US3444365A - Continuously evacuated corpuscularray apparatus,such as electron microscope,having a cryogenically cooled specimen cartridge - Google Patents

Description

May 13, 1969 .LOEBE 3,444,365

CONTINUOUSLY EVACUATED PUSCULAFPRAY APPARATUS, SUCH AS ELECTRON MICROSCOPE, HAVING A CRYOGENICALLY COOLED SPECIMEN CARTRIDGE Filed Oct. 19, 1965 CONTINUOUSLY EVACUATED CORPUSCULAR- RAY APPARATUS, SUCH AS ELECTRON MICRO- SCOPE, HAVING A CRYOGENICALLY COOLED SPECIMEN CARTRIDGE Wolfram Loebe, Berlin, Germany, assignor to Siemens Aktiengesellschaft, Munich, Germany, a corporation of Germany Filed Oct. 19, 1965, Ser. No. 497,894 Claims priority, applicgtion Gigi-many, Nov. 25, 1964, 94,31 Int. Cl. H01j 37/26, 37/20; G01n 23/20 U.S. Cl. 25049.5 6 Claims ABSTRACT OF THE DISCLOSURE Combination with a vacuum vessel in a corpuscularray apparatus of a specimen cartridge having first and second coaxial component portions, a specimen carrier held by the first portion and surrounded by the second portion, both portions being mutually thermally insulated except for heating-conducting structure interconnecting the same and being formed of at least one elongated metal element disposed about the cartridge axis, and a cryogenic device in cooling contact with only the second of the portions, the heat-conducting structure having a conductance whereat the specimen carrier steady state temperature is higher than that of the second portion.

My invention relates to corpuscular ray apparatus, particularly electron microscopes which, during operation, are continuously exhausted by a vacuum pump and have a specimen-accommodating cartridge connected with a cryogenic cooling device. Such corpuscular-ray apparatus are described in the copending application of K. H. Herrmann and W. Loebe, Ser. No. 392,088, filed Aug. 26, 1964, and assigned to the assignee of the present invention.

In order to prevent in corpuscular-ray apparatus the specimen from being soiled by precipitation of atoms or molecules from residual gases and vapors, the environment of the specimen should be cooled in such a manner that the specimen will never form the coldest point within the vacuum vessel. For that purpose the above-mentioned copending application proposes to provide a specimen cartridge composed of two coaxial portions. The first portion accommodates the specimen carrier, usually a centrally apertured diaphragm. In the vicinity of this object carrier, the first portion is surrounded by the second portion of the cartridge which is in good heat-conducting connection with a cryogenic device. In the corpuscular-ray apparatus heretofore disclosed, care is taken that the two cartridge portions are heat-insulated from each other and that the first cartridge portion with the object carrier is in good heat-conducting connection with those parts of the vacuum vessel structure that are normally at least approximately at room temperature.

Such a construction of the specimen accommodating means in corpuscular-ray apparatus affords the assurance that essentially only the vicinity of the specimen will be cooled, thus preventing the occurrence of detrimental soiling at the specimen. However, it is not feasible with such devices to apply low-temperature cooling to the specimen itself, although this is often required or desirable, for example in the electron-microscopic investigation of various objects.

For that reason, an attempt has been made to modify the above-described construction so as to afford also cooling the object, by providing between the two mutually heat-insulated portions of the cartridge a heat-conducting structure in the form of a spiral spring of copper. This, however, is a diflicult matter because the heat contact at United States Patent 3,444,365 Patented May 13, 1969 "ice the two ends of the copper spiral is unreliable and there occurs a nonrotationally symmetrical force between the two cartridge portions which tend to introduce uncertainties as to the correct positioning of the specimen.

My invention, relating to corpuscular-ray apparatus, preferably but not exclusively to electron microscopes, generally of the above-mentioned type in which a two-part specimen cartridge is cryogenically cooled, has for its purpose to permit retaining the desired protection against soiling of the specimen, while also permitting low-temperature cooling of the specimen, without any deficiency as regards heat transfer conditions or positioning of the specimen.

