Precision alignment system using laser moire sensor

The authors have previously investigated precision automatic mask-to-wafer alignment techniques using moire signals. The calculated results of the moire signals are compared with actual experimental results, and good agreements are obtained. The signals used in the experiment were obtained from two pairs of gratings arranged 180 degrees out of phase with each other. A differential moire signal between the two measured moire signals is used as the control signal. In-phase noise in the moire signals is cancelled and the S/N ratio of the control signal is improved. In this way, a high alignment accuracy of +/-4nm is obtained.