Photon source, metrology apparatus, lithographic system and device manufacturing method

An inspection apparatus comprising:

• a support configured to support a substrate, the substrate having a structure thereon;
• an optical system configured to illuminate the structure under predetermined illumination conditions and to detect radiation diffracted by the structure under the predetermined illumination conditions;
• and
• a processor configured to process information characterizing the detected radiation to obtain a measurement of a property of the structure, wherein the optical system includes a photon source apparatus comprising:
• a container configured to contain a gaseous atmosphere;
• a driving system configured to form a driving radiation beam focused on a plasma forming zone within the container and maintain, by the driving radiation beam, a plasma;
• a collecting optical system configured to collect photons emitted from the plasma along a longitudinal axis and to form the collected photons into at least one beam of output radiation;
• and
• a reflector configured to reflect photons emitted in at least one of a direction reverse to the longitudinal axis and a direction transverse to the longitudinal axis back into the plasma.

A method comprising:

• generating a driving radiation beam focused on a plasma forming zone within a container containing a gaseous atmosphere;
• maintaining, by the driving radiation beam, a plasma;
• reflecting, by a reflector, photons emitted in at least one of a direction reverse to a longitudinal axis and a direction transverse to the longitudinal axis back into the plasma;
• collecting, by a collecting optical system, photons emitted from the plasma along the longitudinal axis;
• and
• forming the collected photons into at least one beam of output radiation.