Moiré signals in reflection

One of the recent applications of the moiré interferometric technique is in mask alignment for lithography. Usually it is necessary to use reflection moiré systems for alignment. We have analysed theoretically and experimentally the behaviour of moiré signals in reflection, as well as in transmission, with the gap as parameter. A periodic variation of the contrast of the moiré signals was observed when the gap was varied within the Fresnel region. The moiré signals were found to be superimposed on a background which varies with the gap. This variation of background was different for reflection and transmission. However, in both cases the alignment signal itself in the alignment method we are proposing, is independent of small variations in the gap between the two gratings.