Determination Of Optical Constants From Photometric Measurements

We discuss and compare several photometric methods for the evaluation of optical constants of thin absorbing films. Optical constants of such films are frequently troublesome to assess with conventional photometric techniques. For the commonly used (T,R) method, where near normal transmittance T and reflectance R of a thin film on a transparent sub-strate are measured, specific problems occur when the real part n of the refractive index is of the same order of magnitude as the imaginary part k. By combining instead T and Rm, where Rm is the reflectance of the film on a metallized part of the substrate, we obtain the (T,Rm) method, which has a greatly increased sensitivity. It is further shown that the (T,Rmb) combination, where Rmb is the substrate side reflectance of the thin film with an optically thick metal overcoating, yields additional advantages. A detailed evaluation of the accuracy obtained for the (T,R), (T,Rm) and (T,Rmb) methods is presented.