The present invention relates to a magnetic field immersion type electron gun for controlling electrons emitted from an electron gun by an electric field lens and further by a magnetic field lens formed by ion pump magnets of a coaxial ion pump.
In the electron gun used for an electron microscope, a large intensity is required for an electron beam emitted from the electron gun, and further the electron beam is required to be controlled by a lens of a small spherical aberration. Although the electron beam emitted from the electron gun is usually controlled by an electric field lens, it has been also known that when a magnetic field lens is superimposed upon the electric field lens, it is possible to reduce the spherical aberration effectively.
As the magnetic field lens as described above, a magnetic field formed by permanent magnets of an ion pump is usually used. The electron gun provided with the magnetic field lens as described above is referred to as a magnetic field immersion type electron gun. The ion pump of the magnetic field immersion type electron gun is a vacuum pump for evacuating a predetermined vessel of an apparatus (e.g., an electron microscope) to which an electron gun is attached.
In the ion pump, electrons are emitted from a cathode thereof against particles within the vacuum vessel for ionization, and the ionized charged particles are trapped on an anode of the ion pump as getter to evacuate the vessel. In the above-mentioned ion pump, the charged particles in the vessel are moved as by a cyclotron based upon the magnetic field of the permanent magnets, so that the frequency of collisions can be increased effectively to obtain a high vacuum. As the ion pump as described above, a coaxial ion pump disposed coaxially with the optical axis of the electron gun is widely used.
In the case where the coaxial ion pump is used, a magnetic field lens is formed by the utilization of the magnetic field generated by the permanent magnets of the coaxial ion pump, and the formed magnetic field lens is superimposed upon the cathode of the electron gun or the electric field lens to construct the magnetic field immersion type electron gun. In this case, it is possible to improve the aberration of the electron gun of field emission type, in particular.
Accordingly, the prior art techniques related to the present invention are both the magnetic field immersion type electron gun and the coaxial ion pump.
In the field emission type electron gun, an electron beam emitted at a large solid angle from an apex of emitter cathode must be converged by an emitter cathode. The electron beam is converged conventionally by use of an electric field lens. However, there exists such a drawback that the diameter of the beam increases due to the spherical aberration of the electric field lens and thereby the average intensity of the electron beam tends to be lowered.
To overcome this problem, a method of obtaining a complex lens of both the magnetic field lens and the electric field lens by superimposing a magnetic field upon the electron gun lens formed by an electrostatic field lens or by replacing a part of the electron gun lens with a magnetic field lens has been so far proposed, which is referred to as a magnetic field immersion type electron gun (e.g., M. Troyon: High current efficiency field emission gun system incorporating a pre-accelerator lens. Its use in CTEM. Optik, 57, 401 (1980)). In this document, Troyon has improved the current density by about 6 times by replacing a first anode of a three-electrode electron gun with a magnetic field lens generated by an electromagnet. In this case, the magnetic field lens must be positioned within the electron gun chamber. However, in order to operate the electric field emission type electron gun stably, since a baking (at 200.degree. to 300.degree. C.) is required to evacuate the electron gun chamber less than an ultra-high vacuum of 10.sup.-9 Torr, the magnetic field lens must be proof against the baking temperature. However, it is practically difficult to form the magnetic field lens so as to be proof against this high baking temperature.
To solve this problem, a relatively practical structure of the magnetic field immersion type electron gun has been proposed, in which an electromagnet is disposed outside the electron gun chamber so that the magnetic field can be applied in the vacuum vessel from the outside thereof (J. R. A. Cleaver; Field emission electron gun systems incorporating single-pole magnetic field lenses. Optik, 52,293 (1978/79). FIG. 4 shows a prior art structure proposed by Cleaver, in which a single pole magnetic field lens 5 is mounted on an electron gun chamber. The electron gun is of three electrode structure composed of a cathode 1, a extraction electrode (Wehnelt) 2, and an electron gun anode 3. A single pole magnetic field lens 8 is mounted on the upper wall of the vacuum vessel of the electron gun as a magnetic field lens. As another method of applying a magnetic field effectively from the outside of the electron gun chamber, A. Takaoka et al. have proposed a structure in which a magnetic field is superimposed from the side of the electron gun chamber (A. Takaoka et al.: Improvement of beam characteristics by superimposing a magnetic field on a field emission gun. J. Electron Microsc. 38, No. 2,83 (1989)).
