Metrology method and apparatus, computer program and lithographic system

1. A method, comprising:

obtaining a plurality of first measurements, wherein the plurality of first measurements comprise measurements of structural asymmetry of a plurality of first structures and the plurality of first measurements are obtained using a plurality of measurement radiations;

obtaining a plurality of second measurements, wherein:

the plurality of second measurements comprise measurements of a target asymmetry of a plurality of targets,

each of the plurality of second measurements corresponds to one of the plurality of first measurements,

each of the plurality of targets comprises one of the plurality of first structures and a second structure overlaid thereon, and

the measurements of the target asymmetry comprise a first part which is independent of the structural asymmetry and a second part that is due to the structural asymmetry;

determining a relationship function describing a relationship between the plurality of first measurements and the plurality of second measurements; and

determining, using the relationship function, a corrected overlay value, the corrected overlay value being based on the measurements of the target asymmetry corrected for the structural asymmetry.

2. The method of claim 1 , wherein the first plurality of measurements is obtained based on a plurality of values of a first parameter.

3. The method of claim 2 , wherein the first parameter corresponds to a position on a surface of a substrate.

4. The method of claim 2 , further comprising:

determining simultaneous equations relating corresponding pairs of the plurality of first measurements and the plurality of second measurements in terms of a corresponding relationship function; and

solving the simultaneous equations to obtain solutions for the relationship functions, each solution for the relationship functions corresponding to one of the plurality of measurement radiations.

5. The method of claim 4 , wherein each of the simultaneous equations additionally relates corresponding pairs of the plurality of first measurements and the plurality of second measurements based on the first part which is independent of the structural asymmetry.

6. The method of claim 1 , further comprising:

determining a set of corrected overlay values for each of the plurality of measurement radiations;

determining a set of measurement recipe independent overlay values describing the first part independent of the structural asymmetry;

determining a plurality of difference metrics, each difference metric relating to a different subset of the plurality of measurement radiations, each of the plurality of difference metrics describing a difference between corresponding overlay values of the set of measurement recipe independent overlay values and different pairs of the sets of corrected overlay values.

7. The method of claim 6 , wherein the corresponding overlay values comprise overlay values corresponding to the same location on a substrate.

8. The method of claim 1 , wherein:

the relationship function comprises a constant,

the plurality of first measurements and the plurality of second measurements are obtained using a pair of measurement radiations,

the relationship function describes the relationship between the first measurements as performed with one of the pair of measurement radiations and a difference of the second measurements as performed with a first one of the pair of measurement radiations and a second one of the pair of measurement radiations.

9. The method of claim 8 , wherein the determining a relationship function comprises making a 3-dimensional plot of one of the first measurements as performed with the first one of the pair of measurement radiations against another one of the first measurements as performed with a second one of the pair of measurement radiations against a difference of the second measurements as performed with the first one of the pair of measurement radiations and the second one of the pair of measurement radiations.

10. The method of claim 9 , wherein data points on the 3-dimensional plot substantially correlate to define a flat plane and wherein a first relationship function is described by a slope of the flat plane relative to an axis of the one of the first measurements as performed with the first one of the pair of measurement radiations and a second relationship function is described by the slope of the flat plane relative to an axis of the other one of the first measurements as performed with the second one of the pair of measurement radiations.

11. The method of claim 8 , wherein the determining a relationship function comprises determining a relationship function which describes a substantially linear relationship between:

a first difference, comprising a difference between one of the first measurements as performed with a first one of the pair of measurement radiations and another one of the first measurements as performed with a second one of the pair of measurement radiations; and

a second difference, comprising a difference between the second measurements as performed with the first one of the pair of measurement radiations and the second one of the pair of measurement radiations.

12. The method of claim 1 , further comprising:

performing an initial optimization to determine a target from a plurality of candidate targets, the performing an initial optimization comprises:

obtaining a plurality of sets of first measurements and second measurements, for the plurality of candidate targets and a plurality of candidate combinations of measurement radiations, each set of the plurality of sets of first and second measurements relating to a different combination of one of the plurality of candidate targets and one of the plurality of candidate combinations of measurement radiations;

determining a degree of correlation between the first measurements and the second measurements for each of the plurality of sets of first and second measurements; and

selecting the target from the plurality of candidate targets based upon the determined degree of correlation.

13. The method of claim 12 , wherein selecting the target comprises selecting a combination of one of the plurality of candidate targets and one of the plurality of candidate combinations of candidate measurement radiations based upon the determined degree of correlation.

14. The method of claim 1 , wherein at least the obtaining a plurality of first measurements, the obtaining a plurality of second measurements, and the determining a relationship function are performed during a calibration process and wherein the method further comprises determining an updated value for the relationship function from a production substrate during a production process.

15. A method of selecting a target from a plurality of candidate targets, the method comprising:

obtaining a plurality of sets of measurements using a plurality of candidate measurement radiations and the plurality of candidate targets, wherein:

each candidate target comprises a first structure overlaid with a second structure,

each set of measurements corresponds to a different combination of one of the plurality of candidate targets and one of the plurality of candidate measurement radiations, and

each set of measurements comprises:

a plurality of first measurements comprising measurements of structural asymmetry of the first structure of the candidate target using the plurality of candidate measurement radiations; and

a plurality of second measurements comprising measurements of target asymmetry of the candidate target using the plurality of candidate measurement radiations;

for each set of measurements, determining a degree of correlation between the plurality of first measurements and the plurality of second measurements;

selecting the target from the plurality of candidate targets based upon the determined degree of correlation for each set of measurements.

16. The method of claim 15 , wherein the selecting the target comprises selecting a combination of one of the plurality of candidate targets and one of the plurality of candidate measurement radiations based upon the determined degree of correlation for each set of measurements.

17. The method of claim 16 , wherein the plurality of candidate measurement radiations comprises a pair of candidate measurement radiations and the selecting the combination comprises selecting a combination for which a corresponding set of measurements defines a flat plane on a corresponding plot of one of the first measurements as performed with a first one of the pair of candidate measurement radiations against another one of the first measurements as performed with a second one of the pair of candidate measurement radiations against a difference of the second measurements as performed with the first one of the pair of candidate measurement radiations and a second one of the pair of candidate measurement radiations.

18. A non-transitory computer program product comprising machine-readable instructions, when run on a processor controlled apparatus, cause the processor controlled apparatus to perform a method, comprising:

obtaining a plurality of first measurements, wherein the plurality of first measurements comprise measurements of structural asymmetry of a plurality of first structures and the plurality of first measurements are obtained using a plurality of measurement radiations;

obtaining a plurality of second measurements, wherein:

the plurality of second measurements comprise measurements of target asymmetry of a plurality of targets,

each of the plurality of second measurements corresponds to one of the plurality of first measurements,

each of the plurality of targets comprises one of the plurality of first structures and a second structure overlaid thereon, and

the measurements of the target asymmetry comprise a first part which is independent of the structural asymmetry and a second part that is due to the structural asymmetry;

determining a relationship function describing a relationship between the plurality of first measurements and the plurality of second measurements; and

determining, using the relationship function, a corrected overlay value, the corrected overlay value being based on the measurements of the target asymmetry corrected for the structural asymmetry.