 549

Thin Film Characterization - Optical Techniques

Ohring Chapter 6 Section 2.2

Ellipsometry

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
- 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
  
  We often use these to define reflection coefficients:
  
  r_p= E^r_p / Eⁱ_p and r_s = E^r_s / Eⁱ_s
  
  In ellipsometry we define two other parameters (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 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.

Interferometry

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

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