Ellipsometry

Ellipsometry is a non-contact and non-destructive technique used to measure thin films as well as bulk materials. This technique was first used in 1888 by Paul Drude. Although the technique was not called ellipsometry at this time, Drude is credited with deriving the fundamental equations which are still used today. It wasn’t until 1945 that Alexandre Rothen coined the term ellipsometry.

Ellipsometry_Diagram
Ellipsometry is commonly used in the semiconductor and thin film industry. Its main application is to determine the optical properties of thin films or stratified layers of films. Ellipsometry consists of reflecting linearly polarized monochromatic light off of a sample at oblique angles. After interacting with the material, the reflected light becomes elliptically polarized. It is because of this change in the polarization of the light that we are able to extract the optical properties of the material. By combining ellipsometry with other common measurement techniques such as Atomic Force Microscopy or Raman Spectroscopy, we can interpret the structure and composition of any given sample with a very high degree of accuracy.

The fundamental equation of ellipsometry discussed earlier has the two most important variables used in ellipsometry, Ψ and Δ. It is shown below:

ρ=tan(Ψ)e(iΔ)=rp/rs

From the equation, rs and rp represent the reflectance ratios of the amplitude of the electric fields polarized perpendicular and parallel to the plane of incidence. The ratio of these two coefficients gives us the value for ρ which is the complex reflectance ratio. Also from our fundamental equation we have the value Ψ which measures the relative amplitude change of the electric fields parallel and perpendicular to the plane of incidence. Δ represents the phase difference of the two electric fields before and after being reflected off the sample being examined. From these values we are able to get the thickness of the sample.