Optical element with alignment mark, and optical system having such optical element

A device manufacturing method, comprising the steps of:

• exposing a substrate with a device pattern by use of light having a wavelength λ and by use of an exposure apparatus having a diffractive optical element, the diffractive optical element including (1) a diffractive surface for diffracting predetermined light corresponding to the wavelength λ, and (2) 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 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 a second wavelength λ′ different from the 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, whereby a position of the alignment mark is detected using the light of the second wavelength;
• and
• developing the exposed substrate.