1. A method of determining one or more features of a structure formed on a semiconductor wafer, the method comprising: a) obtaining a profile model of the structure, the profile model being defined by profile parameters that characterize the geometric shape of the structure; b) obtaining a set of values for the profile parameters; c) generating a set of simulated diffraction signals using the set of values for the profile parameters, each simulated diffraction signal characterizing the behavior of light diffracted from the structure; d) training a support vector machine using the set of simulated diffraction signals as inputs to the support vector machine and the set of values for the profile parameters as expected outputs of the support vector machine; e) obtaining a measured diffraction signal off the structure; f) inputting the measured diffraction signal into the trained support vector machine; and g) after f), obtaining values of profile parameters of the structure as an output from the trained support vector machine; further comprising: normalizing values of profile parameters, wherein the set of simulated diffraction signals are generated using the normalized values of profile parameters; and de-normalizing the values of profile parameter obtained in g).