A method for determining the centroid of a wafer target. In one embodiment, the
method comprises a series of steps in a stepper, starting with the step of receiving
a wafer, having a target set formed therein. Next, a signal is passed over the
target set and over a material separating target shapes in the target set. Then
a return signal is reflected, and received, from the surface of the target shapes
and the material separating them. A location of at least one maxima point of the
return signal is identified. Finally, a centroid is determined as the median of
the locations of at least one maxima point.
