Patent ID: 6856392

Claim:
A diffractive optical element to be used with a design wavelength λ and providing for detection of an alignment mark using light of a second wavelength, comprising: a diffractive surface for diffracting predetermined light of the design wavelength λ; and an alignment mark formed in a portion of the diffractive surface, wherein, with regard to the predetermined light, a phase difference corresponding to a multiple, by an integer, of the design wavelength λ is produced between (i) a light ray, of the predetermined light, as transmitted through or reflected by the alignment mark and (ii) a light ray, of the predetermined light, as transmitted through or reflected by a portion adjacent to the alignment mark, and that, with regard to second light of the second wavelength λ′ different from the design wavelength λ, no phase difference corresponding to a multiple, by an integer, of the second wavelength λ′ is produced between (a) a light ray, of the second light, as transmitted through or reflected by the alignment mark and (b) a light ray, of the second light, as transmitted through or reflected by a portion adjacent to the alignment mark.