Scan strategies to minimize the charge effects and radiation damage of the charged particle beam metrology system

摘要

Apparatus and methods are disclosed for performing overlay metrology on a target having at least two layers formed thereon. A target is provided having a plurality of periodic structures for measuring the overlay in at least two overlay directions. The charged particle beam is scanned in a first direction across a plurality of scan swaths of the target and with a first tilt relative to the target such that each edge of the periodic structures is scanned at an angle. The charged particle beam is scanned across the plurality of scan swaths in a second direction opposite to the first direction and with a second tilt of 180° from the first tilt. The first and second direction scanning operations are then repeated for different plurality of scan swaths and different first and second tilts of the target so that the target is scanned symmetrically. Images 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 the analysis of the combined image.