US3659097A

US3659097A - Magnetic lenses

Info

Publication number: US3659097A
Application number: US115847A
Authority: US
Inventors: Richard Bassett; Thomas Mulvey
Original assignee: NAT RES DEV
Current assignee: NAT RES DEV; NATIONAL RESEARCH DEVELOPMENT CORP
Priority date: 1971-02-16
Filing date: 1971-02-16
Publication date: 1972-04-25
Anticipated expiration: 1989-04-25

Abstract

Apparatus having a generator of a beam of charged particles and magnetic focusing means in which a beam of charged particles is brought to a focus between the source and the focusing means so that radiation from a target placed at the focus can be received by a radiation detector within a substantial solid angle, bounded by said magnetic focusing means and the beam of charged particles. Preferably the focusing means is an electrically conducting coil of substantially flat or shallow conical form having a conical half-angle of not less than 75*.

Description

o l-@5 72 XR 3,659,097

United States Patent 1151 3,659,097 Bassettetal. [4 1 M11225, 1972 [54] MAGNETIC LENSES 3,008,044 11/1961 Buchhold ..250/49.5

3,189,953 6/1965 Smith,.lr. .219/121EBX [721 lnvemms Rich! 3 9* Cheltenham; Thlmas 3,417,224 12/1968 Steigerwald et al... ....219/121 EB Blrmmgham, bmh Englarld 3,437,734 4/1969 Roman et al ..219/121 EB x [73] Assignee: National Research Development Corporai L d E l d Primary Examiner-William F. Lindquist Attorney-Cushman, Darby & Cushman [22] F1led: Feb. 16, 1971 g [21] Appl. No.: 115,847 ABSTRACT Related Application Data Apparatus having a generator of a beam of charged particles and magnetic focusing means in which a beam of charged par- [63] Continuation of Ser. No. 781,219, Dec. 4, 1968, abanticles is brought to a focus between the source and the focusdoneding means so that radiation from a target placed at the focus can be received by a radiation detector within a substantial 219/121 solid angle, bounded by said magnetic focusing means and the 250/495 PE beam of charged particles. [51] Int. Cl. ..l-l0lj 37/26 58 Field of Search ..219/121 EB; 250/495 A, 49.5 B, Preferably the focusmg means 18 an elecmcally conductmg 250 495 D, 9 5 313/34 coil of substantially flat or shallow conical form having a conical half-angle of not less than 75.

[56] References cued 11 Claims, 7 Drawing Figures UNITED STATES PATENTS I 2,058,914 10/1936 Riidenberg ..250/49.5

PATENTEDAPR 2 5 I972 sum 2 OF 3 FIG4 I I I I I I I I f iv Invmtar 8 Horny;

PATENTEDAPR 25 m2 SHEET 3 OF 3 FIG] ' MAGNETIC LENSES This application is a continuation of application, Ser. No. 781,219 filed on Dec. 4, 1968, now abandoned.

. This invention relates to magnetic lenses for the focusing of beams of charged particles, in particular of beams of electrons.

It is well known to focus a beam of electrons by causing it to pass axially through a magnetic field of symmetrical distribution, the magnetic field being produced by a current carrying coil positioned around the beam. The focusing of electron beams is required, for example, in electron beam cutting or welding apparatus, in the illuminating system of electron microscopes of the transmission type, in scanning electron microscopes and in electron probe X-ray micro-analysis apparatus. In all these forms of apparatus, but particularly in the last named form of micro-analysis apparatus an electron beam is required to be focused on a small target area. It is desirable that the area in which the beam is concentrated should be very small and compact, and it is therefore'essential that the aberrations of the lens should be as small as possible.

Magnetic lenses used hitherto have been positioned around the path of the beam and may occupy a considerable distance along the beam. Such lenses so positioned can impose undesirable structural and design limitations on the apparatus in which they are used. Thus, for example, in electron microscopes, magnetic condenser lens systems are employed which are closely positioned around the electron beam so that a very powerful pumping system is required to maintain the desired vacuum conditions within the constricted passageways.

