US3900808A

US3900808A - Magnetic lens assemblies for corpuscular ray devices which operate under vacuum

Info

Publication number: US3900808A
Application number: US648623A
Authority: US
Inventors: Helmut Zerbst; Reinhard Weyl; Isolde Dietrich
Original assignee: Individual
Current assignee: Individual
Priority date: 1966-09-21
Filing date: 1967-06-26
Publication date: 1975-08-19
Anticipated expiration: 1992-08-19

Abstract

A corpuscular ray device having a magnetic lens assembly. Although the invention is particularly applicable to objective lens assemblies of electron microscopes which operate under vacuum, it is also applicable to lens assemblies for ion microscopes, electron-diffraction devices, or other corpuscular ray apparatus wherein the rays are situated in tubular enclosures. The magnetic lens assembly includes a pair of shielding cylinders which are coaxial and spaced from each other, which are made of a superconductive material, and which have their common axis coinciding with a lens axis of the lens assembly, these shielding cylinders being surrounded by a lens winding which may also be made of a superconductive material and which serves to generate the magnetic field which is concentrated by the shielding cylinders in the region of the corpuscular ray. These shielding cylinders have directed toward each other a pair of end faces which are spaced from each other and define between themselves a lens gap which is devoid of any shielding components made of superconductive material. The aperture error constant of the lens assembly depends upon the maximum value of the field intensity in the lens gap and upon the field gradient along the lens axis in the lens gap. The magnitude of the lens gap is chosen in such a way that with a predetermined value of field intensity outwardly beyond the lens gap and the shielding cylinders the maximum value of the field intensity in the lens gap and the field gradient in the lens gap along the lens axis provide for the lens assembly an aperture error constant which is less than a predetermined value of aperture error constant.

Description

United States Patent 1 Zerbst et al.

[ MAGNETIC LENS ASSEMBLIES FOR CORPUSCULAR RAY DEVICES WHICH OPERATE UNDER VACUUM [75] Inventors: Helmut Zerbst; Reinhard Weyl;

[solde Dietrich, all of Munich, Germany [73] Assignee: Siemens Aktiengesellschaft,

Berlin and Munich. Germany [22] Filed: June 26, 1967 [2|] Appl. No.1 648,623

[30] Foreign Application Priority Data Sept, 2t. 1966 Germany 5105968 [52] US. Cl. 335/210; 250/396; 335/216 [5 1] Int. Cl. H01! 7/00 [58] FieldofSearch ..335/2l0,2ll,2l2,2l3, 335/214, 216; 250/495 [56} References Cited UNITED STATES PATENTS 3.008.044 ll/l96l Buchhold 335/210 X 135L754 l H1967 Dietrich et al 335/210 X Primary E.\uminer-George Harris Armrney, Agent, or Firm-Curt M. Avery; Arthur E. Wilfond; Herbert L. Lerner [57] ABSTRACT A corpuscular ray device having a magnetic lens assembly. Although the invention is particularly applica- 1 51 Aug. 19, 1975 ble to objective lens assemblies of electron microscopes which operate under vacuum, it is also applicable to lens assemblies for ion microscopes, electron diffraction devices. or other corpuscular ray apparatus wherein the rays are situated in tubular enclosures. The magnetic lens assembly includes a pair of shielding cylinders which are coaxial and spaced from each other, which are made of a superconductive material. and which have their common axis coinciding with a lens axis of the lens assembly, these shielding cylinders being surrounded by a lens winding which may also be made of a superconductive material and which serves to generate the magnetic field which is concentrated by the shielding cylinders in the region of the corpuscular ray. These shielding cylinders have directed toward each other a pair of end faces which are spaced from each other and define between themselves a lens gap which is devoid of any shielding components made of superconductive material. The aperture error constant of the lens assembly depends upon the maximum value of the field intensity in the lens gap and upon the field gradient along the lens axis in the lens gap. The magnitude of the lens gap is chosen in such a way that with a predetermined value of field intensity outwardly beyond the lens gap and the shielding cylinders the maximum value of the field intensity in the lens gap and the field gradient in the lens gap along the lens axis provide for the lens assembly an aperture error constant which is less than a predetermined value of aperture error constant.

