An algorithm is described whereby the thickness of polycrystalline silicon (polysilicon) films on oxidized silicon substrates may be determined from the magnitude and wavelength positions of extrema in the visible reflectance spectra. The underlying layer structure, which can consist of two or more layers, is combined mathematically with the silicon into an effective homogeneous substrate to facilitate analysis. Polysilicon layers from about 20 nm to about 1 μm in thickness are measurable for underlying layers in approximately the same range. Proper account is taken of absorption in the polysilicon layer, and interference effects in the underlying layers when they are sufficiently thick. Reflectance lowering caused by scattered light is corrected with the aid of the measured reflectance at 400-nm wavelength, where the polysilicon is opaque. The algorithm is practically insensitive to overlying oxide layers up to 55 nm in thickness, so that buried polysilicon layers are also measurable. When implemented on an automated spectrophotometer, the algorithm provides thickness values in essentially real time, with a precision of about 0.1 nm.
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