US2914675A - Element for correcting electron-optical systems - Google Patents

Element for correcting electron-optical systems Download PDF

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Publication number
US2914675A
US2914675A US571458A US57145856A US2914675A US 2914675 A US2914675 A US 2914675A US 571458 A US571458 A US 571458A US 57145856 A US57145856 A US 57145856A US 2914675 A US2914675 A US 2914675A
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pole
electron
axis
pole members
members
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US571458A
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English (en)
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Adrianus Cornelis Van Dorsten
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US Philips Corp
North American Philips Co Inc
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US Philips Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement or ion-optical arrangement
    • H01J37/153—Electron-optical or ion-optical arrangements for the correction of image defects, e.g. stigmators

Definitions

  • stigmators 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
  • 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
  • Fig. 10 is a diagrammatic cross-sectionalview of an electron microscope in accordance with the invention.
  • 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.
  • 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.
  • 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;
  • 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.
  • Fig. 2f is, 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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
  • a magnetizing coil 47 is arranged 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. 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.
  • FIG. 7 illustrates the shape of the pole faces 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.
  • 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.
  • 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.
  • a curve is obtained from which the highest harmonics have disappeared and as the radius is made smaller more harmonics will disappear.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the correcting element 68 which serves to eliminate the astigmatism of the objective in an optimum manner.
  • 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.
  • 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.
  • 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).
  • 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.
  • 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.
  • 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.

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  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Electron Beam Exposure (AREA)
  • Electron Sources, Ion Sources (AREA)
US571458A 1955-03-15 1956-03-14 Element for correcting electron-optical systems Expired - Lifetime US2914675A (en)

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NL339297X 1955-03-15

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US (1) US2914675A (en:Method)
BE (1) BE546040A (en:Method)
CH (1) CH339297A (en:Method)
DE (1) DE1098634B (en:Method)
FR (1) FR1148643A (en:Method)
GB (1) GB793664A (en:Method)
NL (2) NL195609A (en:Method)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3046397A (en) * 1959-06-17 1962-07-24 Tesla Np Device for compensating axial astigmatism of electron-optical systems
US3150258A (en) * 1962-07-05 1964-09-22 Philips Electronic Pharma Electromagnetic stigmators for correcting electron-optical deficiencies in the lenses of electron beam instruments
US3221133A (en) * 1963-04-02 1965-11-30 Japan Electron Optics Lab Co L Electron microscope with means for treating and observing specimens
US3223837A (en) * 1961-07-10 1965-12-14 First Pennsylvania Banking And Beam probe system and apparatus
US3223871A (en) * 1961-08-22 1965-12-14 Gen Electric Electron optical system
US3287558A (en) * 1961-09-08 1966-11-22 High Voltage Engineering Corp Charged particle deflecting device consisting of sequentially positioned uniform and non-uniform magnetic field sectors

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2200039A (en) * 1937-11-01 1940-05-07 Emi Ltd Permanent magnet device for producing axially symmetrical magnetic fields
US2486856A (en) * 1947-04-12 1949-11-01 Gen Electric Electron lens
US2520813A (en) * 1947-12-10 1950-08-29 Rudenberg Reinhold Electron optical system
US2580675A (en) * 1947-06-26 1952-01-01 Csf Correction device for microscopes of the reflection mirror type
US2586559A (en) * 1950-02-23 1952-02-19 Gen Electric Multiple element electron lens arrangement

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2200039A (en) * 1937-11-01 1940-05-07 Emi Ltd Permanent magnet device for producing axially symmetrical magnetic fields
US2486856A (en) * 1947-04-12 1949-11-01 Gen Electric Electron lens
US2580675A (en) * 1947-06-26 1952-01-01 Csf Correction device for microscopes of the reflection mirror type
US2520813A (en) * 1947-12-10 1950-08-29 Rudenberg Reinhold Electron optical system
US2586559A (en) * 1950-02-23 1952-02-19 Gen Electric Multiple element electron lens arrangement

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3046397A (en) * 1959-06-17 1962-07-24 Tesla Np Device for compensating axial astigmatism of electron-optical systems
US3223837A (en) * 1961-07-10 1965-12-14 First Pennsylvania Banking And Beam probe system and apparatus
US3223871A (en) * 1961-08-22 1965-12-14 Gen Electric Electron optical system
US3287558A (en) * 1961-09-08 1966-11-22 High Voltage Engineering Corp Charged particle deflecting device consisting of sequentially positioned uniform and non-uniform magnetic field sectors
US3150258A (en) * 1962-07-05 1964-09-22 Philips Electronic Pharma Electromagnetic stigmators for correcting electron-optical deficiencies in the lenses of electron beam instruments
US3221133A (en) * 1963-04-02 1965-11-30 Japan Electron Optics Lab Co L Electron microscope with means for treating and observing specimens

Also Published As

Publication number Publication date
BE546040A (en:Method)
GB793664A (en) 1958-04-23
DE1098634B (de) 1961-02-02
NL195609A (en:Method)
NL97470C (en:Method)
FR1148643A (fr) 1957-12-12
CH339297A (de) 1959-06-30

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