US2777958A

US2777958A - Magnetic electron lens

Info

Publication number: US2777958A
Application number: US266298A
Authority: US
Inventors: Poole Jan Bart Le
Original assignee: Hartford National Bank and Trust Co
Current assignee: Hartford National Bank and Trust Co
Priority date: 1951-02-10
Filing date: 1952-01-14
Publication date: 1957-01-15
Anticipated expiration: 1974-01-15
Also published as: BE509097A; DE911878C; GB753562A; FR1058851A; CH302299A

Description

Jan. 15, 1957 J. B. LE POOLE MAGNETIC ELECTRON LENS 2 Sheets-Sheet 1 Filed Jan. 14, 1952 INVENTOR Jon Barf le Poole y WW Ag nt Jan. 15, 1957 J. B. LE POOLE 2,777,958

MAGNETIC ELECTRON LENS Filed Jan. 14, 1952 2 Sheets-Sheet 2 Fig. 3a

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I [I H I 28 INVENTOR. Fig. 40 JAN BART LE POOLE GENT United States Patent MAGNETIC ELECTRON LENS Jan Bart Le Poole, Delft, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application January 14, 1952, Serial No. 266,298

Claims priority, application Netherlands February 10, 1951 7 Claims. (Cl. 250-495) An electron beam can be circularly deflected by a homogeneous magnetic field, of which the lines of force are at right angles to the beam. It is known that such a field has a focussing effect on the electron beam in a radial direction. This effect is utilised in certain electron-spectrometers.

According to the invention, a transverse field magnetic lens for electrons or other energized particles, is characterised in that one of the two pole surfaces is a sector having an angle (lens angle) of not more than 270 of a plane of revolution and in that a plane at right angles to the axis of revolution, intersecting with one of the tangents of the generatrix of the plane of revolution under an angle of 30 or less at a distance from the axis of revolution which is equal to that of the tangent point, is the plane with respect to which the second pole surface is the reflection of the first pole surface or else forms the counterpole surface itself. The counterpole surface is defined to mean a pole surface located either along the plane which is perpendicular to the axis of revolution or located to constitute a mirror image of the first pole surface with respect to that plane, the two surfaces defining the region through which the charged beam passes and is acted upon by the magnetic field existing between the two surfaces.

As will be seen from the further explanation, a lens of particular quality is obtained, if the generatrix of the plane of revolution forms part of a parabola, the apex of which lies in the axis of revolution. Each tangent of this curve intersects the equatorial plane at a point being equally spaced apart from the axis of revolution as the tangent point.

However, with practically sufficient approximation, 21 parabola may be replaced by a tangent which gives the plane of revolution the shape of a conical surface. The tangent which, as far as its inclination with respect to the equalatorial plane and its point of intersection with this plane are concerned, fulfills the aforesaid condition,

coincides in this case with the generatrix.

The optical axis of the lens according to the invention is a circle section, of which the center lies on the axis of revolution and of which the radius varies with the magnetic field strength and the velocity of the particles. With a lens having conical pole surfaces the size of this radius acts upon the astigmatism of the lens. It is in this case a minimum, if the spacing between the pole surfaces in the area of the optical axis is twice the spacing between their point of intersection with the axis of revolution (the cone tops).

The strength of the lens is determined, among other things, by the size of the lens angle. Suitable values for this angle to ensure a strong lens lie between 120 and 135. The maximum lens strength is obtained at an angle of 127% It is usually inefficient to energise a magnetic electron lens, of which the lines of force extend primarily in the direction of the electron paths (longitudinal field lens) by means of a permanent magnet. This would be useful, if the electrons should have a velocity which is materially lower than that usually employed in electron microscopes. Since the transverse field lens, owing to the focussing efiect, requires a materially smaller magneto-motive force, the energisation is effected here by permanent magnetism, even at considerably higher electron velocities. This implies a simplification of the construction (no energising current circuit, no cooling) and an economy in losses.

The optically operating field of the electron lens must be arranged in a vacuous space. The energising winding of the radially symmetrical longitudinal field lens can be readily arranged outside the vacuous space, which is surrounded by the cylindrical yolre. The shape of the transverse field lens is less suitable for a similar arrangement. lt'is easier to arrange this lens as a whole in the vacuous space. The permanent magnet eliminates the disadvantage involved in the arrangement of an energising winding in the vacuous space.

