Importance

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Abstract

Disclosed are apparatus and methods for performing overlay metrology upon a target having at least two layers formed thereon. A target having a plurality of periodic structures for measuring overlay in at least two overlay directions is provided. A charged particle beam is scanned in a first direction across a plurality of scan swaths of the target and at a first tilt with respect to the target so that each edge of the periodic structures is scanned at an angle. The charged particle beam is scanned in a second direction, which is opposite the first direction, across the plurality of scan swaths and at a second tilt that is 180° from the first tilt. The first and second direction scanning operations are then repeated for different first and second tilts and a different plurality of scan swaths of the target so that the target is scanned symmetrically. Images that are generated by the first and second direction scanning operations are combined to form a combined image, and an overlay error of the target is determined and reported based on analyzing the combined image.

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First Claim