US3870891A

US3870891A - Magnetic lenses

Info

Publication number: US3870891A
Application number: US393532A
Authority: US
Inventors: Thomas Mulvey
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1972-09-04
Filing date: 1973-08-31
Publication date: 1975-03-11
Anticipated expiration: 1992-03-11
Also published as: GB1395201A

Abstract

Apparatus in which a beam of charged particles is brought to a focus by means of a magnetic lens situated behind the target position, leaving the area in front of the target clear, the magnetic lens comprising an electrically conducting coil arranged around the axis of the beam and a magnetic polepiece which extends along the axis of the beam at least within the space surrounded by the coil. Preferably the polepiece protrudes and at least partly tapers towards the target position.

Description

1 1 Mar. 11, 1975 1 1 MAGNETIC LENSES [75] lnventor: Thomas Mulvey, Birmingham,

England [73] Assignee: The National Research Development Corporation, London, England 22 Filed: Aug. 31, 1973 21 Appl. No.: 393,532

[301 Foreign Application Priority Data Sept, 4, 1972 Great Britain 40888/72 [52] U.S. C1. 250/398, 250/396 [51] Int. Cl. H0lj 37/26 [58] Field of Search 250/396, 398; 313/89 56] References Cited UNITED STATES PATENTS 2,438,971 4/1948 Hiller 250/398 3,560,739 2/1971 Wolff 250/398 3,587,013 6/1971 Dietrich et a1 250/396 3,629,578 12/1971 Le Poole 250/398 3,687,716 8/1972 Steigerwald 250/398 Primary Examiner-Archie R. Borchelt Assistant E.\'aminerB. C. Anderson Attorney, Agent, or Firm-Elliott l. Pollock [57] ABSTRACT Apparatus in which a beam of charged particles is brought to a focus by means of a magnetic lens situated behind the target position, leaving the area in front of the target clear, the magnetic lens comprising an electrically conducting coil arranged around the axis of the beam and a magnetic polepiece which extends along the axis of the beam at least within the space surrounded by the coil. Preferably the polepiece protrudes and at least partly tapers towards the target position.

9 Claims, 3 Drawing Figures MAGNETIC LENSES This invention relates to magnetic lenses for the focusing of beams of charged particles, .in particular 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 microanalysis 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.

Conventional magnetic lenses which are used in these-forms of apparatus are positioned about the path of the beam and may accupy 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 aroundv 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 normal almost to 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 hashad, 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 analysing the emitted X-rays into their various components.

A magnetic lens system, as described in British Pat. Specification No. 1,253,652 having a conducting wire coil positioned behind the target position with respect to the beam of particles, was invented to overcome these mechanical difficulties. A magnetic lens system has now been devised which in its optimal configuration provides a substantial improvement over the aforementioned magnetic lens system.

According to the invention, an apparatus comprises a source arranged to produce a beam of charged particles and a magnetic lens which provides substantially the only means for focusing the said beam onto a target position between the said source and magnetic lens, the magnetic lens comprising an electrically conducting wire coil arranged around said axis and a polepiece composed of material of high magnetic permeability which extends along said axis within the space surrounded by the coil. The term electrically conducting wire coil includes a single coil or a plurality of coils electrically connected so that, when suitable excited, they will produce the magnetic field required to focus the beam onto the aforesaid target position.

Preferably the coil is circular and the magnetic polepiece is of circular cross-section. The polepiece may lie partially or wholly within the space surrounded by the coil and in the majority of applications it will occupy at least half this space. The cross-sectional area of the polepiece is preferably as large as the dimensions of the coil will allow and optionally at least part of the polepiece tapers in the direction of the source of charged particles. The total angle of taper is preferably greater than in a number of preferred applications, the polepiece may have a snout which projects beyond the side of the wire'coil facing the source of charged particles. The snout may be shaped symmetrically about the axis of the beam of charged particles so as to induce a magnetic field configuration required for focusing the beam of charged particles in a desired manner. Such a magnetic field configuration may for example demand a high localized flux density and it will be recognized by a person skilled in the art that care must be exercised to ensure that the magnetic material of the polepiece is not unnecessarily driven into saturation in such a situation. If it is found to be necessary, the polepiece may be tapered or stepped along its length to overcome problems of saturation. This may be required, for example, if the snout projects a considerable distance beyond the side of the coil facing the source of charged particles.

