Scatterometer and method of scatterometry using acoustic radiation
An acoustic scatterometer 502 has an acoustic source 520 operable to project acoustic radiation 526 onto a periodic structure 538 and 540 formed on a substrate 536 . An acoustic detector 518 is operable to detect the −1st acoustic diffraction order 528 diffracted by the periodic structure 538 and 540 while discriminating from specular reflection (0th order 532 ). Another acoustic detector 522 is operable to detect the +1st acoustic diffraction order 530 diffracted by the periodic structure, again while discriminating from the specular reflection (0th order 532 ). The acoustic source and acoustic detector may be piezo transducers. The angle of incidence of the projected acoustic radiation 526 and location of the detectors 518 and 522 are arranged with respect to the periodic structure 538 and 540 such that the detection of the −1st and +1st acoustic diffraction orders 528 and 530 discriminates from the 0th order specular reflection 532.
1. A scatterometer comprising:
an acoustic source comprising first and second acoustic sources arranged to project acoustic radiation comprising first and second acoustic radiation at an acoustic angle comprising first and second angles of incidence onto a periodic structure to produce an acoustic diffraction order comprising first and second acoustic diffraction orders; and
an acoustic detector configured to detect the first and second acoustic diffraction orders while discriminating from specular reflection and to produce a detection signal,
wherein the scatterometer is configured to determine a property of the substrate based on the detection signal, and
wherein the first and second acoustic sources are configured to project the first and second acoustic radiation at different times and with the first and second angles of incidence selected such that the acoustic detector is configured to separately detect the first and second respective diffraction orders.
2. The scatterometer of claim 1 , wherein the angle of incidence of the projected acoustic radiation and location of the detector are arranged with respect to the periodic structure such that the detection of the acoustic diffraction order discriminates from specular reflection.
3. The scatterometer of claim 1 , further comprising a couplant system to provide a couplant to facilitate transmission of the acoustic radiation between the acoustic source and the acoustic detector via the periodic structure.
4. The scatterometer of claim 1 , wherein the scatterometer is configured to determine the property of the substrate based on at least one of amplitude, phase and direction of the acoustic diffraction order.
5. The scatterometer of claim 1 , wherein the property of the substrate comprises a physical property of the periodic structure.
6. The scatterometer of claim 1 , wherein the scatterometer is configured to determine a structural asymmetry of the periodic structure based on asymmetry of the first and second corresponding opposite detected acoustic diffraction orders diffracted by the periodic structure.
7. The scatterometer of claim 1 , wherein:
the periodic structure comprises a grating overlaid over another grating; and
the scatterometer is configured to determine an overlay error based on asymmetry of the first and second corresponding opposite detected acoustic diffraction orders diffracted by the gratings.
8. The scatterometer of claim 7 , wherein the overlay error is determined based on asymmetry of at least one of amplitude, phase and direction of the first and second corresponding opposite detected acoustic diffraction orders diffracted by the gratings.
9. The scatterometer of claim 1 , wherein the acoustic source is configured to project continuous wave acoustic radiation onto the periodic structure while the acoustic detector is configured to detect the acoustic diffraction order diffracted by the periodic structure.
10. The scatterometer of claim 1 , comprising an acoustic transceiver configured as the acoustic source to project pulsed acoustic radiation and configured as the acoustic detector to detect a backscattered acoustic diffraction order diffracted by the periodic structure and arising from the pulsed acoustic irradiation.
11. A method of scatterometry comprising:
projecting acoustic radiation comprising first and second acoustic radiation at an angle of incidence comprising first and second angles of incidence onto a periodic structure formed on a substrate, wherein the first and second acoustic radiation is projected at different times;
producing an acoustic diffraction order comprising first and second acoustic diffraction orders based on the first and second acoustic radiation interacting with the periodic structure;
detecting first and second acoustic diffraction orders diffracted by the periodic structure while discriminating from specular reflection; and
determining a property of the substrate based on the acoustic diffraction order.
12. The method of claim 11 , comprising arranging the angle of incidence of the acoustic radiation and location of the detecting with respect to the periodic structure such that the detecting of the acoustic diffraction order discriminates from the specular reflection.