Patent ID: 8296688

Claim:
A method for evaluating the quality of an assist feature placement in a post-optical proximity correction (post-OPC) mask layout, the method comprising: receiving a set of target patterns which represent a set of polygons in a pre-OPC mask layout; receiving an assist feature placement for the post-OPC mask layout; constructing a focus-sensitive cost function based on the target patterns, wherein the focus-sensitive cost function represents an amount of movement of post-OPC contours of the target patterns in response to changes in focus condition of the lithography system, and wherein the contours of the target patterns substantially coincide with the edges of set of the polygons; determining, by computer, a cost-covariance field (CCF field) based on the focus-sensitive cost function, wherein the CCF field is a two-dimensional (2D) map representing changes to the focus-sensitive cost function due to an addition of a pattern at a given location within the post-OPC mask layout; and using the CCF field to evaluate the quality of the assist feature placement for the post-OPC mask layout.