In a given application, it is advantageous to reduce the reluctance of the magnetic circuit linking the coil as much as possible without altering the essential focusing properties of the coil. Thus it is usual to minimize the length of the magnetic circuit in air or in vacuo by confining the magnetic circuit to a layer of magnetic material for that part of its length where it does not contribute to the focusing of the beam of charged particles provided both that the magnetic field required for focusing the beam can be achieved with the given polepiece configuration and that the given polepiece is mechanically suitable for the given application.

According to a preferred feature of the invention therefore, the magnetic lens has a layer of material of high magnetic permeability arranged on the side of the coil remote from the source of charged particles. The layer of magnetic material is preferably continuous within the periphery of the coil and is integral with or is in intimate contact with the central cylindrical polepiece.

The layer of magnetic material may also extend around the periphery of the coil and in some circumstances this peripheral magnetic layer may project beyond the side of the coil facing the source of charged particles. In a further embodiment, the aforesaid layer of magnetic material may extend to cover an outer portion, but not the whole, of the side of the coil facing the source of charged particles. The peripheral, or extended, magnetic layer may be suitably shaped so as to modify the magnetic field distribution.

In some applications, an apparatus according to the invention is contained within a single vacuum chamber. In such applications, the magnetic lens is preferably constructed to have vacuum properties compatible with the conditions required within the chamber. This may conveniently be done by encapsulating the electrically conducting wire coil in a vacuum-tight enclosure, which enclosure may be formed by either a part or the whole of the combination of the aforementioned layer of magnetic material and the magnetic polepiece together with a layer of non-magnetic material (for example brass or non-magnetic stainless steel) joined together with vacuum-tight seals to complete the said enclosure. However, it is an important feature of the invention that the magnetic lens may be situated outside the vacuum chamber behind the target position, thereby simplifying the construction of the magnetic lens which is no longer required to be compatible with the vacuum conditions existing within the chamber.

Preferably the elctrically conducting wire coil of the magnetic lens is made as small as possible consistent with the satisfactory operation of the invention. This is not essential, particular when in a preferred application the magnetic lens is so constructed that the magnetic circuit is confined by a layer of magnetic material for the greater portion of its length in such a case, the magnetic-field will be approximately uninfluenced by the geometry of the coil and the magnetic field strength on the axis of the lens will depend chiefly on the product of the number of turns of the coil and the electric current passing through them. However it is normally advantageous to minimize the size of the coil as this enables the total size of the magnetic lens to be kept to a minimum, thereby reducing the possibility of setting up stray magnetic fields which might affect the electron optical properties of the system.

The electrically conducting wire coil may be wound either with wire of any convenient cross-sectional shape or with thin metal strip. Anodized aluminum tape may conveniently be employed as in this form of construction 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 coil may be in the form ofa thick disc or plate asdescribed in British Pat. Specification No. 1,253,652 as the superior cooling properties of such a coil allow higher electric currents to be used. The size of the magnetic lens may be significantly reduced by comparison with conventional lenses by providing the coil with cooling means located, for example, within the layer of magnetic material or within the structure of the coil, thereby allowing a required magnetic field to be generated by a larger electric current flowing in a smaller number of turns of wire.

In a preferred application, in which the wire coil is encapsulated in a water tight enclosure, water is circulated around the wire of the coil and the electrical connections to the coil may conveniently be made via insulated wires which are taken out of the enclosure via the water inlet and outlet pipes. This method of construction may be preferable when the enclosure is also vacuum tight and the coil is contained in a vacuum chamber together with the remainder of the apparatus.

