Metrology method and lithographic apparatuses

1. A method of metrology comprising:

obtaining, using a sensor having sensor optics, a pupil plane measurement dataset at a pupil plane relating to scattered radiation resultant from a measurement of a structure, wherein the pupil plane measurement dataset comprises an intensity distribution;

obtaining a sensor term from calibration data, wherein the sensor term corresponds to the sensor optics used to perform the measurement;

processing the intensity distribution to obtain an intensity term; and

determining a correction term for a pupil plane measurement using the intensity term and the sensor term or determining a measurement value using the pupil plane measurement, sensor term, and intensity term;

wherein the sensor term and intensity term correspond to at least one scattering angle at a position in the pupil plane within an angle range comparable with an angular spread of light propagating in an optical system.

2. The method of claim 1 , wherein the method of metrology comprises a method of position metrology in aligning an object, and the measurement value comprises an alignment value.

3. The method of claim 1 , wherein the determining comprises applying a linear regression model to the intensity term and the sensor term.

4. The method of claim 1 , wherein the sensor term relates to aberration in the sensor optics as a function of position in the pupil plane.

5. The method of claim 1 , wherein the intensity distribution comprises an intensity distribution within the pupil plane relating to a single higher diffraction order.

6. The method of claim 5 , wherein the determining is preformed separately for each diffraction order of a corresponding pair of higher diffraction orders and further comprises averaging the results corresponding to each respective diffraction order to obtain the measurement value or correction therefor.

7. The method of claim 1 , wherein the intensity distribution comprises an intensity distribution within the pupil plane describing a distribution of averages of pairs of corresponding pixels, the pairs of corresponding pixels comprising a first pixel in a first diffraction order of a corresponding pair of higher diffraction orders and a corresponding second pixel in a diffraction order of the corresponding pair of higher diffraction orders.

8. The method of claim 7 , wherein the pairs of corresponding pixels comprise pixels in symmetrically opposite locations within the pupil plane.

9. The method of claim 7 , wherein the intensity distribution takes into account a symmetry of the sensor.

10. The method of claim 9 , wherein the pairs of corresponding pixels comprise pairs of pixels that interfere to generate a fringe pattern imaged in performing the measurement, the measurement value being derived from the fringe pattern.

11. The method of claim 10 , wherein the pairs of corresponding pixels comprise pixels in symmetrically opposite locations within the pupil plane.

12. The method of claim 10 , wherein the pairs of corresponding pixels comprise pairs of pixels having a corresponding displacement within the pupil plane with respect to respective chief rays.

13. The method of claim 1 , wherein the intensity distribution comprises a normalized intensity distribution.

14. The method of claim 1 , comprising projecting the pupil plane measurement dataset on a suitable basis to reduce a dimensionality of the determining.

15. The method of claim 14 , wherein the suitable basis comprises Zernike polynomials.

16. The method of claim 1 , comprising an initial calibration step to calibrate the sensor term for the sensor optics.

17. The method of claim 16 , wherein the calibration step comprises:

obtaining diverse calibration pupil datasets using the sensor optics; and

performing an inversion to calibrate the sensor term from the pupil plane measurement dataset derived from each calibration pupil dataset.

18. The method of claim 17 , wherein the diverse calibration pupil datasets comprise respective known measurement values or known corrections therefor and the step of performing an inversion further uses the respective known measurement values or known corrections therefor.

19. The method of claim 17 , wherein the diverse calibration pupil datasets relate to measurements of a calibration structure of interest measured on a calibration substrate, the calibration substrate also comprising one or more calibration reference structures with known and/or low sensitivity characteristics; and the calibration step further comprises:

determining measurement value offset data describing offsets between a measurement value for the calibration structure of interest and measurement values for each of the one or more calibration reference structures; and

using the determining measurement value offset data in the step of performing an inversion.

20. The method of claim 19 , wherein the one or more calibration reference structures comprise one or more parameter values chosen to minimize sensitivity to layer thickness.

21. The method of claim 20 , wherein the one or more parameter values chosen to minimize sensitivity to layer thickness relate to one or both of:

a subsegmentation of the structure or features thereof, and

periodicity or pitch of repeated features of the structure.

22. The method claim 17 , wherein the method comprises obtaining diverse calibration image plane datasets corresponding to at least a subset of the diverse calibration pupil datasets; and

using the diverse calibration image plane datasets in the step of performing an inversion to calibrate the sensor term.

23. The method of claim 22 , wherein effects in the diverse calibration image plane datasets attributable at least partly to crosstalk from surrounding structures adjacent the structure are correlated to corresponding effects in the diverse calibration pupil datasets, so as to distinguish these effects when performing the inversion.

24. The method of claim 22 , wherein each of the diverse calibration image plane datasets comprise sets of one or both of:

an intensity value or distribution measured at an image plane; and

local fringe amplitude and/or fringe phase relating to surrounding structures, measured at an image plane.

25. The method of claim 22 , wherein the step of performing an inversion further comprises inverting for the sensor term: a function comprising a first term comprising the sensor term and the diverse calibration pupil datasets and a second term comprising the sensor term and the diverse calibration image plane datasets, so as to optimize the sensor term in terms of correcting the calibration pupil plane datasets.

26. The method of claim 1 , wherein the pupil plane measurement dataset relates to regions other than one or more diffraction orders from the structure; the determining step also using this additional pupil plane measurement dataset.

27. The method of claim 1 , comprising performing the measurement to obtain the pupil plane measurement data and measurement value.

28. The method of claim 1 , wherein the measurement value and the pupil plane measurement dataset each relate to different respective wavelengths, and/or polarizations of measurement radiation and/or repetition directions of a repeating pattern of the structure.

29. A computer program comprising computer readable instructions operable to perform the method of claim 1 .

30. A processor and associated storage medium, the storage medium comprising the computer program of claim 29 .

31. A metrology device comprising the processor and associated storage medium of claim 30 .

32. A lithographic apparatus comprising the metrology device of claim 31 .

33. The lithographic apparatus of claim 32 , comprising:

a patterning device support for supporting a patterning device; and

a substrate support for supporting a substrate,

wherein the metrology device is operable to determine an aligned position for one or both of the patterning device support and substrate support.