US2259994A - Electron microscope - Google Patents

Description

Oct. 21, 1941; H-. BOERSCH ET AL ELECTRON MIGROS COPE Filed Nov. 25, 1940 Fig.2.

Fig. 5.

Inventors: Hans Boersch Ernst Bnu'che Hans Mahl, ye 52W Then Attorney.

Patented Oct. 21, 1941 ELECTRON Mrcnoscorn Hans Boersch, Berlin-Charlottenburg, and Ernst Briiche and Hans Mahl, Berlin-Reini'ckendorf,

Germany, assignors .to General Electric Com-' ,pany, a corporation of New York Application November 25, 1940, Serial No. 367,138 1 In Germany March 17,1939

3 Claims.

The present invention relates to improvements in focused beam discharge devices'and is' primarily applicable in connection with so called electron microscopes. v j

The electron microscope is a device in which magnification effects, analogousto those realized with an optical microscope, are obtained by appropriate refraction of an electron beam which is caused to be reflected from, or projected through, the object to be magnified. In accordance with known principles, the desired electron refraction may be obtained bypassing the elec tron beam through an electrostatic or magnetic field of such character as to produce strong focusing of the beam.

In connection with magnetic "beamfocusing it has been proposed to accomplish the desired results by means of a magnetic lens system comprising a coil which concentrically surrounds the beam path. This arrangement has been found to occasion considerable difficulty, especially in connection with .the positioning of an object to be magnified at a proper point within the lens system. It is an object of the present invention to obviate this difficulty by the provision of a system in which the magnetic field-producing coil is positioned relatively remote from the path of the electron beam. In accordance with the invention, this is accomplished by the use of a,

magnetic structure having opposed tubular magnetic parts which concentrically surround the beam and which are connected in an appropriate manner with a coil-supporting core offset from the beam path.

The features which we desire to protect hereinare pointed out with particularity in the appended claims. The invention itself, together With further objects and advantages thereof may.

best be understood by reference to the following description taken in connection with the drawing in which Fig. 1 represents a discharge tube embodying the invention; Fig. 2 is afragmentary view indicating a possible modification of the invention; Fig. 3 isa transverse cross-section of a second modification, and Figs. 4 and 5 conjointly illustrate still a third modification.

Referring particularly to Fig. 1 there is shown a discharge device of the cathode ray type which is provided at one end with-means for producing the impingement of electrons thereof, and thus to provide a visible image Whose nature is determinedby the characterof the impinging electron stream. I a

In the region between the electrode structure and the viewing window l5 there are provided two generally tubular metal elements I! and I8 which, in the case illustrated, form parts of the dischargeenvelope. In accordance with the invention, these elements are constituted of a ferro-magneticmaterial, for example, iron or ironchrome alloy, and are positioned in end-to-end relationship so as to be traversed successively by the electron beam. 1

At their adjacent extremities the tubular elements l1 and I8 are respectively providedwith inwardly tapered portions 19 and 20 which converge toward central aperturesZl' and 22. It is the function of the elements and of the tapered portions thereof to provide pole'piecesfor the establishment of an axial magnetic field in the gap which separates the two elements. Due to the axial symmetry of the elements, the field thus provided is itself axiallysymmetrical and is, therefore, well adapted to the-purposesof focusing an electron stream in such fashion as to produce magnification effects.

In order to complete the closure of the discharge device and to render the same vacuum tight, the space between the'tubular elements I1 and I8 is closed by a metal ring 25 which is of non-magnetic character and of such form as snugly to engage the surfaces of the elements. The metal ring 25' is shown as being provided witha removable member, showndiagrammettically at 26, which serves for the introduction into the tube of a mounting fixture for an object which is to be investigated by the use of the electron microscope. The presence of such a mounting is illustrated diagrammatically at 21.

At a point relatively remote from the beam path and non-concentric therewith, there is provided a coil 30 having a suificient number of turns to permit the establishment of a strong magnetic field upon energization thereof. A low reluctance path for the flux established by the coil 30 is provided by means of a U-shaped core structure 3| which is joined at its extremities to the individual elements i1 and, I8, as indicated at 32 and 33respe'ctively. The function of this core, which should be of magnetic material, is to permit a strong magnetic field to :be established in the gap between the magneticelements l1 and !8 in spite of the fact thatthe exciting coil itself is appreciably displaced from the gap. As a result of this disposition of parts the removal of the opening member 26:from:the wall of the discharge vessel for the purpose of introduction or withdrawal of the object mounting 21 is possible without the preliminary dismantling of the netic flux such, for instance, as a permanent magnet.

