Intensity measurements using off-axis illumination

An integrated optical device, comprising:

• a first illumination system configured to:
• generate a first radiation beam at a first wavelength, and
• transmit the first radiation beam toward a region of a surface of a substrate at a first incident angle;
• a second illumination system configured to:
• generate a second radiation beam at a second wavelength, and
• transmit the second radiation beam toward the region at a second incident angle;
• a first detection system configured to:
• measure a first diffracted radiation beam at the first wavelength and diffracted from the region at a first diffraction angle in response to a first illumination of the region by the first radiation beam, and
• generate a first measurement signal based on the first diffracted radiation beam;
• a second detection system configured to:
• measure a second diffracted radiation beam at the second wavelength and diffracted from the region at a second diffraction angle in response to a second illumination of the region by the second radiation beam, and
• generate a second measurement signal based on the second diffracted radiation beam;
• a third detection system configured to:
• measure a third diffracted radiation beam at the first wavelength and/or second wavelength and diffracted from the region at a third diffraction angle that is normal to the surface of the substrate, and
• generate a third measurement signal based on the third diffracted radiation beam;
• and
• a controller configured to generate an electronic signal based on the third measurement signal or on the first measurement signal and the second measurement signal.