Computer Controlled Mask Aligner Using Modified Moire Technique

The operation of mask alignment is one of the most important steps in the process of fabrication of integrated circuits. In this paper we report a computer controlled mask alignment system using modified moire’ technique. In this technique the alignment is controlled in high slope region of the moire’ signal by using a single pair of grating alignment marks. Two methods have been proposed to obtain alignment. The error signal for alignment is taken as the difference of moire’ signal and a secondary signal computed from the moire’ signal. In one method this secondary signal is the inverted moire’ signal while in other it is the average value calculated from the maximal and minimal value of moire’ signal. Experiments were conducted using He-Ne laser of 0.633 μm wavelength and 25μm grating pitch. The alignment accuracy of 0.06μm has been achieved.