Advanced Computer Drawing Envelopes of Transmittance Spectra of Thin Film Specimens

Abstract

Drawing accurate envelopes of a transmittance spectrum is the main factor limiting the accuracy of characterization of one thin film on transmitting substrate specimens, using the method of Swanepoel. An advanced algorithm is proposed for computer drawing accurate envelopes, over the entire spectrum, which takes into account the following issues not considered in the existing algorithms: the wider spectrum for UV/visible/NIR compared to UV/visible spectrophotometers, the absorption in the substrate, and the increasing demand for accuracy in the spectral region of strong absorption in the film. This algorithm uses only one-step smoothing the spectrum, and piecewise cubic Hermite polynomial interpolation which is shown to be superior to cubic spline interpolation for constructing envelopes of such spectra. Boundary points, additional points, and supplementary points are included in the interpolation. The performance of the algorithm is demonstrated for two model specimens. Envelopes are also computer drawn for experimental spectra of two specimens of a-Si film on glass substrates.