Ellipsometers is used to measure the index of bulk materials and to simultaneously measure film thickness and refractive index, there are major differences in the underlying measurement techniques and in the accuracy and the application range of the two instruments. As a result, ellipsometry and prism coupling can be viewed as complementary techniques, with the strengths of one supplementing the weaknesses of the other, and vice versa.
In ellipsometers, the intensity and polarization state of monochromatic light reflected from the film or bulk sample yields the measured parameters. Prism coupling measurements are based on techniques developed in the field of integrated optics, treating the thin film to be measured as an optical waveguide. The prism coupling technique works by measuring the angles at which the thin film waveguide will propagate light, and then calculating film thickness and refractive index from the observed mode angles.
For any given application, the film thickness range to be measured is the single most important factor determining whether ellipsometry or prism coupling is the preferred technique. In general, ellipsometry is the clear choice for measuring films whose thickness is thinner than 300-400 nm or for measuring film structures with three or more layers. For measuring bulk materials and single or dual layers of moderate-to-thick films with thickness ranging from a few hundred nm to tens or hundreds of microns, prism coupler measurements are typically more accurate and straightforward.
