ellipsometry

Ellipsometry is an optical characterization technique that measures the change in polarization of light upon reflection from a material surface. By analyzing the complex reflectance ratio of the p- and s-polarized components, typically expressed as rho = rp/rs = tan(psi) e^{i delta}, ellipsometry provides information about thin films and surfaces. The method relies on Fresnel equations and optical modeling to extract thickness and complex refractive index (n + ik) of layered structures.

Instrumentation typically includes a light source, polarizer, compensator or quarter-wave plate, analyzer, and detector. In spectroscopic ellipsometry, measurements are made over a range of wavelengths. Configurations such as null ellipsometry, rotating compensator, and rotating analyzer are used to determine psi and delta with high accuracy. The data are interpreted with optical models of the film stack to obtain thickness and optical constants.

Applications span semiconductors, photovoltaics, optical coatings, polymer films, and biological layers. Ellipsometry is highly sensitive to sub-nanometer thickness changes and is non-destructive, non-contact, and capable of determining refractive index n and extinction coefficient k as a function of wavelength. Limitations include reliance on an appropriate optical model, potential parameter degeneracies, and sensitivity to surface roughness and substrate properties.

Variants include spectroscopic ellipsometry (SE) across a broad spectrum and imaging ellipsometry that maps spatial variations. For anisotropic or complex films, Mueller matrix ellipsometry may be used. Common samples include silicon dioxide on silicon, oxide and polymer thin films, and multilayer coatings. The technique provides essential data for materials science, microelectronics, and surface science.