X-ray micro-analysis depends on the detection of X-rays emitted from the region of a specimen bombarded by electrons, the X-rays being characteristic of elements present at the region of bombardment. The X-rays are emitted in all directions relative to the surface of the specimen, from nonnal almost to a grazing angle. It has been a characteristic of previously known electron probe X-ray micro-analysis apparatus that the X-ray emission has been intercepted over an appreciable solid angle by the lens structure through which the electron beam has had to pass to be focused on the specimen. This has caused low sensitivity of detection of the emitted X- rays, and mechanical difficulties in the placing of devices for viewing the specimen and of X-ray spectrometers for analyzing the emitted X-rays into their various components.

\ The present invention provides a magnetic lens system for focusing a beam of charged particles which can have highly satisfactory optical properties, in particular low spherical aberration, and which can be so shaped and positioned with respect to the target position that considerable improvements can be achieved in the overall design of apparatus using existing magnetic lens systems.

The present invention is based on the discovery that a magnetic lens system having highly satisfactory optical properties can be provided by a conducting wire coil positioned close to the target position relative to its diameter, even when the coil is positioned behind the target position with respect to the beam of particles.

In accordance with the invention, an apparatus which produces a beam of charged particles is provided with a magnetic lens system for focusing the beam onto a target position which comprises an electrically conducting wire coil which is positioned around the axis of the beam either in front of or behind the target position by a working distance which is less than the mean diameter of the coil. The mean diameter of the coil is the mean between the inner and outer diameters of the coil winding.

When the wire coil is positioned in front of but close to the target position as above defined, a desirably low value of spherical aberration may be achieved by constructing the coil with the difference between its outer and inner annular radii at least twice its axial thickness and with the outer radius of its annulus at least twice the inner radius. The axial thickness of a coil is the average distance through the actual coil winding in an axial direction. Either or both of these conditions may be advantageously applied to a wire coil placed in a negative position, that is to say, a position behind the target position.

Such coils may be said to be in the form of thick discs or plates and although the coils have preferably flat circular faces, it should be understood that the coils may be shallowly coned or dished so that in position they present a concave face to the source of charged particles. The conical half-angle should not be less than 75', preferably not less than although when the coil is behind the target position the halfangle may usefully be as low as 60.

A magnetic lens in accordance with the invention is preferably a coil having a disc or even pancake shape with its outer radius at least three times its inner annular radius and the outer radius at least five times its axial thickness. The whole of such a coil can be positioned close to the target position either behind or in front of the target position and exert a powerful focusing action on the beam. The disc shaped coil can be conveniently fitted into apparatus leaving the whole space between the source of charged particles and the target position around the beam of charged particles, available for siting a radiation detector or any other device or structure. One important practical advantage of siting the coil behind the target position is that the coil may be conveniently placed in cryogenic apparatus so that it is maintained at a temperature at which it is superconducting, thus permitting very high energizing currents and hence very strong magnetic focusing fields to be employed.

According to an optional feature of the invention, the magnetic lens is in the form of a coil which has a layer of material of high magnetic permeability applied to the side remote from the source of charged particles. The layer of magnetic material may be continuous within the periphery of the coil and preferably is provided with a removable central portion concentric with the coil so that the coil may be positioned around the beam of charged particles.

The layer of magnetic material may also extend round the periphery of the coil and cover at least a portion of the side facing the source of charged particles.

According to a further optional feature of the invention, the magnetic backing provides an electrical connection to the center of the coil.

Although one important advantage of the disc shaped magnetic lens coil is that relatively high current densities may be used to produce strong magnetic focusing fields as it has relatively large surfaces which may be used to cool the coil, the coil may be cooled or further cooled by cooling means located, for example, within the layer of magnetic material or within the structure of the coil.

