Patent ID: 9239523
Filing Date: 2016-01-19
CPC Classification: G03F

Claim Text:
1. A method comprising: illuminating a target with incident radiation, the target having a plurality of diffraction-based alignment pads; detecting signals diffracted from each of the plurality of diffraction-based alignment pads after the incident radiation interacts with the plurality of diffraction-based alignment pads; testing a linearity of an overlay error using the signals without using signals from other targets by determining if the signals have a linear response to the overlay error, wherein determining if the signals have a linear response is based on a ratio of a signal difference between a first pair of diffraction-based alignment pads and a signal difference between a second pair of diffraction-based alignment pads that is different than the first pair of diffraction-based alignment pads; and determining an empirical diffraction-based overlay measurement of the overlay error using the signals when the signals have the linear response to the overlay error.