Scanning electron microscope and method of detecting electrons therein

A method of detecting electrons produced from a specimen in a scanning electron microscope having an inner polepiece, an outer polepiece, an objective lens for producing a magnetic lens field leaking onto a surface of a specimen located below lower end surfaces of the polepieces, a first opening formed in the inner polepiece above the lower end surface of the inner polepiece, and a secondary electron detector for detecting secondary electrons passed through the first opening, said detector being located outside said inner polepiece, said objective lens acting to focus an electron beam onto the specimen, said method comprising the steps of:

• applying a negative potential to the specimen to form a decelerating electric field near the surface of the specimen to decelerate the electron beam;
• scanning the specimen with the electron beam under this condition;
• confining electrons emitted from the specimen by said magnetic lens field;
• causing the confined electrons to move upward into a location where the magnetic lens field is weak;
• causing the electrons having relatively high energies to move toward surroundings from said location;
• causing the electrons moving toward the surroundings to impinge on a conversion electrode located around an electron beam passage inside said inner polepiece;
• guiding secondary electrons having relatively low energies produced from said conversion electrode to said secondary electron detector via said first opening;
• and detecting the secondary electrons by said secondary electron detector.