Method for determining optical constants prior to film processing to be

Abstract

A method for determining the thickness of a film in a film stack using reflectance spectroscopy is provided in which one of the films in the stack has unknown optical constants. Conventional methods of using reflectance measurements to determine the thickness of a film require knowledge of the thicknesses and optical constants of all underlying films. An embodiment involves forming a test layer across a substrate having a known thickness and known optical constants. The thickness of the layer is determined using reflectance measurements. A first layer of the same material is then formed across a second layer at the same conditions that the test layer was formed. Thus, the test layer and the first layer can be assumed to have the same thicknesses. A spectral response curve may be determined for the first layer. The first layer is then processed so that its thickness is no longer known. The second layer has unknown optical constants, making it difficult to use reflectance measurements to find the unknown thickness. The values of the unknown optical constants may be guessed and used with the known thickness of unprocessed first layer to create a model spectral response curve. The optical constants may be repeatably varied until the model response curve matches the measured response curve. When this happens, the reasonable optical constants have been found and may be used in determining the thickness of the processed first layer.