General Background

A basic requirement for using optical methods for determining the thickness of a film is that the light must be able to reach the bottom of the film and interact with the underlying layer. Transparent materials such as most oxides qualify throughout the above-mentioned thickness range up to a few micrometers. Many semiconductors are nearly transparent and also easily qualify. Metals, however, must be very thin for these optical methods to be applicable.

The intensity change upon reflection for very thin films is quite small. On the other hand, the phase shift differences are significant. For this reason, ellipsometry is one of the most powerful techniques for very thin films. Other traditional thickness measuring techniques such as stylus profilometry and interferometry are ineffective for films thinner than 50 nm. Electrical phenomena. such as eddy currents are sometimes used for thickness measurements of metal films. These techniques require significantly greater calibration efforts, however, and always depend on assumptions wit regard to microstructure.

Ellipsometry and reflectometry basically involve the measurement of changes in phase shift or intensity, respectively, as light reflects from material. Optical properties and film thicknesses of the samples are then calculated based on a chosen model. Regression software is able to determine the thicknesses that give the best fit to within a few angstroms. In many cases, the regression software also gives a "goodness-of-fit" parameter. The "goodness-of-fit" parameter provides a test of the model describing the sample. If the value is outside the acceptable range, then another model may be required. In any event, thickness is not determined directly; the assumption of a model allows calculation of the ellipsometry or reflectometry parameters and subsequent. comparison of calculated values and measured values. Differences between the results of an analysis and reality are more often due to an oversimplified model than to limitations in the capability of the measuring instrument.

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