Patent ID: 7298481

Claim:
A method of determining an overlay error between two layers of a multiple layer sample, the method comprising: for each of a plurality of periodic targets that each have a first structure formed from a first layer and a second structure formed from a second layer of the sample, measuring a plurality of optical signals at a plurality of incident angles, wherein there are predefined offsets between the first and second structures; and determining and storing, in memory, an overlay error between the first and second structures by analyzing the measured optical signals at the plurality of incident angles from the periodic targets using a scatterometry overlay technique based on the predefined offsets without using a calibration operation or a model-based regression technique, wherein the scatterometry overlay technique is a phase based technique that includes representing each of the first and second optical signals as a set of periodic functions having a plurality of known parameters, and 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.