US2347982A - Electron lens - Google Patents

Description

WWW-Wm MayZ, 1944. c. H. B-ACHMAN 2,347,982

ELECTRON LENS Filed-Dec. 31, 1941 Fig; l.

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Patented 2, 1944 ELECTRON LENS Charles H. Bacliman, Scotia, N. Y assignor to Generai Electric Company, a corporation New York Application December 31, 1941, Serial No. 425,143

11 Claims. (Ci. 250-495) The present invention relates to an improved electrode arrangement for use in electron-optical apparatus.

It is known that the component rays of a beam of charged particles can be focused by the action of apertured electrodes spaced along the beam path and supplied with suitable potentials, this combination being conventionally designated an electron lens. The focusing produced by such a lens is a function of the strength and form of the electrostatic fields existing between the various lens electrodes and is analogous to the focusing of a light beam by an optical lens. It has been usefully employed in one instance in the so-called electron microscope, which is an apparatus for obtaining a greatly enlarged electron-optical image of a minute object desired to be investigated.

To obtain electron optical lens effects of significant magnitude it is ordinarily necessary that a high potential difference, of the order of several thousand volts, be impressed between the electrodes of the system. This has required that great care be exercised in the construction of the electrode elements to minimize the danger of electrical breakdown between them. It has been necessary, for example, to take such precautions as highly polishing the electrode surfaces, avoiding sharp comers and small radii, and providing relatively long insulation paths between adjacent elements. The use of long insulation paths and large radii of curvature makes it difficult to obtain lenses of short focal length and is therefore objectionable for many uses.

It is an object of the present invention to avoid the limitations refer ed to in the foregoing by providing an improved electron lens construction which may be made of relatively short axial length (and -consequently of short focal length) without thereby incurring substantial danger of electrical breakdown. In this connection an important feature of the invention consists in an arrangement in which at least one of the lens electrodes is embedded in a dielectric medium of greater insulating strength than that through which the charged particles to be affected by the lens are projected. This construction not only leads to greater electrical strength, but with a proper configuration of the dielectric medium also assists in shaping the lens field in a manner calculated to minimize aberrational effects.

The aspects of the invention which I desire to protect herein are 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 is a sectional view of an electron microscope suitably embodying the invention; Fig. 2 is a view showing an intermediate step in the fabrication of an electron lens according to the invention; Fig. 3 shows a finished lens made in accordance with the procedure indicated in Fig. 2; Fig. 4 illustrates the application of the invention to a multiple lens; and Figs. 5 and 6 illustrate modifications of the invention.

Referring particularly to Fig. 1 there is shown an electron microscope comprising an elongated vacuum-tight container which consists mainly of a cylindrical metal part I. At one end the container is closed by a glass window ll having afluorescent material I2 on its inner surface. and at the other end of the container there is provided a glass insulator l4 which serves to support an electron source in the form of a filamentary cathode IS.

The cathode is surrounded by an apertured cup-shaped metal member l6 which confines the emitted electrons to a narrow beam and which is cooperatively positioned with respect to an apertured diaphragm supported in contact with the main envelope part III. In the normal use of the apparatus the envelope l0 and the apertured electrode l8 are maintained at ground potential while the cathode is held at several thousand volts below ground potential (e. g., by connection to a potential source 20) so that electrons emitted from the cathode are projected toward the fluorescent screen l2.

In using the apparatus as an electron microscope it is desired to cause the electron stream proceeding from the cathode l5 to produce in the plane of the screen l2 an enlarged electronoptical image of a minute object to be investigated. To this end there is provided a suitable means for supporting an object of the type in question in the path of the electron stream, such means being illustrated as a metal diaphragm 22 provided with a central opening 23 and having a fine mesh screen 24 covering this opening. The object to be investigated (not shown) is applied to the screen 24 at a point which is traversed by the longitudinal axis of the microscope, and the microscope is thereafter evacuated by connection to an appropriate pumping system.

Between the object support 22 and the selected imaging plate (i. e., the screen l2) there is provided an electron lens for exerting refractive forces on the electron rays proceeding from the object. In accordance with the present invention, this comprises in the first instance an annular body 28 of a dielectric material having insulating strength greater than that of the vacuous space through which the electron beam is projected. To this end the material 28 may suitably comprise an insulating vitreous or ceramic substance such as glass, quartz, porcelain or the like, or it may be alternatively constituted of a finely divided inorganic material,

' such as mica or asbestos, cemented or bonded together with a glassy binder, for example, leagl borate. Embedded in the insulating body there is provided a lens electrode 32 in the form of an annular disk or washer having at its inner peripheral edge a ring 32' which is of circular cross-section and which serves to improve the distribution of the electron field in the vicinity of the electrode surface.