The above-mentioned proposed method are of the type in which the magnetic field lens is formed by an electromagnet. In this method, however, since the vacuum system and the lens system are both provided as different systems from each other, the structure is not only complicated, but also the requirements are contradictory to each other as explained in further detail below, thus causing one of problems when the magnetic field immersion type electron gun cannot be put to practical use.
For instance, to realize high performance electron optical characteristics (mainly with respect to the aberration characteristics), it is preferable that the magnetic field intensity of the magnetic field lens superimposed upon the electric field lens is high, that is, the lens intensity is large. The best way of increasing the magnetic field intensity of the magnetic field lens is to place the electromagnet for forming the magnetic field lens near the electron gun. In this case, however, it is necessary to place the electromagnet for forming the magnetic field lens within the high vacuum of the electron gun chamber.
However, this method causes a reduction of the degree of vacuum due to gas emitted from the electromagnet. Or else, heat resistances of the various elements for constituting the electromagnet must be taken into account during the baking for evacuation, which are not preferable.
When a high intensity magnetic field is required to be applied from the magnetic field lens disposed outside the vacuum chamber, it is necessary to increase the exciting current and the number of windings (i.e., ampere-turns) of the electromagnet for forming the magnetic field lens, with the result that the size of the magnetic field lens is inevitably increased extremely.
The degree of vacuum required to operate the field emission type electron gun stably is 10.sup.-9 Torr in the case of the thermal electric field emission type and 10.sup.-10 to 10.sup.-11 Torr in the case of the cold cathode electric field emission type. To obtain the ultra-high vacuum as described above in a small sized structure, the use of a coaxial ion pump has been proposed (M. Miyoshi and Okumura U.S. Pat. No. 4,890,029. Electron beam apparatus including a plurality of ion pump blocks, Dec. 26, 1989, and M. Miyoshi and Okumura U.S. Pat. No. 5,021,702. Electron beam apparatus including a plurality of ion pump blocks, Jun. 4, 1991). In the coaxial ion pump as disclosed in these Patents, some permanent magnets for constructing the ion pump are arranged coaxially with the optical axis of the electron gun and the electric field lens in symmetry about the same optical axis, and further the evacuating portions of the electron gun and the ion pump are formed integral with each other.
FIGS. 5 and 6 are cross-sectional views showing the prior art coaxial ion pump. FIG. 5 shows a first example of the prior art coaxial ion pump. In a cylindrical outer casing 10 for constructing a vacuum vessel of the coaxial ion pump, an electron gun 11 is disposed at the central axis thereof, and further an evacuation operation portion (i.e., ion pump portion) 12 is disposed coaxially with the optical axis of the electron gun 11 so as to enclose the electron gun 11.
The electron gun 11 is provided with an electron gun body 13, a cathode 14 mounted on the lower tip portion of the electron gun body 13, and a hollow cylindrical anode 15 formed with a hole through which an electron beam B emitted from an apex portion (the lower end cathode 14) of the electron gun body 13 is passed. The electron gun lens portion, that is, the electric field lens is composed of the cathode 14 and the anode 15. The electron beam B emitted from the cathode 14 at a wide angle is converged by the electric field lens and then introduced into another apparatus arranged below.
The ion pump portion 12 is provided with a cylindrical inside permanent magnet 16 disposed coaxially with the central axis, an outside permanent magnet 17 also disposed coaxially with the central axis, and an ion getter portion 18 interposed between the inside permanent magnet 16 and the outside permanent magnet 17 also coaxially therewith. The inside permanent magnet 16 and the outside permanent magnet 17 generate a magnetic field in a radial direction of the outer casing 10, and the intensity of the magnetic field is about 1500 to 2000 gauss.
The ion getter portion 18 is composed of a cylindrical ion pump anode 19 disposed coaxially with the optical axis of the electron gun 11, and a plurality of cylindrical ion pump cathodes (gettering electrodes) 20 and 26 formed of titanium so disposed as to sandwich the ion pump anode 19 between both inside and outside thereof.
In the coaxial ion pump, since the electron beam b is self-shielded by the inside permanent magnet 16, it is possible to reduce the influence of the permanent magnets 16 and 17 upon the electron beam b. However, the fact that the electron beam b is self-shielded from the magnetic field of the inside permanent magnet 16 is not appropriate from the structural point of view when considering the object that the magnetic field must be positively superimposed upon the electron beam b emitted from the magnetic field immersion type electron gun 11.
