MEASUREMENT TARGET SIMULATION

A method of simulating an electromagnetic response of a metrology target, the metrology target comprising a first grating and a second grating, where the second grating is located below the first grating, the method comprising: receiving a model that (i) defines the first grating to have a first number of grating lines within a pitch, and (ii) defines the second grating to have a second number of grating lines within a pitch; each grating line in the first number of grating lines is spaced by a first pitch; and (ii) defining the second grating to have a second number of grating lines within the pitch, each of the second predetermined number of grating lines being spaced apart by a second pitch; simulating characteristics of the first grating using the model and the first pitch, and generating a first scattering matrix; simulating characteristics of the second grating using the model and the second pitch, and generating a second scattering matrix; and generating a scattering matrix defining characteristics of