According to an optional feature of the invention, the magnetic polepiece of the lens may be provided with a small circular axial hole throughout its length to permit the passage of any portion of the beam of charged particles which has been transmitted through the target onto which it has been focused by magnetic lens. The hole may be of uniform diameter along its length or it may he stepped or tapered to allow for any spreading in the beam of transmitted charged particles. The hole will normally be of a diameter small enough to cause little or no disturbance of the magnetic field used to focus the beam. In a preferred configuration in which the magnetic lens has a layer of magnetic material applied to the side of the coil remote from the source of charged particles and the layer is continuous within the periphery of the coil, the aforementioned hole is extended through this layer along the axis of the lens. A magnetic lens provided with an axial hole may be used as a barrier between two volumes within a vacuum chamber containing an apparatus in accordance with the invention. The dimensions of the hole may be such that the hole has a low conductance to air at room temperature under conditions of molecular flow and thus it may be possible to maintain a pressure differential across the hole.

An important practical advantage of the invention is that a magnetic lens, as herein described may be conveniently fitted into an electron beam or like apparatus so as to leave 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 the like.

A further practical advantage of the invention is that a magnetic lens as herein described, in focusing a beam of charged particles, can readily introduce a lower level of spherical aberration into the electron optical system of the apparatus than can be achieved using conventional lenses. This is because the aberration depends predominantly upon the dispositioning of the polepiece which can be accurately machined and positioned, rather than on the coil winding which requires great care to be evenly wound. However in some applications requiring a very high level of symmetry of the magnetic field, it may be useful to use sheets of conducting material having radial symmetry to provide electrical connections to the wire coil, as described in U.S.. Pat. specification No. 1,253,652.

In addition, a magnetic lens as herein described provides an essentially radial magnetic field to focus a beam of charged particles. This may be particularly useful as it allows, for example, secondary electrons displaced from the target position to be collected at a point about the axis of the beam close to the lens without the need for a relatively large electrostatic field of attraction to overcome the effect of the focusing magnetic field on the secondary electrons.

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

FIG. 1 is a diagrammatic sectional view of apparatus showing the relative position of a magnetic lens, a target and a beam of charged particles and FIGS. 2 and 3 illustrate in section along their axis two examples of magnetic lenses incorporating a number of the preferred features of the invention.

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 the beam 12, produced, and in a plane normal thereto, is situated a coil 13 of. annular shape, having a front face 14 facing the source 11 and a back face 15. A cylindrical magnetic polepiece 16, manufactured from eg soft iron or other magnetic material of high permeability, occupies the whole of the volume surrounded by the coil 13. Polepiece 16 projects from the coil 13, a short distance towards the source 11 and is provided with a flange'16A, also of magnetic material, which backs the coil 13. The magnetic lens system defined by the coil 13 and the

magnetic material

16 and 16A can be suitably excited to cause the beam 12 of electrons to be brought to a focus at a target position F situated at a point on the axis of the beam 12 between the front face 14 of the coil and the source 11. A specimen 17 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 re striction with magnetic lenses of known form. An X-ray detecting device 18 is shown in block form, receiving radiation. along a direction indicated by the broken line 19. Also shown as an alternative is a collecting device 20, of annular form, which may be positioned to be coaxial with the beam 12 and held at a positive potential with respect to the specimen 17 to collect secondary electrons produced by interaction of the beam 12 with the specimen 17.

A more sophisticated example of the magnetic lens as herein desscribed is illustrated in FIG. 2. This lens is constructed to have a circular coil 13, which is a combination of two

annular coils

21 and 22, and a cylindrical magnetic polepiece 16 which has a snout 23 which is symmetrical about the axis of the polepiece l6 and which projects beyond the front face 14 of the coil 13 towards the source 11. Integral with the polepiece 16 is a layer 24 of material of high magnetic permeability which is applied to the back of the coil 13 and which extends around the periphery of the coil 13. The lens is positioned to be coaxial with the beam 12 and can be suitably excited to cause the beam 12 of electrons to be brought to a focus at a target position F situated at a point on the axis of the beam 12 between the front face 14 of the coil 13 and the source 11.

The polepiece 16 and the layer of magnetic material 24 are provided with a small hole 25 drilled along the axis of the lens to allow for the passage of any part 26 of the beam 12 which may be transmitted through a specimen 17 placed so that F lies in its surface. That part 26 of the beam 12 may be collected by a suitable detector 27.

