US3374349A - Electron probe having a specific shortfocal length magnetic lens and light microscope - Google Patents

Description

March19, 1968 G A RES I 3,374,349

ELECTRON PROBE HAVING A SPECIFIC SHORT-FOCAL LENGTH MAGNETIC LENS AND LIGHT MICROSCOPE ori inal Filed Jan. 51, 1961 2 Sheets-Sheet 1 IN VEN TOR. VIC T 0/? 6. MAC/P55 PATENT AGENT March 19, 1968 3,374,349 FOCAL LENGTH MAGNETIC LENS AND LIGHT MICROSCOPE Original Filed Jan. 31, 1961 V. G. MACR ES ELECTRON PROBE HAVING A SPECIFIC SHORT 2 Sheets-Sheet 2 INVENTOR. VICTOR G. MACPES PATENT AGENT United States Patent 3,374,349 ELECTRON PROBE HAVING A SPECIFIC SHORT- FOCAL LENGTH MAGNETIC LENS AND LIGHT MICROSCOPE Victor G. Viacres, 3385 Louis Road, Palo Alto, Calif. 94303 Continuation of applications Ser. No. 368,448, May 11,

1964., and Ser. No. 86,200, Jan. 31, 1961. This application Nov. 14, 1966, Ser. No. 594,275

4 Claims. (Cl. 250-495) ABSTRACT OF THE DISCLOSURE An electron probe including a support for a specimen to be probed, an electron beam directed at said specimen and focused thereon by an adjacent short-focal length lens, and a light microscope for observing the probed specimen.

The present application constitutes a continuation of applicants prior applications, Ser. No. 86,200, filed Jan. 31, 1961, and Ser. No. 368,448 filed May 11, 1964, both applications having now been abandoned.

The present invention relates generally to electron optics, and more particularly, to electron probes for purposes of microscopic analysis, micro-machining, topical heat treatment or other application involving irradiation of a specimen or sample by a beam of high velocity electrons.

Since 1913 when Moseley determined the irradiation of materials by high velocity electrons produced X-radiation characteristic of the particular material being irradiated, many varied devices embodying such principle have beenconstructed for spectrographic analysis. For purposes of microscopic analysis, Hillier proposed the focusing of the electrons to a minute area on the irradiated material, and the utilization of such microprobe in conjunction with more or less conventional X-ray spectrographic equipment, this proposal being the subject of United States Patent No. 2,418,029. More recently, various microprobes have been constructed, primarily for application as a microscopic analytic tool, but with the concommitant realization that the electron irradiation of material could also be employed for other purposes such as micro-matching or topical heat treating of the irradiated material with but minor operational variations principally in the electron beam intensity.

In the practicalconstruction of electron microprobes, for microscopic analysis or other purposes, serious performance limitations have appeared because of the necessity for certain practical design compromises. As a particular example, it is necessary in a practical electron microprobe not only to focus the electron beam at the desired point on the sample or specimen, but it is also highly desirable to provide for optical viewing of'the irradiated specimen, and finally, it is necessary to provide for egress of the X-radiation emitted from the specimen or sample. Thus,-it will be seen that not only an electron beam but light and X-radiation converge at a single microscopic area, and the obvious conflicting space requirements have necessitated certain of the mentioned design compromises of existing microprobe equipment. Among others, the following limitations have been noted:

(1) a long focal length of the objective electron lens resulting in excessive spherical aberration of the focused electron beam at the irradiated area of the specimen" (2) Extremely complex reflecting optical systems that (3) Low take-off angle of the emitted X-radiation so that detection inaccuracies result.

Accordingly, it is a general object of the present invention to provide an electron probe capable of excellent electron focusing, enabling simple optical viewing, producing highly accurate X-ray detection when used for microscopic analysis, and generally facilitating both construction and operation.

It is a particular feature of the invention to provide an electron probe including an electron focusing lens of short focal length and a lens geometry such that spherical aberration is reduced to a practical minimum.

A related feature is the particular design of the electron -focusing lens so that although its focal length is short, and the specimen to be radiated is consequently in physical proximity thereto, the specimen is positioned in a substantially field-free region.

Another related feature is the design of the electron focusing lens so that a simple, refractive-type light microscope can be accommodated enabling direct optical viewing of the specimen without, however, any interference with the electron beam or its focus.

