Metrology apparatus and method for determining a characteristic of one or more structures on a substrate

A method for using radiation scattered by a pair of structures comprising a first grating structure and a second grating structure, both disposed on a substrate, to yield a characteristic of interest, the method comprising:

• determining a first electric field relating to first radiation scattered by the first grating structure;
• determining a second electric field relating to second radiation scattered by the second grating structure;
• and
• using the determined first and second electric fields to determine a characteristic of interest relating to the pair of structures.

A system to determine a characteristic of interest of a substrate, the system comprising:

• an illumination system configured to direct radiation at a pair of structures comprising a first grating structure and a second grating structure, both disposed on the substrate;
• a measurement system configured to receive radiation scattered by the first and second gratings and output signals corresponding thereto;
• a controller configured to:
• receive the signals;
• determine a first electric field relating to first radiation scattered by the first grating structure;
• determine a second electric field relating to second radiation scattered by the second grating structure;
• and
• determine the characteristic of interest relating to the pair of structures using the determined first and second electric fields.