ellipsometer

An ellipsometer is a non-destructive optical instrument used to characterize thin films by measuring changes in the polarization state of light upon reflection. It yields information about film thickness and optical constants by comparing the polarization of incident and reflected beams.

The measurement relies on the complex reflectance ratio, rho = r_p / r_s = tan(Psi) e^(i Delta), where r_p and r_s are the Fresnel reflection coefficients for p- and s-polarized light, Psi is the amplitude ratio and Delta is the phase difference between the two polarization components. By analyzing Psi and Delta, often across a range of wavelengths and incidence angles, the properties of layered materials can be inferred.

Ellipsometers vary in configuration and mode. Common variants include null ellipsometry, rotating compensator or phase-modulation systems, and spectroscopic ellipsometry, which collects data over multiple wavelengths. Instrumentation typically includes a light source, polarization optics (polarizer, compensator or retarder, analyzer), a sample stage, and a detector. Data are presented as Psi and Delta as functions of wavelength or angle.

Data interpretation requires an optical model of the sample, describing the layer structure and the complex refractive index (n and k) of each layer. The model is fitted to the measured Psi and Delta to extract thicknesses and optical constants. Applications span semiconductor oxide and metal films, dielectric coatings, photovoltaics, and biological interfaces. Ellipsometry offers high sensitivity to sub-nanometer thickness changes and is non-contact, but results depend on accurate modeling and assumptions about layer uniformity and roughness.

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