Metrology apparatus, lithographic system, and method of measuring a structure

A metrology apparatus configured to measure a structure formed on a substrate to determine a parameter of interest, the metrology apparatus comprising:

• an optical system configured to focus radiation onto the structure and direct redirected radiation from the structure to a detection system, wherein:
• the optical system comprises a beam splitter configured to split a radiation beam into a first radiation beam and a second radiation beam and comprises an optical path comprising a first branch and a second branch, the first radiation beam propagating from the beam splitter to the substrate along the first branch and at least part of the first radiation beam propagating from the substrate back to the beam splitter along the second branch, and the second radiation beam propagating from the beam splitter to the substrate along the second branch and at least part of the second radiation beam propagating from the substrate back to the beam splitter along the first branch,
• the optical system is configured to apply a plurality of different offsets of an optical characteristic to radiation before and/or after redirection by the structure, such that a corresponding plurality of different offsets are provided to redirected radiation derived from a first point of a pupil plane field distribution relative to redirected radiation derived from a second point of the pupil plane field distribution, and
• the detection system is configured to detect a corresponding plurality of radiation intensities resulting from interference between the redirected radiation derived from the first point of the pupil plane field distribution and the redirected radiation derived from the second point of the pupil plane field distribution, wherein each radiation intensity corresponds to a different one of the plurality of different offsets.

A method of measuring a structure on a substrate, the method comprising:

• splitting a radiation beam into a first radiation beam and a second radiation beam using a beam splitter;
• propagating the first radiation beam from the beam splitter to the structure along a first optical path branch;
• propagating the second radiation beam from the beam splitter to the structure along the second branch;
• propagating at least part of the first radiation beam from the substrate back to the beam splitter along the second branch;
• propagating at least part of the second radiation beam from the substrate back to the beam splitter along the first branch;
• using a detection system to detect redirected radiation from the at least part of the first radiation beam and at least part of the second radiation beam, wherein:
• a plurality of different offsets of an optical characteristic are applied to radiation before and/or after redirection by the structure, such that a corresponding plurality of different offsets are provided to redirected radiation derived from a first point of a pupil plane field distribution relative to redirected radiation derived from a second point of the pupil plane field distribution, and
• the detection system detects a corresponding plurality of radiation intensities resulting from interference between the redirected radiation derived from the first point of the pupil plane field distribution and the redirected radiation derived from the second point of the pupil plane field distribution, wherein each radiation intensity corresponds to a different one of the plurality of different offsets.

A non-transitory computer-readable medium comprising instructions therein, the instructions, upon execution by a computer system, configured to cause the computer system to at least:

• cause provision of radiation onto a structure on a substrate;
• and
• cause use of a detection system to detect redirected radiation from the structure in order to determine a parameter or interest, wherein:
• a plurality of different offsets of an optical characteristic are applied to radiation before and/or after redirection by the structure, such that a corresponding plurality of different offsets are provided to redirected radiation derived from a first point of a pupil plane field distribution relative to redirected radiation derived from a second point of the pupil plane field distribution, and
• the detection system detects a corresponding plurality of radiation intensities resulting from interference between the redirected radiation derived from the first point of the pupil plane field distribution and the redirected radiation derived from the second point of the pupil plane field distribution, wherein each radiation intensity corresponds to a different one of the plurality of different offsets.