Optimization of a lithographic projection apparatus accounting for an interlayer characteristic

A method to improve a lithographic process of imaging a portion of a design layout onto a substrate using a lithographic apparatus, the method including computing a multi-variable cost function. The multi-variable cost function represents an interlayer characteristic, the interlayer characteristic being a function of a plurality of design variables that represent one or more characteristics of the lithographic process. The method further includes reconfiguring one or more of the characteristics of the lithographic process by adjusting one or more of the design variables and computing the multi-variable cost function with the adjusted one or more design variables, until a certain termination condition is satisfied.