Particle-optical apparatus with a permanent-magnetic lens and an electrostatic lens

A lens assembly for a particle optical apparatus, comprising:

• a magnetic lens including a permanent magnet for producing a magnetic field for focusing a charged particle beam;
• an electrostatic lens, the electrostatic lens producing as electric field for focusing the charged particle beam and altering its energy;
• the magnetic lens end the electrostatic lens being configured such that the focal position of the lens assembly remains substantially constant as the landing energy of the charged particle beam is varied.

A method of making a charged particle beam system, comprising:

• providing a magnetic lens including a permanent magnet to produce a magnetic field for focusing a charged particle beam;
• providing an electrostatic lens, the electrostatic lens producing an electric field for focusing the charged particle beam and altering its energy;
• configuring the magnetic lens and the electrostatic lens such that the focal position of a lens assembly remains substantially constant as the landing energy of the charged particle beam is altered by the electrostatic lens.

A method of operating a charged particle beam system, comprising:

• directing a beam of charged particles through a magnetic focusing field originating from a permanent magnet;
• directing the beam of charged particles through a focusing electrostatic field, the electric field increasing the energy of the charged particles, the electrostatic field and the magnetic field overlapping, the fields being configured such that the focal plane of the charged particle beam system remains substantially unchanged as the energy of the charged particle beam is altered.