To this end, and in accordance with my invention, I provide a corpuscular-ray apparatus of the type continuously evacuated when in operation, with an object cartridge composed of two coaxial portions substantially in the above-mentioned manner, but provide as heat conducting components a number of metal springs and distribute them within the cartridge and about the ray axis of the cartridge in a rotationally symmetrical arrangement. The rotationally symmetrical heat-conducting components can also consist of at least one wire, tape or braid. The heatconductances of these springs are so chosen that, during cryogenic cooling of the cartridge in the above-described manner, the resulting steady-state temperature at the specimen carrier and at the specimen is higher than the temperature of the cartridge portion that coductively connects it with the cryogenic cooling means. The necessary heat conductance of the rotationally symmetrical spring arrangement is obtained by giving the heat-conducting springs consisting, for example, of copper or copper alloys, a proper cross section and an effective length.

By providing such heat-conducting spring structures, an apparatus according to the invention affords the assurance that, for any cooling of the specimen carrier with the specimen, there will always occur a temperature gradient between the cooled second cartridge portion on one hand, and the specimen on the other hand, so that the temperature of the specimen, even if the latter is also being cooled, is always at least slightly higher than the temperature of the surrounding second cartridge portion.

The specimen, therefore, is never the coldest spot in the vacuum space of the apparatus so that the atoms and molecules of residual gases and vapors cannot precipitate upon the specimen. The heat-conducting structures further afford the assurance that in any event a good heat-conducting contact exists between these structures, on the one hand, and the two cartridge portions, on the other hand, and that any forces or moments exerted between the two cartridge portions by the heat-conducting structures are rotaitonally symmetrical with respect'to the axis of the corpuscular-ray so that there is no tendency of affecting the positioning of the specimen.

The springs are prefer-ably formed by tongues extending from an annular marginal member in the radially inward direction. The tongues may be produced by cutting radial slits into the annular spring and are preferably given an individually arcua-te shape curving away from the plane of the marginal annular member. The springs may also be curved alternately in opposed axial directions. Such ring-type springs need not be fastened to either cartridge portion but may simply be placed between them before the removable first portion is seated in proper position against the second portion.

It is preferable to make certain that atoms and/or molecules of residual gases or vapors as may be located in the vacuum space near the specimen carrier are reliably exhausted. For this purpose it is advisable to provide the first portion with openings in the region surrounded by the second cartridge portion. Any such gas residues can then be exhausted through the openings.

Such openings are produced in a particularly simple manner by supporting the specimen carrier on heat-conducting struts into which the adjacent end of the first portion is subdivided, the spaces between the struts then forming the exhaust openings.

The invention will be further described with reference to the accompanying drawing showing an electron microscope of which only a portion involving the invention proper is represented.

FIG. 1 is an axial section through the objective-lens portion of the electron microscope comprising a specimenaccommodating cartridge device according to the invention; and

'FIG. 2 shows in schematical perspective a portion of a ring-shaped multi-tongue spring which forms part of the device illustrated in FIG. 1.

The illustrated portion of the electron microscope comprises an objective lens assembly 3 with two coaxial and axially spaced pole shoes 1 and 2. Coaxially mounted above the pole shoes is an object cartridge 4 composed of two portions 5 and 6. The first portion 5 is seated in a partly illustrated specimen table or stage 7. The second portion 6 of the cartridge is in good heat-conducting connection with a cooling rod 8 whose other end is immersed into a cryogenic vessel (not illustrated). The second cartridge portion 6 is rigidly and firmly attached to the lens system 3 but is thermally insulated therefrom by insulating inserts 9 consisting, for example, of sleeves made of synthetic plastic.

The first cartridge portion 5 has a conical top member 10 which serves for removably inserting the portion 5 into a hollow conical seat or neck of the specimen stage 7. Two tubular members 12 and 13 are separately join-ted with the conical top member 10 by being screwed together therewith. Memember 12 constitutes the holder for the object carrier 14, and part 13 is an elongated tubular insert which forms a holder for a centrally aper-tured diaphragm 15.

The two cartridge portions 5 and 6 have respective front faces of annular shape which lie opposite each other and extend perpendicularly to the ray axis and hence to the direction of seating motion of the first cartridge portion 5. The two annular faces serve as abutments for a good conducting connecting structure constituted by a multitongue annular spring member 16 separately shown in FIG. 2.

