Further Increasing the Accuracy of Characterization of a Thin Dielectric or Semiconductor Film on a Substrate from Its Interference Transmittance Spectrum

Abstract

Three means are investigated for further increasing the accuracy of the characterization of a thin film on a substrate, from the transmittance spectrum T(lambda) of the specimen, based on the envelope method. Firstly, it is demonstrated that the accuracy of characterization, of the average film thickness (d) over bar and the thickness non-uniformity Delta d increment over the illuminated area, increases, employing a simple dual transformation utilizing the product T(lambda)x(s)(lambda), where T-sm(lambda) is the smoothed spectrum of T(lambda) and x(s)(lambda) is the substrate absorbance. Secondly, an approach is proposed for selecting an interval of wavelengths, so that using envelope points only from this interval provides the most accurate characterization of (d) over bar and Delta d, as this approach is applicable no matter whether the substrate is transparent or non-transparent. Thirdly, the refractive index n(lambda) and the extinction coefficient k(lambda) are computed, employing curve fitting by polynomials of the optimized degree of 1/lambda, instead of by previously used either polynomial of the optimized degree of lambda or a two-term exponential of lambda. An algorithm is developed, applying these three means, and implemented, to characterize a-Si and As98Te2 thin films. Record high accuracy within 0.1% is achieved in the computation of (d) over bar and n(lambda) of these films.