The electron lens further includes electrodes 3| and 33 which are respectively disposed on op-- posite sides of the electrode 32 and which are also of annular form. These latter electrodes may be appropriately provided by depositing on the end surfaces of the insulating part 28 a thin metallic coating applied by vaporization of metal or by some equivalent method, such as platinizing. These electrodes are in contact with the main envelope part I and are accordingly maintained at ground potential. The electrode 32, on the other hand, is connected to the negative terminal of the battery through an insulating bushing 35, so that lens fields are provided in the regions between the electrodes. Assuming appropriate configuration of the parts and a proper choice of the potential difference between the lens electrodes, the efiect of the lens may be made such as to project upon the screen I 2 an enlarged electron-optical image of the object supported by the holder 22.

Because of the high dielectric strength of the part 28, the axial spacing of the electrodes 3!, 32 and 33 can, in the arrangement of Fig. 1, be made considerably smaller than would be permissible if the various electrodes were separated only by a vacuum space. Accordingly, a lens of extremely short focal length may be produced without thereby incurring any substantial danger of electrical breakdown.

In order to permit the electrode 32 to be successfully incorporated in the material of the part 28, the electrode should be constituted of a material having appropriate expansion and sealing properties. For example, if the part 28 is to be constituted of glass, it will be desirable to form the part 32 of a metal such as platinum which can be sealed to glass without likelihood of fracture of the resulting joint. The electrodes 3| and 33, of course, need not fulfill this requirement to the same degree and may be formed of a large variety of conductive materials, for example, of metals such as silver, platinum and copper, or of anon-metallic conductor such as carbon applied in an aqueous suspension.

In a construction such as that illustrated in Fig. 1 the shape of the lens field in the part of the ner it is possible to modify the distribution of the lens field in a manner calculated'to minimize aberrational effects such as a. spherical aberration. Configurations having this tendency are illustrated in the various figures of the drawing, the configurations shown being generically related in that they all involve tapering of the aperture of the solid dielectric in the region between the embedded and the external electrodes.

A typical method of fabricating a lens of the character represented by the present invention is illustrated in Fig. 2. In this figure, which shows certain of the lens Darts in their proper relative positions but 'prior to assembly, there are illustrated two cooperatively shaped insulating elements 40 and 4|, consisting, for example, of glass, positioned on opposite sides of a metallic ring electrode 42. In assembling these elements the Various parts are heated to a temperature corresponding to the softening temperature of the particular glass employed, and the parts are thereafter pressed into engagement, the result ing structure being illustrated in Fig. 3. The lens of Fig. 3 is completed by the application to the upper surface of the part 40 of a layer of conductive material comprising, for example, silver deposited by vaporization.

The construction of Fig. 3 represents a twoelement electron lens of a known type and may be used without further modification by impressing potential between the electrodes 42 and 44. If it is desired to use the same method of fabrication in connection with the construction of a multiple lens, this may be done by combining two elements of the character shown in Fig. 3 to produce the composite construction represented in Fig. 4. In this latter figure, the lens system shown includes an annular ceramic part 41 having external electrodes 48 and 48 applied to its external surfaces and having additional electrodes 50 and 5| embedded within its structure. In the use of this lens structure, electrodes 50 and 5| may be connected to a common terminal as indicated at 53 and the electrodes 48 and 49 may also be joined as indicated at 55.

A modification of the invention which further illustrates the possibility of shaping the exposed surface of a ceramic lens part to obtain improved distribution of the lens field is shown in Fig. 5. In this figure, there is provided an annular ceramic body 51 having embedded in its central region an electrode 58 and having cooperating electrodes 59 and 60 applied to its external surfaces. In this case the ceramic 51 is provided with a central aperture 63 having a smoothly curved surface at its respective extremities. The electrodes 59 and 60, which may consist of vaporized metal, extend part way into the ceramic aperture as indicated at '65 and 66, and are shaped to conform to the surface by which the aperture is bounded. This permits the distribution of the lens field to be controlled Jointly by the form of the aperture and by the disposition of the electrode metal.

