Patent ID: 7968258

Claim:
A method for performing photolithography in a photolithography system, comprising: providing a first mask and a second mask for patterning a wafer, the first mask having a first set of marks and the second mask having a second set of marks; defining a first reference focus plane for the first mask based on the first set of marks; determining a best focus of the first mask using a calibration routine of the photolithography system; defining a second reference focus plane for the second mask based on the second set of marks; determining a Z direction difference between the first reference focus plane and the second reference focus plane; determining a best focus of the second mask based on the best focus of the first mask and the Z direction difference between the first and second reference focus planes; providing a third mask for patterning the wafer, the third mask having a third set of marks; defining a third reference focus plane of the third mask based on the third set of marks; determining a Z direction difference between the first reference focus plane and the third reference focus plane; and determining a best focus of the third mask based on the Z direction difference between the first and third reference focus planes.