Patent ID: 8921803

Claim:
A system, comprising: an electrostatic lens positioned in a path between a charged-particle source and a charged-particle detector; the electrostatic lens comprising: a first electrode having a first aperture positioned in the path and aligned with a first axis; a second electrode positioned in the path between the first electrode and the charged-particle detector, the second electrode having a second aperture positioned in the path and aligned with a second axis, the second axis being parallel to the first axis and displaced from the first axis along a first direction; a third electrode positioned in the path between the first electrode and the second electrode, the third electrode extends between the first electrode and the second electrode, the third electrode comprising a tubular electrode having a first end and a second end, a cross section of the first end is identical in shape to a cross section of the second end; and a potential generator coupled to the first, second, and third electrodes, wherein during operation; the potential generator applies a first potential to the first electrode, a second potential to the second electrode, and a third potential to the third electrode so that the electrostatic lens directs a beam of charged particles from the charged-particle source propagating along the first axis to propagate along the second axis, the potential generator applies the first potential to the first electrode, the second potential to the second electrode, and the third potential to the third electrode so that the beam of charged particles converges, at a first location along the second axis, to a first minimum beam cross section measured along both the first direction and a direction perpendicular to the second axis, and the beam of charged particles converges, at a second location along the second axis, to a second minimum beam cross section measured along both the direction perpendicular to the second axis and a direction perpendicular to the first direction, the first location and the second location are outside the electrostatic lens.