Inspection apparatus and methods, substrates having metrology targets, lithographic system and device manufacturing method

A pair of patterning devices for use in a lithographic process, the patterning devices defining patterns which when applied sequentially to a substrate will produce a substrate provided with metrology targets formed by a lithographic process, each of the metrology targets comprising:

• a plurality of structures arranged to repeat with a spatial period in at least a first direction;
• and
• wherein a plurality of sub-structures are of a size several times smaller than the spatial period and arranged to repeat in the first direction, wherein subsets of the plurality of sub-structures are arranged in between adjacent structures of the plurality of structures, the plurality of sub-structures comprising a first population of sub-structures interleaved with a second population of sub-structures,
• wherein the first and second populations of sub-structures are formed in a first layer of the substrate in different steps of the lithographic process with a positional offset between the two interleaved populations of sub-structures, wherein the positional offset comprises a combination of a known offset and an unknown offset, the known offsets being different for different ones of the targets, and wherein each sub-structure in the second population is shifted by the positional offset towards or away from an adjacent sub-structure in the first population in the first layer of the substrate.

A method of forming metrology targets by a lithographic process on a substrate, the method comprising:

• forming a plurality of structures arranged to repeat with a spatial period in at least a first direction;
• and
• forming a plurality of sub-structures of a size several times smaller than the spatial period and arranged to repeat in the first direction, wherein subsets of the plurality of sub-structures are arranged in between adjacent structures of the plurality of structures, the plurality of sub-structures comprising a first population of sub-structures interleaved with a second population of sub-structures, wherein:
• the first and second populations of sub-structures are formed in a first layer of the substrate with a positional offset between the two interleaved populations of sub-structures,
• the positional offset comprises a combination of a known offset and an unknown offset, and
• each sub-structure in the second population is shifted by the positional offset towards or away from an adjacent sub-structure in the first population in the first layer of the substrate.