Adjustable permanent magnetic lens having shunting device

An electromagnetic lens including

• a charged particle lens and a sleeve insert inserted into the passage space along the longitudinal axis,
• wherein the charged particle lens is configured to modify a charged-particle beam of a charged particle optical system, the lens being provided with a beam passage extending primarily along a longitudinal axis and allowing the passage of a charged-particle beam, said lens including a magnetic circuit assembly comprising:
• at least one permanent magnet;
• a yoke body;
• and
• at least one shunting device,
• said yoke body being composed of at least two yoke components, of which a first yoke component realizes an inner yoke shell, arranged surrounding the passage space, and a second yoke component realizes an outer yoke shell which is arranged surrounding the inner yoke shell, said yoke components being arranged circumferentially around the longitudinal axis and comprise a ferromagnetic or ferrimagnetic material;
• said at least one permanent magnet being arranged between the at least two yoke components and circumferentially around the inner yoke shell, said at least one permanent magnet comprising a permanent magnetic material being magnetically oriented with its two magnetic poles towards respective yoke components;
• wherein in the magnetic circuit assembly, the at least one permanent magnet and the yoke body form a closed magnetic circuit but having at least two gaps formed between respective axial faces of different yoke components, wherein the closed magnetic circuit is configured to direct a magnetic flux density coming from said at least one permanent magnet through the yoke body and, in said gaps, induce a magnetic field, which reaches inwards into the beam passage, and
• said at least one shunting device comprising at least one magnetically permeable shunting component and being arranged in a defined position between the at least two components of the yoke body, said at least one shunting device enabling a defined portion of the magnetic flux density to bypass at least parts of the magnetic circuit assembly;
• said sleeve insert surrounding a beam passage having a radius smaller than the radius of the beam passage of the charged-particle lens, the sleeve insert extending along a longitudinal axis,
• said sleeve insert comprising a mounting body, which is at least partially electrically conductive, and at least one electrically conductive electrode element,
• said at least one electrode element being configured to be applied an electric potential via a power supply so as to generate an electrostatic field within the beam passage of the charged particle lens;
• wherein the at least one electrode element includes a beam aperture element forming a delimiting opening with a defined radius around the longitudinal axis,
• said delimiting opening being configured to limit the lateral width of a charged-particle beam propagating along the longitudinal axis;
• and
• said beam aperture element being connected to a current measurement device configured to measure an amount of the charged-particle beam absorbed at the beam aperture element.