Patent ID: 6973636

Claim:
A method of identifying forbidden pitches for a lithographic exposure tool, comprising: selecting a set of exposure conditions; performing a simulation of an actual lithographic exposure process using a computer model, wherein the performing of the simulation comprises: selecting a set of pattern pitches for a certain target critical dimension, and running the computer model using the selected set of exposure conditions for at least some of the pattern pitches from the set of pattern pitches to obtain a corresponding set of depth-of-focus (DOF) values for each pitch ran; determining whether the DOF values are greater than a focus budget of the exposure tool for a set of frequently used pitches; if the DOF values are not greater than the focus budget of the exposure tool for the set of frequently used pitches, selecting a new set of exposure conditions and re-performing the simulation using the new set of exposure conditions; if the DOF values are greater than the focus budget of the exposure tool for the set of frequently used pitches, performing a verification test comprising: providing a test mask having formed therein at least one of the pattern pitches used in a last performed simulation, exposing a wafer through the test mask using the exposure tool and using the set of exposure conditions of the last performed simulation, and determining an actual DOF value for at least one of the exposed pattern pitches; for at least some of the exposed pattern pitches, comparing the corresponding actual DOF value with the focus budget of the exposure tool; and if the actual DOF value is not greater than the focus budget of the exposure tool, designating that actual DOF value's corresponding pattern pitch as a forbidden pitch.