Particle-optical apparatus comprising a detector for secondary electrons

A particle-optical apparatus comprising:

• a particle source for producing a primary beam of electrically charged particles which propagate along an optical axis of the apparatus;
• means for deflecting the beam to cause it to scan a specimen to be examined;
• focusing means for bringing the beam into focus in the vicinity of said specimen, said focusing means being a combination of a gap lens and a monopole lens having lens fields which are shifted relative to one another along said optical axis;
• a detection device for detecting electrically charged particles originating from the specimen, at least a part of said detection device being situated within said focusing means;
• a screening plate between the gap lens and the monopole lens for establishing a substantially field-free space between the lens fields thereof;
• and
• said part of the detection device which is situated within said focusing means is also situated within said field-free space.