Patent ID: 8665417

Claim:
An apparatus for measuring properties of a substrate, comprising: a radiation source configured to direct radiation onto targets of corresponding fields of the substrate, wherein a first one of the targets has at least: a first grating and a second grating having respective predetermined offsets, the predetermined offset of the first grating being in a direction opposite the predetermined offset of the second grating, and a third grating and a fourth having respective predetermined offsets, the predetermined offset of the third grating being in a direction opposite the predetermined offset of the fourth grating; a detector configured to detect the radiation reflected from the first one of the targets and to obtain an asymmetry value for the first one of the targets from the detected radiation; and a controller configured to (i) determine an overlay value for the first one of the targets based on at least the obtained asymmetry value and the predetermined offsets and (ii) determine a polynomial fit across the remaining targets of a corresponding field based on a relationship between the obtained asymmetry value and the determined overlay value of the remaining targets.