1. A computer-implemented method for creating a metrology target structure design for a reticle layout, comprising: simulating how one or more initial metrology target structures will be formed on a wafer at different values of one or more parameters of one or more fabrication processes that will be used to form a metrology target structure on the wafer based on the one or more fabrication processes, one or more parameters of the wafer, and one or more initial metrology target structure designs; simulating one or more spectra that will be produced by a predetermined metrology system configuration for each of the simulated one or more initial metrology target structures; and creating the metrology target structure design based on the one or more spectra, wherein said creating comprises determining if the one or more initial metrology target structure designs are suitable for a layer to be formed on the wafer during the one or more fabrication processes and, if none of the one or more initial metrology target structure designs are determined to be suitable for the layer, creating one or more different initial metrology target structure designs expected to be more suitable for the layer for use as the created metrology target structure design, and wherein said creating the one or more different initial metrology target structure designs is performed using a genetic algorithm.