Automatic evaluation of optical constants and thickness of thin films: application to thin dielectric layers

We briefly describe the principle of a completely automatic spectrophotometer which measures optical properties (R, R', T) in the spectral range 2 000-7 000 A. The results obtained for normal incidence are then used by computer to determine the optical constants (n, k) and thickness d of a thin film. We study in detail the computer program which is based on successive iterations. The parameters d, n, k must be successively determined. First iterations are performed to compute thickness and mean values of n and k over a large spectral range. It remains to determine the complex refractive index, the accuracy being only limited by experimental errors measurements. We present some of our results. For certain layers, the curve n(λ) obtained shows characteristic discontinuities and we conclude that the layer has defects of structure. On the other hand, it is possible to obtain practically perfect layers with given materials and with the values of optical constants differing little from one layer to another if evaporation conditions are identical.