Illumination source for an inspection apparatus, inspection apparatus and inspection method

An inspection apparatus for measuring a target structure on a substrate, comprising:

• an illumination source comprising:
• a high harmonic generation medium;
• a pump radiation source configured to emit a beam of pump radiation having a profile comprising no pump radiation in a central region of the beam, the beam of pump radiation configured to excite the high harmonic generation medium so as to generate high harmonic radiation;
• a spatial filter located beyond a focal plane of the beam of pump radiation, configured to block the beam of pump radiation,
• wherein the beam of pump radiation and the generated high harmonic radiation are spatially separated beyond the focal plane of the beam of pump radiation,
• wherein the high harmonic radiation is configured to be used as measurement radiation, and
• wherein the pump radiation source comprises a plurality of sub-beam sources.

A method of generating high harmonic measurement radiation optimized for filtration of pump radiation therefrom, the method comprising:

• generating the high harmonic measurement radiation using a beam of the pump radiation, wherein the high harmonic measurement radiation and the beam of pump radiation are separated spatially beyond a focal plane of the beam of pump radiation;
• and
• coherently combining a plurality of sub-beams of pump radiation using a tiled aperture geometry,
• wherein a central region of the tiled aperture geometry is devoid of the sub-beams.

A method for measuring a parameter relating to a structure formed by a lithographic process, comprising:

• generating high harmonic measurement radiation optimized for filtration of pump radiation using a beam of the pump radiation, wherein the high harmonic measurement radiation and the pump radiation are separated spatially beyond a focal plane of the beam of pump radiation;
• coherently combining a plurality of sub-beams of pump radiation using a tiled aperture geometry, wherein a central region of the tiled aperture geometry is devoid of the sub-beams, the coherently combining being used to generate high harmonic measurement radiation;
• and
• measuring the structure using the high harmonic measurement radiation.