Method and apparatus for determining a process model that models the impact of CAR/PEB on the resist profile

A computer-readable storage medium storing instructions that when executed by a computer cause the computer to perform a method for determining a process model, wherein the computer-readable storage medium does not include a transmission medium or a carrier wave, the method comprising:

• receiving a first optical model which models an image that is formed by an optical system of a photolithography process at a first distance which is equal to the optical system's best focus distance;
• receiving a second optical model which models a second latent image that is formed by the optical system at a second distance which is different from the first distance;
• receiving a third optical model which models a third latent image that is formed by the optical system at a third distance which is different from the first distance and the second distance;
• receiving process data which is obtained by subjecting a test layout to the photolithography process;
• determining an uncalibrated process model by combining the first optical model, the second optical model, and the third optical model;
• and
• fitting the uncalibrated process model using the process data.