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1. (WO2019001873) METHODS AND PATTERNING DEVICES AND APPARATUSES FOR MEASURING FOCUS PERFORMANCE OF A LITHOGRAPHIC APPARATUS, DEVICE MANUFACTURING METHOD
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CLAIMS

1. A method of measuring focus performance of a lithographic apparatus, the method comprising:

(a) using the lithographic apparatus to print at least one focus metrology pattern on a substrate, the printed focus metrology pattern comprising at least a first periodic array of features,

(b) using inspection radiation to measure asymmetry between opposite portions of a diffraction spectrum for the first periodic array in the printed focus metrology pattern; and

(c) deriving a measurement of focus performance based at least in part on the asymmetry measured in step (b),

wherein said first periodic array comprises a repeating arrangement of first features interleaved with second features, a minimum dimension of each first feature being close to but not less than a resolution limit of the printing step (a), a maximum dimension of each second feature in the direction of periodicity being at least twice the minimum dimension of the first features;

wherein each first feature is positioned between two adjacent second features such that a spacing in the direction of periodicity between the first feature and its nearest second feature is between one half and twice the minimum dimension of the first features.

2. A method as claimed in claim 1 wherein each first feature in the periodic array has said minimum dimension in the direction of periodicity or wherein each first feature in the periodic array has said minimum dimension in a direction transverse to the direction of periodicity.

2. A method as claimed in any of claims 1 or 2 wherein each second feature in the periodic array further includes sub-features having minimum dimensions close to but not less than a resolution limit of the printing step in a direction transverse to said direction of periodicity.

3. A method as claimed in any preceding claim wherein the printed focus metrology pattern comprises at least first and second periodic arrays of features, each periodic array of features having a form as specified in said preceding claim, wherein there is a programmed asymmetry within each periodic array, the asymmetry of the second periodic array being opposite to that of the first periodic array, and wherein step (b) includes measuring asymmetry of each of the first and second periodic arrays and step (c) determines said measure of focus performance by combining the asymmetries measured for the periodic arrays; and wherein, optionally, said sub-features are arranged such that each second feature is asymmetric within regard to the direction of periodicity, and wherein the asymmetry of each second feature in the second periodic array of features is opposite to that in the first periodic array of features.

4. A method as claimed in any of claims 1 to 3 wherein the spacing of each first feature between two adjacent second features is unequal in the direction of periodicity, and wherein the unequal spacing of the first features between the adjacent second features in the second periodic array of features is opposite to that in the first periodic array of features, and wherein, optionally, a distance between each first feature and one of said adjacent second features in said direction of periodicity is between one and two times the distance between the first feature and the other one of said adjacent second features.

5. A method as claimed in any of claims 1 to 4 wherein the measurement in step (b) is performed using radiation having a wavelength much longer than said minimum dimension of the first features, and wherein, optionally, the measurement in step (b) is performed using radiation having a wavelength longer than 150 nm while said minimum dimensions of the first features is less than 40 nm.

6. A method as claimed in any of claims 1 to 6 wherein the measurement in step (b) is performed using radiation having a wavelength longer than 150 nm while a wavelength of radiation used by the lithographic apparatus to print the said focus metrology pattern is less than 20 nm.

7. A method as claimed in any of claims 1 to 6 wherein the period of each of said periodic arrays of features in said focus metrology pattern is greater than 350 nm.

8. A method of measuring focus performance of a lithographic apparatus, the method comprising:

(a) using the lithographic apparatus to print at least one focus metrology pattern on a substrate, the printed focus metrology pattern comprising an array of features that is periodic in at least one direction;

(b) measuring a property of the printed focus metrology pattern; and

(c) deriving a measurement of focus performance from the measurement of said property, wherein the focus metrology pattern comprises at least a first periodic array of features, a dimension of each feature being close to but not less than a resolution limit of the printing step (a), wherein said features are arranged in pairs and a spacing between adjacent pairs of features within the focus metrology pattern in the direction of periodicity is much greater than both the dimension of each first feature and the spacing between first features within a pair.

9. A method as claimed in claim 8 wherein the focus metrology pattern comprises a periodic array of groups of pairs of features, and wherein a spacing between adjacent groups of pairs of

features within the focus metrology pattern in the direction of periodicity is much greater than both the dimension of each feature and the spacing between features within a pair.

10. A method as claimed in claim 8 or 9 wherein the dimensions of the features within each pair and the dimension of the spacing between the features within each pair are of the same order of magnitude, in the direction of periodicity, and wherein, optionally, the dimension of the spacing between the features within each pair is between one half and two times the average dimension of the features within each pair, in the direction of periodicity.

11. A patterning device for use in a lithographic apparatus, the patterning device comprising reflective and non-reflective portions to define features of one or more device patterns and one or more metrology patterns, the metrology patterns including at least one focus metrology pattern, the focus metrology pattern comprising a at least a first periodic array of features,

wherein said first periodic array comprises a repeating arrangement of first features interleaved with second features, a minimum dimension of each first feature being close to but not less than a resolution limit of the lithographic apparatus, a maximum dimension of each second feature in the direction of periodicity being at least twice the minimum dimension of the first features; wherein each first feature is positioned between two adjacent second features such that a spacing in the direction of periodicity between the first feature and its nearest second feature is between one half and twice the minimum dimension of the first features.

12. A patterning device for use in a lithographic apparatus, the patterning device comprising reflective and non-reflective portions to define features of one or more device patterns and one or more metrology patterns, the metrology patterns including at least one focus metrology pattern, wherein the focus metrology pattern comprises at least a first periodic array of features, a dimension of each feature being close to but not less than a resolution limit of the lithographic apparatus,

wherein said features are arranged in pairs and a spacing between adjacent pairs of features within the focus metrology pattern in the direction of periodicity is much greater than both the dimension of each first feature and the spacing between first features within a pair.

13. A metrology apparatus for measuring a parameter of a lithographic process, the metrology apparatus being operable to perform steps (b) and (c) of the method of any of claims 1 to 10

14. A lithographic system comprising:

a lithographic apparatus comprising:

an illumination optical system arranged to illuminate a reflective patterning device;

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

a metrology apparatus according to claim 13;

wherein the lithographic apparatus is arranged to use the measurement of focus performance derived by the metrology apparatus when applying the pattern to further substrates.

15. A method of manufacturing devices wherein a device pattern is applied to a series of substrates using a lithographic process, the method including:

using the method of any of claims 1 to 10 to measure focus performance of the lithographic process, and

controlling the lithographic process for later substrates in accordance with the measured focus performance.