US2914675A

US2914675A - Element for correcting electron-optical systems

Info

Publication number: US2914675A
Application number: US571458A
Authority: US
Inventors: Adrianus Cornelis Van Dorsten
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1955-03-15
Filing date: 1956-03-14
Publication date: 1959-11-24
Anticipated expiration: 1976-11-24
Also published as: GB793664A; DE1098634B; NL195609A; BE546040A; FR1148643A; NL97470C; CH339297A

Description

Nov. 24, 1959 A. c. VAN DORSTEN 2,914,675

ELEMENT FOR CORRECTING ELECTRON-OPTICAL SYSTEMS Filed March 14, 1956 5 Sheets-Sheet 1 1: 4s 17 16 g 4 J 4- 3 +1! ii. :0 5a. 4 0

INVENTOR ADRIANUS GJRNELIS VAN DORSTEN AGENT NOV. 1959 A. c. VAN DORSTEN 2,914,575

ELEMENT FOR CORRECTING ELECTRON-OPTICAL SYSTEMS Filed March 14, 1956 5 Sheets-Sheet 2 ADRIANUS CORNEUS \AN DORSTEN INVENTOR AGT NOV. 24, 1959 c, VAN DQRSTEN 2,914,675

ELEMENT FOR CORRECTING ELECTRON-OPTICAL SYSTEMS Filed March 14, 1956 3 Sheets-Sheet 3 EEO-t0.

INVENTOR ADRIANUS CORNEUS VAN DORSTEN AGENT it i 7 2,914,675 Jj ELEMENT FOR CORRECTING ELECTRON- OPTICAL SYSTEMS.

Adrianus Cornelis van Dorsten, Eindhoven, Netherlands,

assignor, by mesne assignments, toNorth American Philips Company, 'Inc., New York, .N.Y., a corporation of Delaware 1 Application March 14, 1956, Serial No. 571,458 Claims priority, application Netherlands March 15, 1955 a 9 Claims. (c1. 2s0 49.5

-When the field in an electron optical system is not truly radially symmetrical, the image produced by such a system exhibits astigmatism. As is well-known the detrimental influence 'of the asymmetry .can be eliminated by means of Correcting elements which are referred to as stigmators. The invention relates to a novel stigmator, which offers certain advantages over the known stigmators, as will beseen from the description.

A fstigmator' in accordance with the invention comprises four or a greater even number of pole pieces of electrically conductive magnetizable material which are arranged symmetrically about the same part of an axis and each are insulated electrically from .thetwo adjacent 2,914,675 Patented Nov. 24,1959

ance with the invention provided with concave polefaces, Fig. 8 is a longitudinal sectional view of the element shown in Fig. 7 taken along the line VIIIVIII.

Fig. 9 illustrates the last-mentioned embodiment, and

Fig. 10 is a diagrammatic cross-sectionalview of an electron microscope in accordance with the invention.

In Figs. 1 and 2, the four convex pole pieces are designated 1, 2, 3 and 4. They are made of a soft steel frequently used to manufacture magnetic electron lenses.

They are interconnected magnetically by ferromagnetic coil-

cores

5, 6, 7 and 8. Said cores are made of a ferromagnetic insulating material, such as for example, one of the ferrites commercially available under the trade 'name .ferroxcube. These materials are fully described in US. Patents 2,452,529, 2,452,530 and 2,452,531. They have the composition MFe O in which M is a bivalent ones. 'Said pole-pieces are-providedwith cylindrical pole faces the generatrices of which are parallel to'the axis. The stigmator must be so arranged that its axis coin-l cideswi'th the optical axis of the system to be connected.

The pole "pieces mustbe magnetized to anequal extent so that the magnetic polarity ofeach pole-face is opposed to t hat of the twof adjacent ones; In addition, an equal electric potential must be applied between each pair of adjacent pole pieces so that polefaces ofequal magnetic polarity areat the same potential. i i f i controlling the potential difierencs between the pole" pieces and the magnetic'field strength, astigmatisms in'any direction and of difleientivalues can be eliminated without the correcting elements" or partther'eof being mo ed-J,

"The line along which the pole faces intersect a plane attriglitangles to the axis may be shaped into various forms." It is of advantage for the pole pieces to be' con vex. It has been found that the optimurn efi'ect of the stigmator in accordance with the invention is ensured by shaping the cross section of the pole faces into the form .of two associatedequilateral hyperbolas. I l

fA stigmator in accordance with the invention comprising four pole piecesbehaves as a combination of two cylindrical lenses in thesame space, an electrostatic and a magnetic lens; I Thews'agittal planes or said cylindrical lenses are at an angle of 45 "withone another.

