Patent ID: 7061615

Claim:
An overlay target for measuring the alignment of layers on a substrate, said overlay target comprising: a first symmetrical grating on a first layer disposed over said substrate, said first symmetrical grating being a diffraction grating; a second symmetrical grating on a second layer disposed over said first symmetrical grating, said second symmetrical grating being a diffraction grating, wherein the second symmetrical grating is nominally positioned over the first symmetrical grating so that there is an asymmetry between said first symmetrical grating and said second symmetrical grating in the direction of the periodicity of the first symmetrical grating and the second symmetrical grating; and a third symmetrical grating disposed over said substrate, said third symmetrical grating being a diffraction grating comprised of a third plurality of parallel lines; and a fourth symmetrical grating disposed over said third symmetrical grating, said fourth symmetrical grating being a diffraction grating comprised of a fourth plurality of parallel lines substantially parallel to said third plurality of parallel lines, said third symmetrical grating and said fourth symmetrical grating are asymmetrical relative to each other, and said third plurality of parallel lines and said fourth plurality of parallel lines are substantially perpendicular to said first plurality of parallel lines and said second plurality of parallel lines.