Thin Film and Material Properties

Film porosity (void percentage)

In case where the porosity is randomly distributed the film is represented by a mixture of voids (air pores) and material. The percentage of each component can be determined by spectroscopic ellipsometry.

The term film density is also used. As soon as voids are present in a layer it signifies that the layer is less dense resulting in a decrease of its refractive index.

The graph represents the comparison of refractive index of Al2O3 films:
: dense (100%) Al2O3 layer
: less dense (68%) Al2O3 layer

Percentage of mixed materials forming an interface

There is a thin interface between the poly-Si and SiO2 layer composed of 82.2% of poly-Si and 17.8% of SiO2.

Composition of compound alloys including: AlxGa(1-x)As ; [AlxGa(1-x)](0.5)In(0.5)P ; In(1-x)GaxAsyP(1-y) ; Si(1-x)Gex ; Hg(1-x)CdxTe

Graded alloy composition can also be characterized, showing variation of composition within the film.

Depolarization coefficient (P)

The measurement of the depolarization coefficient (P) is useful for demonstrating that the measured sample depolarizes the incoming light. When P<1, the sample is depolarizing.
Common sources of depolarization are:

  • Incoherent reflections (case of transparent substrate measurement)
  • Roughness
  • Scattering
  • Inadequate spectral resolution
  • Inhomogeneity

Spectroscopic Polarimetry

The measurement of the 16 Mueller Matrix coefficients is useful to accurately characterize anisotropic samples. Mueller matrix measurement enables to identify the axis of anisotropy. In case the sample exhibits biaxial anisotropy, the high number of unknown parameters requires to increase the number of independant experimental data, which is provided by the measurement of the full mueller matrix.