Shielding strategy for mitigation of stray field for permanent magnet array

1. An inspection system comprising:

an array of electron beam columns, each comprising an electron source configured to emit electrons toward a stage;

a first permanent magnet array configured to condense the electrons from each electron source into an array of electron beams, wherein the first permanent magnet array is arranged at a first end of the array of electron beam columns; and

a plurality of shielding plates extending across the array of electron beam columns downstream of the first permanent magnet array in a direction of electron emission;

wherein the plurality of shielding plates each comprise a plurality of apertures and have a thickness between 1 μm and 1 mm, and the array of electron beams pass through the plurality of apertures; and

whereby the plurality of shielding plates reduce stray magnetic field in a radial direction of the array of electron beams.

2. The inspection system of claim 1 , further comprising:

a second permanent magnet array configured to focus the array of electron beams toward a target on the stage, wherein the second permanent magnet array is arranged at a second end of the array of electron beam columns.

3. The inspection system of claim 2 , wherein the plurality of shielding plates comprises:

a first shielding plate arranged proximate to the first permanent magnet array; and

a second shielding plate arranged proximate to the second permanent magnet array.

4. The inspection system of claim 3 , further comprising:

a detector configured to detect electrons reflected from the target, wherein the detector is arranged between the first permanent magnet array and the second permanent magnet array; and

wherein the plurality of shielding plates further comprises:

a third shielding plate arranged proximate to the detector.

5. The inspection system of claim 4 , wherein the plurality of shielding plates further comprises:

a fourth shielding plate arranged proximate to the detector, opposite to the third shielding plate.

6. The inspection system of claim 5 , wherein the plurality of shielding plates further comprises:

at least one supplemental shielding plate arranged between the first shielding plate and the second shielding plate.

7. The inspection system of claim 1 , wherein the plurality of shielding plates comprises at least seven shielding plates.

8. The inspection system of claim 1 , wherein at least one of the plurality of shielding plates is arranged where the stray magnetic field is at a highest magnitude in the radial direction of the array of electron beams.

9. The inspection system of claim 1 , wherein the thickness of each of the plurality of shielding plates is at least 120 μm.

10. The inspection system of claim 1 , wherein the plurality of shielding plates are comprised of a magnetic nickel-iron alloy.

11. A method of stray field mitigation applied to a multi-column inspection system including an array of electron beam columns, each electron beam column comprising an electron beam source configured to emit electrons toward a stage, the method comprising:

directing the electrons from each electron source through a first permanent magnet array to condense the electrons into an array of electron beams, wherein the first permanent magnet array is arranged at a first end of the array of electron beam columns; and

directing the array of electron beams through a plurality of shielding plates extending across the array of electron beam columns downstream of the first permanent magnet array;

wherein the plurality of shielding plates each comprise a plurality of apertures and have a thickness between 1 μm and 1 mm, and the array of electron beams pass through the plurality of apertures; and

whereby the plurality of shielding plates reduce stray magnetic field in a radial direction of the array of electron beams.

12. The method of claim 11 , further comprising:

directing the array of electron beams through a second permanent magnet array to focus the array of electron beams toward a target on the stage, wherein the second permanent magnet array is arranged at a second end of the array of electron beam columns, downstream of the plurality of shielding plates.

13. The method of claim 12 , wherein directing the array of electron beams through a plurality of shielding plates comprises:

directing the array of electron beams through a first shielding plate arranged proximate to the first permanent magnet array; and

directing the array of electron beams through a second shielding plate arranged proximate to the second permanent magnet array.

14. The method of claim 13 , further comprising:

directing the array of electron beams through a detector configured to detect electrons reflected from the target, wherein the detector is arranged between the first permanent magnet array and the second permanent magnet array; and

wherein directing the array of electron beams through a plurality of shielding plates further comprises:

directing the array of electron beams through a third shielding plate arranged proximate to the detector.

15. The method of claim 14 , wherein directing the array of electron beams through a plurality of shielding plates further comprises:

directing the array of electron beams through a fourth shielding plate arranged proximate to the detector, opposite to the third shielding plate.

16. The method of claim 15 , wherein directing the array of electron beams through a plurality of shielding plates further comprises:

directing the array of electron beams through at least one supplemental shielding plate arranged between the first shielding plate and the second shielding plate.

17. The method of claim 11 , wherein the plurality of shielding plates comprises at least seven shielding plates.

18. The method of claim 11 , wherein at least one of the plurality of shielding plates is arranged where the stray magnetic field is at a highest magnitude in the radial direction of the array of electron beams.

19. The method of claim 11 , wherein the thickness of each of the plurality of shielding plates is at least 120 μm.

20. The method of claim 11 , wherein the plurality of shielding plates are comprised of a magnetic nickel-iron alloy.