Compensating optical system for nonuniform surfaces, a metrology system, lithographic apparatus, and methods thereof

A method comprising:

• irradiating a target structure with a radiation beam;
• controlling, using an optical element, respective positions of one or more scattered beams, wherein the target structure is configured to produce scattered radiation comprising the one or more scattered beams;
• receiving a portion of the position-controlled scattered radiation at a detector;
• generating a detection signal based on the received scattered radiation;
• and
• determining a property of the target structure based on at least the detection signal.

A lithography apparatus comprising:

• an illumination apparatus configured to illuminate a pattern of a patterning device;
• a projection system configured to project an image of the pattern onto a substrate;
• and
• a metrology system including:
• a radiation source configured to generate a radiation beam,
• an optical system configured to:
• direct the radiation beam toward a target structure, wherein the target structure is configured to produce scattered radiation comprising one or more scattered beams, and
• receive the scattered radiation,
• an optical element configured to control a position of the one or more scattered beams,
• a detection system configured to receive a portion of the position-controlled scattered radiation and to generate a detection signal, and
• a processor configured to determine a property of the target structure based on at least the detection signal.