The individual springs 16a form curved tongues which extend radially inwardly from a ring-shaped marginal portion of the spring member 16. These spring tongues have the effect that when the cryogenic cooling device of which the cooling rod 8 forms part, is in operation, the specimen carrier 14 and the specimen mounted thereupon are cooled to a temperature which is always slightly higher than the temperature of the second cartridge portion 6 surrounding the specimen.

Such a temperature gradient will adjust itself also if, as illustrated, additional means are provided for heat-insulating the region of the first cartridge portion 5, where this portion is in cont-act with the specimen carrier 14 and the heat-conducting springs 16, from the other components of the electron microscope, such as the specimen stage 7. For this purpose, the conical top member 10 of the first cartridge portion possesses a zone 17 formed of heat-insulating material. If desired, however, the entire top member 10 may be made of insulating or poorly heat-conducting material, in which case the inner surface of the top member facing the electron ray must be metallized and be electrically connected with metal parts of the equipment in order to prevent the accumulation of static electric charges.

Providing in the above-described manner for heat insulation of the first-cartridge region contacted by the specim-an carrier 14 and the heat-conducting springs 16, from the other components of the corpuscular-ray apparatus, has the advantage of improving the efficiency of the cryogenic cooling operation, in contrast to the introductorily mentioned prior devices in which a particularly good heat conductive contact was provided for between the components of the corpuscular-ray apparatus that are of normal room temperature, on the one hand, and the first cartridge portion with the specimen carrier on the other hand.

The lower end of the first cartridge portion 5 which holds the specimen carrier 14 is subdivided into a number of struts of which only two are visible in FIG. 1 at 12a and 12b. The interspaces between these struts form openings through which the space within the first cartridge portion is continuously exhausted during operation of the electron microscope, so that any atoms or molecules of residual gases or vapors are exhausted from within the first cartridge portion.

A diaphragm 18 is mounted on the second cartridge portion 6, preferably in heat-insulated relation thereto. Such heat insulation is secured by interposition of an insulating insert 19 and serves to prevent excessive cooling of the diaphragm 18 which might result in icing of this diaphragm and consequently, since the diaphragm is located in the electron-optical gap of the objective lens 3, may cause disturbing astigmatism of the lens.

The 'apertured diaphragm 20 of the lens assembly is at least approximately at normal room temperature. This diaphragm is mounted on a slider 21 by means of which the diaphragm is displaceable transversely of the ray axis. If desired, the space surrounding the lens diaphragm 20 may also be cooled, or a further diaphragm mounted at a somewhat lower location on an extension of a second cartridge portion may be cooled, so that the space enclosed within the second cartridge portion approaches as best as feasible the properties of a chamber enclosed within cooled walls.

According to another feature of the invention, it is particularly advantageous to provide the first cartridge portion 5 with a heater in the region where this portion is in contact with the specimen carrier and the heatconducting spring structures. The heater then permits regulating the temperature of the specimen. For such purposes, the temperature may be measured, for exmaple, by thermocouples arranged in the number required for proper temperature measurement. Accordingly the illustrated embodiment has a heater winding 22 coaxially mounted on the tubular member 12. The terminals of winding 22 are to be connected to a controllable source of electric current outside of the microscope structure, so that the temperature of the specimen can be controlled or regulated. For this purpose a thermocouple or other heat sensor (not shown) may be mounted at a suitable locality for response to the temperature of the specimen.

As exemplified by the above-described embodiment, it is preferable to attach the second portion 6 of the cartridge to the objective lens structure of the corpuscularray apparatus, whereas the first cartridge portion 5 with the specimen carrier 14 is made removable from the vacuum vessel for such purposes as exchanging the specimen. The second cartridge portion therefore, can be introduced through a vacuum lock or sluice to be removably seated in the specimen stage 7.

