1. A method for calibrating a photolithography process model for modeling a photolithography process, wherein the photolithography process is followed by an etch process, the method comprising: constructing an etch bias model which models a critical dimension (CD) difference between a measured CD value of a feature after the photolithography process and a measured CD value of the feature after the etch process; receiving measured CD data for a layout which is measured after the photolithography process; and fitting the photolithography process model based at least on the measured CD data and the etch bias model, thereby causing the photolithography process model to be aware of etch effects.