Scanning strategy for minimizing charging effects and radiation damage in charged particle beam measurement systems

摘要

An apparatus and method for performing overlay measurements on a target having at least two layers formed thereon are disclosed.A target having a plurality of periodic structures is provided for measuring overlay at least two overlay directions.The charged particle beam is scanned in the first direction by the first tilt with respect to the target by a plurality of scan swings of the target, and as a result, each edge of the periodic structure is slanted.The charged particle beam is from a plurality of scanning swans to the first tilt ° In the second tilt which is opposite to the first direction is scanned.Next, the scanning operation in the first and second directions is repeated with different first and second tilts and with a plurality of scan SWS different in the target, so that the target is scanned symmetrically.An overlay error of the target is determined and reported based on the synthesized image formed by combining the image generated by the scanning operation in the first and second directions to form a composite image.