Characterization of random rough surfaces from scattered intensities by neural networks |
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Authors: | Ilkka Kallioniemi Ari Niinistö Jyrki Saarinen Ari T. Friberg |
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Affiliation: | 1. Department of Engineering Physics and Mathematics , Helsinki University of Technology , PO Box 2200, FIN-02015 HUT, Espoo, Finland;2. Heptagon Oy , Tekniikantie 12, FIN-02150, Espoo, Finland;3. Department of Physics-Optics , Royal Institute of Technology , SE-10044, Stockholm, Sweden |
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Abstract: | Abstract Optical scatterometry, a non-invasive characterization method, is used to infer the statistical properties of random rough surfaces. The Gaussian model with rms-roughness [sgrave] and correlation length σ is considered in this paper but the employed technique is applicable to any representation of random rough surfaces. Surfaces with wide ranges of Λ and σ, up to 5 wavelengths (λ), are characterized with neural networks. Two models are used: self-organizing map (SOM) for rough classification and multi-layer perceptron (MLP) for quantitative estimation with nonlinear regression. Models infer Λ and σ from scattering, thus involving the inverse problem. The intensities are calculated with the exact electromagnetic theory, which enables a wide range of parameters. The most widely known neural network model in practise is SOM, which we use to organize samples into discrete classes with resolution ΔΛ = Δσ = 0.5λ. The more advanced MLP model is trained for optimal behaviour by providing it with known parts of input (scattering) and output (surface parameters). We show that a small amount of data is sufficient for an excellent accuracy on the order of 0.3λ and 0.15λ for estimating Λ and σ, respectively. |
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