"In order that the invention may be readily put into effect, a number of 'structuralembodiments will now be described in detail with reference to "the. accompanying drawings, in which: i i l Figl lis a part plan view, part sectional view of such an embodimennviewed in the direction of the axis,

,Fig. 2fis, partly a side elevation of the same element, partly asectional view thereof taken along a plane passing through the axis, i

. Figlj}3"' is apart plan view,part sectional view of a second. embodiment of astigmator in accordance with the i nv e n tion, viewed inthe direction of the axis. Fig. 4 is a cross-sectional View of the element shown :Fig57 :is a crossf-sectionalview of a stigmato'r. in accord-f metal such as copper, nickel, zinc, magnesium, etc. These materials exhibit very low magnetic losses and have a high resistivity. Coil windings 9, 10, 11 and 12 are provided on said cores.

The pole faces are arranged symmetricallyabout the axis 13. They are providedwith cylindrical

convex pole faces

14, 15, 16 and 17, the generatrices of which are parallel to the axis 13. In the operation of the stigmator, the current passing through the coil windings 9, 10, 11 and 12 must be so directed that two adjacent coil cores are magnetized in opposite senses. Thus, the successive pole faces 14, 15, 16 and 17 are given alternate polarities: when 14 produces a north pole, 15 produces a south pole, 16 a north pole again and 17 a south poleagain. Consequently the magnetic line of forcewill be symmetrical and at-right angles to the axis 13.

In addition, an electric potential is applied between the pole pieces 1 and 2. An equal potential difference is applied between the pole pieces 3 and 2 and between the pole'pieces 3 and 4'. The pole pieces designated by odd reference numerals are at the, same potential which differs from the common potential of the pole pieces designated by even reference numerals. This resultsin the production of an electric field the lines of force of which coincide with the magnetic lines of force. Since the value of the potential must be different for any direc-. tion and value of the astigmatism to be compensated, the potential is made controllable, for example between a fewhundreds of volts negative and positive. For the same reason the magnetizing current of the coils is also made variable so that the magneto-motive force can be controlled, for example between 100 ampere-turns negative and positive. l i

' The two fields, the electric and the magnetic field, each actas a' cylindrical lens. However, the direction of the refractive force at a predetermined point in space is not the same for the two cylindrical lenses. When the curves at which the pole faces 14, 15, 16 and 17 intersect the common principal plane of the two cylindrical lenses are in the form of two associated equilateral hyperboias, it can be proved mathematically that the sagittal planes of said lenses are at an angle of 45 with one another.

The astigmatism of an optical system, for example of an electron lens, produced by an insufficient degree of symmetry of the'field boundaries can be eliminated with metry of the optical system to be corrected, provided that t the lens is of suitable power. Starting from the optimum effect of this cylindrical lens, the minimum effect is obtained by rotating it through an angle of45 only. The same eifect can be produced by using a system of two cylindrical lenses of adjustable power the azimuthal angles g of which differ by 45. This requirement is satisfied by the insulator 34.

the correcting element in accordance with the invention which has the advantage that it need not be rotated at all. In order to adjust the element to the optimum effect, the power of the two component lenses is changed in opposite senses (so that the resulting lens is rotated) until the remaining astigmatismexhibits a minimum value. Subsequent proportional variation of the strength of the two fields permits of entirely eliminating the astigmatism.

When the cross-section of the pole faces is not shaped 7 into the form of a true hyperbola, the effect of the correcting element is less complete, but nevertheless the use of pole pieces the shape of which does not deviate excessively from the theoretically correct form provides an improvement which is sufficient in practice. The theoretically correct form may, for example, be approximated to by the use of a circular cross section.

In the embodiment shown in Figs. 1 and 2, the coilcores are arranged tangentially. They may alternatively be arranged radially. The latter arrangement is shown in Figs. 3 and 4. The pole pieces are designated 18, 19, 20 and 21. The pole faces 22, 23, 24 and 25 are shaped similarly to those shown in the first embodiment. The coil-

cores

26,27, 23 and 29 in this arrangement are made of the same material as the pole pieces and are interconnected by a yoke 36 made of the same material. In order, however, to permit a voltage to be set up between the pole pieces, the yoke is provided with

gaps

31, 32, 33 and 34 which are filled with electrically insulating magnetizable material. The coil windings are designated 35, 36, 37 and 38. The current must pass through said windings in alternately opposite senses so that the magnetic flux of, for example, the core 26, starting from the pole-face 22., is closed for one half through the pole piece 19 and the insulator 31 and for the other half through the pole piece 21 and As an alternative, a stigmator in accordance With the invention may be produced in which the coil turns are at right angles to the axis ofthe element.

