Optical metrology with reduced sensitivity to grating anomalies

A method comprising:

• determining a measurement model of a spectral response of a periodic metrology target to measurement by a metrology system at a plurality of available values of an angle of incidence, azimuth angle, illumination wavelength, polarization state, or any combination thereof;
• selecting a subset of the plurality of available values of an angle of incidence, azimuth angle, illumination wavelength, polarization state, or any combination thereof that reduces the presence of grating anomalies in a spectral response of the periodic metrology target to measurement by the metrology system at the selected subset of the plurality of available values;
• generating a measurement recipe for a measurement of the periodic metrology target at the selected subset of the plurality of available values;
• and
• performing measurements of the periodic metrology target with the metrology system based on the measurement recipe.

A metrology system comprising:

• an illumination source configured to generate an amount of illumination light at a plurality of wavelengths;
• an illumination optics subsystem configured to direct the amount of illumination light from the illumination source to a periodic metrology target disposed on a specimen under measurement at a plurality of available values of angle of incidence, azimuth angle, polarization state, or any combination thereof;
• a detector having a planar, two-dimensional surface sensitive to incident light, wherein the detector is configured to generate a plurality of output signals indicative of a response of the specimen to the amount of illumination light;
• a collection optics subsystem configured to collect an amount of collected light from the specimen and direct the amount of collected light to the surface of the detector;
• and
• a non-transitory, computer-readable medium, comprising:
• code for causing a computing system to determine a measurement model of a spectral response of the periodic metrology target to measurement by the metrology system at the plurality of available values of the angle of incidence, azimuth angle, illumination wavelength, polarization state, or any combination thereof;
• code for causing the computing system to select a subset of the plurality of available values of the angle of incidence, azimuth angle, illumination wavelength, polarization state, or any combination thereof that reduces the presence of grating anomalies in a spectral response of the periodic metrology target to measurement by the metrology system at the selected subset of the plurality of available values;
• and
• code for causing the computing system to generate a measurement recipe for a measurement of the periodic metrology target at the selected subset of the plurality of available values.