Since the radius of curvature of the optical axis of the transverse field lens varies with the velocity of the energised particles, the image produced by this lens shifts in place at a variation of the acceleration voltage. This property of the transverse field lens may be utilised to scan the image; however, the same property involves a disadvantage, when the transverse field lens is used as an electron-optical system in an electron microscope, of which the working voltage is not sufficiently constant. This disadvantage may be mitigated by providing the microscope with a system of two or more transverse field lenses, which are traversed in succession by the electron beam and which deflect the latter in the same plane and in the same sense. These lenses may be arranged relative to one another and to the object in a manner such that the resultant image produced by the lens system does not exhibit any or substantially any chromatic shift.

In a particularly arranged electron microscope according to the invention the electron-optical lens system directs the electron beam on the mirror formed by the electrodes of the electron gun and reproducing on an amplified scale the image on which the beam thus directioned is focussed.

It is efiicient to interconnect the pole shoes of two or more transverse field lenses by means of a common yoke. In this case the co-opera-ting lenses constitute a structural unit.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing by way of example.

Fig. 1 serves to explain the focussing effect of a homogeneous magnetic field on electron paths.

Figure 2 shows a pole shoe of a transverse field lens viewed in the direction of the axis of revolution.

Fig. 3 is a cross sectional view of the transverse field lens, with which the pole shoe shown in Fig. 2 is associated, taken in the plane IIIIII of Fig. 2.

Fig. 3a is a perspective view of the transverse field lens shown in Fig. 3, together with a yoke.

Fig. 4 is a diagrammatical view of the arrangement of two transverse field lenses in an electron microscope according to the invention.

Fig. 4a is a cross sectional view of the diagrammatic arrangement shown in Fig. 4.

Referring to Fig. 1, the cross-hatched part represents a homogeneous magnetic field, of which the lines of force are at right angles to the plane of the drawing. Two electron paths 1 and 2, located in this plane, enter the field under an angle of to the boundary plane 3 of the magnetic field. In the field they are circularly curved by the Lorenz force and emerge from the field again as parallel lines. The circle arcs intersect with one another at point 4. If the boundary plane 3 is not flat, but curved with the axis 18 at point 19.

3 or broken in a manner such that the electrons in the field describe an are which diifers from 180, the initially parallel electron paths emerge from the field at an angle to one another. It is thus possible to perform focussing, which only applies to electron paths in a plane at right angles to the lines of force.

If the pole surfaces between which the electron paths extend are not flat, but if they are shaped in the form of planes of revolution, in accordance with the invention, a non-homogeneous magnetic field is produced between these pole surfaces. If these pole surfaces extend over a sector of 270 or less, a beam of electrons may be tangentially introduced into this field. The field has a focussing effect on this beam both in an axial and a radial direction. There is thus produced a lens.

Referring to Fig. 2, 5 designates a pole shoe of such a transverse field lens, viewed in the direction of the axis of revolution 6. Fig. 3 shows the two pole shoes 5 and 7 in a sectional view in the flat plane IIlllI through the axis 6 of Fig. 2. The sectional area of the pole surfaces 8 and 9 is a parabola, of which the top 10 lies on the axis 6.

The two pole shoes are made of ferromagnetic material and are associated with one magnetic system. They are interconnected by means of a yoke 41 (Fig. 3a.), which is surrounded by an energising winding or which includes a permanent magnet 42. If the system has a north pole at 7, the south pole is at 5. A non-homogeneous magnetic field prevails in the space between these poles, through which a beam of energised particles is passed. The strongly curved part of the pole surfaces near the axis 6, extending beyond the optically operating range of the 'lens, is cut off.

In the equatorial plane 11, which intersects the magnetic lines of force at right angles, one of the particles, which enter the field in a tangential direction, describes a circuit are about the axis 6. This are is to be considered as the optical axis of the lens. It is designated by 12. Paths 13 and 14, which, previously to entering the lens, extend parallel to the optical axis 12 on either side thereof, are curved in the magnetic field in a manner such that they intersect with the axis 12 at point 15. This point, which lies at the boundary of the field in the example shown, is the focal point of the lens.

