Ellipsometry is a powerful tool in thin film field to characterize film properties, e.g. refractive index (n), extinction coefficiency (k), surface roughness, crystallinity, etc.
Ellipsometers measure ∆ and Ψ and calculated n, k, d through suitable models.
- Delta, the phase difference induced by the reflection; if δ1 is the phase difference before, and δ2 the phase difference after the reflection, ∆=δ1-δ2; its range is 0-360 degree or -180 to 180 degree.
- Psi, the ratio of the amplitude diminutions with range of 0-90 degree, defined as tan Ψ =|Rp|/|Rs|.
- The fundamental equation of ellipsometry is
The setup of a typical ellipsometer is as the following:
Basically, Ellipsometers can be classified into two general families: null ellipsometer and non null ellipsometer. The non-null ellipsometers can be classified into three groups: rotating polarizers or analyzers, rotating compensators and phase modulated. The main difference among different ellipsometry technologies is the way to modulate signals and obtain optical data of Muller matrix as defined as the following, assuming no depolarization and isotropic media.
S and C sometimes are written as Is, Ic in Horiba ellipsometer. The above calculation is based on the assumption that no optical element depolarizes the light. Typically it is true. However some depolarization process do occur, including (1) sample film thickness(es) is (are) not uniform (2) light reflection of the back surface of a transparent substrate, which contribute a not phase-related intensity component. (3) Very rough surface. If the sample is very rough, then some of the light reaching the PSD will not have an identifiable polarization state or cross-polarization can occur in nominally isotropic systems.