The invention will be further described, by way of example only, in relation to the accompanying drawings in which:

FIGS. 1 and 2 are diagrammatic sectional views of apparatus showing the relative position of a lens coil, a target and a beam of charged particles;

FIG. 3 illustrates in section along its axis a lens including a magnetic backing for the coil and means for cooling the coil;

FIG. 4 illustrates in section along its axis an alternative form of coil construction and cooling there for, and an alternative construction of the magnetic backing;

FIG. 5 illustrates in section a mode of making a connection to the inner end of the coil;

FIG. 6 illustrates in section an arrangement for maintaining the coil at a temperature at which it is superconducting; and

FIG. 7 illustrates in section an embodiment of the invention in which the coil is positioned in front of the target.

Referring to FIG. 1, 11 represents diagrammatically a source of charged particles, hereinafter referred to for convenience as electrons, collimated by means not illustrated into a beam 12. Concentric with the axis 'of beam 12, produced, and in a plane normal thereto, is situated a coil 13, having a front 14 facing the source 11 and a back 15. The coil is of generally annular shape, having an inner radius R1 an outer radius R2 and an axial thickness T. The mean diameter MD is (R2 R1) and, as shown, is many times greater than the working distance W, the distance between the front of the coil and the focus F. By preferably making the difference (R2 R1) between the inner and outer annular radii of the wire coil at least twice its axial thickness T, the outer annular radius R2 of the wire coil at least twice and preferably three times its inner radius R1, and the ratio of the outer radius R1 of the annulus of the wire coil not less than five times its axial thickness T it has been found that the magnetic lens system has highly satisfactory optical properties and can produce strong magnetic focusing fields which can focus high energy electrons without bringing about excessive difficulty with dissipation of heat from the wire coil. Suitable excitation of the coil 13 will cause the beam 12 of electrons to be brought to focus at a target position F, at a distance W, the working distance, in front of the inner boundary of the coil 13. A specimen 16; may be placed so that F lies in its surface. The specimen may be microwelded or investigated, for example, as the object of electron microscopical examination or by electron probe X- ray micro-analysis. It is for the latter purpose that the present invention is particularly advantageous, since there is very little restriction of the angle over which the emitted X-rays may be detected, whereas, as explained above, there is considerable restriction with magnetic lenses of known form.

In FIG. 1 the coil has been shown substantially flat in shape, but similar properties are exhibited by coils of a shallow conical shape, e.g. as shown diagrammatically in FIG. 2. FIG. 2 also shows, in block form, an X-ray detecting device, 38, receiving radiation along a direction indicated by the broken line 39, from the specimen 16. The detector 38 may be placed anywhere in the solid angle subtended at the focus F by the periphery 40 of the coil 13, except, of course, for the relatively small solid angle occupied by the electron beam 12.

An improved performance of the lens may be obtained by providing the coil, on the back face, with a plate of magnetic material, indicated by 17 in FIG. 3. This magnetic material may, by way of example, be soft iron, transformer steel or mumetal. By reducing the reluctance of the magnetic circuit through the coil it enables higher flux densities to be employed and more energetic electron beams to be focused. It is possible to reduce the reluctance further by extending the backing of magnetic material round the periphery of the coil through a ring 18 of magnetic material, and, if desired, over the front face of the coil in the form of an annular plate 19. In FIG. 3 the plate 19 has been shown with the same inner radius as that of the coil, but the inner radius of the annular plate may be greater than this.

It may be convenient from other design consideration for the backing of magnetic material to be tapered in thickness towards the rim, as shown at 20 in FIG. 4. This presents no magnetic disadvantage since in a plate of uniform thickness the flux density will be considerably greater near the middle than at the rim. The plate may be radially tapered so as to maintain the flux density substantially constant through the material of the plate.

The distribution of the magnetic field strength along the coil axis may conveniently be described by reference to the halfwidth. This is the distance, measured along the coil axis, between the points on the distribution curve at which the field strength is half the maximum field strength. For the coil configuration herein described, the half-width of the axial magnetic field distribution is typically in the range 2R1 to 10R1, where R1 is the inner radius of the coil as indicated in FIG. 1.

