Multi-axis magnetic lens for focusing a plurality of charged particle beams

A multi-axis magnetic lens, comprising:

• an upper magnetic conductor plate, comprising:
• a first upper through round hole;
• and
• a second upper through round hole;
• a lower magnetic conductor plate, planar and parallel to said upper magnetic conductor plate, comprising:
• a first lower through round hole, coaxial with said first upper through round hole;
• and
• a second lower through round hole, coaxial with said second upper through round hole;
• a first upper magnetic ring, positioned inside and coaxial with said first upper through round hole, wherein a first upper radial gap is between said upper magnetic conductor plate and said first upper magnetic ring;
• a first lower magnetic ring, positioned inside and coaxial with said first lower through round hole, wherein a first lower radial gap is between said lower magnetic conductor plate and said first lower magnetic ring;
• a second upper magnetic ring, inside and coaxial with said second upper through round hole;
• a second upper radial gap, between said upper magnetic conductor plate and said second upper magnetic ring;
• a second lower magnetic ring, inside and coaxial with said second lower through round hole;
• a second lower radial gap, between said lower magnetic conductor plate and said second lower magnetic ring;
• and
• a common excitation coil, located between said upper magnetic conductor plate and said lower magnetic conductor plate;
• wherein said first upper through round hole, said first upper magnetic ring, said first upper radial gap, said first lower round hole, said first lower magnetic ring and said first lower radial gap function as a first magnetic sub-lens module for focusing a first charged particle beam;
• wherein said second upper through round hole, said second upper magnetic ring, said second upper radial gap, said second lower round hole, said second lower magnetic ring and said second lower radial gap function as a second magnetic sub-lens module for focusing a second charged particle beam;
• wherein said common excitation coil provides magnetic flux to said first magnetic sub-lens module and said second magnetic sub-lens module;
• wherein one of said first upper radial gap and said first lower radial gap comprises a first annular multilayer.

A permeability-discontinuity unit, comprising:

• a first plurality of annular layers, comprising at least one weakly-magnetic annular layer;
• and
• a second plurality of annular layers, comprising at least one magnetic annular layer;
• wherein one magnetic annular layer is immediately enclosed by one weakly-magnetic annular layer and/or immediately encloses one weakly-magnetic annular layer, and the innermost annular layer is a magnetic annular layer,
• wherein permeability of each weakly-magnetic annular layer is smaller than that of each magnetic annular layer which adjoins said weakly-magnetic annular layer.

A permeability-discontinuity unit, comprising:

• a plurality of first annular layers with a first permeability range;
• and
• a plurality of second annular layers with a second permeability range, wherein said second permeability range is higher than said first permeability range;
• wherein each of said plurality of first annular layers is interleaved within said plurality of second annular layers, and an innermost annular layer is one of said second annular layers.