system in this embodiment comprises two inter-engaging equal parts. Said parts eachconsist of a flat annular member (39, 40) having two diametrically opposed projecting thickened portions (41, 42 and 43, 44 respectively) the size of which in theaxial direction also exceeds that of the remaining part of the member and the cross-section of which in a plane at right angles to the axis is shaped in the form of the two curves of an equilateral hyperbola cut off'by a concentric circle 45. The diameter d of the circle 45 is less than the inner diameter d of the members 39 and'4t). The members are arranged coaxially with one another, one being reversed relatively to the other, and one pair of thickened portions is arranged symmetrically with respect to the other pair. A ferromagnetic annular member 46 made from insulating material spaces the members 39 and 40 apart. The inner diameter d of the member 46 which is arranged coaxially with the members 39 and 40 is larger than d In the space between the member 46 and the parts of the thickened portions 41 to 44 which project beyond the surface of the members 39 and 40, a magnetizing coil 47 is arranged. Since d is less than d there is a gap between the thickened portion 41 and 42 and the inner wall of the member 39 and likewise between the thickened

portions

43 and 44 and the member 40. As a result, magnetic poles are producedwhen a' current passes through the coil 47. The hyperbolic planes of 41 and 42 are given one magnetic polarity and those of 43 and 44 the other magnetic polarity, so that a magnetic cylindrical lens is produced. Between themembers39 and 40 an electricpotentialcan beset up in order to make'the system into an electrostatic cylindrical lens.

alternative embodiment of a stigmator in accordance with the invention, in which the shape of the pole faces is considerably different from that in the embodiments so far described, is shown in Figures 7 and 8. In this embodiment, the pole faces 48, 49 and 50 and 51 are shaped into a circular cylinder. Fig. 9 illustrates the reason why satisfactory results cannevertheless be obtained with this arrangement.

When of the field produced between the hyperbolic pole faces of the above-described embodiments the field strength in the points of a circle situated in a plane at right angles to thejaxis and having the centre in said axis is shown in a graph on rectangular coordinates as a function of the angle between the radius vector and a fixed diameter, a sinusoidal line is produced.

The variation of the potential along the circular crosssection of the pole faces 48 to 51 can be represented by the rectangular wave-form line 52 of Fig. 9. The function represented by said line can be resolved according to Fourier into a sinusoidal function and a plurality of harmonics. For a circle having a slightly smaller radius, a curve is obtained from which the highest harmonics have disappeared and as the radius is made smaller more harmonics will disappear. In a circle having a very small radius theyhave disappeared almost entirely, so that substantially only a sinusoidal functions 53 will be left having the same passages through zero as the initial line 52. Consequently, in the proximity of the axis the variation of the field strength is approximately equal to that which is found throughout the entire field with hyperbolic pole faces. Thus, when the radius of the cylinder constituted by the pole faces is large compared with that of the crosssection of the electron beam upon which the fields must act, the shape shown in Fig. 7 is useful in practice.

The stigmator shown in Figs. 7 and 8 substantially comprises two

concentric cylinders

54 and 55 made of ferro-magnetic metal. The outer cylinder tapers to- Figs. 5 and 6 "show an embodiment of this arrangement. The magnetic wards the top and at this top. is provided with two windows which each occupy an amount of space slightly larger than one quarter of the periphery. In said windows tags 56 and 57 are arranged which project upwards beyond-the edge of the inner cylinder and each occupy an amount of space slightly smaller than one quarter of the periphery. The tops of the tags are slightly spaced away from the upper edge of the windows. Hence the two cylinders are not in contact with one another, although the diameters of their upper parts are equal.

The space between the cylinders, in which space a magnetizing winding 58 is arranged, is closed by a ferro-magnetic electrically insulating annular member 59. This member establishes a direct connection between the cylinders for the magnetic flux which produces magnetic poles at the faces 48 to 51.

It will be obvious that in all the embodiments de scribed the entire ferro-magnetic may consist of electrically insulating material (ferroxcube), provided that the pole faces are coated with a conductive material so that an electric potential can be set up between said faces.

Fig. 10 is a sectional view of an electric microscope provided with a stigmator in accordance with the invention.

The outer wall of said microscope is a steel tube 60 into the upper end of which an electrode system 61 is introduced which acts to produce the electron beam which passes through the entire microscope along the axis of the tube 60. The microscope is provided with three magnetic lenses: a condenser 62, an objective 63 and a projection lens 64. The design and the operation of said members are assumed to be known. In the objective 63, an object 65 is arranged an image of which is produced by the electron beam upon the glass window 66 which is coated with a layer 67 of fluorescent material.

According to the invention, between the objective and the projection lens provision is made of the correcting element 68 which serves to eliminate the astigmatism of the objective in an optimum manner.

secured to this element and passed out through the wall 60. In addition, provision is made of electric conductors '70 which act to supply current to the magnetizing coils. By adjusting the potential and the current the power of the two cylindrical lenses produced by the element can be adjusted. This electric adjustment replaces the known adjustment by means of displacement in the tube of metal correcting members. This means a considerable simplification.

