Apparatus of plural charged particle beams

1. An electromagnetic compound lens comprising:

an electrostatic lens provided on an optical axis of the compound lens, and comprising a first electrode and a second electrode;

a magnetic lens provided on the optical axis, wherein the magnetic lens includes a permanent magnet surrounding the optical axis, the permanent magnet having an annular shape; and

a first magnetic pole piece and a second magnetic pole piece, the first magnetic pole piece and the second magnetic pole piece being magnetically coupled to opposite magnetic poles of the permanent magnet, wherein

the first magnetic pole piece, the second magnetic pole piece and the permanent magnet are between the first electrode and the second electrode along the optical axis.

2. The compound lens of claim 1 , wherein:

the permanent magnet is between the first magnetic pole piece and the second magnetic pole piece along the optical axis; and

a gap is formed between the first magnetic pole piece and the second magnetic pole piece so that a magnetic field originally generated by the permanent magnet is directed to leak through the gap toward the optical axis, the gap having an annular shape.

3. The compound lens of claim 2 , wherein the gap is formed on a radially inner side of the permanent magnet.

4. The compound lens of claim 2 , wherein the gap is formed close to one end of the permanent magnet along the optical axis.

5. The compound lens of claim 1 , wherein the first magnetic pole piece and the second magnetic pole piece directly contact the permanent magnet.

6. The compound lens of claim 1 , wherein an inner diameter of the first magnetic pole piece is different from an inner diameter of the second magnetic pole piece.

7. The compound lens of claim 1 , wherein the first magnetic pole piece is a third electrode of the electrostatic lens and the second magnetic pole piece is a fourth electrode of the electrostatic lens.

8. The compound lens of claim 2 , wherein the electrostatic lens further comprises a third electrode between the first electrode and the second electrode along the optical axis.

9. The compound lens of claim 8 , wherein the third electrode is provided in the gap.

10. The compound lens of claim 9 , wherein the third electrode is provided completely within the gap.

11. The compound lens of claim 8 , wherein the third electrode has an inner diameter smaller than an inner diameter of the first magnetic pole piece and an inner diameter of the second magnetic pole piece.

12. The compound lens of claim 8 , wherein the third electrode covers inner surfaces of the first magnetic pole piece and the second magnetic pole piece facing the optical axis.

13. A charged particle optical system comprising:

a beam separator provided on a first optical axis, the beam separator configured to separate a plurality of beamlets of a primary charged particle beam generated by a source from a plurality of secondary charged particle beams emitted from a sample in response to illuminations of the plurality of beamlets, wherein the plurality of secondary charged particle beams travel along a second optical axis after passing through the beam separator;

a secondary imaging system configured to focus the plurality of secondary charged particle beams onto a detector along the second optical axis, wherein

the secondary imaging system includes an electromagnetic compound lens comprising:

an electrostatic lens provided on the second optical axis; and

a magnetic lens provided on the second optical axis, wherein the magnetic lens includes an annular permanent magnet.

14. A method to configure an electromagnetic compound lens with an optical axis, comprising:

forming a magnetic lens with a permanent magnet;

using two magnetic pole pieces to direct a magnetic field of the magnetic lens toward the optical axis, the two magnetic pole pieces being magnetically coupled to opposite magnetic poles of the permanent magnet; and

forming an electrostatic lens with two end electrodes surrounding the two magnetic pole pieces along the optical axis,

wherein a first dimension of a first end electrode of the two end electrodes in a direction perpendicular to the optical axis is greater than a second dimension of the first end electrode of the two end electrodes in a direction parallel to the optical axis.

15. The compound lens of claim 1 , wherein the electromagnetic compound lens is configured to operate in a positive mode wherein one or both of the first electrode and the second electrode are set to a first potential and one or both of the first magnetic pole piece and the second magnetic pole piece are set to a second potential higher than the first potential.

16. The compound lens of claim 15 , wherein the first electrode and the second electrode are set to the first potential.

17. The charged particle optical system of claim 13 , wherein the electromagnetic compound lens includes:

a first magnetic pole piece and a second magnetic pole piece, the annular permanent magnet being between the first magnetic pole piece and the second magnetic pole piece along the second optical axis, the first magnetic pole piece and the second magnetic pole piece being magnetically coupled to opposite magnetic poles of the permanent magnet; and

a first electrode and a second electrode, wherein the annular permanent magnet, the first magnetic pole piece and the second magnetic pole piece are between the first electrode and the second electrode along the second optical axis.

18. The method of claim 14 , further comprising:

operating the electrostatic lens in a positive mode wherein one or both of the end electrodes are set to a first potential and one or both of the magnetic pole pieces are set to a second potential higher than the first potential.

19. The method of claim 18 , wherein the first potential is higher than ground potential.