Another feature of the invention relates to the support of the specimen or sample to be irradiated in a position such that good optical viewing is enabled and a relatively high angle of emergence of X-radiation is enabled to thus provide accurate X-ray detection.

Yet a further feature of the invention is to provide a specimen support or stage which can support a plurality of specimens for alternative movement into irradiation position.

A related feature is the provision of a fluorescent specimen on the specimen stage so that the characteristics of the focused electron beam can be visually indicated through utilization of the light optical system.

Yet another feature of the invention is the provision of a main housing arranged to accommodate the specimen stage, the light microscope and other units in a manner permitting ease of removal and also arranged to function as a mounting jig so that appropriate disposition is obtained upon reassembly.

These as well as other objects and features of the invention will become more apparent from a perusal of the following description of the accompanying drawing wherein:

FIG. 1 is a perspective view of an electron probe apparatus embodying this invention, parts thereof being broken away to illustrate interior details of construction,

FIG. 2 is a schematic diagram of the electron probe physically illustrated in FIG. 1, and

FIG. 3 is an enlarged fragmentary perspective view of the specimen stage included as an element of the BIG. 1 structure.

With reference to the drawings, the electron microprobe is housed within a generally tubular body 10 that is somewhat enlarged at its lower end, as shown in FIG. 1. At the upper end of the tubular body 10, an electron gun is mounted to direct a beam of electrons substantially axially through the tubular body. Such electron gun is of conventional nature, being of the type employed in an ordinary electron microscope, and is thus not illustrated physically in FIG. 1, but is only schematically illustrated in FIG. 2. As so illustrated, the electron gun, generally indicated at 12, includes a filament 14 which is suitably heated so as to emit electrons, a control grid 16, and an appropriately formed anode 18 operating at a relatively high positive potential so as to effect acceleration of the emitted electrons into a generally pencil-like beam indicated at E. A suitable control indicated at 20 is provided so that the grid bias can be adjusted to provide the required electron source characteristics.

A toroidal magnetic beam-condensing lens 22 is mounted in axial alignment with and spaced relation to the electron gun 12 so as to focus and condense the beam of electrons E also in a manner normally employed in conventional electron microscopes so that further detailed structural or operational description is not warranted and such condensing lens is only illustrated schematically in FIG. 2.

The initially condensed electron beam E is thereafter collimated by an apertured plate 24 interposed in the beam path and preferably composed of brass or other non-magnetic material. The size of the aperture 24a in such plate 24 determines what portion of the beam E is transmitted therethrough and what portion is directed against the adjoining surface of the plate and thus stopped from further transmission. Preferably, as shown in FIG. 1, four distinct plates 24 are employed, each of which radiates from a central juncture point or hub so as to form a unitary structure which has the shape of a cross in transverse section. The multiple-plate structure is carried at the inner end of a member that extends through the wall of the tubular body 10, is rotatably supported thereby, and has a suitable knurled handle 26 at its outer extremity enabling manual rotation of one or another of the plates 24 into beam-collimating position with its aperture 24a aligned with the axis of the tubular body 10. Various size apertures 24a can thus be chosen for a particular microprobe operation, and if desired, one of the plates 24, rather than being apertured, can have a small amount of fluorescent material placed on its surface and suitable cross-hair line indicia placed thereover, as indicated at 24b in FIG. 1. If such plate 24 is then brought into operative position on the axis of the tubular body 10, a rough indication of the desired alignment of the electron beam E is visually apparent. So that this surface can be viewed easily by the operator, a suitable window 28 is mounted in the side of the tubular body 10.

The collimated electron beam E in its continuing traverse downward through the tubular body next passes through an electrostatic deflecting device generally indicated at 30 which includes adjustably-supported deflection plates 32 whose potential can be adjusted so as to maintain the electron beam symmetrically centered on its longitudinal axis, or can be varied to produce scanning of the beam across a specimen S.

As schematically illustrated in FIG. 2, the electron beam E thereafter continues in its traverse, diverging slightly in its advance, until it passes through a second magnetic objective electron lens, generally indicated at 34, whose general purpose is to focus the beam into substantially a point at the surface of a specimen S supported therebeyond.

