Patent ID: 7858404

Claim:
A method of semiconductor manufacturing, comprising: forming an overlay offset measurement target including a first feature and a third feature located on a first layer and a second feature and a fourth feature located on a second layer, wherein the first feature and the second feature have a first predetermined overlay offset and the third feature and the fourth feature have a second predetermined overlay offset, different than the first predetermined overlay offset; irradiating the target with a radiation beam having a single wavelength; determining a first reflectivity measurement associated with the first and second feature at the single wavelength; determining a second reflectivity measurement associated with the third and fourth feature at the single wavelength; generating a reflectivity profile which has a first axis including reflectivity at the single wavelength and a second axis including predetermined overlay offset, wherein the generating the reflectivity profile includes: plotting the first reflectivity measurement; generating a first line fitted to the plotted first reflectivity measurement; plotting the second reflectivity measurement; and generating a second line fitted to the plotted second reflectivity measurement; and calculating an overlay offset for the first layer and the second layer by determining a value on the second axis where the first line and the second line intersect, wherein the value provides a predetermined overlay offset that corresponds to an actual overlay offset of zero.