Aberration correction in charged particle system
A lens element of a charged particle system comprises an electrode having a central opening. The lens element is configured for functionally cooperating with an aperture array that is located directly adjacent said electrode, wherein the aperture array is configured for blocking part of a charged particle beam passing through the central opening of said electrode. The electrode is configured to operate at a first electric potential and the aperture array is configured to operate at a second electric potential different from the first electric potential. The electrode and the aperture array together form an aberration correcting lens.
1. A lens element of a charged particle system, the lens element comprising:
an electrode having a central opening,
wherein the lens element is configured for functionally cooperating with an aperture array that is located directly adjacent to the electrode, wherein the aperture array comprises a plurality of apertures and wherein the plurality of apertures and the electrode are configured such that a charged particle beam passing through the central opening of the electrode interacts with each of the plurality of apertures to form a plurality of beamlets from the charged particle beam,
wherein the electrode is configured to operate at a first electric potential and the aperture array is configured to operate at a second electric potential different from the first electric potential, thereby enabling the electrode and the aperture array together to form an aberration correcting lens, and
wherein the aperture array comprises an upper surface facing towards the electrode that is located upstream of the aperture array, the surface having a central portion lower than an outward portion.
2. The lens element according to claim 1 , wherein the first electric potential is set to a voltage of about 1 kV.
3. The lens element according to claim 1 , wherein the second electric potential is set to a ground potential.
4. The lens element according to claim 1 , wherein the aberration correcting lens is for a spherical aberration correction on the charged particle beam.
5. The lens element according to claim 1 , wherein the lens element is a part of a collimator system comprising multiple electrodes for collimating the charged particle beam.
6. The lens element according to claim 1 , wherein the aperture array is located downstream of the electrode of the lens element, downstream being relative to the direction of the charged particle beam.
7. The lens element according to claim 1 , wherein the outward portion is slanted to be closer to the electrode as a distance from the center portion increases.
8. A collimator system for collimating a charged particle beam, the collimator system comprising:
an electrode stack comprising multiple electrodes; and
a lens element comprising one of the electrodes,
wherein the electrode of the lens element having a central opening, wherein the lens element is configured for functionally cooperating with an aperture array that is located directly adjacent to the electrode, wherein the aperture array comprises a plurality of apertures, wherein the plurality of apertures and the electrode are configured such that a charged particle beam passing through the central opening of the electrode interacts with each of the plurality of apertures to form a plurality of beamlets from the charged particle beam, and wherein the electrode is configured to operate at a first electric potential and the aperture array is configured to operate at a second electric potential different from the first electric potential, thereby enabling the electrode and the aperture array together to form an aberration correcting lens, and
wherein the aperture array comprises an upper surface facing towards the electrode that is located upstream of the aperture array, the surface having a central portion lower than an outward portion.
9. The collimator system according to claim 8 , wherein the first electric potential is set to a voltage of about 1 kV.
10. The collimator system according to claim 8 , wherein the second electric potential is set to a ground potential.
11. The collimator system according to claim 8 , wherein the aberration correcting lens is arranged for a spherical aberration correction on the charged particle beam.
12. The collimator system according to claim 8 , wherein the aperture array is located downstream of the electrode of the lens element, downstream being relative to the direction of the charged particle beam.
13. The collimator system according to claim 12 , wherein the aperture array is a structural component of the collimator system.
14. The collimator system according to claim 12 , wherein the aperture array is a part of a condenser lens module arranged in a projection column located directly downstream from a beam generator module, downstream being relative to the direction of the charged particle beam, and wherein the beam generator module comprises the collimator system.
15. The collimator system according to claim 8 , wherein the outward portion is slanted to be closer to the electrode as a distance from the center portion increases.
16. The collimator system according to claim 8 , further comprising a center electrode configured to operate at a third electric potential that is higher than the first electric potential and higher than the second electric potential.
17. The collimator system according to claim 16 , wherein the central electrode is configured for refracting the charged particle beam and wherein the third electric potential is set to a voltage between 15 kV and 25 kV.
18. The collimator system according to claim 8 , wherein one or more of the multiple electrodes are configured to operate at a negative electric potential and are included in the collimator system at a position upstream of electrode of the lens element, upstream being relative to the direction of the charged particle beam.
19. The collimator system according to claim 18 , wherein the negative electric potential is arranged for deflecting secondary electrodes and/or backscattered electrodes originating from a region downstream of the collimator electrode stack, downstream being relative to the direction of the charged particle beam.
20. A charged particle system comprising:
a beam source for generating an electron beam;
a collimator system according to claim 8 for collimating the electron beam; and
an aperture array for forming a plurality of beamlets from the electron beam.
21. A method of operating a charged particle beam generator, the charged particle beam generator comprising a collimator system according to claim 8 and a beam source, the method comprising:
generating a charged particle beam with the beam source;
projecting the generated charged particle beam along an optical axis (A) through apertures of a collimator electrode stack;
applying electrical potentials onto collimator electrodes of the collimator electrode stack, wherein a first collimator electrode is kept at ground potential, a middle collimator electrode is kept at a highest positive electric potential, and a last collimator electrode is kept at a low positive electric potential.
22. The method according to claim 21 , wherein a second collimator electrode preceding the middle electrode is kept at a negative electric potential.
23. The method according to claim 21 , wherein at least one of two intermediate collimator electrodes directly preceding the last collimator electrode is kept at low negative electric potentials.
24. The method according to claim 21 , wherein at least one of two intermediate collimator electrodes directly preceding the last collimator electrode is kept at a fixed electric potential with a value of −300 Volts to −500 Volts.
25. The method according to claim 21 , wherein the second collimator electrode is kept at a fixed electric potential with a value of −3 kilovolts to −4 kilovolts.
26. The method according to claim 21 , wherein the middle collimator electrode is kept at a fixed electric potential with a value of +20 kilovolts to +30 kilovolts.
27. The method according to claim 21 , wherein a last collimator electrode is kept at a positive potential in a range of +500 Volts to +1100 Volts.