Apparatus using multiple charged particle beams
The present disclosure proposes an anti-rotation lens and using it as an anti-rotation condenser lens in a multi-beam apparatus with a pre-beamlet-forming mechanism. The anti-rotation condenser lens keeps rotation angles of beamlets unchanged when changing currents thereof, and thereby enabling the pre-beamlet-forming mechanism to cut off electrons not in use as much as possible. In this way, the multi-beam apparatus can observe a sample with high resolution and high throughput, and is competent as a yield management tool to inspect and/or review defects on wafers/masks in semiconductor manufacturing industry.
1. An anti-rotation lens having a focusing power for focusing a charged particle beam, comprising:
a first magnetic lens configured to generate a first magnetic field being aligned with an optical axis of the anti-rotation lens; and
a second magnetic lens configured to generate a second magnetic field being aligned with the optical axis, wherein the focusing power of the anti-rotation lens is adjustable by varying the first magnetic field and the second magnetic field, and the first magnetic field and the second magnetic field have opposite directions on the optical axis.
2. The anti-rotation lens of claim 1 , wherein the focusing power is adjustable while keeping a rotation angle of the charged particle beam unchanged or substantially unchanged.
3. The anti-rotation lens of claim 2 , wherein the rotation angle is zero.
4. The anti-rotation lens of claim 1 , wherein the anti-rotation lens comprises a third magnetic lens configured to generate a third magnetic field aligned with the optical axis, the focusing power and a principal plane of the anti-rotation lens are adjustable by varying the first magnetic field, the second magnetic field, and/or the third magnetic field, and two of the first magnetic field, the second magnetic field, and the third magnetic field have opposite directions on the optical axis.
5. The anti-rotation lens of claim 4 , wherein the focusing power and the principal plane are adjustable while keeping a rotation angle of the charged particle beam unchanged or substantially unchanged.
6. A multi-beam apparatus, comprising:
an electron source configured to generate a primary-electron beam;
a condenser lens configured to focus the primary-electron beam;
a source-conversion unit configured to form a plurality of images of the electron source by a plurality of beamlets of the primary-electron beam; and
an objective lens configured to focus the plurality of beamlets onto the surface and form a plurality of probe spots thereon;
wherein the multibeam apparatus further comprises an anti-rotation lens according to claim 1 .
7. The multi-beam apparatus of claim 6 , further comprising an electron detection device having a plurality of detection elements configured to detect a plurality of secondary beams generated by the plurality of probe spots from the sample for observing the sample.
8. A method to configure an anti-rotation lens for focusing a charge particle beam, the method comprising:
generating a first magnetic field by a first magnetic lens that is aligned with an optical axis of the anti-rotation lens;
generating a second magnetic field by a second magnetic lens that is aligned with the optical axis; and
generating a focusing power of the anti-rotation lens by the first magnetic field and the second magnetic field, wherein the first magnetic field and the second magnetic field have opposite directions on the optical axis;
wherein the method further comprises adjusting the focusing power by adjusting the first magnetic field and the second magnetic field while keeping a rotation angle of the charged particle beam unchanged or substantially unchanged.
9. The anti-rotation lens of claim 1 , wherein a ratio between an amplitude of the first magnetic field and an amplitude of the second magnetic field is one.
10. The anti-rotation lens of claim 1 , wherein a distribution of the first magnetic field and the second magnetic field is substantially similar.
11. The anti-rotation lens of claim 2 , wherein the rotation angle of the charged-particle beam is based on an interaction between a charged-particle source configured to generate the charged-particle beam and the first and the second magnetic fields.