A possible modification of the invention is lustrated in Fig. 2 which represents the central portion of a cathode ray tube-ethat is, a tube with the electrode structure and theviewing win: dow and its associated bulb removed. It is the purpose of this figure to show an arrangement by which a series of magnetic lenses having the characteristics of my invention may be arranged in cascade. This may be accomplished in one way by the use of a generally cylindrical magnetic. member in combination with two additional elements 35 and 31 of similar cross-section, which are positioned adjacent the respective extremities of the element 35. The magnetic lenses which are formed by these elements, and especially by the inwardly tapered end portions thereof, are energized by a pair of magnetic coils 39 and 45. These in turn are associated with the base portions of a complex core structure which comprises, in efiect, apair of U-shaped cores arranged in side-by-side relationship. It will be readily understood that if the coils 39 and 40 are energized with the proper polarity, the passage of magnetic flux will be such as to establish fields in the gaps between the elements 35, 3'6, and 31 and thus to create magnetic lens effects in these gaps. For the. utilization of these eiiects, an object support may bepositioned in one of the gaps as indicated at 43.

A further aspect of the invention which is also illustrated in Fig. 2 comprises the utilization of articulated joints in the magnetic structure, as indicated at and 4.3. By virtue, of the provision of such joints it becomes readily possible to separate the parts of the discharge vessel (i. e. the elements 35, 36 and 31) without completely dismantling the magnetic structure.

A further variation. of the invention is illustrated in Fig. 3 which represents a transverse cross-section taken through a discharge device of the general character of that shown in Fig. 1. In this connection it is to be understood that the element represents in section a tubular magnetic part such as the part I! of- Fig. 1. Connected with this part, and extending radially therefrom in quadrangularly displaced positions, there are provided a number of magnetic core components indicated respectively at 5|, 52, '53, and 54. These elements are all associated with additional core components to 58 inclusive) which extend perpendicularly thereto. Each of the core structures is assumed to be closed by the provision of additional leg portions which respectively connect with a second cylindrical element positioned in end-to-end relationship with the element 50. Moreover, each of the core portions 55 to 58 is provided with a coil winding as shown at 60, BI, 62 and 63. The resultant construction, therefore, represents a multiplication of the arrangement of Fig. 1 in which four U-shaped'core structures, each excited by its own exciting coil, are provided at ninety degree positions around the axis of the discharge device and in which all such structures contribute to the energizae tion of a single magnetic lens of which the element 5B constitutes one component.

Figs. 4 and 5 illustrate a still further variation of the invention. In this case, tubular pole pieces f extending cylinder 69 of magnetic material.

65 and 66 are respectively connected with annular magnetic core. parts Bland fi8-which extend outwardly, therefrom. These. corei parts are joined at their outer peripheries by an axially Y providing a coil winding either externally or internally of the cylinder 59 (or both), it is possible to establish an axial magnetic field in the gap which separates the elements 65 and 66. However, this last described arrangement is not preferred for the reason that it does not afiord ready accessibility to the internal structure of the lens system in the manner which characterizes the embodiments previously described.

While we have described our invention in connection with particular embodiments thereof, it

will baunderstood by thoseslrilled in the ajrtthat;

numerous modifications may be'niade Without. departing from the invention. We, therefore, aim in the appended claims, to cover all such equivalent variations as come within the true.

spirit, and scope of the foregoing disclosure.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a discharge device including means for producing a beam of moving charges, a vacuum-tight discharge enclosure providing an elongated chamber to be traversed bythe beam, said enclosure being-iormed in part of a pair of tubular magnetic members positionedv n mu u y c en -end e ation t p vide a lens space between them, magnetic fieldproducing means associated with the members for establishinga magnetic, flux through the said lens space to produce a desired focusing efiect on the said beam, and means externally closing the gap between the members to maintain the vacuum-tight quality of the enclosure.

2. In combination, a, discharge device includ: ing means for producing a beam of moving charges, a vacuum-tight discharge enclosure providing a relatively elongated chamber to be traversed by the beam, said enclosure being formed in part of a pair of tubular magnetic members positioned in mutually spaced end-to-end relation to provide a lens space between them, a magnetic structure externally connected between the members for establishing'a mag'netic flux through the said space to produce a desired focusing effect on the said beam, and non-magnetic means peripherally sealing the gap between the members to maintain the vacuum -tight quality of the enclosure.

3. In combination, a discharge device including means for producing a beam of moving charges, a vacuum-tight discharge enclosure formed in part of a pair of tubular magnetic members arranged to be successively traversed by the beam, said members having conically tapered extremities which are positioned in juxtaposed relation to provide a lens space between them, magnetic field-producing means associated with the'said members for establishing a magnetic flux through the lens space adapted to produce a desired focusing efiect on the said beam, and a ring of nonmagnetic material interposed between the'opposed conically tapered faces of said members for hermetically sealing the gap between them.

' I I-ANS BOERSCH. ERNST BRiicHE. HANS MAHL.

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