12 Claims, l2 Drawing Figures PATENTEDAUGISIQTS 3800808 Fig.1

III

9. CD b (11G) Fig.2

Fig.3

PATENTED AUG 1 9 I975 saw u 0F 4 Fig. 12

MAGNETIC LENS ASSEMBLIES FOR CORPUSCULAR RAY DEVICES WHICH OPERATE UNDER VACUUM Our invention relates to electromagnetic lens assemblies for corpuscular ray devices, and in particular to objective lens assemblies for electron microscopes.

Our invention in fact is a further development of the invention disclosed in copending US. Pat. application Ser. No. 389,089, filed Aug. 12, 1964 now U.S. Pat No. 3,351,754.

Thus, it has already been proposed to provide a lens assembly which includes along the lens axis a pair of coaxial mutually spaced shielding cylinders made of superconductive material and having their common axis coinciding with the lens axis, these shielding cylinders being thermally connected with a cryogenic refrigerating medium. The shielding cylinders which are thus spaced from each other and range one after the other along the lens axis which coincides with the common axis of the shielding cylinders serve to concentrate in the region of the corpuscular ray a magnetic field which is generated by a suitable number of lens windings also made preferably of superconductive material and through which current flows.

In this proposed construction the two shielding cylinders, the end faces of which are directed toward each other and spaced from each other, form parts of a magnetic shielding structure which has generally spoken the shape of a hollow ring and contains in its hollow space the lens windings. This magnetic shielding structure is madeof superconductive material and has its inner wall which is situated adjacent the corpuscular beam divided in two shielding cylinders by a ringshaped opening lying in a plane perpendicular to the beam. This ring-shaped opening serves to accommodate an apertured disc which is superconducting and which has a substantially central aperture provided for the corpuscular beam. Because of the presence of this ring-shaped opening in the inner wall of the magnetic shielding structure said inner wall consists of a pair of shielding cylinders which are spaced from each other along the lens axis and which have end faces spaced from and directed toward each other.

It is a primary object of our invention to improve the structure at the region of the lens gap.

In particular, it is an object of our invention to provide a construction where the space between the end faces of the shielding cylinders which define the lens gap is completely devoid of any components made of superconductive material.

Also, it is an object of our invention to provide a construction which will reliably maintain the aperture error constant below a predetermined value.

In addition, it is an object of our invention to provide shielding cylinders with constructions at their mutually spaced ends which define the lens gap which will greatly enhance the formation of a desired magnetic field.

Thus, our invention relates primarily to a particular configuration of the arrangement defined by the shielding cylinders which are made of the superconductive material. With the lens assembly of our invention the pair of end faces of the coaxial shielding cylinders which are directed toward each other and which define between themselves the lens gap by limiting the ends of the lens gap coact with a lens gap which is completely devoid of any shielding elements made of superconductive material. The magnitude of the lens gap is chosen in such a way that at a predetermined value of the field intensity outwardly beyond the lens gap and the shielding cylinders, the maximum value of the field intensity in the lens gap and the field gradient in the lens gap along the lens axis provide an aperture error constant of the lens assembly which is less than a predetermined value of this latter constant.

With the already proposed construction of the type referred to above, the shielding cylinders created by the ring-shaped opening in the ring-shaped shielding structure serve only to prevent the formation of the field in the region of the corpuscular ray beyond a predetermined location while a further superconducting element in the form of the above-mentioned apertured disc serves to define this predetermined location for the influencing of the corpuscular beam by the magnetic lens field. In contrast, however, with our invention the construction of the shielding cylinders is such that the lens gap and the shape of the magnetic field within the lens gap are determined solely by the two shielding cylinders. The functions performed by an apertured disc of superconductive material of the type referred to above are instead taken over by the two shielding cylinders themselves, since it is these shielding cylinders of our invention which determine the field relationships at the corpuscular ray.

As a result, the construction of the lens assembly of our invention is simpler. At least by suitably dividing the lens windings it is possible to have easy access to the lens gap. Moreover, where an apertured disc of superconductive material is used, as in the already proposed construction, it is essential to refrigerate the disc with a cryogenic refrigerating medium which is operatively connected to the disc from the outside of the assembly, so that it is possible to collect the field lines in the region of the corpuscular ray, and of course all of these inconveniences and complications are avoided with our construction.

