A method and apparatus measure properties of two layers of a damascene structure
(e.g. a silicon wafer during fabrication), and use the two measurements to identify
a location as having voids. The two measurements may be used in any manner, e.g.
compared to one another, and voids are deemed to be present when the two measurements
diverge from each other. In response to the detection of voids, a process parameter
used in fabrication of the damascene structure may be changed, to reduce or eliminate
voids in to-be-formed structures.
