Apparatus for multiple charged-particle beams
Systems and methods for observing a sample in a multi-beam apparatus are disclosed. A charged particle optical system may include a deflector configured to form a virtual image of a charged particle source and a transfer lens configured to form a real image of the charged particle source on an image plane. The image plane may be formed at least near a beam separator that is configured to separate primary charged particles generated by the source and secondary charged particles generated by interaction of the primary charged particles with a sample. The image plane may be formed at a deflection plane of the beam separator. The multi-beam apparatus may include a charged-particle dispersion compensator to compensate dispersion of the beam separator. The image plane may be formed closer to the transfer lens than the beam separator, between the transfer lens and the charged-particle dispersion compensator.
1. A charged particle optical system comprising:
a first deflector array configured to deflect a plurality of beamlets of a primary charged particle beam generated by a source;
a first lens configured to focus the plurality of beamlets to form a plurality of images of the source on an image plane;
an objective lens configured to project the plurality of images onto a sample and form a plurality of probe spots thereon; and
a beam separator configured to separate the plurality of beamlets of the primary charge particle beam and secondary charged particles emitted from the sample due to illumination by the plurality of probe spots, wherein the image plane is at least near the beam separator.
2. The charged particle optical system of claim 1 , wherein the first deflector array is configured to deflect the plurality of beamlets at deflection angles that are set to obtain a predetermined pitch of the plurality of probe spots and to decrease aberrations thereof.
3. The charged particle optical system of claim 1 , further comprising a first aperture array above the first deflector array and configured to limit currents of the plurality of probe spots.
4. The charged particle optical system of claim 3 , further comprising a second lens between the charged particle source and the first aperture array and configured to focus the primary charged particle beam.
5. The charged particle optical system of claim 4 , wherein the second lens is configured to focus the primary charged particle beam as a parallel beam.
6. The charged particle optical system of claim 5 , wherein the second lens is a movable lens configured to change the currents of the plurality of probe spots.
7. The charged particle optical system of claim 5 , further comprising a compensator array configured to compensate aberrations of the plurality of probe spots.
8. The charged particle optical system of claim 7 , further comprising a second aperture array above the second lens, close to the source and configured to cut off portions of the primary charged particle beam not used in the plurality of beamlets.
9. The charged particle optical system of claim 5 , further comprising a main aperture above the second lens, close to the source and configured to cut off a peripheral portion of the primary charged particle beam not used in the plurality of beamlets.
10. The charged particle optical system of claim 9 , further comprising a second aperture array between the main aperture and the first aperture array and configured to cut off portions of the primary charged particle beam not used in the plurality of beamlets.
11. The charged particle optical system of claim 10 , wherein the second aperture array is above the second lens and close to the source.
12. The charged particle optical system of claim 4 , further comprising a second deflector array configured to deflect the plurality of beamlets to be normally incident onto the first aperture array.
13. A method of forming a plurality of probe spots on a sample, the method comprising:
deflecting a plurality of beamlets of a beam generated by a charged particle source;
focusing the plurality of beamlets, by a lens, to form a plurality of images of the source on an image plane;
projecting the plurality of images onto a sample to form a plurality of probe spots thereon; and
adjusting the lens to locate the image plane at least near a beam separator configured to separate the plurality of beamlets of the beam and secondary electrons emitted from the sample due to illumination by the plurality of probe spots.