In a preferred emboridment of the type described, the heat-conducting springs 16a are bindingly stressed between the two cartridge portions when the first portion 5 is properly seated in the specimen stage. This facilitates designing the two cartridge portions as mechanically independent structural units so that an exchange of the specimen or the removal of diaphragms for cleaning purposes or the like becomes possible simply by sluicing only the first cartridge portion out of the vacuum vessel. This is a relatively simple matter because the first cartridge 5 portion is not directly in connection with the cryogenic cooling device. Consequently no additional manipulations or expedients are necessary in order to re-establish the necessary heat-conducting connection etween the two cartridge portions after inserting a new specimen or diaphragm into the apparatus.

If desired, the springs of heat-conducting material, in the preferred embodiment of the invention, may also be arranged on surface areas of the respective cartridge portions that extend parallel to the direction of motion of the first cartridge portion 5 when the latter is being inserted into the specimen stage 7, so that this cartridge portion during seating is virtually guided by the springs. As a rule, however, care must be taken that during such seating of the cartridge portion no excessive forces are imposed upon the springs as this may impair the proper centering of the specimen with respect to the corpuscularray. It is, therefore, preferable, according to another feature of the invention and as illustrated, to provide the two cartridge portions 5, 6 with the above-described faces transverse to the seating direction and ray axis, and to use these faces as abutments for the intermediately located heat-conducting springs. In this case, the springs virtually constitute a limit stop for the inserted first cartridge portion.

It will be understood that the invention is not limited to the particular example of the embodiment illustrated. Thus, the two portions of the specimen cartridge need not be designed as separable units in the illustrated manner. For example, the second cooled cartridge portion may be given a shape which upwardly surrounds the upper cartridge portion where the diaphragm 15 is supported by means of a cooled tube.

Upon a study of this disclosure, such and other 'modifications will be readily available to those skilled in the art and are indicative of the fact that the invention may be given embodiments other than particularly illustrated and described herein, without departing from the essential features of my invention and within the scope of the claims annexed hereto.

I claim:

1. In a corpuscular-ray apparatus comprising a specimen stage and an objective lens system, and a Vacuum vessel structure surrounding the specimen stage and the objective lens system, the combination of a specimen-accommodating device within said vessel structure comprising a specimen cartridge having first and second coaxial component portions, said first cartridge portion being removably seated in said specimen stage, a specimen carrier held by said first cartridge portion, said second cartridge portion surrounding said first cartridge portion in the region of said carrier and being attached to said lens system, and heat-conducting structure interconnecting said cartridge portions which are otherwise thermally insulated from each other, said heat-conducting structure comprising springs symmetrically disposed about the axis of said cartridge, said springs being under stress when said first portion has been inserted into said specimen stage, and a cryogenic device in cooling contact with only the second one of the cartridge portions, said heat-conducting structure having a heat conductance whereat the steady state temperature of said first cartridge portion and said specimen carrier held thereby is higher than that of said second cartridge portion cooled by contact with said cryogenic device.

2. In corpuscular-ray apparatus according to claim 1, said two cartridge portions having respective faces extending transverse to the axis and being located opposite and in axially spaced relation to each other, said two faces forming respective abutments engaged by said springs.

3. In corpuscular-ray apparatus according to claim 2, said springs forming respective legs of an annular member having a marginal ring portion from which said springs extend radially inwardly, said individual springs having arcuate shape curving out of the plane of said marginal ring portion.

4. In corpuscular-ray apparatus according to claim 2, said first cartridge portion having exhaust openings at 10- calities surrounded by said second portion so that gas residues may be exhausted through said openings.

5. In corpuscular-ray apparatus according to claim 4, said first cartridge portion having its end adjacent to said specimen carrier subdivided so as to form mutually spaced struts, said exhaust openings being formed by the spaces intermediate said struts, and said specimen carrier being held by and between said struts.

6. In corpuscular-ray apparatus according to claim 1, said first cartridge portion having a heater winding mounted in the region of said specimen carrier and said heatconducting structure.

References Cited UNITED STATES PATENTS 2,543,825 3/1951 Ben et al. 250-5l.5 3,171,955 3/1965 Cardile. 3,226,542 12/ 1965 Craig et al.

RALPH G. NILSON, Primary Examiner.

A. L. BIRCH, Assistant Examiner.

US. Cl. X.R.

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