Patent Number: 048067679
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, the invention will be described in detail in conjunction with an illustrative embodiment by referring to FIGS. 3 and 4. FIG. 3 is a sectional view of an electron lens assembly according to an embodiment of the invention. Parenthetically, it is to be noted that like elements or parts are attached with like reference characters throughout all the drawings, and description is not repeated, unless it is necessary. Referring to FIG. 3, a lower yoke member 2 and an electron beam passage defining pipe 9' are welded together by way of a non-magnetic member (interconnecting member) 20 by Heliarc or the like welding at portions where the yoke member 2 and the pipe 9' are located close to each other, as indicated by 21 and 22, whereby the lower yoke member 2 and the pipe 9' are implemented in an integral structure. The non-magnetic member 20 may be implemented in a sufficient thickness to prevent penetration of gases therethrough (e.g. thickness of about 1 to 1.5 mm). On the other hand, an O-ring 13' which is disposed between the lower yoke member 2 and the spacer 10' for the purpose of realizing a vacuum tight sealing is constituted by a metal O-ring. In this connection, it should be mentioned that when a metal gasket such as of copper, aluminum or the like is employed, a clamping force of very great magnitude is required and besides an additional clamping must be performed later on in view of poor resiliency. In contrast, the metal O-ring exhibits a high resiliency and requires no additional clamping, to advantage. FIG. 4 shows schematically in a fragmental perspective view a exemplary structure of the metal O-ring suited advantageously for the use in the electron lens assembly according to the present invention. As will be seen in the figure, the metal O-ring is constituted by a cylindrical pipe 28 of aluminum having a slot-like cut-out of notch extending longitudinally of the pipe. A helical spring 29 of stainless steel is disposed within the cylindrical pipe 28. (Parenthetically, this type of metal O-ring is commercially available from Cefilac Company of France and Usui Kokusai Sangyo K.K. of Japan under designation "Helicoflex".) Turning back to FIG. 3, reference numeral 24 denotes a metal flanged pipe, and 11" denotes a transverse bore for mounting the objective aperture, cold fingers and others. In the case of the illustrative embodiment under consideration, the objective aperture or other member is mounted on the flange of the flanged pipe 24 with an O-ring or the like being interposed while the metal flanged pipe 24 is welded to the spacer 10 by Heliarc or the like welding as indicated by 25 for maintaining the vacuum. An O-ring 12' for sealing vacuum tightly the connection between the upper yoke member 1 and a portion of the column (not shown) disposed on the yoke member 1 is also constituted by a metal O-ring similarly to the O-ring 13'. On the other hand, the electron beam passage defining pipe 9' is realized in an integral structure with the lower yoke member 2. In that case, if the pipe 9' projects beyond the bottom surface of the lower yoke member 2, inconvenience will be involved in the handling. Accordingly, in the case of the illustrative embodiment, the electron beam passage defining pipe 9' is so implemented as not to project beyond the bottom surface of the lower yoke member 2, wherein the lower end of the pipe 9' is coupled to a lower spacer 27 with an O-ring 26 being interposed therebetween. For the purpose of degassing by heating, there is disposed a heater 18 at a location adjacent to the pipe 9', while another heater 19 is disposed on the bottom side of the spacer 10, as shown in FIG. 3. Additionally, cooling pipes 23 and 23' are disposed above and below the coil 3 for dissipating heat generated by the coil 3 when it is electrically energized. With the structure described above, the vacuum state within the specimen chamber in which a specimen 8 is disposed can be sustained satisfactorily by means of the welds formed by Heliarc or the like welding and the metal O-rings. Since the metal O-ring can well withstand a high temperature of 200.degree. C. or higher, it is possible to produce the vacuum on the order of 10.sup.-10 Torr. It should be mentioned that a Xe-lamp or the like may be disposed within the space adjacent to the spacer 10 for degassing purpose in addition to the degassing heaters 18 and 19. Further, the Heliarc welding for realizing the vacuum tight seal may be replaced by other method such as oven blazing or other. In particular, the bonding between the non-magnetic member 20 and the lower yoke member 2 can be advantageously realized by the oven blazing method according to which a blazing material such as silver or the like is disposed in a ring-like pattern on the portions to be bonded and heated subsequently within an oven filled with an inert gas such as argon or the like. In a modification, the electron beam passage defining pipe 9' may be formed integrally with the non-magnetic member 20 and welded to the lower yoke member 2 at the portion indicated by 22. Alternatively, the electron beam passage defining pipe 9' may be directly welded to the lower yoke 2 without resorting to the use of the interconnecting non-magnetic member 20. The electron lens assembly of the structure described above can well withstand the heating at a temperature of about 200.degree. C. and allows the ultra-high vacuum to be easily produced.