Superconductive materials are used in the electromagnetic lens assemblies because the aperture error of the lens assembly which influences the resolving power of the lens or corpuscular ray device is maintained smaller the larger the magnetic field intensity in the region of the corpuscular ray and the smaller the length of this field-filled region. With conventional electromagnetic lenses which operate at normal temperatures and which are provided with iron components for the return flux path as well as pole shoes, these requirements, which actually conflict with each other in these conventional constructions, cannot be fulfilled to a degree sufficient for high resolutions because of the magnetic properties of the iron. in the event that it is desired to achieve high magnetic field intensities in the region of the corpuscular ray, with such conventional constructions, then in order to avoid saturation of the iron large cross sections of the iron are essential, so that the axial length of that region in which the magnetic lens field acts on the corpuscular ray is undesirably enlarged.

In contrast, however, when superconducting materials are used it is possible to achieve much higher magnetic field intensities without increasing the axial length of the region where the magnet field acts on the corpuscular ray to any appreciable degree.

The aperture error constant C, of an electromagnetic lens assembly depends not only on the maximum value H, of the field intensity in the lens gap, which is to say the region where the field acts on the corpuscular ray, but also on the field gradient along the lens axis in the lens gap.

If an at least approximately bell-shaped field is taken as an illustrative example, then the field gradient for a given maximum value I-I of field intensity in the gap has its magnitude determined by the width at one half the maximum intensity.

Our invention is illustrated by way of example in the accompanying drawings which form part of this application and in which:

FIG. 1 is a graphic illustration of the relationship between field intensity and width;

FIG. 2 is a schematic sectional elevation of one possible embodiment of a structure of our invention;

FIG. 3 is a sectional elevation of another embodiment of a structure of our invention;

FIGS. 4-9 respectively illustrate different configurations at the inner ends of the shielding cylinders; and

FIG. is a longitudinal sectional elevation schematically illustrating yet another embodiment of a construction according to our invention.

FIGS. 11 and 12 show in diametrical section two further embodiments of shielding cylinders according to the invention.

Thus, referring to FIG. 1, it will be seen that the width 2d at half the maximum field intensity is shown in the graph of FIG. 1. Thus, FIG. 1 shows the horizontal coordinate z which corresponds to the lens axis and which has its starting point 0 forming the central plane of the lens gap, while the behavior of the field intensity I-I/I-I, in the gap, normalized on the maximum value H is indicated along the lens axis in FIG. 1. The width at half the maximum value of field intensity 2d is to be understood, by definition, as the field width where H/H is equal to 0.5.

It is apparent, therefore, that the width at half maximum field intensity depends upon the dimensions of the pair of shielding cylinders used according to our invention to form the lens gap, at the region of the end faces of these cylinders which are directed toward each other. This dependency applies not only with respect to the distance between the pair of shielding cylinders, which is to say the length of the gap, but also with respect to the configuration of the shielding cylinders in the region of their end faces which are directed toward each other. Since the shielding cylinders operate in such a way that over their length they prevent the approach of the electromagnetic lens field to the corpuscular ray, while at the same time they are also to solve the task of the above-mentioned apertured disc of superconductive material, that is to provide for entrance of the magnetic field to the largest possible extent into the lens gap, a preferred construction of our invention provides for the end faces of the shielding cylinders configurations which conform to the course taken by the field lines. In this way high values of field intensity at critical locations of the material are avoided so as to also avoid the danger of flux jumping. Moreover, it is possible to provide for the shielding cylinders end faces which are formed differently from each other so that the field is not symmetrical in the gap.

For the same reasons a further construction of the shielding cylinders of our invention provides for the bore of the shielding cylinders through which the corpuscular ray travels a diameter which at least at the lens gap region is as small as possible. By reducing the crosssectional area of the bore of the shielding cylinders through which the corpuscular ray passes, at least in the region of the lens gap, this size of the bore being reduced to the minimum value required for passage of the corpuscular ray, a very sharply defined guiding of the magnetic field in the lens gap and in particular in the region immediately surrounding the corpuscular ray with a definite field gradient is assured.

