Patent ID: 8125615

Claim:
An optical focus sensor arranged to generate a focus error signal indicative of a position of a substrate relative to a focal plane of an objective lens, the optical focus sensor comprising: a depolarizer constructed and arranged to convert polarized light into a non-polarized light beam; and a splitter constructed and arranged to split the non-polarized light beam into a first sub-beam and a second sub-beam associated with respectively a first and a second optical branch, the splitter being further constructed and arranged to direct the first sub-beam to a first detector via a first aperture, and to direct the second sub-beam to a second detector via a second aperture wherein, the first aperture is positioned between a first back focal plane of the objective lens in the first optical branch and the first detector, and the second aperture is positioned between the objective lens and a second back focal plane of the objective lens in the second optical branch.