Automatic angular alignment in mask aligner using computer controlled moire technique

A novel method for the 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 /spl 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. A computer controlled moire sensor is employed to obtain the angular alignment accuracy of the order of /spl plusmn/130 nanoradians. This high accuracy is maintained for more than 5 minutes. Feasibility experiments are performed for wafers of various sizes and very high accuracy is obtained in each case.