Metrology target simulation

A computer implemented method of simulating an electromagnetic response of a metrology target having a multi-layer structure comprising a first grating and a second grating, wherein the second grating is below the first grating in the multi-layer structure, the method comprising:

• receiving a model representing the multi-layer structure, the model defining (i) the first grating as having a first predetermined number of grating lines within a pitch of the multi-layer structure, each of the first predetermined number of grating lines separated by a first pitch;
• and (ii) the second grating as having a second predetermined number of grating lines within the pitch of the multi-layer structure, each of the second predetermined number of grating lines separated by a second pitch;
• using the model and the first pitch to simulate the transmission and reflection properties of the first grating and generate a first scattering matrix associated with the first grating;
• using the model and the second pitch to simulate the transmission and reflection properties of the second grating and generate a second scattering matrix associated with the second grating;
• and
• generating a scattering matrix defining the scattering and reflection properties of the multi-layer structure by combining the first scattering matrix and the second scattering matrix.