Ellipsometry

Abstract Ellipsometry is an optical measurement technique that involves generating a monochromatic or quasimonochromatic light beam in a known polarization state and reflecting it from a sample having a planar, specularly reflecting surface. By measuring the polarization state of the reflected beam, the ellipsometric angles (ψ, Δ) can be determined, and upon detailed analysis, these angles yield information about the sample. Such information includes the optical properties, that is, the index of refraction and extinction coefficient, for a bulk sample or the thickness and optical properties for one or more thin films on the sample surface having plane‐parallel interfaces. For an isotropic sample, the angles (ψ, Δ) are defined by tanψexp( i Δ) = r p / r s , where r p and r s are the complex amplitude reflection coefficients for linear p and s polarization states, for which the electric field vibrates parallel and perpendicular to the plane of incidence, respectively. Several variations of the ellipsometry experiment have been developed. In spectroscopic ellipsometry (SE), (ψ, Δ) are measured continuously versus the wavelength of the light beam, and in real‐time ellipsometry (RT‐E), (ψ, Δ) are measured continuously versus time at fixed wavelength. The latter two modes can be combined to yield real‐time SE (RT‐SE), utilizing an instrument with a linear photodiode array for parallel detection at many wavelengths simultaneously. In imaging ellipsometry (IE), (ψ, Δ) are measured over a two‐dimensional (2‐D) area of a nonuniform sample surface using an instrument with a 2‐D charge‐coupled device (CCD) as the detection system. The most widely used instruments for SE and RT‐SE span the spectral range from the ultraviolet (200–300 nm) to the near infrared (800 nm). Over this spectral range, the optical properties deduced from SE provide information on the processes of absorption and dispersion originating from the valence electrons in semiconductors and dielectrics and from the conduction electrons in metals. Specialized SE equipment designed for the infrared range can be more effective for chemical identification of thin films, based on the absorption processes originating from the vibrational modes of chemical bonds.