Paths extending in the plane going through the optical axis 12 at right angles to the equatorial plane and being parallel to one another before they enter the lens also curved in the non-homogeneous magnetic field in a manner such that they intersect with the axis 12 at point 15. Consequently complete focussing take place, i. e. about each point of the optical axis the focussing effect is the same in two directions at right angles to one another.

A focussing which is generally sufficient for practical purposes may be obtained by replacing the parabolic sectional areas of the pole surfaces by

straight lines

16 and 17 i. e. by rendering the pole surfaces conical. The astigmatism produced by this approximation is minimized, if, in the area of the optical axis 12 the spacing between these lines is twice the spacing between their points 'of intersection with the axis of revolution 6, i. e. if a=2b. The straight lines 16 and ll7 are tangents to the parabolic lines 8 and 9 at points 34 and 35.

The position of the optical axis, i. e. the radius of the path in the equatorial plane, which is circularly curved,

'varies with the magnetic field strength and with the velocity of the particles. If the voltage which accelerates the particles increases, the radius of this path also increases and the optical axis shifts in position, for example, to occupy the circle are 18. The paths 13 and 14, which are both located on the same side of the new optical axis, are then curved in a different manner, so that they intersect It follows therefrom that at a variation of the acceleration voltage the image produced by the transverse field lens shifts in position. With a transverse field lens having conical pole surfaces this variation furthermore affects the image sharpness, since with this lens astigmatism is only absent, if the aforesaid condition with respect to the position of the optical axis is fulfilled.

If the angle between the tangents or generatrices becomes large, the curvature of the magnetic lines of force becomes so strong, that the lens will exhibit optical errors. It has been found that as long as the inclination of these lines remains below 30, these errors are, in most cases, permissible.

The strength of the lens varies with the angle 0- between the end surfaces 20 and 21 of the pole shoes, which may be termed the lens angle. The maximum lens strength is obtained, if this angle is 127%". The focal distance of a transverse field lens having a lens angle of this maximum value is R x/Z where R designates the radius of the are, which is formed by the optical axis of the lens. The pole shoe shown in Fig. 2 has a lens angle 0 of the said optimum value, so that the focal points of the lens provided with such pole shoes are located in the boundary planes 2t) and 21. With a lens having conical pole surfaces this only applies if the condition a=2b is fulfilled.

The maximum limit of 270 for the lens angle of the lens disclosed in this application is set, since with angles exceeding 270 it is not always possible to center the elec tron beam about the optical axis.

The property of an image producing lens, according to the invention, that at variation of the electron velocity, the image shifts in position, i. e. the chromatic displacement, is a disadvantage for an electron microscope provided with such a lens. If the lens should have a separating power of A., the relative variation of the working voltage may not exceed This requirement is severe and often difficult to fulfill. However, the chromatic displacement may be reduced and even completely obviated, if the electron beam emerging from the lens is collected in a second transverse field lens, which curves the electron beam in the same sense. At a variation of the working voltage a displacement of the optical axis also occurs in this second lens, but owing to the reversal of the image, the displacement of the optical axis of the second lens corrects the displacement of the image. This is readily comprehensible by considering how, with a system of two positive glass lenses the course of the light rays varies when the system is displaced in a direction at right angles to the optical axis. For this purpose the case is considered in which the first lens projects a real, reversed image in front of the focal point of the second lens, this image being reproduced again in a reversed manner by the latter.

By a suitable choice of the arrangement of the second lens, and, if required, of one or more further lenses, the chromatic displacement may be eliminated, at least as far as it is a lens error of the first order. The chromatic displacement which is left as a lens error of higher order is so small that requirements for the constance of the working voltage and the magnetic field strength are less severe than with a longitudinal field lens having the same separating power. A relative variation of is permissible in many cases.

Figs 4 and 4a show an arrangement of two transverse field lenses for an electron microscope according to the invention. These figures also show diagrammatically the electron gun of the microscope. The latter is formed by a filament cathode 22, a Wehnelt cylinder 23 and an anode 24. The system formed by these electrodes emits a ray of electrons 25, if suitable voltages are applied thereto. This ray enters the transverse field lens 26 at right angles to one of the boundary surfaces designated 1 by 20 and 21 in Fig. 2; the ray is curved therein in a manner such that it emerges from this lens in the direction 27. It is assumed that the axis of the electron ray coincides with the optical of the lens 26, which'may be ensured by a suitable choice of the voltage or of the magnetic field strength and a suitable arrangement of the lens. This lens produces an amplified image at point 29 of a speciman arranged near the lens 26 at point 28. This image constitutes in its turn the object for a second transverse field lens 30, in which the ray is again curved in the direction 31.

