Automatic alignment system for optical projection printing

An automatic alignment method is described which has been applied in a projection system for pattern definition by direct step and repeat on the wafer. The method is based upon the imaging through the projection lens of relief gratings on the wafer onto gratings on the mask and photoelectric detection of the resulting Moiré signal. The alignment accuracy is better than ±0.1 µm and the method can be used for all integrated circuit processing steps. The advantages of the use of phase structures and electrooptical detection are discussed and practical results are given.