Automatic angular alignment in mask aligner using computer controlled moire technique

A novel method for automatic alignment of masks with respect to a wafer in a mask-aligner is developed and reported. Alignment marks are used in the form of gratings of 25 mu m pitch. The effect of various parameters on moire fringes is studied theoretically and experimentally, and the results are found to be in good agreement. Computer controlled moire sensor is employed to obtain the angular alignment accuracy of the order of +/-130 nanoradians. This high accuracy is maintained for more than 5 minutes. The feasibility experiments are performed for wafers of various sizes and very high accuracy is obtained in each case.