Selecting unit cell configuration for repeating structures in optical metrology

A method of determining profile parameters of a repeating structure formed on a wafer using an optical metrology model, the optical metrology model having profile parameters associated with a top-view of the structure and profile parameters associated with a cross-sectional view of the structure, the method comprising:

• a) defining a plurality of unit cell configurations of the repeating structure, each unit cell configuration defined by one or more unit cell parameters, wherein each of the unit cell configurations differs from one another in at least one unit cell parameter;
• b) selecting a unit cell configuration from the plurality of unit cell configurations based on one or more selection criteria;
• c) characterizing a top-view profile of the repeating structure using the selected unit cell configuration;
• d) for the selected unit cell configuration, optimizing metrology device variables for diffraction signal sensitivity;
• e) selecting profile parameters to represent variations in the top-view profile of the structure corresponding to the selected unit cell configuration;
• f) selecting profile parameters associated with a cross-sectional view profile of the structure;
• g) integrating the selected profile parameters representing the top-view profile and the cross-sectional view profile of the structure into an optical metrology model;
• h) optimizing the optical metrology model;
• i) creating a set of profile parameters and simulated diffraction signals using the optimized optical metrology model;
• j) extracting a best match simulated diffraction signal using the set of created simulated diffraction signals and one or more measured diffraction signals;
• k) when the best match simulated diffraction signal and the measured diffraction signals do not match within one or more matching criteria, revising the characterization and/or selection of profile parameters;
• and
• l) iterating e), f), h), h), i), j) and k) until the best match simulated diffraction signal and the measured diffraction signal match within the one or more matching criteria.

A computer-readable storage medium containing computer executable instructions for causing a computer to model a repeating structure formed on a wafer for optical metrology, comprising instructions for:

• a) defining a plurality of unit cell configurations of the repeating structure, each unit cell configuration defined by one or more unit cell parameters, wherein each of the unit cell configurations differs from one another in at least one unit cell-parameter;
• b) selecting a unit cell configuration from the plurality of unit cell configurations based on one or more selection criteria;
• and
• c) characterizing a top-view profile of the repeating structure using the selected unit cell configuration.