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US2880324A - Variable magnetic electron lens - Google Patents

Variable magnetic electron lens Download PDF

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Publication number
US2880324A
US2880324A US379716A US37971653A US2880324A US 2880324 A US2880324 A US 2880324A US 379716 A US379716 A US 379716A US 37971653 A US37971653 A US 37971653A US 2880324 A US2880324 A US 2880324A
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United States
Prior art keywords
insert
lens
magnetic
astigmatism
pole
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Expired - Lifetime
Application number
US379716A
Inventor
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
    • 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
    • H01J37/10—Lenses
    • H01J37/14—Lenses magnetic
    • 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
    • H01J37/10—Lenses
    • H01J37/14—Lenses magnetic
    • H01J37/141—Electromagnetic lenses

Definitions

  • the invention relates to magnetic electron lenses for use in electron microscopes and similar apparatus It provides means of varying in a simple manner the magnetic fieldof such a lens and of correcting-certain defects of the lens.
  • a movable, cylindrical, magnetic insert is arranged axially in the bore of a pole piece for use in a magnetic electron lens. It has been found that varying the position of such an insert enables the properties of the lens to be varied. Thus, axial shifting of theinsert permits of varying the focal length of the lens.
  • Ser No. 578,990 filed June 16, 1954, the effect of an insert in thecase of magnetic saturation'is described, if spaceis available between the insert and the bore wall.
  • the focal length to be varied by means of a sliding insert which fits into the pole piece bore.
  • Such an exactly fitting insert does not provide the magnetic screening effect of a disengaged tube, since it is saturated together with the surrounding material.
  • the movability of the magnetic insert may be useful in view of some defects occurring in the magnetic electron lens. It is known, for example, that there is a relationship between the form of the field curve (the curve showing the magnetic field strength in the lens axis as afunction of the distance along the axis) and the spherical aberration. Thus, adjustment of the axially movable inserts (which may be difierent for either pole piece) enables the spherical aberration to be corrected.
  • the movable insert permits of compensating for or correcting the astigmatism of the lens, which is a departure from rotation symmetry.
  • Fig. 1 is a cross-sectional view of a pole piece according to the invention taken along a plane passing through the axis, and
  • Figs. 2, 3 and 4 show separate insert cylinders which may be used to provide correction for astigmatism.
  • pole pieces 1 and 7 of a magnetic electron lens for use in an electron microscope or an electron diffraction apparatus are shown.
  • the front face 2 of the pole pieces 1 and 8 of pole piece 7 constitute the pole faces. They are separated by a spacer 9 of non-magnetic material.
  • a copper tube 3 passes through the bore of the pole pieces. It acts to exclude the ambient atmosphere from a space around the axis, which is done by the tube being evacuated so as to allow the electrons to travel through it.
  • the pole pieces 1 and 7 are connected through a yoke 4 extending between them and which surrounds an energizing winding 10 which produces a magnetic flux in the yoke.
  • a cylindrical insert movably arranged according to the invention surrounds the tube 3.
  • the cylindrical 2,880,324 Patented Mar. 31, 195? insert is extended at the end more remote from the pole face 2 and provided with external screw thread by means of which it fits into the wider part of the pole piece 1.
  • An enlarged portion 6 projecting beyond the pole piece enables the insert to be actuated so as to screw it outwardly or inwardly and thus to vary the magnetic field at the pole face adjacent the axis.
  • FIG. 2 A cylindrical insert 5 by which this correction effect is ensured is shown in Fig. 2 viewed in the axial direction.
  • the outer wall of this cylinder is truly circular but the inner wall thereof is elliptical. However, the centre of the elliptical inner cross-section coincides with the centre of the circular outer cross-section and this ensures symmetry with' respact to two planes passing through the axis at right angles to each other. Thus, the wall has a varying thickness which is at its minimum in one plane of symmetry and at its maximum in the other. Rotation of this cylinder through enables the resultant astigmatism to assume any desired value.
  • the edge of the end of the cylindrical insert nearer the pole face may be given a height varying along the circumference. This will generally work out as a length of the cylindrical insert varying along the circumference. Also in this case there must be symmetry with respect to two planes passing through the axis at right angles to each other.
  • Embodiments of a cylindrical insert 5 formed into such a shape are shown in Figs. 3 and 4.
  • the profile may be toothed as shown in Fig. 3, its developed surface being bounded by a broken line. It may alternatively be a curve or exhibit a form as shown in Fig. 4, the developed surface of which exhibits a curve, for example, a sinusoidal line.
  • the elevations and depressions are identical in form and two cylinders as shown in Fig. 3 or 4 have interlocking profiled edges. This assists in an even variation of the astigmatism and consequently in easy adjustment.
  • the astigmatism of the lens may generally be looked upon as the combination of a fundamental departure from normal and one or more departures of higher order. To correct the latter, provision may be made of separately adjustable cylinders co-axial with the others. Since each of these components has a sinusoidal variation, this form is most suitable for the profile of the edge.
  • a magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a gap therebetween, said pole shoes having axially-aligned bores extending therethrough and communicating with said gap, and a hollow, ferromagnetic insert member rotatably mounted within the bore of one of the pole shoes and axially movable therewithin, said ferromagnetic insert having an end portion, communicating with said gap, that has a center of symmetry and is also symmetrical with respect .to orthogonal planes having the and having a center of symmetry at the bore axis and .having mirror symmetry with respect to each of orthogonal planes having the bore axis on their line of .inter .section, whereby rotation ofsaid insert member changes the field distribution in the gap and enables the lens astigmatism to be corrected.
  • a magnetic electron lens comprising apair of spaced apart ferromagnetic pole shoes defining a gap ttherebe- "tween, said pole shoes having axially-aligned bores extending therethrough and communicating with said gap, and a soft ferromagnetic substantially cylindrical hollow insert member rotatably mounted within the bore of one of the pole shoes, said insert having a.hollow end portion, communicating with said gap, whose outer and inner periphery have the shape of a circle and ellipse,
  • said insert also .having mirror symmetry with respect'to each of orthogonal planes having the bore axis as their-line of intersection, whereby rotation of said insert member enables astigmatism of the lens to be corrected.
  • a magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a gap therebetween, said pole shoes having axially-aligned bores extending therethrough and communicating with said gap, and a ferromagnetic substantially cylindrical hollow insert member rotatably mounted within the bore of one of the pole shoes, said insert having a hollow end portion, communicating with said gap, with axially-projecting portions of different length, said end portion also having mirror symmetry with respect to each of orthogonal planes having the bore axis as their line of intersection, whereby rotation of said insert enables astigmatism of the lens to be corrected.

