Patent ID: 7460231

Claim:
An alignment tool comprising: at least one reference mark; an illuminator configured to direct an alignment radiation beam onto a diffractive alignment mark on a substrate such that the alignment radiation beam is diffracted by the diffractive alignment mark into sub-beams, each sub-beam having a different diffraction order; a deflection element configured to deflect the sub-beams from the alignment mark to the at least one reference mark according to the diffraction order of the sub-beams, wherein the at least one reference mark is configured to further diffract the sub-beams such that each sub-beam comprises an indication of a position of the alignment mark with respect to said at least one reference mark; and a detector configured to inspect the sub-beams diffracted from said at least one reference mark in order to determine the position of the alignment mark with respect to the at least one reference mark, wherein the deflection element comprises a sub-wavelength grating of structures, wherein a distance between adjacent structures is less than the wavelength of the sub-beams.