Disclosed are methods and apparatus for analyzing the quality of overlay
targets. In one embodiment, a method of extracting data from an overlay target
is disclosed. Initially, image information or one or more intensity signals of
the overlay target are provided. An overlay error is obtained from the overlay
target by analyzing the image information or the intensity signal(s) of the overlay
target. A systematic error metric is also obtained from the overlay target by analyzing
the image information or the intensity signal(s) of the overlay target. For example,
the systematic error may indicate an asymmetry metric for one or more portions
of the overlay target. A noise metric is further obtained from the overlay target
by applying a statistical model to the image information or the intensity signal(s)
of the overlay target. Noise metric characterizes noise, such as a grainy background,
associated with the overlay target. In other embodiments, an overlay and/or stepper
analysis procedure is then performed based on the systematic error metric and/or
the noise metric, as well as the overlay data.
