*Ronald Willey, Willey Optical, Consultants, Charlevoix, MI
The index of refraction values of optical thin film layers are dependent upon the detail deposition parameters. It is therefore necessary to characterize these values of the indices before producing a designed film, in order to obtain films which agree adequately with the designed films. Various procedures to extract the n and k of a film from spectrophotometric measurements are provided. Cautions to minimize errors in the procedures are given, and processes to confirm the accuracy of the obtained results are described. Fitting of %T and %R spectral data are often used successfully but tend to provide less than satisfactory results in the presence of large absorptance. In cases of metal films where high absorptance is present, piecewise fitting is beneficial. However, in some cases, there are multiple solutions (similar to the case of a square root), and the proper solution depends upon having starting values for n and k which are close to the real values. A technique for obtaining good starting values is discussed.