Method of measuring a structure, inspection apparatus, lithographic system and device manufacturing method

1. A method of determining a property of at least a first part of a target structure formed by a lithographic process, the method being based on radiation diffracted by periodic features within the target structure, the method comprising:

forming, using processing circuitry of a detection system, a first image of the target structure when illuminated with radiation having a first angular illumination distribution, the first image being formed using a selected portion of radiation diffracted by the target structure in a first direction and a selected portion of radiation diffracted by the target structure in a second direction, the first and second directions being defined relative to the target structure and being non-parallel;

forming, with the processing circuitry, a second image of the target structure when illuminated with radiation having a second angular illumination distribution, first and second angular illumination profiles being oriented differently to one another, relative to the target structure;

combining, with the processing circuitry, intensity values from the first image and the second image so as to discriminate between radiation diffracted in the first direction by a first part of the target structure and radiation diffracted in the second direction by the same first part of the target structure; and

determining, with the processing circuitry and based at least partly on the discrimination, the property of the first part of the structure.

2. The method of claim 1 , wherein based on the discrimination, the determining determines whether the first part of the target structure is configured to cause significant asymmetrical diffraction in the second direction.

3. The method of claim 1 , wherein based on the discrimination, the determining determines the property of the first part of the target structure based on the radiation diffracted in the first direction by the first part of the target structure and disregarding the effect of the radiation diffracted in the second direction by the same first part of the target structure.

4. The method of claim 3 , wherein the property is related to asymmetry of the first part of the target structure in the first direction.

5. The method of claim 4 , further comprising calculating the property related to asymmetry of the part of the target structure in the first direction based at least partly on a difference of intensity between complementary portions of the first image and complementary portions of the second image, the complementary portions in each image being images of the first part of the target structure formed using opposite diffraction orders of the radiation diffracted in the first direction.

6. The method of claim 5 , further comprising:

determining the property related to asymmetry for at least a second part of the target structure; and

calculating a measurement of a first performance parameter of the lithographic process based on the property determined for the first part and the second part of the target structure and based on known bias properties of the first part and the second part of the target structure in the first direction.

7. The method of claim 5 , further comprising determining a second property of the first part of the target structure, the second property being related to asymmetry in the second direction.

8. The method of claim 7 , further comprising:

determining the second property related to asymmetry of at least a second part of the target structure in the second direction; and

calculating a measurement of a second performance parameter of the lithographic process based on the properties determined for the first part and the second part of the target structure and based on known bias properties of the first part and the second part of the target structure in the second direction.

9. The method of claim 8 , further comprising:

combining intensity values from the first image and the second image so as to discriminate between radiation diffracted in the second direction by a third part and a fourth part of the target structure and radiation diffracted in the first direction by the same third part and fourth part of the target structure; and

calculating, based on the discriminating between radiation diffracted in the second direction by a third part and a fourth part, properties of the third and fourth parts of the target structure related to asymmetry of the third and fourth parts of the target structure in the second direction, and calculating a measurement of a third performance parameter of the lithographic process based on the properties determined for the third and fourth parts of the target structure and based on known bias properties of the third and fourth parts of the target structure.

10. The method of claim 1 , wherein the first angular illumination distribution of radiation is derived from a segmented illumination profile having illuminated regions and dark regions, each illuminated region being symmetrically opposite a dark region, when reflected in the first direction and when reflected in the second direction.

11. An imaging device comprising:

processing circuitry configured to:

receive at least first and second images of a target structure,

derive a measurement of one or more properties of one or more parts of a target structure by:

combining intensity values from the first image and the second image so as to discriminate between a selected portion of radiation diffracted in a first direction by a first part of the target structure and a selected portion of radiation diffracted in a second direction by the same first part of the target structure, and

determining, based at least partly on the discrimination, the property of the first part of the structure, and

automatically control the operation of the imaging device to cause capture of the first and second images.

12. A non-transitory computer readable storage medium configured to store machine-readable instructions, wherein when executed, the machine-readable instructions cause processing circuitry to perform operations comprising:

receiving at least first and second images of a target structure;

deriving a measurement of one or more properties of one or more parts of the target structure by:

combining intensity values from the first image and the second image so as to discriminate between a selected portion of radiation diffracted in a first direction by a first part of the target structure and a selected portion of radiation diffracted in a second direction by the same first part of the target structure, and

determining, based at least partly on the discrimination, the property of the first part of the structure; and

automatically controlling the operation of an inspection apparatus to cause capture of the first and second images.

13. The non-transitory computer readable storage medium of claim 12 , wherein the

automatically controlling comprises:

forming a first image of the target structure when illuminated with radiation having a first angular illumination distribution, the first image being formed using a selected portion of radiation diffracted by the target structure in a first direction and a selected portion of radiation diffracted by the target structure in a second direction, the first and second directions being defined relative to the target structure and being non-parallel, and

forming a second image of the target structure when illuminated with radiation having a second angular illumination distribution, first and second angular illumination profiles being oriented differently to one another, relative to the target structure.

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

measuring one or more properties of one or more structures formed as part of or beside the device pattern on at least one of the substrates by:

forming, using a detection system, a first image of the target structure when illuminated with radiation having a first angular illumination distribution, the first image being formed using a selected portion of radiation diffracted by the target structure in a first direction and a selected portion of radiation diffracted by the target structure in a second direction, the first and second directions being defined relative to the target structure and being non-parallel;

forming, using the detection system, a second image of the target structure when illuminated with radiation having a second angular illumination distribution, first and second angular illumination profiles being oriented differently to one another, relative to the target structure,

combining intensity values from the first image and the second image so as to discriminate between radiation diffracted in the first direction by a first part of the target structure and radiation diffracted in the second direction by the same first part of the target structure, and

determining, based at least partly on the discrimination, the property of the first part of the structure; and

controlling the lithographic process for later substrates in accordance with the result of the measuring.