Electron beam energy filter

An energy filter consisting of at least one magnet assembly and mounted within a vacuum created inside an electron microscope, said energy filter arranged to receive incident electrons including electrons having a given energy, said energy filter acting to pass the electrons having said given energy, said energy filter comprising:

• said magnet assembly having a pair of polepiece bases having large planar surfaces;
• each of said polepiece bases being provided with a coil groove recessed from said large planar facing surfaces thereof;
• portions of said pole piece bases surrounded by the coil grooves in said polepiece bases defining polepieces, respectively, such that each portion defines one polepiece;
• coils received in said coil grooves, respectively;
• said polepiece bases being so positioned that said polepieces are located opposite to each other;
• first and second nonmagnetic spacers each having a large planar surface;
• a yoke mounted to side surfaces of said polepiece bases;
• sealing grooves formed in said spacers recessed from said large planar surfaces of said spacers, respectively, to hermetically seal said coils in said coil grooves in said polepiece bases;
• said first and second spacers being overlapped, abutting upon each other, and inserted between said polepiece bases such that planar surfaces of said spacers abut planar surfaces of said polepieces except at the location of coil and sealing grooves;
• at least one electron passage gap formed between said spacers to form an electron passage;
• and
• sealing members received in said grooves, respectively, to permit said coils to be located outside said vacuum.