1. A method comprising: calibrating an optical proximity correction (OPC) model for a structure to be formed in a substrate; simulating a critical dimension (CD) of the structure at at least two locations along a substrate profile of the structure using the OPC model; comparing the simulated CD of the structure at the at least two locations along the substrate profile against a measured CD at the at least two locations along the substrate profile; recalibrating the OPC model based on the comparing of the simulated CD against the measured CD at the at least two locations along the substrate profile; repeating the steps of simulating, comparing, and recalibrating until comparing at a first of the at least two locations along the substrate profile converges to a first criteria and comparing at each other of the at least two locations along the substrate profile converges to a corresponding criteria; forming the structure in the substrate by photolithography using the recalibrated OPC model, wherein the OPC model comprises an OPC submodel corresponding to each of the at least two locations along the substrate profile, the method comprising recalibrating the OPC model for each of the at least two locations along the substrate profile by: determining a difference between the simulated CD and the measured CD; adjusting the first OPC submodel if the difference is equal to or greater than a corresponding criteria; generating a new simulated CD at the location along the substrate profile according to the adjusted corresponding OPC submodel; and iterating the steps of determining, adjusting, and generating for the location along the substrate profile until the difference between the new simulated CD at the location along the substrate profile and the measured CD at the location along the substrate profile is less than the corresponding criteria.