ellipsometriaa

Ellipsometriaa is a technique used to characterize thin films by measuring how the polarization state of light changes when it reflects from a sample. In practice, ellipsometry (the more common term) provides information about film thickness, optical constants, and layering by analyzing the change in the amplitude ratio and phase between the p- and s-polarized components of light.

The basic principle relies on the complex reflectance ratio, rho = rp/rs, which can be expressed as rho = tan(psi) exp(i Delta). Here psi is the amplitude ratio and Delta is the phase difference between the reflected p- and s-polarized waves. By modeling the sample with Fresnel equations and known layer structures, these measured parameters are fitted to extract parameters such as film thickness and refractive index (or extinction coefficient) of each layer. Measurements are often performed as a function of wavelength (spectroscopic ellipsometry) to improve robustness and resolve dispersive properties.

Instrumentation typically includes a broadband or laser light source, a polarizer, a compensator, an analyzer, and a detector. In spectroscopic ellipsometry, psi and Delta are recorded across a range of wavelengths. Generalized and imaging ellipsometry extend the technique to anisotropic materials and spatially resolve thickness and optical constants across a surface.

Applications span semiconductor device fabrication, thin-film coatings, photovoltaic cells, metal oxides, and biological films. Ellipsometry is valued for being non-destructive, highly sensitive to sub-nanometer thickness changes, and capable of determining complex refractive indices. Its accuracy depends on constructing a valid optical model that represents the real multilayer structure and surface roughness.