In accordance with the present invention, this magnetic objective lens 34 is formed to provide a short focal length, to permit optical viewing of the irradiated specimen S by a simple refractive-type light microscope, and to permit egress of the emitted X radiation. Like the magnetic condensing lens 22, the magnetic objective lens 34 is of generally toroidal configuration including a coil 35 surrounded by a shield 36. More particularly, the internal surface of the torus as formed by the core shield 36 tapers inwardly in the direction of electron motion so as to converge on the electron axis at an angle slightly greater than 30. One pole piece 38 of the lens 34 is formed at the inner extremity of the shield 36 and the other pole piece 40 of the lens 34 is formed in tapered alignment therewith at the inner extremity of the lower core shield 42 that lies in a plane that is substantially perpendicular to the axis of the electron beam. Consequently, as can be readily visualized by reference to FIG. 2, the magnetic center of the lens 34 is located but a very short distance from its lowermost physical extremity.

As a result of the disposition of the magnetic center of the objective lens 34, the mentioned speciment S to be irradiated can be positioned in relatively close proximity thereto. In turn, the lens 34 can thus be designed to have a relatively short focal length. In practice, a focal length of 16 millimeters has been successfully utilized. Since it is known that spherical aberration of an electron optical system increases with an increasing focal length, it will be clear that in the present structure, it is reduced to a practical minimum.

The lower pole piece 40 of the magnet is provided with an inward and downward taper, as indicated at 40a, which serves to restrict the fringing fields of the magnetic lens 34 from the space occupied by the specimen S. Again, in practice, a restriction of the field to less than 10 gauss at the specimen has been obtained. In view of such magnetic field restriction, the specimen S can constitute a ferromagnetic material yet will not interfere to any appreciable degree with the proper focusing of the electron beam.

The described taper or internal convergence of the objective lens 34 permits the introduction of the tubular lens-supporting body 52 of an adjustable light microscope, generally indicated at 50, in a manner such that the light optical axis intersects the electron optical axis at the point or minute area (approximately 1 micron diameter) of irradiation of the specimen S. As shown best in FIG. 1, the light microscope is mounted as a unit on a circular plate 54 arranged for sealing juncture to the circular lip at the extremity of a cylindrical protuberance 58 projecting angularly from the tubular body 10 of the electron probe. When the plate 54 is secured in position by suitable bolts, conventional viewing binoculars 60 are available to the operator of the device, and the lens-supporting body 52 projects angularly inwardly toward the electron beam axis, the lower extremity of the tubular lens-supporting body terminating just shy of intersection with the moving electrons. The microscope incorporates a light source and illumination arrangement such that critical illumination can be realized so as to provide maximum contrast and thus allow the realization of the inherent resolution capabilities of the light optical microscope.

To avoid interference with the proper operation of the magnetic objective lens 34, the tubular lens-supporting body 52 of the light microscope 50 is formed of brass or other non-magnetic material and preferably a small metallic cylindrical shield 62 is attached to the extremity of the light microscope body so as to isolate the electron beam and thus preclude the influence of electric charges which may accumulate on the non-conductive glass lens of the light microscope 50 which accumulation might otherwise produce a distorting effect on the electron focusing action. Preferably, the axis of the light optical system is approximately 30 relative to the electron beam axis and preferably, as shown, the specimen S is tilted so that its exposed surface lies in a plane perpendicular to the light optical axis so as to enhance optical viewing.

Such tilted position of the specimen S also enhances the accuracy of X-ray detection of associated spectrographic equipment when the electron probe hereinabove described is utilized for microscopic chemical analysis. More particularly, as shown in FIG. 2, even though the specimen S is located physically quite close to the magnetic objective lens 34, a relatively large X-radiation emergence angle is obtainable. As shown at X in FIG. 2, X-radiation angles up to 35 or 40 are achievable, and as shown in FIG. 1, a suitable window 64 is positioned in the side of the body .10 to permit passage of the X- radiation to any associated spectrographic equipment. It may be mentioned that such spectrographic equipment forms no part of the present invention, in and of itself, and therefore is not described. However, conventional spectrographic equipment such as mentioned in the Hillier Patent No. 2,418,029 can be employed in association with the present electron probe apparatus when microscopic analysis is to be performed.

