Patent ID: 8423917

Claim:
A method for determining image intensity at a location in a photoresist (PR) layer on a wafer, wherein a photolithography process uses a light source and a first mask to expose the PR layer, wherein the wafer includes a multilayer structure underneath the PR layer, and wherein the multilayer structure includes one or more patterned layers which were created by one or more previous applications of the photolithography process, the method comprising: receiving a set of masks which were used to generate the one or more patterned layers of the multilayer structure, wherein a patterned layer in the one or more patterned layers includes a set of reflectors on a top surface of the patterned layer, which correspond to patterns in a patterned-layer mask in the set of masks, wherein a reflector reflects light from the light source during the photolithography process; generating, by computer, a first virtual mask based at least on the first mask and the patterned-layer mask, wherein the first virtual mask uses a clear area to model a reflector in the set of reflectors of the patterned layer on the wafer; and determining the image intensity value at the location in the PR layer by: computing a first image intensity component due to the light source and the first virtual mask, which involves: receiving a process model which models the photolithography process; adjusting the process model to take into account the position of the patterned layer; and convolving, at the location in the PR layer, the adjusted process model with the first virtual mask; and estimating the image intensity value by combining the first image intensity component with an image intensity component due to the light source and the first mask.