The lens 30 could be caused to produce an image on a collecting screen struck by the ray 31. As an alternative, the microscope may be provided with a third electronoptical system, if this is required for the control of the amplification or for an increase in amplification.

The fact that the ray can be curved through a total angle of more than 180 by the two

lenses

26 and 30 provides the possibility of arranging the lenses in a manner such that the ray of electrons 31, emerging from the lens 30, enters into the electro-static field of the

electrode system

23, 24. From an electron-optical point of view this system then operates as a convex mirror. If the image plane of this lens 30 lies behind this mirror, for example, at 32, the

mirror

23, 24 reproduces the image on an enlarged scale in a plane located, for example, at 33'(see Fig. 4a). By means of a collecting screen (fluorescent screen or photographic film) arranged in this plane this image may be rendered visible. The mirror formed by the elements 23 and 24 (Fig. 4a) causes the beam to curve around and strike the screen 33. The plane 32 (Fig. 4) indicates the plane in which an image would be formed if the

mirror

23, 24 was not functioning. With respect to the

mirror

23, 24, therefore, the imaginary image formed in the plane 32 may be regarded as the virtual object for producing the final image on the screen 33.

The ray 31 need not necessarily be exactly directed to the aperture of the anode 24 (Fig. 4), through which the electrons initially emerge. As an alternative, at the side of the exit aperture of the anode for the ray 25 (Fig. 4a), a second aperture may be provided for the ray 31, behind which is located the electrode 23, which is at a low negative potential with respect to the electron source 22, when the microscope operates. Even this modified embodiment provides the advantage that the mirror system does not require separate fastening means and supply conductors.

The electron ray may be given its correct course by suitable choice of the distance of the lens 26 from the electron gun and of the spacing between the

lenses

26 and 30. As an alternative, the ray emerging from the transverse field lens may be deflected in direction by varying the lens angle. An efiicient distance between the lens 26 and the electron gun is 50 centimeters. The spacing between the image plane 33 and the

electrode system

23, 24 may be of the same order.

The chromatic displacement is eliminated, if the following requirement is fulfilled:

R V 1 where V1 and V designate the amplifications produced by the lenses 26 and 3t) respectively and R1 and R2 desigmate the radii of curvature of the optical axes of the first and the second lens respectively. If the two lenses are caused to produce the same amplification V, R2 must be equal to VRl. It has been found that practical results are obtained by an amplification of each lens of approximately 7 and by an approximately hundredfold amplification produced by the

mirror

23, 24. This yields in total an amplification of about 5000, i. e., 7 7 i00=4900, but owing to variations of the various factors affecting the total amplification, the result may be an amount which strongly diverges therefrom. A practical value of the radius of curvature R1 is, for

example, 3 mms. Then, in order to fulfill the aforesaid requirement of elimination of the chromatic displacement, the radius of curvature R2 must be 21 mms., if it is assumed that the two lenses produce a sevenfold amplification.

The

lenses

26 and 30 may be provided with a common yoke, so that a lens system is obtained which excells in simplicity and easy arrangement, particularly, if the lenses can be arranged closed to one another by choosing the sum of the two lens angles to be only slightly in excess of 180.

What I claimed is:

1. In an electron microscope including an electron beam source, and viewing means disposed in the path of the electron beam for producing a visible image therefrom, a magnetic lens for deflecting and focussing said beam on said viewing means comprising a pair of opposed spaced ferromagnetic pole members defining tw opposed mirror-symmetrical pole surfaces each being a surface of revolution formed by a generatrix approximating a part of a parabola the apex of which lies on the axis of revolution and shaped in the form of a sector of a circle having an angle of not less than and not more than 270, and a ferromagnetic yoke connecting said pole members and forming therewith a closed magnetic circuit including an air-gap between the polemembers.

