Ellipsometry is an optical technique that measures the change in polarization state of light reflected from a surface. It is the primary method for determining thin film thickness and optical constants of thin films, with sensitivity down to sub-angstrom thickness for high-quality films.
The technique measures two parameters: psi (Ψ) and delta (Δ). Ψ is the amplitude ratio of the p- and s-polarized components of the reflected light. Δ is the phase difference between them. These parameters are related to the Fresnel reflection coefficients, which depend on the angle of incidence, the wavelength of light, the optical constants of the substrate and film, and the film thickness.
Spectroscopic ellipsometry measures Ψ and Δ over a range of wavelengths, typically from the ultraviolet to the near-infrared. Multiple wavelengths provide sufficient information to determine both thickness and optical constants simultaneously. Variable angle spectroscopic ellipsometry adds angular data for complex multi-layer structures.
Data analysis requires an optical model of the sample structure and a regression procedure to fit the calculated Ψ and Δ to the measured values. The Levenberg-Marquardt algorithm minimizes the mean squared error. The model includes the number of layers, their approximate composition, and the optical constants parameterized by dispersion formulas. The Cauchy model describes transparent materials. The Tauc-Lorentz model describes amorphous semiconductors. The Drude model describes metals.
Ellipsometry is used extensively in the semiconductor industry for measuring gate oxide thickness, photoresist layer thickness, and critical dimensions. In materials research, it characterizes organic thin films, self-assembled monolayers, polymer coatings, and 2D materials. In biology and medicine, ellipsometry measures protein adsorption on surfaces, biofilm formation, and cell attachment. Imaging ellipsometry provides spatial mapping of film thickness with micrometer lateral resolution.
In situ ellipsometry monitors film growth in real time during deposition processes. Dynamic ellipsometry follows adsorption kinetics, surface reactions, and film degradation. The technique operates in air, vacuum, or liquid environments.
Limitations include the need for an appropriate optical model, ambiguity in determining thickness and optical constants for very thin or transparent films, and lateral resolution limited by the probe spot size. Multiple angles and wavelengths reduce parameter correlation.
