Illumination source for an inspection apparatus, inspection apparatus and inspection method

1. An illumination source for generating measurement radiation for an inspection apparatus, configured to generate at least first measurement radiation and second measurement radiation such that said first measurement radiation and said second measurement radiation interfere to form combined measurement radiation modulated with a beat component, wherein said illumination source is a high harmonic generation source, and wherein the first measurement radiation is centered on a first wavelength and the second measurement radiation comprises at least one component centered on a second wavelength, the second wavelength being different than the first wavelength.

2. The illumination source of claim 1 , wherein said first measurement radiation and said second measurement radiation are spectrally coherent.

3. The illumination source of claim 1 , wherein the difference between said first wavelength and said second wavelength is smaller than 0.01 nm.

4. The illumination source of claim 1 , wherein the frequency difference between the frequency of the first measurement radiation and the frequency of the second measurement radiation is less than 10 MHz, less than 100 kHz, or less than 1 kHz.

5. The illumination source of claim 1 , wherein each high harmonic of the first measurement radiation and a corresponding harmonic of the second measurement radiation interfere to form said combined measurement radiation modulated with a beat component.

6. The illumination source of claim 5 , wherein a wavelength difference of each high harmonic of the first measurement radiation and the corresponding harmonic of the second measurement radiation is smaller than the bandwidth of each of said corresponding high harmonics.

7. The illumination source of claim 5 , comprising:

at least one pump radiation source configured to generate a first pump radiation beam centered on a first pump wavelength and a second pump radiation beam comprising at least one component centered on a second pump wavelength; and

a high harmonic generating medium;

wherein said at least one pump radiation source is arranged such that said first pump radiation beam excites said high harmonic generating medium to generate said first measurement radiation and said second pump radiation beam excites said high harmonic generating medium to generate said second measurement radiation.

8. The illumination source of claim 7 , wherein said at least one pump radiation source comprises a first pump radiation source configured to generate said first pump radiation beam and a second pump radiation source configured to generate said second pump radiation beam; said first pump radiation source and said second pump radiation source being mode-locked or being phase-locked.

9. The illumination source of claim 7 , wherein said at least one pump radiation source comprises a single pump radiation source; and

said illumination source comprises:

a beam splitting element to form said first pump radiation beam and said second pump radiation beam each centered on said first pump wavelength; and

a modulator device configured to modulate said second pump radiation beam, centering it on at least said second pump wavelength.

10. The illumination source of claim 7 , wherein said first pump radiation beam and said second pump radiation beam are configured to excite different locations within said high harmonic generating medium at substantially the same time.

11. The illumination source of claim 7 , wherein said first pump radiation beam and said second pump radiation beam are configured to excite the substantially same location within said high harmonic generating medium at different times.

12. An illumination source for generating high harmonic radiation configured to generate at least first high harmonic radiation and second high harmonic radiation, such that said first high harmonic radiation and said second high harmonic radiation interfere to form combined high harmonic radiation modulated with a beat component.

13. The illumination source of claim 12 , wherein said first high harmonic radiation and said second high harmonic radiation are spectrally coherent.

14. The illumination source of claim 12 , wherein the first high harmonic radiation is centered on a first wavelength and the second high harmonic radiation comprises at least one component centered on a second wavelength.

15. The illumination source of claim 14 , wherein the difference between said first wavelength and said second wavelength is smaller than 0.01 nm.

16. The inspection apparatus, comprising:

an illumination source configured to generate at least first measurement radiation and second measurement radiation such that said first measurement radiation and said second measurement radiation interfere to form combined measurement radiation modulated with a beat component, wherein said illumination source is a high harmonic generation source, and wherein the first measurement radiation is centered on a first wavelength and the second measurement radiation comprises at least one component centered on a second wavelength, the second wavelength being different than the first wavelength;

an illumination system configured to illuminate a target structure with the measurement radiation resulting in scattered radiation modulated with the beat component;

a detector configured to detect the scattered radiation; and

a processor configured to use said beat component to spectrally resolve said scattered radiation.

17. A method of measuring a target structure on a substrate comprising:

generating at least a first measurement radiation and a second measurement radiation such that said first measurement radiation and said second measurement radiation interfere to form combined measurement radiation modulated with a beat component in a high harmonic generation source; and wherein the first measurement radiation is centered on a first wavelength and the second measurement radiation comprises at least one component centered on a second wavelength, the second wavelength being different than the first wavelength;

illuminating the target structure with the measurement radiation resulting in scattered radiation modulated with the beat component;

detecting the scattered radiation; and

processing the detected scattered radiation, said processing comprising using said beat component to spectrally resolve said scattered radiation.

18. A non-transitory computer readable medium comprising machine readable instructions which, when run on a suitable processor, cause the processor to perform a method for measuring a target structure on a substrate comprising:

generating at least a first measurement radiation and a second measurement radiation such that said first measurement radiation and said second measurement radiation interfere to form combined measurement radiation modulated with a beat component in a high harmonic generation source, and wherein the first measurement radiation is centered on a first wavelength and the second measurement radiation comprises at least one component centered on a second wavelength, the second wavelength being different than the first wavelength;

illuminating the target structure with the measurement radiation resulting in scattered radiation modulated with the beat component;

detecting the scattered radiation; and

processing the detected scattered radiation, said processing comprising using said beat component to spectrally resolve said scattered radiation.

19. An illumination source for generating measurement radiation for an inspection apparatus comprising:

at least one pump radiation source configured to generate a first pump radiation beam centered on a first wavelength and a second pump radiation beam centered on a second wavelength; and

a high harmonic generating medium;

wherein said at least one pump radiation source is arranged such that said first pump radiation beam excites said high harmonic generating medium to generate first measurement radiation and said second pump radiation beam excites said high harmonic generating medium to generate second measurement radiation, and such that said first measurement radiation and said second measurement radiation interfere to form combined measurement radiation modulated with a beat component.