Integrated circuit step and repeat inspection using spatial filtering

An improved method for photomask inspection employing spatial filtering is described. Previous techniques were not accurate enough for checking step and repeat photoplates over the field of the photomask. The technique described can measure pattern step and repeat by correlating it with a fringe pattern and using spatial filtering analyze the result. Results indicate a precision of about 0.1 mil over 1 cm field; the field should able to be extended to 5 cm with further experimental investigation.