Metrology method and apparatus, computer program and lithographic system

1. A metrology apparatus comprising:

an optical system comprising a sensor, the optical system configured to measure a target on a substrate by illuminating the target with radiation and, using the sensor, detecting the radiation scattered by the target; and

an array of lenses configured to form an image on the sensor, the image comprising a plurality of sub-images, such that each of the sub-images is formed by a corresponding one of the lenses;

wherein the optical system is further configured to:

measure intensity asymmetries in respective higher orders of the sub-images; and

determine, from the intensity asymmetries, a local target asymmetry, and

wherein said sensor is located at the image plane defined by said array of lenses.

2. The metrology apparatus of claim 1 , wherein said array of lenses is located at a pupil plane of the optical system.

3. The metrology apparatus of claim 1 , wherein said array of lenses is located between a pupil plane of the optical system and the image plane, and at a distance from the pupil plane equal to the focal length of each lens of said array of lenses.

4. The metrology apparatus of claim 2 , wherein said optical system comprises an output lens and an objective lens, said output lens being operable to form an image of a back focal plane of the objective lens at said pupil plane.

5. The metrology apparatus of claim 3 , wherein said optical system comprises an output lens and an objective lens, said output lens being operable to form an image of a back focal plane of the objective lens at said pupil plane.

6. The metrology apparatus of claim 1 , wherein said image is a plenoptic image.

7. The metrology apparatus of claim 1 , wherein at least some of said sub-images are composed of higher orders of said scattered measurement radiation.

8. The metrology apparatus of claim 1 wherein said metrology apparatus is configured to:

measure intensity asymmetry in corresponding higher orders of the scattered measurement radiation from said sub-images; and

determine, from each of the measurements of intensity asymmetry, a local measurement of target asymmetry in a target being measured.

9. The metrology apparatus of claim 8 , further configured to combine said local measurements of target asymmetry in the target being measured, to obtain an overall measurement of target asymmetry.

10. The metrology apparatus of claim 9 , wherein said combining of said local measurements of target asymmetry includes weighting said local measurements of target asymmetry based on the deviation of the positions of the corresponding sub-images relative to a regular 2D grid.

11. The metrology apparatus of claim 10 , wherein said local measurements of target asymmetry are given greater weighting the closer the positions of its corresponding sub-image is to the regular 2D grid.

12. The metrology apparatus of claim 8 , further configured to:

obtain, in a single measurement step, a plurality of different values for intensity asymmetry from different corresponding pairs of sub-images of a target comprising a first grating and a second grating, wherein the target asymmetry of the first grating comprises a structural contribution due to structural asymmetry, a first imposed target asymmetry and an overlay error and the target asymmetry of the second grating comprises, the structural contribution due to structural asymmetry, a second imposed target asymmetry and the overlay error; and

determine a value for the overlay error from said plurality of different values for intensity asymmetry.

13. The metrology apparatus of claim 12 , wherein said structural contribution due to structural asymmetry results in an offset in the relationship between target asymmetry and intensity asymmetry described by an offset term, and said determining a value for the overlay error comprises determining a value for said offset term.

14. The metrology apparatus of claim 12 , wherein said determining a value for the overlay error comprises plotting the intensity asymmetry measurements obtained from inspection of said first grating against asymmetry measurements obtained from inspection of said second grating.

15. The metrology apparatus of claim 8 , further configured to determine whether said optical system is correctly focused based on said deviation of the positions of the sub-images relative to a regular 2D grid.

16. The metrology apparatus of claim 1 , further configured to:

obtain an image for each of a plurality of focus settings;

determine the relationship between focus and said deviation of the positions of the sub-images comprised with said images relative to a regular 2D grid; and

for subsequent images, determine focus from the deviation of the positions of the sub-images comprised within each subsequent image relative to a regular 2D grid and said determined relationship.

17. The metrology apparatus of claim 16 , further configured to determine a static component in said deviation of the positions of the sub-images relative to a regular 2D grid and correcting for this static component.

18. The metrology apparatus of claim 17 , further configured to: calculate a pupil image from intensities of the sub-images;

determine a phase distribution from said pupil image; and

determine aberrations in the optical system from said phase distribution.