METROLOGY METHOD AND APPARATUS, SUBSTRATES FOR USE IN SUCH METHODS, LITHOGRAPHIC SYSTEM AND DEVICE MANUFACTURING METHOD

摘要

A substrate has a plurality of overlay gratings formed thereon by a lithographic process. Each overlay grating has a known overlay bias. The values of overlay bias include for example two values in a region centered on zero and two values in a region centered on P/2, where P is the pitch of the gratings. Overlay is calculated from asymmetry measurements for the gratings using knowledge of the different overlay bias values, each of the overall asymmetry measurements being weighted by a corresponding weight factor. Each one of the weight factors represents a measure of feature asymmetry within the respective overlay grating. The calculation is used to improve subsequent performance of the measurement process, and/or the lithographic process. Some of the asymmetry measurements may additionally be weighted by a second weight factor in order to eliminate or reduce the contribution of phase asymmetry to the overlay.

主权项

1. A method of measuring a parameter of a lithographic process, the method comprising:
illuminating a plurality of target structures of a target formed by a lithographic process on a substrate, each target structure comprising overlaid periodic structures and each having a known overlay bias, and detecting radiation scattered by each target structure to obtain for that target structure a measurement representing an overall asymmetry that includes contributions due to (i) the known overlay bias, (ii) overlay performance of the lithographic process, and (iii) feature asymmetry within one or more of the periodic structures; and using the overall asymmetry measurements for a plurality of target structures to calculate a measure of the overlay error, the calculation being performed using the known overlay bias values and the overall asymmetry measurements, wherein each of the overall asymmetry measurements is weighted by a corresponding weight factor, each one of the weight factors representing a measure of feature asymmetry within the respective one or more periodic structures.