Mask alignment technique using phase-shifted moire signals

Moire technique with different variations has been successfully used for mask alignment, with very high accuracies. In this paper we report a new approach to computer controlled mask alignment using modified moire technique. In this technique alignment is controlled in the higher slope region of the moire signal using a single pair of grating alignment marks. In the present case a phase shifted signal is generated by the computer using the input moire signal. The point at which this phase shifted signal becomes equal to the moire signal is treated as the alignment point. The error signal for controlling alignment is obtained by computing the difference of instantaneous moire signal from the intensity of this point. Computer simulation studies as well as experimental studies were conducted on this approach. The results of these studies are presented.