It is desirable that the magnetic field produced by the coil 13 be as symmetrical as possible. Any disturbance of symmetry is most likely to arise with the connection to the inner radius of the coil, and FIG. illustrates one way in which this may be made. Between the coil 13 and the magnetic backing 17 may be placed a thin conducting layer 22 which may consist, e.g. of copper foil. To this may be connected, by soldering, for example, the lead 23 to the inside of coil 13. Connection to the foil may be made at a number of equally spaced locations around the periphery, which approximates to that of the magnetic backing, so that the flow of current to the inner connection to the coil may be rendered substantially symmetrical with respect to the coil axis. The connection to the outside of the coil has a negligible influence on the symmetry of the magnetic field produced by the coil, at least in the region near the axis, which is the most important.

FIG. 5 illustrates a coil wound with wire, but it is possible, as an alternative, to construct a continuous spiral of insulated thin metal tape to form a coil of similar overall dimensions. If anodized aluminum tape is employed, there is no need for insulation over and above that provided by the oxide layer since the voltage drop from one turn to the next is relatively small; The almost solid metallic coil winding so produced promotes the removal of heat.

When the most intense magnetic fields are required, the necessary current may produce considerable quantities of [heat within the coil structure. In order to avoid damage, cooling is desirable. This may be effected, by way of example, by means of passages in the magnetic backing through which cooling fluid may be made to flow. FIG. 3 illustrates an example of such construction. The face 15 of the coil 13 is bonded closely to the magnetic backing 17 so that heat flow between them is promoted. This may be effected with, for example, an epoxy resin loaded with powdered metal or other substances of high thermal conductivity. In the magnetic backing is formed a series of passageways, 24, having an inlet 25 and an outlet 26. The passages may be in the form of a spiral and are situated as near as practicable to the surface of the magnetic backing adjacent to the coil. The passages 24 may be in the form of tubes cast into the magnetic backing. As one alternative they could be milled into the surface and covered with a thin sheet of bonded on material, which could either be magnetic, or if non-magnetic could be so thin as not appreciably to increase the reluctance of the magnetic circuit. It may be desirable to cool the coil from the front face as well and this may be effected with a similar set of passages 27 having inlet and

outlet

28 and 29 respectively. The plate 19 in which these passages are formed may be wholly or partly of magnetic material, or may be wholly of non-magnetic material. If desired, the plate 17 may also be of non-magnetic material if the design of the lens does not demand a low reluctance path.

For lenses of the highest power it may be desirable to adopt a different coil construction in order to assist cooling to a greater degree, as illustrated in FIG. 4. The coil is wound in a number of concentric sections 30, offset from each other alternately in an axial direction so as to provide passages 31 between the coil sections. The coils are mounted alternately on

annular plates

32, 33 and the whole is sealed by inner and

outer tube members

34, 35 respectively. The passages 31 are supplied with cooling liquid through inlet 36 and outlet 37. In order to avoid interference with the symmetry of the magnetic field produced by the lens the inlet pipe 36 is made of nonmagnetic material. As in this construction the cooling fluid is in contact with the coil sections it is desirable that it be an inert substance with good dielectric properties. A suitable fluid is transformer oil.

Heating effects in the coil may be avoided altogether by working under superconducting conditions. In these conditions the mode of working in which an electron beam is brought to a focus between the electron source and the lens coil, as illustrated in FIG. 1, is particularly advantageous since the bulky cryogenic equipment does not obstruct the X-rays emitted from the specimen 16 as in the case when the lens coil is situated between the electron source and the electron beam focus as in more conventional forms of lens.

Apparatus according to the invention, for working under super-conducting conditions may be arranged as illustrated diagrammatically and by way of example in FIG. 6. The source, 11, of charged particles, the specimen l6 and the X- ray detecting device 38 are arranged within an evacuable enclosure 41, supported by plate 48, the electrical connections to the source and detecting device not being shown. The coil 13, the electrical connections to which are not shown, is sup ported from plate 49 a short distance below the base of enclosure 41 so that both faces may be kept in contact with liquid helium, 42, in a double-walled container 43, for which the plate 49 provides a cover. For working under super-conducting conditions the conducting wire of the coil is preferably made of an alloy of niobium and zirconium or an alloy of niobium and titanium or of niobium stannide (Nb Sn). The vessel 43 may be filled through pipe 46, and filling assisted by the provision of a vent pipe 47. Either or both pipes may be connected to apparatus, (not shown) which functions to collect and store for re-use, helium evaporating from the main body of liquid 42. In order to reduce heat transfer to the vessel 43 and its contents, that vessel may be immersed in liquid nitrogen 44 in an outer double walled container 45 for which the plate 48 forms a cover.

