Patent ID: 7876440

Claim:
A method for determining overlay between a plurality of first structures in a first layer of a sample and a plurality of second structures in a second layer of the sample, the method comprising: providing targets A, B, C and D that each include a portion of the first and second structures, wherein the target A is designed to have an offset Xa between its first and second structures portions, wherein the target B is designed to have an offset Xb between its first and second structures portions, wherein the target C is designed to have an offset Xc between its first and second structures portions, wherein the target D is designed to have an offset Xd between its first and second structures portions, wherein each of the offsets Xa, Xb, Xc and Xd is different from zero, Xa has an opposite sign and differs from Xb, and Xc has an opposite sign and differs from Xd, wherein the offsets Xa, Xb, Xc and Xd are selected so that an overlay error, including the respective offset, is within a linear region of a plurality overlay values, which include pairs of selected offset Xa, Xb, Xc or Xd and any overlay error; illuminating the targets A, B, C and D with electromagnetic radiation to obtain radiation output signals S A , S B , S c , and S D from targets A, B, C, and D, respectively; and determining any overlay error between the first structures and the second structures using a linear or phase based scatterometry technique based on the obtained radiation output signals S A , S B , S C , and S D , wherein the offsets Xa, Xb, Xc, and Xd are selected so that a slope of a negative region of the overlay values equals a slope of a positive region of the overlay values.