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

A non-transitory computer program product comprising machine-readable instructions for causing a processor to cause performance of a method for obtaining a computationally determined interference electric field describing scattering of radiation by a pair of structures comprising a first grating structure and a second grating structure on a substrate, 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
• computationally determining the interference of the first electric field and second electric field, to obtain a computationally determined interference electric field.

A processing apparatus comprising machine-readable instructions for causing a processing device to cause the performance of a method for obtaining a computationally determined interference electric field describing scattering of radiation by a pair of structures comprising a first grating structure and a second grating structure on a substrate, 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
• computationally determining the interference of the first electric field and second electric field, to obtain a computationally determined interference electric field.