FIG. 6 shows a second prior art example of the prior art coaxial ion pump. In this structure,.a plurality of coaxial ion pump anodes 31 are arranged around an electron gun 13, and a pair of annular ion pump cathodes (gettering cathodes) 32 are disposed so as to sandwich the ion pump anode 13 between both upper and lower sides of the ion pump anode 31. Further, outside a vacuum vessel 30, two cylindrical permanent magnets 33 and 34 are disposed on both the upper and lower sides of the ion pump cathodes 32, respectively, so that a magnetic field can be applied in parallel to the arrangement direction of the ion anode 31.
In the coaxial ion pump of this structure, it is possible to construct the magnet field immersion type electron gun by superimposing the magnetic field generated by the permanent magnets 33 and 34 upon the electric field lens of the electron gun from the structural point of view. The theoretical analysis results of a prototype electron gun of this type are explained in a paper (Y. Yemazaki, M. Miyoshi, T. Nagai and Okumura: Development of the field emission electron gun integrated in the sputter ion pump, J. Vac. Sci, Techno., B9(6), November/December 2967 (1991).
Further, FIGS. 7 to 9 show another prior art example of the magnetic field immersion type electron gun of the structure in which the coaxial ion pump and the field emission type (FE) electron gun are integrated with each other. FIG. 7 is a cross-sectional view showing the magnetic field immersion type electron gun of the structure, in which the coaxial ion pump and the electric field emission type (FE) electron gun are integrated with each other. FIG. 8 shows the distribution of the magnetic field intensity along the central axis of the magnetic field immersion type electron gun in correspondence to the shape of the magnetic field lens thereof; and FIG. 9 shows the magnetic fields generated by the permanent magnets 43 and 44.
The magnetic field immersion type electron gun shown in FIG. 7 is basically the same in structure as with the case shown in FIG. 6. In FIG. 7, two cylindrical permanent magnets 43 and 44 are disposed outside a vacuum vessel 40 under atmospheric pressure. To prevent the magnetic field from being leaked toward the outside and further, to form a closed magnetic circuit as perfect as possible, a malleable iron yoke 45 formed with a cylindrical hollow portion and formed into U-shape in cross section is provided in such a way that two permanent magnets 43 and 44 are attached to the upper inside surface and the lower inside surface of the yoke 45.
Within a vacuum vessel 40, a cylindrical ion pump anode 46 is disposed between the two permanent magnets 43 and 44. These permanent magnets 43 and 44, the ion pump anode 46, and the yoke 45 are all arranged coaxially with the central axis 49 of the electron gun body 47 and the electron gun lens 48. At the end of the electron gun 47, cathode 42 of the electron gun is attached to the electron gun 47.
In this structure, it is possible to obtain the magnetic field distribution as shown in FIG. 8 along the central axis thereof. However, there exists a problem in that the magnetic field intensity direction is reversed at points A; C and B, respectively.
The reason thereof will be explained with reference to FIG. 9. Here, the assumption is made that the two permanent magnets 43 and 44 are mounted upper and lower sides in such a way that the magnetic poles are arranged as S.fwdarw.N.fwdarw.S.fwdarw.N. Then, the magnetic field As directed from the permanent magnet 43 to the permanent magnet 44 near the ion pump anode 46, and further leaks (deviates) largely toward the central axis 49, as shown in FIG. 9. This leaked magnetic field forms the magnetic field at the middle peak point B (the maximum intensity at the middle height position along the central axis 49) in the magnetic field distribution shown in FIG. 8. On the other hand, in the vicinity of the edge portions 43a and 44a of the respective permanent magnets 44 and 43, leaked magnetic fields flow toward the yoke 45, respectively. The direction (upward) of this magnetic field is opposite to that (downward) of the magnetic field at the middle portion. Accordingly, there exists the opposite magnetic field intensity distribution having sub-peak points A and C both above and below the main peak B, as shown in FIG. 8.
In the structure as shown in FIG. 6 or 7 although the magnetic field distribution near the central axis as described above can be modified to some extent by design, it is impossible to basically eliminate the distribution as shown in FIG. 8.