As has already been pointed out above, the aperture error constant C of a lens assembly is very strongly influenced by the maximum value H, of the magnetic field intensity in the lens gap. It is therefore also a feature of our invention to situate in the lens gap additional field-generating lens windings so as to increase the maximum value H, of the field intensity in the lens gap, these additional windings thus augmenting the lens windings which surround the shielding cylinders which define the lens gap. Such additional windings may also be provided to contribute to a predetermined field configuration, to reduce hysteresis phenomena, or to make it possible to change the focal length of the lens by generating a counter-field.

As has already been pointed out above, the shielding cylinders prevent formation of the magnetic field in the vicinity of the corpuscular ray, except for the region of the lens gap. For these reasons the length of the shielding cylinders is such that their ends which are directed away from each other preferably extend into a region of negligible field intensity.

It may be desirable, however, in many cases, for the purpose of saving space within the vacuum chamber of the corpuscular ray device, for example, to connect to the pair of shielding cylinders further shielding structure made of superconductive material, so as to reduce the extension of the magnetic field generated by the lens windings. Preferably, these additional shielding structures take the form of shielding structures which surround at all sides the surfaces of the lens windings which are arranged around the shielding cylinders except the surfaces being opposite the shielding cylinders. This shielding structure is thus comparable to the ringshaped shielding of the already proposed construction.

In order to avoid a considerable increase in the length of the field-filled region of the corpuscular ray at the particular field intensity which is utilized, the shielding action of the pair of shielding cylinders is required. Therefore, it is necessary for these shielding cylinders to have a thickness selected according to the desired field intensity.

Sintered Nb Sn has proved to be suitable as a material to be used for the shielding cylinders and also for any further shielding structure. However, where the structure has relatively thick walls for the shielding cylinders which are made of this sintered material, in order to avoid flux jumping, care must be taken to provide for the magnetic field intensity Fl, outwardly beyond the shielding cylinders and the gap a magnitude which increases at only a relatively slow rate after the structure is put in operation. It is possible, however, to avoid this factor which reduces the speed of operation with a corpuscular ray device which has this type of lens assembly, if porous sintered material is used or if the shielding cylinders and also in given cases the further shielding structure take the form of preferably disc-shaped carrier bodies made of heat-resistant material and coated with Nb Sn, this heat-resistant material being, for example, Nb, Pt or heat-resistant steel.

The superconductive materials can be used in connection with materials of normal electrical and thermal conductivity by arranging the superconducting and normal conducting components one after the other in alternating sequence.

FIGS. 2-10 illustrate the more important features of different embodiments of lens assmeblies according to our invention.

In the embodiments of our invention illustrated in FIGS. 2 and 3 there is only a single field-generating lens winding 1. This lens winding 1 surrounds the shielding cylinders 2a and 3a of FIG. 2 which have a common axis coinciding with and extending along the coordinate 2 which represents the lens axis. The shielding cylinders 2a and 3a are arranged one after the other along the lens axis and are spaced from each other so that they have end faces 4 and 5, respectively, which are spaced from and directed toward each other and which form between themselves the lens gap S shown in FIG. 2. The pair of shielding cylinders 2a and 3a of superconductive material thus prevent the formation of the magnetic lens field generated by the winding 1 in the region of the lens axis 2, and thus in the region of the corpuscular ray, except for the space which forms the gap S, and at this latter gap the shielding cylinders provide for the formation of a field of the greatest possible intensity.

It will be seen that the structure of FIG. 3 corresponds generally to that of FIG. 2. However, in order to facilitate the formation of the field in the region of the corpuscular ray within the lens gap S, FIG. 3 shows a construction provided, as contrasted with that of FIG. 2, with shielding cylinders 2b and 3b which have end faces 6 and 7, respectively, directed toward and spaced from each other. These end faces 6 and 7 respectively have configurations which conform to the course taken by the field lines within the lens gap.

Furthermore, the embodiments of FIGS. 2 and 3 differ from each other in that the shielding cylinders 2b and 3b of FIG. 3 have a wall thickness which is approximately four times as great as the wall thickness of the shielding cylinders 2a and 3a of FIG. 2. Experience has shown that with this construction of FIG. 3 it is possible to have outside of the shielding cylinders a field intensity H, which is double that which is possible with FIG. 2 without fearing any undesired increase in the length of the field-filled region along the corpuscular ray.

