The interference fringe aligner uses interference fringes formed by two crossed laser beams to align each exposure field in a step and repeat camera. The alignment mark is a diffraction grating with a period equal to a multiple of the laser fringe period. The fringes move back and forth across the grating producing variations in the intensity of the several diffraction orders. This amplitude modulation of the diffracted beams contains sufficient information to determine the relative position between the fringes and the grating. Alignment is accomplished with a feedback loop to the stepper stage that moves the grating lines directly under the fringes to produce a symmetrical pattern. The essential optical components of the input are a beam splitter and a phase modulator to move the fringes; the output consists only of photodiodes. Detection and analysis are all electronic, not optical. Thus the system is simple, very sensitive, and stable. The beams strike the wafer at a shallow angle, allowing the aligner to be attached to the bottom of the stepper lens and illuminate a chip under the lens. Chips are aligned in the ‘‘expose’’ position, eliminating stage positioning errors that occur when the chip is aligned at one site and moved to another for exposure. It is thus easy to retrofit existing steppers that have a working distance >1 cm between the lens and the wafer.