2. In an electron microscope including an electron beam source, and viewing means disposed in the path of the electron beam for producing a visible image therefrom, a magnetic lens for deflecting and focussing said beam on said viewing means comprising a pair of opposed spaced ferromagnetic pole members defining two opposed mirror-symmetrical pole surfaces each being a conical surface of revolution formed by a generatrix which is a tangent to a parabola the apex of which lies on the axis of revolution and shaped in the form of a sector of a circle having an angle of not less than 90 and not more than 270, and a ferromagnetic yoke connecting said pole members and forming therewith a closed magnetic circuit including an air-gap between the pole-members.

3. In an electron microscope including an electron beam source, and viewing means disposed in the path of the electron beam for producing a visible image therefrom, a magnetic lens for deflecting and focussing said beam on said viewing means comprising a pair of opposed spaced ferromagnetic pole members defining two opposed mirror-symmetrical pole surfaces each being a conical surface of revolution formed by a generatrix which is a tangent to a parabola the apex of which lies on the axis of revolution and shaped in the form of a sector of a circle having an angle of not less than and not more than and a ferromagnetic yoke connecting said pole members and forming therewith a closed magnetic circuit including an air-gap between the polemembers.

4. In an electron microscope including an electron beam source, and viewing means disposed in the path of the electron beam for producing a visible image therefrom, a magnetic lens for deflecting and focussing said beam on said viewing means comprising a pair of opposed spaced ferromagnetic pole members defining two opposed mirror-symmetrical pole surfaces each being a conical surface of revolution formed by a generatrix which is a tangent to a parabola the apex of which lies on the axis of revolution and shaped in the form of a sector of a circle having an angle of about 127%. and a ferromagnetic yoke connecting said pole members and forming therewith a closed magnetic circuit including an air-gap between the pole-members.

5. In an electron microscope including an electron beam source, and viewing means disposed in the path of the electron beam for producing a visible image therefrom, a magnetic lens for deflecting and focussing said beam on said viewing means comprising a pair of opp sed. ,sp ed, ferro a net polemembersv defining two opposed mirron-symnietfical pole surfaces ;ea ch being. ,a conical sur face .of revolution formed by a generatrix which is a tangent to a parabola the apex of whichlies on thevaxis-of revolution and shaped inthe form of a sector of a circle having an angle of not, less. than 90 and not more than 270,.and a permanentflmagnetyoke connectingv said pole, membersv and forming ,therewith ,a closed magnetic'circuit including. an, air-gap between the pole-members.

6. In an electron microscope including an electron beam source, and viewing meanstdisposed in the path of the electron beam for producing ,a visible image therefrom, a magnetic lensSYStem for. deflecting and focussing said beam on said viewing meanscomprising two pairs of opposed spaced ferromagnetic pole members successively positioned inthe pathof ,theelectron beam, each pair of pole members defining two Opposed mirrorsymmetrical pole surfaces eachbeing a surface of revolution formed by a generatrix approximating a part of a parabola the apex of which lieson the, axis of revolution and shaped in the form of a sector of a circle having an angle of not less than 90 and not more than 270, and a ferromagnetic yoke connecting said pole members and forming therewith a closed magnetic circuit including an air-gap between the pole-members.

7. In an electron microscope including an electron beam source having an acceleratingelectrode, and viewing means'disposed in the pathof the electronbeam for producing a visible image therefrom, a magneticlens SYSIQmiQrUdefleQting andfocussingusaidbeam on s i view ugzmeans comprising tw pairs of ppaomdsuace ferromagnetic lpole, members successively positioned in the pathoftheelectron beam, each pair oflpole members defining two opposed,mirror-symmetrical pole surfaces eachbeinga 'surfacelof revolutionformed by a generatrixapproximating apart of a parabola the apex of which lieson the ,axis of revolution and shaped in theform of a sector of a circle having an angle of, not less than 90 and, not more than 270", and aferromagnetic yoke connecting said pole, members andforming therewith a closed magnetic circuit including an air-gap between the polemembers, said respective pairs of pole-members being further positioned to direct the electron beam toward said accelerating electrode whereby the latter jdefiects said beam toward said viewing means.

References Cited in theifile of this patent UNITED STATES PATENTS IHewitt Apr. 28, 1953