35. A system to select a profile model for use in examining a structure formed on a semiconductor wafer using optical metrology, the system comprising: an optical metrology device configured to provide a measured diffraction signal; a first machine learning system trained using an initial profile model having a set of profile parameters that characterize the structure to be examined, the first machine learning system configured to generate a simulated diffraction signal for an optimized profile model having a set of profile parameters with the same or fewer profile parameters than the initial profile model, wherein the optimized profile model is modified by selecting at least one profile parameter of the optimized profile model to eliminate or fix to a value and the first machine learning system generates another simulated diffraction signal using the modified optimized profile model.
35. A system to select a profile model for use in examining a structure formed on a semiconductor wafer using optical metrology, the system comprising: an optical metrology device configured to provide a measured diffraction signal; a first machine learning system trained using an initial profile model having a set of profile parameters that characterize the structure to be examined, the first machine learning system configured to generate a simulated diffraction signal for an optimized profile model having a set of profile parameters with the same or fewer profile parameters than the initial profile model, wherein the optimized profile model is modified by selecting at least one profile parameter of the optimized profile model to eliminate or fix to a value and the first machine learning system generates another simulated diffraction signal using the modified optimized profile model. 46. The system of claim 35 , wherein if one or more termination criteria are met, at least one profile parameter of the optimized profile model is selected and set to a determined value.