Of course, insofar as the end faces of the shielding cylinders which are directed away from each other may happen to still be situated in the regions of relatively high magnetic field intensity, it is possible either to increase the length of the shielding cylinders or to provide additional shielding, and it is possible to increase the length so that the outer ends of the cylinders which are directed away from each other are situated at regions of negligible field intensity. In some cases it is possible, particularly at very high field intensities, to resort to both of these measures, namely increasing the length of the shielding cylinders and providing additional shielding structure.

It will be noted that with the embodiment of FIG. 3, the end faces 6 and 7 of the shielding cylinders 21: and 3b, respectively, have a configuration which conforms to the course taken by the field lines in the lens gap S.

FIGS. 4-9 illustrate various possible constructions of this type which have proved in practice to be highly favorable.

Thus, FIG. 4 shows the shielding cylinder 2!; of FIG. 3 and in particular the end face 6 thereof provided with the bevelled frustoconical exterior surface portions 60, as is also illustrated in FIG. 3, so that in this way the end face 6 will conform to the configuration of the field lines. Of course, the other shielding cylinder has its end face 7 constructed in the same way, and in FIGS. 5-9 it is to be understood that the illustrated shielding cylinders coact with identical shielding cylinders which have oppositely directed end faces.

Thus, in the case of FIG. 5 the shielding cylinder 2c is provided with an end face having a frustoconical surface 6b forming part ofa cone whose apex angle is substantially smaller than the apex angle of a cone which includes the surface 6a of FIG. 4. Particularly in those cases where the field intensity H is relatively high outside of the gap, this construction of FIG. 5 results in an increase in the maximum value H of the field intensity. However, this latter advantage is achieved only with a small increase in the length of the field-filled region along the corpuscular ray.

In the embodiment of FIG. 6, the shielding cylinder 2d has a pair of exterior frustoconical surface portions 60 and 6d which have different angles so as to conform to an even greater degree to the course taken by the field lines. In the embodiment of FIG. 7, the shielding cylinder 2e has a rounded convex end face 6f surrounding the bore of the shielding cylinder and situated at the end of a frustoconical surface 6e which conforms generally to the surface 6b of FIG. 5.

On the other hand, in the embodiment of FIG. 8, the end face 63 of the shielding cylinder 2f is completely flat and situated in a plane which is normal to the lens axis. However, in this embodiment the bore of the shielding cylinder terminates in the region of the lens gap in a reduced bore portion 1% which has an extremely small diameter just sufficiently great to permit the corpuscular ray to pass therethrough. This construction of FIG. 8 results in a large maximum value H, of the field intensity in the gap together with a small length of the field-filled region along the corpuscular ray.

FIG. 9 shows a variation of the embodiment of FIG. 8 according to which the shielding cylinder 2g of FIG. 9 has the same bore portions 10a and 101: as those shown in FIG. 8 while including the same exterior surface portions 62 and 6f as those shown in FIG. 7.

As is apparent from the features of FIGS. 4-9, the distance S between the sheilding cylinders, the length thereof, and the configuration of their end faces which define and limit the lens gap must to a large extent be empirically determined in accordance with the special relationships which are encountered in the particular corpuscular ray device.

FIG. 10 shows an embodiment of our invention where the lens assembly is provided with shielding

cylinders

22 and 23 respectively corresponding to the above shielding cylinders, such as those shown in FIGS. 2 and 3. However, it will be noted that whereas in FIGS. 2 and 3 a single lens winding 1 surrounds both shielding cylinders, in the embodiment of FIG. 10 a pair of separate spaced

lens windings

8 and 9 respectively surround the pair of shielding

cylinders

22 and 23. These shielding

cylinders

22 and 23 of FIG. 10 are provided with additional superconductive shielding structures and 11.

In all embodiments the shielding cylinders are in thermal conductivity with a cryogenic refrigerating medium, and in FIGS. 2, 3 and 10, this cryogenic refrigerating means takes the form of containers c in which a cryogenic medium such as liquid helium is located, for example.

The division of the lens winding means into a pair of

individual lens windings

8 and 9 as shown in FIG. 10 provide a gap S which is of easy accessibility. It is thus possible to arrange in the gap S additional windings 12, as indicated in FIG. 10. However, it is also possible to situated in this gap S a suitable specimen, a diaphragm, or a stigmator element, these elements being capable of introduction into the gap S from the side. According to FIG. 11, the shielding cylinders may be made by arranging disc-shaped components 30 of superconducting material and components 31 of normal conductivity one after the other in alternating sequence.

