Alignment system for a Half-Field Dyson projection system

An alignment system for a stepper employing a Half-Field Dyson projection system;

• wherein said alignment system may be used to align an image of at least one key disposed on a reticle with at least one target disposed on a wafer, in which said one key includes a key diffraction-grating pattern having a given angular orientation and a given line spacing and said one target includes a target diffraction-grating pattern having an angular orientation corresponding to said given angular orientation and a line spacing substantially equal to a predetermined integer multiple of said given line spacing;
• and wherein said alignment system comprises:
• first means including said Half-Field Dyson projection system for imaging alignment light diffracted from said one-key diffraction-grating pattern onto said wafer in response to said one-key diffraction-grating being illuminated with alignment light comprising a given range of wavelengths;
• and
• second means for detecting coincidence in the position of said one wafer target and the position on said wafer of the image of said one reticle key, wherein said second means includes third means shaped, sized and positioned to have incident thereon only diffracted alignment light of at least two certain selected orders reflected from said one-target diffraction-grating pattern and substantially no diffraction-order alignment light reflected from said one-key diffraction-grating pattern, and said second means detects changes in the intensity of alignment light incident on said third means as said wafer and said reticle are moved relative to one another, whereby said one wafer target substantially coincides with the image on said wafer of said one reticle key when the intensity of alignment light incident on said third means is at a maximum.