Lithographic apparatus, metrology apparatus, optical system and method

An optical system for directing a beam of radiation having a first polarization and a second polarization that is perpendicular to the first polarization, the optical system comprising:

• a diffractive element;
• first and second mirrors;
• and
• first and second reflective elements,
• wherein:
• the diffractive element is configured to direct the beam toward the second mirror through a first opening in the first mirror and then through a second hole in the first mirror toward the first reflective element at a first angle of incidence,
• the first reflective element is configured to reflect the beam toward the second reflective element at a second angle of incidence,
• the second reflective element is configured to reflect the beam toward a substrate, and
• the first and second angles of incidence are configured to reduce a variation of a ratio of intensities of the first polarization to the second polarization, the variation being based on a property of a layer of the first and/or second reflective elements.

A metrology system comprising:

• a radiation source configured to generate a beam of radiation having a first polarization and a second polarization that is perpendicular to the first polarization;
• an optical system configured to direct the beam of radiation toward a substrate, the optical system comprising:
• a diffractive element;
• first and second mirrors;
• and
• first and second reflective elements,
• wherein:
• the diffractive element is configured to direct the beam toward the second mirror through a first opening in the first mirror and then through a second hole in the first mirror downstream toward the first reflective element at a first angle of incidence elements,
• the first reflective element is configured to reflect the beam toward the second reflective element at a second angle of incidence,
• the second reflective element is configured to reflect the beam toward the substrate, and
• the first and second angles of incidence are configured to reduce a variation of a ratio of intensities of the first polarization to the second polarization, the variation being based on a property of a layer of the first and/or second reflective elements;
• and
• a detector configured to receive radiation scattered by the substrate and generate a signal based on the received radiation, wherein the signal comprises information of a height position of the substrate.