Integrated optical system for scalable and accurate inspection systems

A lithographic apparatus comprising:

• an illumination system configured to illuminate a pattern of a patterning device;
• a projection system configured to project an image of the pattern onto a wafer;
• and
• an inspection system comprising:
• a radiation source configured to generate radiation to irradiate a target;
• an integrated optical system comprising:
• a substrate;
• first, second, third, and fourth waveguides disposed on the substrate;
• first and second grating couplers disposed on the substrate, wherein
• the first grating coupler is configured to couple first and third portions of radiation scattered by the target respectively into the first and third waveguides, and
• the second grating coupler is configured to couple second and fourth portions of radiation scattered by the target respectively into the second and fourth waveguides;
• a first combiner coupled to the first and second waveguides, disposed on the substrate, and configured to combine the first and second portions of radiation;
• and
• a second combiner coupled to the third and fourth waveguides, disposed on the substrate and configured to combine the third and fourth portions of radiation;
• a first detector configured to receive the combined first and second portions of radiation from the first combiner and to generate a first measurement signal based on the combined first and second portions of radiation;
• and
• a second detector configured to receive the combined third and fourth portions of radiation from the second combiner and to generate a second measurement signal based on the combined third and fourth portions of radiation.