Method and metrology tool for determining information about a target structure, and cantilever probe

A method of determining information about a target structure disposed on a substrate using a lithographic process, comprising:

• positioning a cantilever probe adjacent to the target structure, the cantilever probe comprising a cantilever arm and a probe element, the probe element extending from the cantilever arm towards the target structure, wherein the target structure comprises:
• a first sub-target and a second sub-target, wherein the first sub-target and the second sub-target each comprise two sub-structures with a deliberately applied overlay bias between the two sub-structures;
• and
• a third sub-target and a fourth sub-target, wherein the third sub-target and the fourth sub-target each comprise two sub-structures with a deliberately applied overlay bias between the two sub-structures,
• wherein the overlay biases of the first sub-target, second sub-target, third sub-target and fourth sub-target are all different from each other;
• generating ultrasonic waves in the cantilever probe, the ultrasonic waves propagating through the probe element into the target structure and reflecting back from the target structure into the probe element or into a further probe element extending from the cantilever arm;
• detecting the reflected ultrasonic waves;
• and
• determining information about the target structure from the detected reflected ultrasonic waves,
• wherein the information about the target structure comprises information about overlay between the two sub-structures of the first, second, third, and/or fourth sub-targets, and
• wherein the overlay represents a degree of misalignment between the two sub-structures.