Optical system for a plurality of primary beamlets, charged particle multi-beam apparatus and method of focusing a plurality of primary beamlets

A charged particle multi-beam apparatus having a plurality of primary beamlets focused on a specimen, comprising:

• a charged particle emitter for a primary charged particle beam;
• an aperture lens array for generating a plurality of primary beamlets from the primary charged particle beam;
• and
• an optical system configured to focus a plurality of primary beamlets of a multi-beam apparatus, the optical system comprising:
• a first optical component, comprising:
• a first multipole arrangement with a plurality of first multipoles of an order of four or more, each first multipole of the plurality of first multipoles is configured to provide a first astigmatism;
• and
• a second multipole arrangement with a plurality of second multipoles of an order of four or more, each second multipole of the plurality of second multipoles is configured to provide a second astigmatism, wherein the first astigmatism and the second astigmatism compensate each other;
• the optical system further comprising:
• a second optical component, comprising:
• three or more electrodes forming a plurality of electrostatic lenses for the plurality of primary beamlets, each electrode comprising:
• an electrode body and a plurality of openings in the electrode body for guiding one primary beamlet of the plurality of primary beamlets through one opening of the plurality of openings;
• and
• an electrical connection to provide a potential to the electrode body at the plurality of openings;
• wherein a first multipole of the plurality of first multipoles and a second multipole of the plurality of second multipoles form a pair of corresponding multipoles configured for individual focus adjustment of a primary beamlet of the plurality of primary beamlets.