Nondestructive Depth-Profiling of Multilayer Structures by Spectroscopic Ellipsometry

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Abstract
Spectroscopic ellipsometry (SE) is the newest nondestructive and nonperturbing technique for characterizing surfaces, interfaces, and multilayer structures. The technique was originally developed and perfected by Aspnes of Bell Laboratories and a commercial instrument is currently available. After a brief description of the basic principles involved in this technique, one of its many applications — the depth profiling of multilayer structures — is described. Further details about SE and its other applications can be found elsewhere.The automated spectroscopic ellipsometer that has been built in our laboratory is based on the design of Aspnes and Studna. A schematic diagram of the instrument is shown in Figure 1. It is basically a rotating analyzer ellipsometer operated by an on-line Apple computer, and it has spectroscopic scanning capability. Plane polarized monochromatic light is allowed to be incident on the sample at a chosen angle of incidence. The characteristics of the elliptically polarized reflected light is analyzed by a rotating analyzer. As Budde has shown, Fourier analysis of the detector output in a rotating analyzer ellipsometer yields the desired ellipsometric parameters Δ and ψ characterizing the material under study. Aspnes has pointed out that with proper design, alignment and operation, the precision attained is very high, and accuracy of the data is also as high as that attainable with null ellipsometers. The results of our own studies confirm these conclusions. Such measurements are carried out at a number of discrete wavelengths (˜100) distributed uniformly in the UV-visible-near IR spectral range.