The specimen S is supported in the described angular disposition on a suitable stage that is illustrated more clearly in FIG. 3 and which generally is arranged to enable selection of specimens S and movement of a selected specimen into appropriate irradiation position. As shown, each specimen S can be mounted on a small disk 72. In turn, a plurality of the small specimen disks 72 can be removably supported for rotation about their own axes on a plurality of seats peripherally located on a larger disk 74. Suitable electrical and mechanical means are provided to raise and lower this larger disk 74 and also to rotate the same so that one or another of the specimens S can be brought into irradiation position. In turn, the rotatable large disk 74 is mounted for translatory motion with a table 76 slidably mounted on transverse bars 78 within the upstanding ends of a carriage 80 and under the control of a micrometer 82 that projects through the tubular body of the device so as to enable the desired adjustment during operation. The mentioned carriage 80 is also mounted on bars or rails 84 for movement in a direction at right angles to the translatory motion of the described table 76 and is again under the control of a micrometer 86 that is manually accessible exterior of the tubular body 10. To facilitate focusing of the electron beam E on the specimens S, one of the latter can be in the form of fluorescent material. Thus, for precision alignment of the electron beam E, the fluorescent specimen S can be brought into irradiation position and appropriately adjusted physically until by visual observation through the light microscope 50, the desired electron optical focus thereon is obtained.

It 'will, of course, be understood that the tubular body 10 must be evacuated to approximately 10* millimeters of mercury and suitable vacuum equipment is accordingly connected thereto in a conventional manner which need not here be described. Also, it will be apparent that all junctures to the tubular body 10 must be appropriately sealed so as to maintain such vacuum during operation.

Quite obviously, various modifications and/or alterations can be made to the described structure without depaitming from the spirit of the invention, Accordingly, the foregoing description of one structure is to the considered as purely exemplary and not in a limiting sense; the actual scope of the invention is to be indicated by reference to the appended claims. 1

What is claimed is:

1. An electron probe which comprises means for forming an electron beam, means for supporting a specimen to be irradiated at a predetermined position on the axis of said electron beam, means including a short focal length magnetic electron lens disposed entirely between said beam forming means and the specimen for focusing said beam on the specimen, said electron lens having an internal aperture of convergent configuration in the direction of electron motion for passage of the beam therethrough and a lens center closely adjacent the specimen, a light microscope supported for viewing of the irradiated specimen, said light microscope having an optical lens supporting tubular body of non-magnetic material projecting partially through the interior of said convergent electron lens aperture with its axis intersecting the axis of said electron beam at the surface of the irradiated specimen so that the light passes through the same lens aperture as the electron beam, and means to analyze and detect X-rays emitted from said specimen.

2. An electron probe according ot claim 1 which comprises an electron shield disposed between said electron beam and the terminal portion of said tubular light microscope body.

3. An electron probe according to claim 1 wherein said specimen-supporting means supports the specimen with its surface 'to be irradiated in a plane perpendicular to the axis of said light microscope.

4. An electron probe which comprises means for forming an electron beam, means for supporting a specimen to be irradiated at a predetermined position on the axis of said electron beam, the surface of the specimen being disposed at an acute angle relative to said beam axis, and means including a magnetic electron lens disposed entirely between said beam forming means and the specimen for focusing said beam on the specimen, said electron lens including a coil with a surrounding core shield terminating in pole pieces having an entirely internal convergent configuration in the direction of electron motion, and means to analyze and detect X-rays emitted from said specimen.

References Cited UNITED STATES PATENTS 2,243,403 5/ 1941 Von Ardenne 25049.5 2,273,235 2/1942 Von Ardenne 25049.5 2,301,303 11/1942 Marton 25049.5 2,392,243 1/ 1946 Hillier 25049.5 2,849,619 8/1958 Eisfeldt 250-49.5 2,862,129 11/1958 Van Dorsten 25049.5 X 2,877,353 3/1959 Newberry 25049.5 2,916,621 12/1959 Wittry 25049.5 2,944,172 7/1960 Opitz et al. 25049.5 X 2,987,641 6/1961 Wegmann 25049.5 X 3,049,618 8/1962 Thome 25049.5

WILLIAM F. LINDQUIST, Primary Examiner.

RALPH G. NILSON, Examiner.

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