Patent ID: 7630070

Claim:
A scatterometer configured to measure a property of a substrate, the apparatus comprising: a high numerical aperture lens configured to project radiation onto the substrate and to project radiation redirected from the substrate towards a back focal plane of the high numerical aperture lens or towards a conjugate of a front focal plane of the high numerical aperture lens; a first detector configured to detect a first cross-sectional area of the redirected radiation having an intensity above a first value; and a second detector configured to detect a second cross-sectional area of the redirected radiation having an intensity above a second value, wherein the first detector is arranged in front of the back focal plane, between the high numerical aperture lens and the back focal plane, and the second detector is arranged behind the back focal plane, or the first detector is arranged in front of the conjugate of the front focal plane, between the high numerical aperture lens and the conjugate of the front focal plane, and the second detector is arranged behind the conjugate of the front focal plane.