Photolithographic mask alignment using moiré techniques

A method is described for quick alignment of photolithographic masks over silicon substrates. Moiré reflection patterns are observed from grids etched on the silicon wafer and grids located on the photolithographic masks. A restricted class of moiré patterns provides information on both the direction and degree of misalignment during all phases of alignment. Alignment accuracy of 0.2 microm has been achieved even when there were separations of 30 microm between the mask and wafer. The moiré patterns can be observed visually through standard mask-alignment facilities. The etched grids are found to retain their diffraction properties during the operations of reoxidation and epitaxial growth.