1. A method comprising: receiving a spectral response of an unfinished, multi-layer semiconductor wafer across a spectral range; determining a plurality of parameter values of an optical dispersion model of one or more layers of the multi-layer semiconductor wafer based at least in part on the spectral response, wherein the optical dispersion model includes a Cody-Lorentz model augmented by one or more oscillator functions sensitive to one or more defects of the unfinished, multi-layer semiconductor wafer; and storing the plurality of parameter values of the optical dispersion model in a memory.