Methods and apparatus for acoustic metrology

A metrology apparatus for determining one or more parameters of a structure having a grating fabricated in or on a semiconductor substrate, the apparatus comprising:

• a transducer array comprising a plurality of transducers positioned in a plane, wherein the plurality of transducers comprise:
• at least one transmitter transducer configured to emit acoustic radiation in a frequency range from 1 GHz to 100 GHz towards the structure, and
• at least first and second receiver transducers configured to receive acoustic radiation reflected and/or diffracted from the structure, and
• a processor configured to:
• obtain, from the first and second receiver transducers, respective first and second signals indicative of an amplitude of the acoustic radiation received by the first and second transducers as a function of time,
• determine a time difference between the first and second signals, and
• use the time difference to determine the one or more parameters of the structure.