Gun lens design in a charged particle microscope

A method of operating a charged particle microscope comprising:

• providing a specimen on a specimen holder;
• using a source to produce a beam of charged particles;
• passing said beam through an illuminator, provided between said source and specimen holder, and comprising in a propagation direction of said beam:
• a source lens;
• and
• a condenser system;
• irradiating the specimen with the beam emerging from said illuminator;
• using a detector to detect radiation emanating from the specimen in response to said irradiation;
• wherein said source lens is a compound lens, comprising in said propagation direction:
• a magnetic lens, comprising permanent magnets disposed outside said vacuum enclosure but producing a magnetic field within it;
• and
• a variable electrostatic lens.

A method of swapping sources in a charged particle microscope, the charged particle microscope comprising:

• a vacuum enclosure;
• a source, for producing a beam of charged particles;
• a specimen holder, for holding a specimen;
• an illuminator, provided between said source and specimen holder, and comprising in a propagation direction of the beam:
• a source lens lens;
• and
• a condenser system;
• a detector, for detecting radiation emanating from the specimen in response to irradiation by said beam;
• a magnetic lens, comprising permanent magnets disposed outside said vacuum enclosure but producing a magnetic field within it;
• and
• a variable electrostatic lens,
• the method comprising:
• removing from the microscope a first assembly, which comprises a first source;
• and
• replacing said removed first assembly by a second assembly, which comprises a second source and a second implementation of said magnetic lens, while leaving said variable electrostatic lens in place.