The lens is constructed to allow for water cooling of the coil 13. A plate 28 of non-magnetic material (for example non-magnetic stainless steel) 'is bolted (by means not shown) to the layer of magnetic material 24 and together with the layer of magnetic material 24 and the polepiece 16 forms an enclosure for the coil 13. O-

ring seals

29 and 30 are provided as shown to make the enclosure water-tight and two non-magnetic

stainless steel pipes

31 and 32 are argon-arc welded into the layer of magnetic material 24 to allow for the inflow and outflow of water to cool the coil 13.

The electrical connections to the

coils

21 and 22 are made via insulated wires (not shown) which are brought out of the lens enclosure via the

water pipes

31 and 32. The

coils

21 and 22 may be electrically connected to power supplies (not shown) in any way suitable to produce the magnetic field required to focus the beam 12 at the target position F.

A further example of a magnetic lens as herein described is illustrated in FIG. 3. This lens is constructed basically the same as the lens shown in FIG. 2, having a coil 33 which is a combination of two

annular coils

34 and 35. The lens however has a radially symmetric polepiece 36 which tapers along the whole of its length in the direction of the source of charged particles 11 and which projects beyond the face 37 of the coil 35 towards the source 11.

The polepiece 36 tapers at a total angle of 45along the larger part 38 of its length and at a total angle of along the smaller part 39 of its length which projects beyond the magnetic lens towards the source 11. The coil 35 is wound to have a smaller inner diameter than its companion coil 34 in order to make better use of the available space.

The polepiece 36 is provided with a circular hole 40 along its axis throughout its length to allow for the passage of any part 26 of the beam 12 which may be transmitted through a specimen 17 placed so that F lies in its surface. The diameter of the hole 40 increases by steps along its length away from the source 11 of charged particles to allow for spreading of the part 26 of the beam 12 transmitted through the specimen 17.

We claim:

1. An apparatus which comprises a source operative to produce a beam of charged particles along a given axis, and a magnetic lens which provides substantially the only means for focusing the said beam onto a target position on the axis between the said source and magnetic lens, the said magnetic lens being operative to provide a substantially radial field in the focusing region, said lens being located on the side of said target position opposite to said source and comprising an electrically conducting wire coil having a front face which lies in a plane perpendicular to the said given axis, the axis of said coil being coaxial with said given axis, said lens further comprising a polepiece composed of material of high magnetic permeability which extends along said core axis towards said source at least within the space surrounded by the coil.

2. An apparatus according to claim 1 wherein said polepiece occupies at least half of the space surrounded by the coil.

3. An apparatus according to claim 1 wherein said polepiece projects beyond the said front face of the electrically conducting wire coil toward the source of charged particles. 1

4. An apparatus according to claim 1 wherein at least part of said polepiece tapers in the direction of the source of charged particles.

5. An apparatus according to claim 4 in which the total angle of taper is not less than 45.

6. An apparatus according to claim 1 in which the magnetic lens has a layer of material of high magnetic permeability arranged on that side of the coil which is opposite to said front face and remote from the source of charged particles, the reluctance between said layer and said polepiece being low.

7. An apparatus according to claim 6 in which said layer of material of high magnetic permeability extends around the periphery of the coil and presents to the source of charged particles an end face having radial symmetry.

8. An apparatus according to claim 7 in which said electrically conducting wire coil is encapsulated in a fluid-tight enclosure which is formed by the combination of said layer of material of high magnetic permeability and said polepiece sealed to a layer of nonmagnetic material to complete the saidenclosure, and

means for circulating fluid through said enclosure throughout its length.

Claims

3. An apparatus according to claim 1 wherein said polepiece projects beyond the said front face of the electrically conducting wire coil toward the source of charged particles.

5. An apparatus according to claim 4 in which the total angle of taper is not less than 45*.

8. An apparatus according to claim 7 in which said electrically conducting wire coil is encapsulated in a fluid-tight enclosure which is formed by the combination of said layer of material of high magnetic permeability and said polepiece sealed to a layer of nonmagnetic material to complete the said enclosure, and means for circulating fluid through said enclosure around the coil in said enclosure to cool said coil.