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The ellipsometric technique has been around since the end of the last century. However, until the introduction of computers in the 1960s mainly nulling techniques were used. These were accurate, but extremely slow, and data analysis capabilities were limited. The use of personal computers permits rapid data acquisition and analysis. Thus the emphasis is now on ellipsometry for materials research and analysis. Ellipsometry is a unique tool in that it is totally non-destructive and can be performed in any ambient, including air, vacuum, high pressure, or liquid. In ellipsometry a polarized, collimated beam of monochromatic light is reflected at a preselected angle of incidence from the surface of a material, and the polarization state of the reflected beam is determined.1'2 For each wavelength and angle of incidence, one pair of numbers is obtained. These can be used to determine at most two materials parameters; such as the real and imaginary parts of the complex dielectric functions. Frequently, however, two or more variables are correlated and cannot be uniquely determined. This is an important caveat for ellipsometry: one always obtains numbers from the experiment, but they may not be correct. It is extremely important for the user to understand the basis of regression analysis, and the possibility for correlated variables when using ellipsometry for any new materials system.3 To minimize this possibility, multiple angles of incidence and wavelength are used to maximize sensitivity and uniquely determine parameter values. In general, ellipsometry determines the real and imaginary part of the optical dielectric function or equivalently the index of refraction and extinction coefficient. For filmed materials systems, one can potentially determine film thickness (with monolayer sensitivity), void fraction (density), alloy fraction, interfacial and/or surface roughness, and other microstructural properties.4 To do these sophisticated levels of microstructural materials analysis requires spectroscopic and variable angle measurements, in order to minimize correlation, increase sensitivity, and maximize total information obtained
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