It is possible to embed all the lens electrodes in solid dielectric material, and a construction which illustrates this aspect of the invention in connection with a two-electrode lens is shown in Fig. 6. The lens of this figure includes an annular ceramic part 68 having embedded in it at symmetrically disposed locations a pair of annular metal washers l0 and H which are adapted to serve as lens electrodes. These electrodes are provided with externally accessible terminals 12 and I3 by means of which a potential may be impressed between them. By virtue'of the complete enclosure of the electrode surfaces in solid dielectric material a relatively very high potential difference may be employed without danger of breakdown.

While the invention has been described by reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. I, 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 I claim as new and desire to secure by Letters Patent of the United States, is:

1. A system for producing electron optical lens effects upon a stream of charged particles projected through a non-obstructive medium, said system comprising spaced electrodes adapted to be maintained at different potentials, the active surfaces of at least one of said electrodes being surrounded by a dielectric medium of greater insulating strength than the medium through which the said stream is projected, said dielectric medium having a tapered surface exposed to said stream to reduce aberrational efiects thereof.

2. Electronic apparatus, including means for projecting a beam of charged particles through a vacuous or gas-filled space and a system for producing electron optical lens efiects on the said beam, said system comprising spaced electrodes which are adapted to be maintained at different potentials and at least one of which is embedded in a dielectric medium of greater insulating strength than the medium through which the said beam is projected, said dielectric medium comprising a tapered surface exposed to said beam and cooperatively positioned relative to the direction of projection thereof to reduce aberrational effects.

3. A system for producing electron optical lens effects upon a stream of charged particles projected through a vacuous or gas-filled space, said system comprising spaced apertured electrodes adapted to be maintained at different potentials, at least one of said electrodes having its active surfaces embedded in a solid dielectric medium having a restricted region through which said stream passes.

4. A system for producing electron-optical lens effects upon a stream of charged particles pro- Jected through a non-obstructive medium, said system comprising a first annular electrode embedded in a solid dielectric medium having a restricted region through which said stream passes and a second annular electrode cooperating with said first electrode and adapted to be maintained at a potential difference with respect to said first electrode.

5. A system for producing electron-optical lens efiects upon a stream of charged particles projected through a vacuous r gas-filled space, said system comprising spaced electrodes adapted to be maintained at different potentials, the active surfaces of at least one of said electrodes being embedded in a ceramic body of high dielectric strength.

6. A system for producing electron-optical lens effects upon a stream of charged particles projected through a vacuous or gas-filled space, said system comprising a first apertured electrode, second and third apertured electrodes symmetricaly disposed on opposite sides of the first electrode and adapted to be maintained at a common potential diii'erence with respect thereto, said first electrode having its active surfaces embedded in a solid dielectric substance having a restricted region through which said stream is projected.

7. An electron lens comprising an annular body of solid dielectric material, an annular lens electrode having its active surfaces embedded in said dielectric material and a second lens electrode comprising an annular conductive layer applied to an exposed surface of said dielectric body.

8. A system for producing electron-optical lens effects upon a stream of charged particles projected through a non-obstructive medium, said system comprising an annular body of solid insulating material, a first lens electrode having its active surfaces embedded in said insulating material and a second lens electrode cooperating with the first electrode and adapted to be maintained at a potential difference with respect thereto, said insulating body having its exposed surfaces shaped to minimize aberrational "effects of the lens.

9. A system for producing electron-optical lens effects upon a stream of charged particles projected through a non-obstructive medium, said system comprising an annular body of insulating materiaha first annular lens electrode having its active surfaces embedded in said insulating body at the central region thereof, and a second and third lens electrode respectively disposed on opposite sides of said first electrode and insulatingly spaced therefrom by said insulating material, said insulating body having its inner peripheral surface in the region between said electrodes shaped to minimize aberrational effects of the lens system.

10. Electronic apparatus comprising means for producing a beam of charged particles, a lens system for affecting the condition of the said beam and means responsive to the condition of the beam for intercepting the beam after its traversal of the said lens, said lens including a first electrode having its active surfaces embedded in a solid dielectric medium and a second electrode cooperating with the first electrode to produce a lens field, said dielectric medium having a restricted region through which said stream is projected.

11. An electron microscope including means for producing a beam of electrons, means for supporting an object desired to be investigated in the path of the electron beam, and an electron lens system for electron-optically refracting the beam after its traversal of the said object, said lens system comprising a first electrode having its active surfaces embedded'in a solid dielectric medium and a second electrode cooperating with the first electrode to produce a lens field between them, said dielectric medium having a restricted region through which said stream is projected.

CHARLES H. BACHMAN.

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