Methods and apparatus for acoustic metrology

1 . 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.

2 . The metrology apparatus of claim 1 , wherein the transducer array is disposed in or on a solid delay line element extending from the transducer array in the direction of the structure and includes a planar face configure to position adjacent to a corresponding planar face of the substrate or a structure fabricated on the substrate.

3 . The metrology apparatus of claim 1 , wherein the plurality of transducers are arranged along a common axis.

4 . The metrology apparatus of claim 1 , wherein the plurality of transducers comprises at least one receiver transducer on either side of the at least one transmitter transducer along the common axis.

5 . The metrology apparatus of claim 3 , wherein the at least one transmitter transducer comprises a plurality of transmitter transducers connected in parallel and arranged to be excited by a single signal.

6 . The metrology apparatus of claim 4 , the apparatus wherein the plurality of transmitter transducers is configured to be operated as a phased-array.

7 . The metrology apparatus of claim 5 , further comprising one or more switches configured to connect and/or disconnect one or more additional transducers of the plurality of transducers to the at least one transmitter transducer.

8 . The metrology apparatus of claim 1 , comprising one or more switches configured to switch one or more of the plurality of transducers from a transmitter transducer to a receiver transducer within a switching time of less than 50 ns.

9 . The metrology apparatus of claim 7 , further comprising a controller configured to control the one or more of the plurality of transducers to be a transmitter transducer or a receiver transducer.

10 . A metrology apparatus of claim 1 , wherein the one or more transmitter transducers is or are configured to emit the acoustic radiation in a pulse having a duration corresponding to a number of cycles of the acoustic radiation in a range from 1 to 1000 cycles.

11 . The metrology apparatus of claim 1 , wherein the transducer array has a pitch in a range from 250 nm to 10 μm.

12 . The metrology apparatus of claim 1 , further comprising a further transducer array comprising a plurality of further transducers coplanar with the transducer array, the plurality of further transducers comprising at least one further transmitter transducer for emitting acoustic radiation in a frequency range from 1 GHz to 100 GHz towards the structure, and at least one further receiver transducer for receiving acoustic radiation reflected and/or diffracted from the structure.

13 . The metrology apparatus of claim 1 , wherein the one or more parameters of the grating is a lateral offset of the grating with respect to the transducer array.

14 . The metrology apparatus of claim 13 , wherein the determining the time difference comprises one or more of:

determining respective times at which each of the signals first crosses a particular amplitude;

determining a respective one or more times at which each of the signals is zero; and

determining a respective one or more times at which a gradient of each of the signals is zero.

15 . The metrology apparatus according to claim 14 , wherein the determining the time difference comprises:

determining a cross correlation function of the signals; and

determining the time difference based on a time for which the cross correlation function of the time-varying signals is maximal.

16 . The metrology apparatus of claim 1 , wherein:

the structure comprises a further grating arranged, such that the grating is positioned between the further grating and the at least one transmitter transducer,

the at least first and second receiver transducers are configured to receive acoustic radiation reflected and/or diffracted from the first and second gratings, and

the grating has a first pitch and the further grating has a second pitch.

17 . The metrology apparatus of claim 1 , wherein:

the structure comprises a further grating spaced apart from the grating along a direction parallel to a normal both gratings, and

the grating has a first pitch and the further grating has a second pitch.

18 . The metrology apparatus of claim 16 , wherein the second pitch is different than the first pitch.

19 . The metrology apparatus of claim 17 , wherein the second pitch is different than the first pitch.