In this case, the magnetic field at the middle main peak B or the lower sub-peak A is to be used as the magnetic field lens of the magnetic field immersion type electron gun. Here, it is preferable to use the main middle peak B when only the lens effect of the magnetic field lens is taken into account, because the magnetic field intensity is large. However, since this main peak B is located as the fairly inner side of the coaxial ion pump (at a central height position of the ion pump anode 46 on principle), it is rather difficult to layout the mechanical elements. Further, since the sub-peak A under the main peak B is superimposed upon the central (optical) axis 49 of the electron beam, two magnetic field lenses are eventually superimposed, so that there exists such a problem in that the analysis is complicated and thereby the design is difficult. Accordingly, in practice, the magnetic field immersion type lens is constructed by superimposing the lower sub-peak A upon the electron gun lens.
Further, in this method, since the magnetic energy of the two permanent magnets 43 and 44 is separated into three peaks A, Band C, so that the utilization efficiency of the magnetic energy is not high. This implies that in order to form a stronger magnetic field lens, a permanent magnet of unnecessarily high surface magnetic flux density (high cost) must be used or else the thickness of the permanent magnet cylinder must be increased to increase the surface magnetic flux density, thus causing another problem in that the weight and the size of the apparatus are inevitably increased.
Further, another prior art method of constructing an ion pump coaxially with the optical axis of the electron gun and further utilizing the magnetic field generated by a permanent magnet of the ion pump as the magnetic field lens is proposed by Wiesner (J. C. Wiesner et al. U.S. Pat. No. 4,397,611. Particle beam instrumentation ion pump, Aug. 9, 1983).
In this method, a pair of ring-shaped permanent magnets are arranged on both upper and lower sides coaxially with the electron gun in such a way as to be supported by a yoke disposed inside the permanent magnets (on the ion pump anode side sandwiched between a pair of the permanent magnets). A magnetic field is applied to a vacuum vessel through the yoke, and the ion pump anode is disposed within the vacuum vessel for vacuum evacuation. In this case, the magnetic field generated by a pair of upper and lower ring-shaped permanent magnets forms the magnetic field lens. The structure is similar to that shown in FIG. 7.
However, this prior art structure involves the following problems:
(1) Since the magnetic field is applied from the permanent magnets through the yoke to the ion pump anode portion at which the strongest magnetic field intensity is required (at which the actual evacuation is effected, and further the evacuation speed is proportional to the square of the magnetic field intensity), the effective magnetic field intensity is attenuated markedly or the magnetic force lines form a magnetic circuit in the yoke, with the result that the evacuation speed is lowered.
(2) Since the ring-shaped permanent magnets and the yoke are combined with each other, in the same-reason as explained with reference to FIGS. 8 and 9, a plurality of magnetic field peaks are inevitably formed around the central axis.
As explained above, the prior art technique related to the magnetic field immersion type electron gun involves the following problems:
(1) In the case of the method of forming the magnetic field lens in the magnetic field immersion type electron gun by the electromagnet of the coaxial ion pump, since a large magnetic field intensity is preferably required to improve the magnetic field lens, in order to increase the magnetic field intensity of the superimposed magnetic field lens, it is necessary to place the electromagnet of the coaxial ion pump near the electron gun or else to use a large electromagnet for generating a fairly strong magnetic field.
In this case, in order to place the electromagnet near the electron gun, the electromagnet must be disposed within the vacuum vessel. As a result, in order to operate the electric field emission type electron gun stably within an ultra-high vacuum, there arise various problems such as gas discharge, heat resistance during baking, etc.
Further, although a large electromagnet can be used when disposed outside the vacuum vessel, since the exciting current and the ampere-turns are both increase, the size of the magnetic field lens inevitably increases.
(2) The method of controllably superimposing the magnetic field of the permanent magnet in the coaxial ion pump upon the electric field lens of the electron gun, that is, the method of forming the magnetic field immersion type electron gun is the best method in practical use. In the prior art method so far proposed, however, since the unitization efficiency of the magnetic field of the permanent magnet is relatively low and further a plurality of magnetic field intensity peaks are generated, it is difficult to design the magnetic field lens. In addition, since the peak position of the maximum magnetic field intensity of the best performance is generated deep inside the ion pump, an electron gun lens of complicated structure must be arranged within a narrow space, so that the mechanical layout is markedly limited.