Metrology method and apparatus and computer program

1. A method of measuring, using a metrology apparatus, a process effect parameter relating to a manufacturing process for manufacturing integrated circuits on a substrate having a structure, the method comprising:

determining, for the structure, using a processor, at least one first quality metric value for a quality metric from a set of measurement values relating to at least a first set of measurement conditions of the metrology apparatus, such that each measurement value relates to a different measurement condition, while cancelling or mitigating an effect of the process effect parameter on the set of measurement values;

determining, for the structure, a second quality metric value for the quality metric from at least one measurement value relating to at least one measurement condition without cancelling or mitigating the effect of the process effect parameter on the at least one measurement value; and

calculating a process effect parameter value for the process effect parameter using the first quality metric value and the second quality metric value.

2. The method of claim 1 , wherein the step of calculating a process effect parameter value comprises calculating a difference of the first quality metric value and the second quality metric value.

3. The method of claim 1 , wherein:

a single measurement value relating to a single measurement condition is used in determining the second quality metric value; and

the single measurement value is the same as one of the different measurement values used in determining the first quality metric value.

4. The method of claim 1 , wherein the second quality metric value is determined from an average of two or more quality metric values that are individually determined from one of the set of measurement values.

5. The method of claim 1 , wherein:

the process effect comprises structural asymmetry; and

the structure comprises a grating and the structural asymmetry comprises grating asymmetry wherein one or more structural features of the grating has an asymmetric profile.

6. The method of claim 1 , comprising using the process effect parameter value to detect an excursion where the process effect parameter has fallen outside of a specified tolerance.

7. The method of claim 1 , further comprising using the calculated process effect parameter value to adjust a parameter of the measuring method or the manufacturing process.

8. The method of claim 1 , wherein:

the quality metric is overlay; and

the structure comprises an overlay target.

9. The method of claim 8 , wherein the overlay target comprises at least two gratings, each grating comprising:

a sub-grating in each of two layers; and

a positional bias between the two sub-gratings;

wherein the positional bias is different for the two gratings.

10. The method of claim 1 , wherein the set of measurement values used in determining the first quality metric value and the at least one measurement value used in determining the second quality metric value are all obtained in a single measurement step comprising multiple measurements on the structure using different measurement conditions.

11. The method of claim 10 , wherein the single measurement step comprises performing a measurement on the structure using measurement radiation comprising more than one wavelength, more than one polarization, and/or more than one angle of incidence on the structure to obtain at least a subset of the set of measurement values simultaneously.

12. The method of claim 1 , further comprising:

determining a set of the first quality metric values, each of the first quality metric values being determined from a different set of measurement values, each value of which relating to a different measurement condition;

determining a monitor metric parameter value from a difference between a pair of the first quality metric values; and

monitoring a validity of the at least one first quality metric value and/or the second quality metric value using the monitor metric parameter value.

13. The method of claim 12 , comprising performing an initial calibration comprising:

determining a reference value for the quality metric;

calculating calibration values for the quality metric for a plurality of different combinations of measurement conditions;

determining a combination of measurement conditions corresponding to a calibration value that most closely matches the reference value; and

selecting a subset of measurement conditions out of the combination of measurement conditions.

14. The method of claim 12 , wherein each quality metric value of the pair of first quality metric values is determined from a different number of measurement values.

15. The method of claim 14 , wherein the measurement values used to determine one of the pair of first quality metric values is a subset of the measurement values used to determine the other of the pair of first quality metric values.

16. A metrology apparatus comprising:

a support for a substrate, the substrate having a structure formed thereon using a manufacturing process, wherein a process effect parameter is associated with the manufacturing process;

an optical system for illuminating the structure with measurement radiation;

a detector for detecting the measurement radiation scattered by the structure; and

a processor configured to:

determine, for the structure, a first quality metric value for a quality metric from a set of measurement values relating to a corresponding set of different measurement conditions while cancelling or mitigating an effect of the process effect parameter on the set of measurement values;

determine, for the structure, a second quality metric value for the quality metric from at least one measurement value relating to at least one measurement condition without cancelling or mitigating the effect of the process effect parameter on the at least one measurement value; and

calculate a process effect parameter value for the process effect parameter from the first quality metric value and the second quality metric value.

17. A non-transitory computer program carrier comprising processor readable instructions which, when run on suitable processor controlled apparatus, cause the processor controlled apparatus to perform a method of measuring a process effect parameter relating to a manufacturing process for manufacturing integrated circuits on a substrate, the method comprising the steps of:

determining, for a structure, at least one first quality metric value for a quality metric from a set of measurement values relating to at least a first set of measurement conditions, such that each measurement value relates to a different measurement condition, while cancelling or mitigating an effect of the process effect parameter on the set of measurement values;

determining, for the structure, a second quality metric value for the quality metric from at least one measurement value relating to at least one measurement condition without cancelling or mitigating the effect of the process effect parameter on the at least one measurement value; and

calculating a process effect parameter value for the process effect parameter using the first quality metric value and the second quality metric value.

18. A method of measuring, using a metrology apparatus, a quality metric relating to a process effect parameter associated with a manufacturing process for manufacturing integrated circuits on a substrate, the method comprising the steps of:

determining, for a structure, a first quality metric value for the quality metric from a first set of measurement values relating to a corresponding first set of different measurement conditions of the metrology apparatus while cancelling or mitigating an effect of the process effect parameter on the first set of measurement values;

determining, for the structure, a second quality metric value for the quality metric from a second set of measurement values relating to a corresponding second set of different measurement conditions of the metrology apparatus without cancelling or mitigating the effect of the process effect parameter on the second set of measurement values; and

monitoring a validity of the first quality metric value and/or the second quality metric value using a monitor metric parameter value determined from a difference of the first quality metric value and the second quality metric value.

19. The method of claim 18 , wherein each of the first quality metric value and the second quality metric value is determined from a different number of measurement values.