A specific example of the invention has the inner coil radius R1 3.3 cm and the outer radius R2 cm, so that R2/Rl 3. If the electron beam has an accelerating voltage of KV, and the coil provides an mrnf of 2,700 ampere turns, a working distance of minus 1 cm is obtained, in the location shown in FIG. 1; that is to say, the beam is brought to focus between the beam source and the focusing coil.

In a second specific example of the invention the inner coil radius R1 2 cm and the outer radius R2 ll. 4 cm, so that R2/R1 5.7. if the electron beam has an accelerating voltage of KV, and the coil provides an mmf of 1,600 ampere turns, a working distance of minus 2 cm is obtained.

FIG. 7 illustrates an embodiment of the invention in which the wire coil 13 is in front of the target 16. The coil is positioned around the electron beam 12, which is brought to a focus at a target position F at a working distance W behind the inner boundary of the coil 13. it has been found that when the wire coil is arranged so that the working distance W is less than the mean diameter MD of the coil, while the difference (R2 R1) between its outer and inner annular radii is at least twice its axial thickness T and the outer radius R2 of its annulus is at least twice the inner radius R1 this magnetic lens system with the wire coil in front of the target also has highly satisfactory optical properties and can produce strong magnetic focusing fields which can focus high energy electrons.

The construction illustrated in FIG. 4, in which the wire coil is provided with a magnetic backing, may be adapted to work, when required, according to the embodiment illustrated in FIG. 7 by the provision of a removable portion 21, at the center of the coil backing, through which the electron beam 12 may pass to reach a focus at the far side of the coil under suitable conditions of coil excitation and electron beam ener- It may be convenient, in particular to promote dissipation of heat generated within the wire coil of the magnetic lens system, to construct the coil so that its axial thickness tapers from the inner radius outward. The axial thickness T of the coil would then be defined as the average axial thickness. Likewise the conical angle of the coil would be the average conical angle between the angles of the inner and outer conical faces of the coil.

We claim:

1. An apparatus which comprises a source arranged to produce a beam of charged particles and a magnetic lens for point focusing the beam onto a target position located between said source and lens, the magnetic lens coilwise consisting of electrically conducting wire coil means positioned around the axis of the beam and disposed fully behind the target position by a working distance which is less than the mean diameter of the coil to effect said point focusing onto said target position.

2. An apparatus according to claim 1 in which the difference between the inner and outer annular radii of the wire coil is at least twice its axial thickness.

3. An apparatus according to claim 1 in which the outer radius of the annulus of the wire coil is at least twice its inner radius.

4. Apparatus according to claim 1 in which the outer radius of the annulus of the wire coil is at least three times its inner radius.

5. Apparatus according to claim 1 in which the outer radius of the annulus of the wire coil is not less than five times its axial thickness.

6. Apparatus according to claim 1 in which the coil has a layer of material of high magnetic permeability applied to the side remote from the source of charged particles.

7. Apparatus according to claim 6 in which the layer of magnetic material is provided with a removable central portion concentric with the coil.

8. Apparatus according to claim 6 in which the coil has cooling means for cooling the coil located within the layer of magnetic material.

9. Apparatus according to claim 1 in which the coil has cooling means located within the structure of the coil for cooling the coil.

10. Apparatus according to claim 1 which includes means for maintaining the coil at a temperature at which it is superconducting.

11. Apparatus according to claim 10 in which the coil is positioned so that it can be contacted by liquid helium over substantially the whole of its surface.