Patent ID: 8749786

Claim:
A method to determine an overlay error on a substrate comprising: projecting a beam of radiation onto at least three targets on a substrate, each of the at least three targets comprising first and second overlapping patterns with a predetermined overlay offset; measuring asymmetry of radiation reflected from each of the at least three targets on the substrate; determining an overlay error not resultant from the predetermined overlay offset for each of the at least three targets based on the measured asymmetry; measuring asymmetry of radiation reflected from a point on the substrate other than the at least three targets; and determining an overlay error for the point on the substrate as a function of the measured asymmetry of the radiation reflected from the point, wherein the function limits an effect of linearity error and is based on the determined overlay error not resultant from the predetermined overlay offset, wherein the linearity error is determined by an offset between differences between measured overlay errors of the at least three targets and the respective predetermined overlay offsets of the at least three targets, and a difference between a smallest measured overlay error of the at least three targets and a respective predetermined overlay offset of the respective target of the at least three targets.