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Infrared Ellipsometry On Silicon Wafers

Ellipsometry is a very useful tool for measuring Si02 film thickness and other surface characteristics of silicon wafers; however, the 633-nm source used in most commercial ellipsometers does not permit measurement of subsurface characteristics. Infrared light can be used to measure thickness and complex index of buried layers regardless of the thickness of the overlayers. However, subtle complications in data collection or data analysis can result from the back surface reflection from the silicon wafer. This problem is described with measurement examples of silicon wafers with thin, transparent metal layers. A technique is also discussed for measuring the thickness of a buried layer in silicon.