Since the electrostatic cylindrical lens exhibits no spherical aberration and the magnetic lens shows only little spherical aberration, the electron beam in the stigmator may have a large cross-section, while a comparatively large distance between the correcting element and the lens with which it cooperates is not inconvenient per se. For the same reason, an increase in the axial size of the element does not adversely afiect the image quality. A comparatively large size in the direction of the axis is even of advantage since it reduces the influence of inconvenient marginal fields.

1 However, it is desirable for other reasons that the distance between the stigmator and the objective be small compared with that between the stigmator and the production lens, for the smaller the latter distance is at a predetermined distance between the objective and the production lens, the stronger the stigmator must be magnetized in order to enable it to eliminate the astigmatism, but the stronger its influence becomes upon the magnification produced by means of the system to be corrected (obviously in one direction only).

By increasing the spacing between the stigmator and the projection lens, the distortion of the image is reduced. For this reason the stigmator is embedded in the ferromagnetic of the objective in the microscope shown in Fig. 10.

What is claimed is:

1. An electron optical system for an electron microscope having a given optical axis comprising electron beam producing means, an electron lens system for focussing the electron beam which introduces astigmatism into the focussedbeam along said axis, a plurality of pairs of spaced pole members each of magnetizable material disposed symmetrically about said optical axis for compensating for the astigmatism introduced into the beam by the electron lens system, each of said pole members having a cylindrical pole face the generatrix of which is parallel to said axis, a member connecting each of said pole members to one another and consisting of an electrically-insulating ferromagnetic material having a composition MFe O in which M is a bivalent metal, means to alternately polarize each of said pole members in an opposite sense, means to apply equal electrical potential difierences between adjacent pole members, and means for varying the potential difierence between and the field strength of successive pairs of pole members.

2. An electron optical system as claimed in claim 1 in which the electron lens system is a magnetic lens system.

3. An electron-optical system as claimed in claim 1 in which the cylindrical pole face is convex.

4. An electron-optical system as claimed in claim 3 in which a section of each pair of pole faces forms two associated hyperbolas in a plane at right angles to the common axis.

5. An electron-optical system as claimed in claim 3 in which the pole-face is a circular cylinder.

6. An electron-optical system as claimed in claim 1 in which the pole member polarizing means is a coil disposed on said pole and has a given axis which is tangent to a circle concentric with the common axis and passes through said pole member.

7. An electron-optical system as claimed in claim 1 in which each pair of pole members together comprises two coaxial electrically conductive annular members secured together by a yoke of electrically insulating ferromagnetic material having a composition MFe O in which M is a bivalent metal.

8. An electron-optical system as claimed in claim 7 in which the pole members are polarized by a coil disposed between the annular members.

I 9. An electron-optical system as claimed in claim 7 in which the pole members are polarized by a coil surrounding the annular members.

References Cited in the file of this patent UNITED STATES PATENTS 2,200,039 Nicoll May 7, 1940 2,486,856 Liebmann Nov. 1, 1949 2,520,813 Rudenberg Aug. 29, 1950 2,580,675 Grivet et al Jan. 1, 1952 2,586,559 Page Feb. 19, 1952

Claims

1. AN ELECTRON OPTICAL SYSTEM FOR AN ELECTRON MICROSCOPE HAVING A GIVEN OPTICAL AXIS COMPRISING ELECTRON BEAM PRODUCING MEANS, AN ELECTRON LENS SYSTEM FOR FOCUSSING THE ELECTION WHICH INTRODUCES ASTIGMATISM INTO THE FOCUSSED BEAM ALONG SAID AXIS, A PLURALITY OF PAIRS OF SPACED POLE MEMBERS EACH OF MAGNETIZABLE MATERIAL DISPOSED SYMMETRICALLY ABOUT SAID OPTICAL AXIS FOR COMPENSATING FOR THE ASTIGMATISM INTRODUCED INTO THE BEAN BY THE ELECTRON LENS SYSTEM, EACH OF SAID POLE MEMBERS HAVING A CYLINDRICAL POLE FACE THE GENERATRIX OF WHICH IS PARALLEL TO SAID AXIS, A MEMBER CONNECTING EACH OF SAID POLE MEMBERS TO ONE ANOTHER AND CONSISTING OF AN ELECTRICALLY-INSULATING FERROMAGNETIC MATERIAL HAVING A COMPOSITION MFE2O4 IN WHICH M IS A BIVALENT METAL, MEANS TO ALTERNATELY POLARIZE EACH OF SAID POLE MEMBERS IN AN OPPOSITE SENSE, MEANS TO APPLY EQUAL ELECTRICAL POTENTIAL DIFFERNECES BETWEEN ADJACENT POLE MEMBERS, AND MEANS FOR VARYING THE POTENTIAL DIFFERENCES BETWEEN AND THE FIELD STRENGTH OF SUCCESIVE PAIRS OF POLE MEMBERS.