Accurate depth profiling of complex optical coatings

The 'DataFurnace' code, based on the simulated annealing algorithm, was use d to extract elemental depth profiles from Rutherford backscattering spectr a of 4-layer and 21-layer anti-reflection coatings and a 22-layer narrow-ba ndpass filter (respectively 240, 1140 and 800 nm thick). The layers are on a sodium glass containing four major elements and the coatings are respecti vely titania/silica and zirconia/silica multilayers and a yttria/zinc sulph ide/siiver multilayer, The DataFurnace code has an execution time that incr eases exponentially with the number of elements in the sample, so to reduce computation time as well as ambiguity we have extended the code to allow f itting for molecules. Accurate fitting requires a proper accounting for the energy straggling of the beam. Multiple spectra from the same sample can a lso reduce both computation time and the ambiguity of the solution obtained . No information about the samples has to be given to the code except the e lements and molecules present. The precision of the extracted depth profile s can be estimated using Bayesian techniques and is 1.5 nm per layer, on av erage, for the 1100 nm film. Greater precision is available by using high-r esolution techniques or a maximum entropy Bayesian prior (neither is done i n this work). Where the depth profiles can be represented by layers of spec ified thickness (in thin-film units), the accuracy is determined largely by the accuracy of the instrumental sensitivity calibration, which is not sub ject to matrix effects and can be as good as 1%, although multiple and plur al scattering effects must be quantified properly for this to be achieved f or thick films (not done in this work). Otherwise, the accuracy of the extr acted depth profiles is determined largely by the accuracy with which the e nergy losses are known. Copyright (C) 2000 John Wiley & Sons, Ltd.