Projection optical system for aligning an image on a surface

A method for aligning alignment images projected by a camera with associated alignment edges etched in the surface of a wafer, comprising the steps of,

• placing a mask having coordinate alignment slits into a projection position on said camera,
• placing said wafer on an alignment table of said camera,
• moving the alignment table in at least two coordinate directions to position the wafer in a dead reckoning alignment position with respect to the mask,
• rotating into an alignment position a light coupling mirror having at least two openings passing therethrough,
• turning on a radiation source to illuminate at least the coordinate alignment slits of the mask,
• transmitting radiation through the alignment slits and associated openings in the light coupling mirror to a portion of a telecentric lens,
• focusing the transmitted radiation as alignment images on the surface of the wafer,
• focusing the radiation reflected from the surface of the wafer through the openings in the light coupling mirror,
• transmitting to the reflective surface of the light coupling mirror the radiation that is diffracted by said alignment edges,
• reflecting the diffracted radiation,
• measuring the intensity of the reflected radiation,
• moving the alignment table in at least two coordinate directions until a peak intensity of reflected radiation is measured.

An automatic method for aligning alignment images projected by a camera with associated alignment images etched in the surface of a wafer, comprising the steps of,

• placing a mask having coordinate alignment slits into a projection position on said camera,
• placing the wafer on an alignment table of said camera,
• moving the alignment table in at least two coordinate directions to position the wafer in a dead reckoning alignment position with respect to the mask,
• rotating into an alignment position a light coupling mirror having at least two openings passing therethrough,
• turning on a radiation source to illuminate at least the coordinate alignment slits of the mask,
• transmitting source radiation through groups of alignment slits in the mask, each group corresponding to a particular coordinate alignment direction,
• deflecting the transmitted source radiation for each group of alignment slits an amount corresponding to the angular position of an associated oscillating transparent plate,
• generating a reference signal for each oscillating transparent plate when the plate is positioned so that its associated source radiation is not deflected,
• passing the deflected source radiation for each alignment group of slits through an associated opening in the light coupling mirror to a telecentric lens,
• focusing the deflected radiation for each alignment group as scanning alignment images on the surface of the wafer,
• focusing the radiation reflected from the surface of the wafer through the openings in the light coupling mirror,
• passing to the reflective surface of the light coupling mirror the radiation that is diffracted by the alignment edges,
• reflecting the diffracted radiation for each alignment group of slits to a corresponding photo-detector,
• defining a registration point for each photo-detector corresponding to the peak intensity of diffracted radiation detected by the detector,
• generating an offset error signal that is proportional to the time delay between the times of occurrence of each reference signal and the associated registration point, and
• applying the offset error signals to move the alignment table in corresponding coordinate alignment directions.