Patent Number: 048067679
Section: abstract

An electron lens assembly comprises at least a pair of magnetic pole pieces disposed in opposition to each other and each having a bore allowing an electron beam to pass therethrough, an exciting coil for producing a magnetic field between the magnetic pole pieces, a yoke coupled to the magnetic pole pieces and constituted by two divided yoke members so that the exciting coil can be accommodated, at least one of the two yoke members being detachably coupled to one of the magnetic pole pieces, and a pipe disposed along the electron beam path for defining a passage for the electron beam except for a space formed between the magnetic pole pieces. A metal O-ring is disposed on a surface of the detachable yoke member so as to prevent the air from entering the space defined between the opposite magnetic pole pieces along the surface of the detachable yoke member from a space accommodating the exciting coil. The electron beam passage defining pipe is coupled integrally to the detachable yoke member. With the electron lens structure, an ultra-high vacuum can be sustained within the column.