Apparatus and methods for determining the position of a target structure on a substrate

A lithography system comprising:

• a sensor comprising:
• a transducer configured to generate acoustic waves;
• a lens assembly configured to transmit and direct the acoustic waves to a target, and to receive at least a part of the acoustic waves, after interaction with the target;
• and
• an optical detector that comprises at least one optically reflective member arranged to receive the at least part of the acoustic waves, wherein the at least one optically reflective member is arranged to be mechanically displaced in response to the received acoustic waves;
• and
• at least one radiation source configured to illuminate the at least one optically reflective member, wherein the at least one radiation source is arranged to provide and to direct a reference beam along a radiation beam reference path in the optical detector.

A metrology system comprising:

• a sensor comprising:
• a transducer configured to generate acoustic waves;
• a lens assembly configured to transmit and direct the acoustic waves to a target, and to receive at least a part of the acoustic waves, after interaction with the target;
• an optical detector that comprises at least one optically reflective member arranged to receive the at least part of the acoustic waves, wherein the at least one optically reflective member is arranged to be mechanically displaced in response to the received acoustic waves;
• and
• at least one radiation source configured to illuminate the at least one optically reflective member, wherein the at least one radiation source is arranged to provide and to direct a reference beam along a radiation beam reference path in the optical detector,
• wherein the metrology system is configured to acquire information of one or more of the targets and to use the information of the one or more of the targets to obtain overlay information.

A method comprising:

• irradiating an object with acoustic waves,
• receiving, at a sensor, at least a portion of the acoustic waves reflected from a target, wherein the senor comprises an optical detector that comprises at least one optically reflective member arranged to receive the at least a portion of the acoustic waves reflected from the target, wherein the at least one optically reflective member is arranged to be mechanically displaced in response to the received acoustic waves;
• illuminating with illumination the at least one optically reflective member using at least one radiation source that is also arranged to provide and to direct a reference beam along a radiation beam reference path in the optical detector;
• measuring the mechanical displacement of the at least one optically reflective member induced by the received acoustic waves using the illumination, and
• deriving characteristics of the target from the measured mechanical displacement.