Patent ID: 7385699

Claim:
A method for determining an overlay error between at least two layers in a multiple layer sample, the method comprising: (a) using an optical system to measure a plurality of measured optical signals from a plurality of periodic targets on the sample, wherein the periodic targets each have a first structure in a first layer and a second structure in a second layer, wherein there are predefined offsets between the first and second structures; and (b) using a scatterometry overlay technique to analyze the measured optical signals of the periodic targets and the predefined offsets of the first and second structures of the periodic targets to thereby determine and store an overlay error between the first and second structures of the periodic targets, wherein the scatterometry overlay technique is a phase based technique that includes representing each of the measured optical signals as a set of periodic functions having a plurality of known parameters and a plurality of unknown parameters that include an unknown overlay error parameter and analyzing the set of periodic functions to solve for the unknown overlay error parameter to thereby determine the overlay error, wherein the number of periodic targets equals the number of unknown parameters.