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, by computer, the photolithography process model based at least on the measured CD data and the etch bias model, wherein said fitting involves: computing a residue of the photolithography process model based at least on the measured CD data; computing a residue fitting target based at least on the etch bias model; and fitting the photolithography process model so that the residue of the photolithography process model is substantially equal to the residue fitting target.