FIG. 12 shows a shielding cylinder avoiding flux jumping after the structure is turned on. This cylinder comprises disc-shaped carrier bodies 40 made of heatresistive material and coated with layers 41 of Nb Sn.

Thus, our invention provides a further development or further generalization of lens assemblies of the type already proposed by arranging and forming the shielding cylinders which are already present in the already proposed assemblies in such a manner that they take over and perform the functions which are now performed by the additional superconducting apertured disc of the already proposed assemblies.

We claim:

I. In a magnetic lens assembly for a corpuscular ray device which is to operate under vacuum, such as an objective lens assembly of an electron microscope, a pair of coaxial shielding cylinders spaced from each other and having a common axis coinciding with a lens axis of the assembly, said shielding cylinders being made of a superconductive material, a cryogenic refrigerating means thermally connected with said cylinders, lens winding means surrounding said cylinders for generating a magnetic field, said cylinders concentrating said field in the region of a corpuscular ray traveling along said lens axis and said shielding cylinders respectively terminating in a pair of end faces which are directed toward each other and which define between themselves a lens gap which is devoid of any shielding components made of superconductive material, said gap having a magnitude which at a predetermined value of field intensity beyond said lens gap and said shielding cylinders provides a maximum value of field intensity in the lens gap and a field gradient in the lens gap along said lens axis resulting in an aperture error constant of the lens assembly which is less than a predetermined value.

2. The combination of claim 1 and wherein said end faces of said shielding cylinders which define said lens gap have a configuration which follows the course of field lines.

3. The combination of claim 1 and wherein said shielding cylinders are respectively formed with axial bores passing therethrough and having at least in the region of said lens gap an extremely small cross sectional area.

4. The combination of claim I and wherein additional field-generating lens windings are situated at said lens gap.

5. The combination of claim 1 and wherein said shielding cylinders respectively terminate distant from said end faces which defined said lens gap in end faces which are directed away from each other and which extend into a region of negligible field intensity.

6. The combination of claim I and wherein additional superconductive shielding material is situated at the exterior of said lens winding means surrounding said shielding cylinders.

7. The combination of claim 1 and wherein said cylinders are of disc-shaped configuration.

8. The combination of claim 1 and wherein said shielding cylinders are made of sintered Nb sn.

9. The combination of claim 1 and wherein said shielding cylinders are made of disc-shaped carrier bodies of heat-resistant material coated with Nb Sn.

10. The combination of claim 1 and wherein components of superconductive material are connected with components which are of normal electrical and thermal conductivity.

11. The combination of claim 6 and wherein the additional superconductive shielding material is surrounding at all sides the lens windings except at the side being directed toward the shielding cylinders.

12. The combination of claim 1 and wherein the lens windings consist of superconductive material.

F l i

Claims

1. In a magnetic lens assembly for a corpuscular ray device which is to operate under vacuum, such as an objective lens assembly of an electron microscope, a pair of coaxial shielding cylinders spaced from each other and having a common axis coinciding with a lens axis of the assembly, said shielding cylinders being made of a superconductive material, a cryogenic refrigerating means thermally connected with said cylinders, lens winding means surrounding said cylinders for generating a magnetic field, said cylinders concentrating said field in the region of a corpuscular ray traveling along said lens axis and said shielding cylinders respectively terminating in a pair of end faces which are directed toward each other and which define between themselves a lens gap which is devoid of any shielding components made of superconductive material, said gap having a magnitude which at a predetermined value of field intensity beyond said lens gap and said shielding cylinders provides a maximum value of field intensity in the lens gap and a field gradient in the lens gap along said lens axis resulting in an aperture error constant of the lens assembly which is less than a predetermined value.

4. The combination of claim 1 and wherein additional field-generating lens windings are situated at said lens gap.

6. The combination of claim 1 and wherein additional superconductive shielding material is situated at the exterior of said lens winding means surrounding said shielding cylinders.

8. The combination of claim 1 and wherein said shielding cylinders are made of sintered Nb3Sn.

9. The combination of claim 1 and wherein said shielding cylinders are made of disc-shaped carrier bodies of heat-resistant material coated with Nb3Sn.