Optimizing image distortion in a multi beam charged particle processing apparatus

A method for adjusting the optical imaging system 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 and being configured to direct the beamlets onto a beam field within a surface of a target provided to said apparatus, according to an imaging whose properties are controllable through a number of operating parameters of respective optical components of the projection optics,
• 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, and
• wherein changes in said one or more operating parameters induce changes in the landing positions and changes in the landing angles,
• the method comprising the steps of:
• determining the image distortion of the beam field, the image distortion describing the deviation of the landing positions and/or landing angles of beamlets from respective nominal values, expressed as functions of the location within the beam field;
• determining fingerprints, each fingerprint representing the change of image distortion effected by a unit change of a respective operating parameter;
• determining optimized values of operating parameters which optimize a corrected total distortion obtained from a superposition of the image distortion and a change of operating parameters effecting a variation of the image distortion expressed by a linear combination of said fingerprints, with the linear combination having coefficients representing the changes in operating parameters;
• applying operating parameters in accordance with the optimized values thus determined to the respective optical elements of the projection optics,
• wherein for each of a pre-selected subset of operating parameters, an amount of change in the landing positions of beamlets associated with a change in the respective operating parameter is determined by applying a pre-determined linear map to an amount of change in landing angles, said pre-determined linear map varying as function of the position in the beam field.