Method and apparatus for measuring a parameter of a lithographic process, substrate and patterning devices for use in the method

A metrology apparatus comprising:

• a support configured to support a substrate on which a target structure comprising first and second targets has been formed, each of the first and second targets comprising a two-dimensional periodic structure formed in a single material layer on the substrate using a lithographic process,
• wherein the lithographic process comprises defining features of grid elements in the first and second targets, modifying the features defined in the first target by removing portions of each grid element in the first target at predefined locations in the first target, each removed portion being offset from a grid element by a first bias amount that is close to one half of a spatial period of the features, and modifying the features defined in the second target by removing portions of each grid element in the second target at predefined locations in the second target, each removed portion from the second target being offset from a grid element by a second bias amount that is close to one half of the spatial period and different from the first bias amount;
• an optical system configured to selectively illuminate the target structure with radiation and collect at least zero order radiation scattered by the target structure;
• a detector configured to detect an angle-resolved scatter spectrum of the target structure using the zero order radiation;
• and
• a processor configured to derive a parameter of the lithographic process using asymmetry of the angle-resolved scatter spectrum of the target structure.

A lithographic system comprising:

• a lithographic apparatus for use in a lithographic process;
• and
• a metrology apparatus comprising:
• a support configured to support a substrate on which a target structure comprising first and second targets has been formed, each of the first and second targets comprising a two-dimensional periodic structure formed in a single material layer on the substrate using the lithographic process, wherein the lithographic process comprises defining features of grid elements in the first and second targets, modifying the features defined in the first target by removing portions of each grid element in the first target at predefined locations in the first target, each removed portion being offset from a grid element by a first bias amount that is close to one half of a spatial period of the features, and modifying the features defined in the second target by removing portions of each grid element in the second target at predefined locations in the second target, each removed portion from the second target being offset from a grid element by a second bias amount that is close to one half of the spatial period and different from the first bias amount
• an optical system configured to selectively illuminate the target structure with radiation and collect at least zero order radiation scattered by the target structure;
• a detector configured to detect an angle-resolved scatter spectrum of the target structure using the zero order radiation;
• and
• a processor configured to derive a parameter of the lithographic process using asymmetry of the angle-resolved scatter spectrum of the target structure,
• wherein the metrology apparatus is configured to measure the parameter of the lithographic process using the target structure formed at least partially using the lithographic apparatus.

A non-transitory computer program product comprising machine readable instructions which, when run on a suitable processor, cause the processor to perform operations comprising:

• illuminating, using an optical system, a target structure with radiation, the target structure comprising first and second targets, each of the first and second targets comprising a two-dimensional periodic structure formed in a single material layer on a substrate using a lithographic process, wherein the lithographic process comprises defining features of grid elements in the first and second targets, modifying the features defined in the first target by removing portions of each grid element in the first target at predefined locations in the first target, each removed portion is offset from a grid element by a first bias amount that is close to one half of a spatial period of the features, and modifying the features defined in the second target by removing portions of each grid element in the second target at predefined locations in the second target, each removed portion from the second target is offset from a grid element by a second bias amount that is close to one half of the spatial period and different from the first bias amount;
• obtaining, using a detector, an angle-resolved scatter spectrum of the target structure;
• and
• deriving, using the processor, a measurement of a parameter using asymmetry found in the angle-resolved scatter spectrum of the target structure.