Patent ID: 7289214

Claim:
A system for measuring a lateral offset between a first layer in proximity to and parallel to a second layer, the system comprising: a) a first grating having first grating lines in the first layer; b) a second grating having second grating lines in the second layer, wherein the first and second grating lines are substantially perpendicular to a common grating axis, wherein the first and second gratings at least partially overlap; c) an optical source providing incident light to the first and second gratings, the incident light having an angle of incidence θ greater than zero and having a plane of incidence making an azimuthal angle φ with the grating axis, wherein φ is selectable to have either of two values φ 1 or φ 2 =φ 1 +180°, wherein φ 1 is not an integer multiple of 90°; d) an optical detector receiving diffracted light from the first and second gratings and providing a first optical response when φ=φ 1 and providing a second optical response when φ=φ 2 ; e) a processor for providing a first difference ΔR 1 between the first and second optical responses, wherein ΔR 1 depends in part on the offset between the first and second layers.