Metrology method and apparatus, lithographic apparatus, and device manufacturing method

1. A metrology apparatus configured to measure a property of a target on a substrate, the apparatus comprising:

a first source configured to emit a first illumination beam of radiation;

a second source configured to emit a second illumination beam of radiation wherein the first illumination beam of radiation differs from the second illumination beam of radiation in at least one of spot size and angular distribution;

an aperture plate having first and second openings configured to pass the first and second illumination beams, respectively;

a beam selection device comprising a tiltable mirror tiltable about two axes, wherein the tiltable minor is positioned such that the first illumination beam of radiation is incident on the tiltable mirror at a first angle of incidence and the second illumination beam is incident on the tiltable mirror at a second angle of incidence and wherein the beam selection device is configured to select one of the first and second illumination beams of radiation as a measurement beam of radiation by tilting the tiltable mirror, wherein the tiltable mirror is positioned in a conjugate plane of the substrate and the aperture plate is positioned in a plane that is a Fourier transform of the conjugate plane of the substrate;

an objective lens configured to direct the measurement beam of radiation onto the target on the substrate and to collect radiation diffracted by the target; and

a sensor configured to detect an angle resolved spectrum in a pupil plane of the objective lens.

2. The apparatus according to claim 1 , wherein the first source comprises:

a broadband radiation source configured to emit a third illumination beam of radiation having a first wavelength range; and

a wavelength selection device configured to output the first illumination beam of radiation by selecting a second wavelength range from the third illumination beam of radiation.

3. The apparatus according to claim 2 , wherein the wavelength selection device comprises at least one of an acousto-optical tunable filter, a dispersive element, or an interference filter.

4. The apparatus according to claim 2 , wherein the broadband radiation source comprises a supercontinuum laser.

5. The apparatus according to claim 1 , wherein at least one of the first and second sources comprises a xenon lamp.

6. The apparatus according to claim 1 , wherein the first source comprises a xenon lamp and the second source comprises:

a broadband radiation source configured to emit a third illumination beam of radiation having a first wavelength range; and

a wavelength selection device configured to generate the second illumination beam of radiation by selecting a second wavelength range from the third illumination beam of radiation.

7. The apparatus according to claim 6 , wherein the wavelength selection device comprises at least one of an acousto-optical tunable filter, a dispersive element or an interference filter.

8. The apparatus according to claim 6 , wherein the broadband radiation source comprises a supercontinuum laser.

9. The apparatus according to claim 1 , wherein each of the first and second sources comprises a xenon lamp.

10. The apparatus according to claim 1 , wherein:

the objective lens has a pupil plane;

the first and second sources are configured so that the first and second illumination beams have respective spatial extents and angular ranges at a plane conjugate with the pupil plane of the objective lens; and

at least one of the spatial extent and angular range of the first illumination beam being different from the spatial extent and angular range of the second illumination beam.

11. The apparatus according to claim 1 , wherein the beam selection device further comprises an optical system configured to direct the first and second illumination beams onto a pivot point of the tiltable mirror and wherein the optical system comprises the aperture plate.

12. The apparatus according to claim 1 , wherein the tiltable mirror is configured so that changes in its orientation change the angle of incidence of the measurement beam of radiation on the substrate.

13. The apparatus according to claim 1 , wherein the beam selection device further comprises a selectable aperture stop.

14. The apparatus according claim 1 , further comprising:

an additional sensor;

a beam splitter configured to direct a part of the radiation collected by the objective lens to the additional sensor; and

an imaging optical system configured to form an image of the target on the additional sensor.

15. A lithographic apparatus comprising:

an illumination optical system arranged to illuminate a pattern;

a projection optical system arranged to project an image of the pattern onto a substrate; and

a metrology device configured to measure a property of a target on the substrate, the metrology device comprising:

a first source configured to emit a first illumination beam of radiation;

a second source configured to emit a second illumination beam of radiation, wherein the first illumination beam of radiation differs from the second illumination beam of radiation in at least one of spot size and angular distribution;

an aperture plate having first and second openings configured to pass the first and second illumination beams, respectively;

a beam selection device comprising a tiltable mirror tiltable about two axes, wherein the tiltable mirror is positioned such that the first illumination beam of radiation is incident on the tiltable mirror at a first angle of incidence and the second illumination beam is incident on the tiltable mirror at a second angle of incidence and wherein the beam selection device is configured to select one of the first and second illumination beams of radiation as a measurement beam of radiation by tilting the tiltable mirror, wherein the tiltable mirror is positioned in a conjugate plane of the substrate and the aperture plate is positioned in a plane that is a Fourier transform of the conjugate plane of the substrate;

an objective lens configured to direct the measurement beam of radiation onto the target on the substrate and to collect radiation diffracted by the target; and

a sensor configured to detect an angle resolved spectrum in a pupil plane of the objective lens.

16. The apparatus according to claim 15 , further comprising:

an exposure station; and

a measurement station,

wherein the substrate table is moveable between the exposure station and the measurement station, and

wherein the metrology device is positioned at the measurement station.

17. A method of measuring a property of a target on a substrate, the method comprising:

generating first and second illumination beams, the first and second illumination beams differing in at least one of spatial extent and range of angles, the first and second beams being incident on a mirror tiltable about two axes, wherein the first illumination beam of radiation is incident on the mirror at a first angle of incidence and the second illumination beam is incident on the mirror at a second angle of incidence, wherein the first illumination beam of radiation differs from the second illumination beam of radiation in at least one of spot size and angular distribution;

passing the first and second illumination beams through first and second apertures of an aperture plate;

tilting the mirror such that one of the first and second illumination beams is directed onto the target, wherein the mirror is positioned in a conjugate plane of the substrate and wherein the aperture plate is positioned in a plane that is a Fourier transform of the conjugate plane of the substrate;

collecting radiation diffracted from the target using an objective lens; and

detecting an angle-resolved spectrum in a pupil plane of the objective lens.

18. The method according to claim 17 , wherein the tilting is based on a size, or type, or both the size and type of the target.