Method for determining focal properties in a target beam field of a multi-beam charged-particle processing apparatus

A method for determining focal properties in a target beam field of a charged-particle multi-beam processing apparatus, said apparatus comprising

• a charged-particle source generating a beam of electrically charged particles,
• a pattern definition device comprising an aperture array composed of a multitude of blanking apertures, said aperture array configured to generate a corresponding multitude of beamlets when said beam is directed at and penetrates the pattern definition device,
• a projection optics including at least one charged-particle optical lens, configured to direct the beamlets onto a beam field within a surface of a target provided to said apparatus,
• wherein the beamlets impinge on the beam field at respective landing positions and with a respective landing angle with respect to the surface of the target, each beamlet forming an aperture image of a respective aperture at a respective location close to the target surface, said aperture image having at least one focal property including a height of focus, which describes the distance of the aperture image from the surface of the target,
• said projection optics being able to tilt the landing angles of the beamlets by modifying at least one operating parameter of at least one optical component of the projection optics,
• the method being characterized by the steps of:
• defining a set of measurement positions in the beam field;
• introducing a shift in at least one operating parameter of an optical component of the projection optics, causing a tilting of the beamlet landing angles;
• determining amounts of displacement of the beamlets at the target surface for said measurement positions as a result of said shift, and determining the change of landing angles at the target surface for said measurement positions as a result of said shift;
• generating a spatially variable map, said map describing a mapping from said change of landing angles to the amounts of displacement as a function of the position within the beam field for an area in the beam field which at least covers the set of measurement positions or the entire beam field;
• extracting, for a plurality of target positions located across the beam field, focal properties from said map, said focal properties including at least one of height of focus, astigmatic length, and a size of blur along at least one directions.