1. A method of determining one or more profile parameters of a structure using an optical metrology model, the optical metrology model comprising a profile model, an approximation diffraction model, and a fine diffraction model, the method comprising: (a) developing a metrology model of a structure, the metrology model including a profile model, the profile model having profile parameters; (b) optimizing the metrology model, the optimized metrology model including an optimized profile model, (c) calculating a simulated approximation diffraction signal based on the approximation diffraction model of the structure; (d) generating a set of simulated fine diffraction signals from a set of profile parameters, the simulated fine diffraction signals generated using the optimized profile model of the structure; (e) calculating a set of difference diffraction signals by subtracting the simulated approximation diffraction signal from each of the simulated fine diffraction signals of the set of simulated fine diffraction signals and pairing each difference diffraction signal with the corresponding profile parameters; (f) training a machine learning system using the pairs of difference diffraction signal and corresponding profile parameters, the machine learning system trained to process the difference diffraction signal as input and generate the profile parameters as output; (g) subtracting the simulated approximation diffraction signal from a measured diffraction signal resulting in an adjusted measured diffraction signal; (h) using the trained machine learning system, inputting the adjusted measured diffraction signal and generating profile parameters; and (i) if one or more matching criteria are met, accessing at least one generated profile parameter.