Method of measuring a parameter of interest, device manufacturing method, metrology apparatus, and lithographic system

1. A method comprising:

a first detecting or estimating of a reference property of radiation (A+) at a first wavelength scattered from a first sub-target of a target structure, the first wavelength being the wavelength at which a derivative of the reference property of the scattered radiation from the first sub-target with respect to wavelength is zero, the first sub-target comprising a first bias;

a second detecting or estimating of the reference property of radiation at a second wavelength (A−) scattered from a second sub-target of the target structure, the second wavelength being the wavelength at which a derivative of the reference property of the scattered radiation from the second sub-target with respect to wavelength is zero, the second sub-target comprising a second bias different than the first bias, and the first wavelength being different from the second wavelength; and

determining a parameter of interest based on the first and second detecting or estimating.

2. The method of claim 1 , wherein the parameter of interest comprises overlay between structures formed in different layers of the target structure.

3. The method of claim 1 , wherein the reference property comprises an intensity asymmetry between different diffraction orders of the scattered radiation.

4. The method of claim 1 , wherein the first bias and the second bias each comprise a deliberate geometrical offset.

5. The method of claim 1 , wherein the first bias and the second bias comprise equal and opposite biases.

6. The method of claim 1 , wherein:

a sensitivity of the reference property of radiation scattered from each of the first sub-target and the second sub-target to change in the parameter of interest varies as a function of the wavelength of the radiation; and

the first wavelength and the second wavelength are selected such that the sensitivity of the reference property of radiation scattered from the first sub-target to change in the parameter of interest is substantially equal to the sensitivity of the reference property of radiation scattered from the second sub-target to change in the parameter of interest.

7. The method of claim 1 , comprising:

detecting the reference property of scattered radiation from the first sub-target at each of a first plurality of wavelengths to obtain a corresponding plurality of first sub-target measurements;

detecting the reference property of scattered radiation from the second sub-target at each of a second plurality of wavelengths to obtain a corresponding plurality of second sub-target measurements;

using the plurality of first sub-target measurements to select the first wavelength; and

using the plurality of second sub-target measurements to select the second wavelength.

8. The method of claim 7 , wherein:

the reference property of radiation at the first wavelength scattered from the first sub-target is detected by illuminating the first sub-target with radiation at the first wavelength and detecting the reference property of radiation at the first wavelength scattered from the first sub-target; and

the reference property of radiation at the second wavelength scattered from the second sub-target is detected by illuminating the second sub-target with radiation at the second wavelength and detecting the reference property of radiation at the second wavelength scattered from the second sub-target.

9. The method of claim 1 , wherein:

the reference property of radiation at the first wavelength scattered from the first sub-target is estimated using the plurality of first sub-target measurements to mathematically predict a variation of the reference property with wavelength and using the predicted variation to determine a value of the reference property at the first wavelength; and

the reference property of radiation at the second wavelength scattered from the second sub-target is estimated using the plurality of second sub-target measurements to mathematically predict a variation of the reference property with wavelength and using the predicted variation to determine a value of the reference property at the second wavelength.

10. The method of claim 1 , wherein the target structure is defined or formed by a lithographic process.

11. A device manufacturing method, comprising:

forming structures on a substrate using a manufacturing sequence including at least one lithography step, the structures comprising a target structure;

measuring a parameter of interest of the target structure, the measuring comprising:

a first detecting or estimating of a reference property of radiation at a first wavelength scattered from a first sub-target of the target structure, the first wavelength being the wavelength at which a derivative of the reference property of the scattered radiation from the first sub-target with respect to wavelength is zero, the first sub-target comprising a first bias;

a second detecting or estimating of the reference property of radiation at a second wavelength scattered from a second sub-target of the target structure, the second wavelength being the wavelength at which a derivative of the reference property of the scattered radiation from the second sub-target with respect to wavelength is zero, the second sub-target comprising a second bias different than the first bias, and the first wavelength being different from the second wavelength; and

determining a parameter of interest based on the first and second detecting or estimating.

12. A metrology apparatus for measuring a parameter of interest of a target structure formed on a substrate, the apparatus comprising:

a processor configured to:

perform a first detecting or estimating of a reference property of radiation at a first wavelength scattered from a first sub-target of the target structure, the first wavelength being the wavelength at which a derivative of the reference property of the scattered radiation from the first sub-target with respect to wavelength is zero, the first sub-target comprising a first bias;

perform a second detecting or estimating of the reference property of radiation at a second wavelength scattered from a second sub-target of the target structure, the second wavelength being the wavelength at which a derivative of the reference property of the scattered radiation from the second sub-target with respect to wavelength is zero, the second sub-target comprising a second bias different than the first bias, and the first wavelength being different from the second wavelength; and

determine a parameter of interest based on the first and second detecting or estimating.

13. The apparatus of claim 12 , wherein the parameter of interest comprises overlay between structures formed in different layers of the target structure.

14. The apparatus of claim 12 , wherein the reference property comprises an intensity asymmetry between different diffraction orders of the scattered radiation at a detector.

15. The apparatus of claim 12 , wherein the first bias and the second bias each comprise a deliberate geometrical offset.

16. The apparatus of claim 12 , wherein the first bias and the second bias comprise equal and opposite biases.

17. The apparatus of claim 12 , wherein:

a sensitivity of the reference property of radiation scattered from each of the first sub-target and the second sub-target to change in the parameter of interest varies as a function of the wavelength of the radiation; and

the processor is configured to select the first wavelength and the second wavelength such that the sensitivity of the reference property of radiation scattered from the first sub-target to change in the parameter of interest is substantially equal to the sensitivity of the reference property of radiation scattered from the second sub-target to change in the parameter of interest.

18. The apparatus of claim 12 , further comprising a measurement system configured to:

detect the reference property of scattered radiation from the first sub-target at each of a first plurality of wavelengths to obtain a corresponding plurality of first sub-target measurements; and

detect the reference property of scattered radiation from the second sub-target at each of a second plurality of wavelengths to obtain a corresponding plurality of second sub-target measurements, wherein the processor is configured to:

use the plurality of first sub-target measurements to select the first wavelength; and

use the plurality of second sub-target measurements to select the second wavelength.

19. A lithographic system comprising:

a lithographic apparatus configured to define structures to be formed on a substrate using a manufacturing sequence including at least one lithography step, the structures comprising a target structure; and

a metrology apparatus for measuring a parameter of interest of a target structure formed on a substrate, the metrology apparatus comprising a processor configured to:

perform a first detecting or estimating of a reference property of radiation at a first wavelength scattered from a first sub-target of the target structure, the first wavelength being the wavelength at which a derivative of the reference property of the scattered radiation from the first sub-target with respect to wavelength is zero, the first sub-target comprising a first bias;

perform a second detecting or estimating of the reference property of radiation at a second wavelength scattered from a second sub-target of the target structure, the second wavelength being the wavelength at which a derivative of the reference property of the scattered radiation from the second sub-target with respect to wavelength is zero, the second sub-target comprising a second bias different than the first bias, and the first wavelength being different from the second wavelength; and

determine a parameter of interest based on the first and second detecting or estimating.