Tracking and dynamic control of the angular alignment position in a photolithographic mask aligner by the moiré interference technique

An automatic tracking of the angular alignment position of mask with respect to the wafer in a photolithographic mask aligner is reported. It is demonstrated that automatic dynamic control with very high accuracy and stability of the angular alignment position is practically possible. The moiré interference fringe method is employed for the present experiments. The alignment marks are used in the form of 25 μm pitch Ronchi ruling gratings. The best angular alignment accuracy obtained with the present experimental conditions is of the order of 2×10−7 rad. An alignment accuracy up to ±3×10−7 rad is reported for more than 30 min which enables the present technique to be used for the real photolithographic mask aligner.