Overlay measurements for a semiconductor wafer are obtained by forming a
periodic grating on the wafer having a first set of gratings and a second
set of gratings. The first and second sets of gratings are formed on the
wafer using a first mask and a second mask, respectively. The first and
second sets of gratings are intended to be formed on the wafer with an
intended asymmetrical alignment. A diffraction signal of the first and
second sets of gratings is measured after the first and second sets of
gratings are formed on the wafer. The misalignment between the first and
second sets of gratings formed on the wafer is determined based on the
measured diffraction signal.