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  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Lenses (AREA)
  • Electron Beam Exposure (AREA)
  • Electron Tubes For Measurement (AREA)

Description

March 31, 1959 A. c. VAN DORSTEN 2,880,324
VARIABLE MAGNETIC ELECTRON LENS Filed Sept. 11, 1953 7 4 INVENTOR.
ADRZANNUS 6612mm Z NDozas TEN A (IL I United States Patent "ice VARIABLE MAGNETIC ELECTRON LENS Adrianus Cornelis van Dorsten, Eindhoven, Netherlands,
Application September 11, 1953, Serial No. 379,716
Claims priority, application Netherlands September 16, 1952 8 Claims. (Cl. 250-495) The invention relates to magnetic electron lenses for use in electron microscopes and similar apparatus It provides means of varying in a simple manner the magnetic fieldof such a lens and of correcting-certain defects of the lens.
, According to the invention a movable, cylindrical, magnetic insert is arranged axially in the bore of a pole piece for use in a magnetic electron lens. It has been found that varying the position of such an insert enables the properties of the lens to be varied. Thus, axial shifting of theinsert permits of varying the focal length of the lens. In my copending application, Serial No. 578,990, filed June 16, 1954, the effect of an insert in thecase of magnetic saturation'is described, if spaceis available between the insert and the bore wall. In the pole piece according to the invention there is no need for said space to be available, it being possible for the focal length to be varied by means of a sliding insert which fits into the pole piece bore.
Such an exactly fitting insert does not provide the magnetic screening effect of a disengaged tube, since it is saturated together with the surrounding material.
The movability of the magnetic insert may be useful in view of some defects occurring in the magnetic electron lens. It is known, for example, that there is a relationship between the form of the field curve (the curve showing the magnetic field strength in the lens axis as afunction of the distance along the axis) and the spherical aberration. Thus, adjustment of the axially movable inserts (which may be difierent for either pole piece) enables the spherical aberration to be corrected.
In addition, the movable insert permits of compensating for or correcting the astigmatism of the lens, which is a departure from rotation symmetry.
The invention will now be described with reference to the accompanying diagrammatic drawing, in which:
Fig. 1 is a cross-sectional view of a pole piece according to the invention taken along a plane passing through the axis, and
Figs. 2, 3 and 4 show separate insert cylinders which may be used to provide correction for astigmatism.
Referring now to Fig. l, pole pieces 1 and 7 of a magnetic electron lens for use in an electron microscope or an electron diffraction apparatus are shown. The front face 2 of the pole pieces 1 and 8 of pole piece 7 constitute the pole faces. They are separated by a spacer 9 of non-magnetic material. A copper tube 3 passes through the bore of the pole pieces. It acts to exclude the ambient atmosphere from a space around the axis, which is done by the tube being evacuated so as to allow the electrons to travel through it.
The pole pieces 1 and 7 are connected through a yoke 4 extending between them and which surrounds an energizing winding 10 which produces a magnetic flux in the yoke.
A cylindrical insert movably arranged according to the invention surrounds the tube 3. The cylindrical 2,880,324 Patented Mar. 31, 195? insert is extended at the end more remote from the pole face 2 and provided with external screw thread by means of which it fits into the wider part of the pole piece 1.
An enlarged portion 6 projecting beyond the pole piece enables the insert to be actuated so as to screw it outwardly or inwardly and thus to vary the magnetic field at the pole face adjacent the axis.
The use of an insert 5 which, although symmetrical with respect to two planes passing through the axis at right angles to each other, i.e., orthogonal planes ha'ving the lens axis on their line of intersection, is not a true solid of revolution allows variation of the astigmatism of the lens. The use of an insert shaped into such a form has the effect of introducing an artificial astigmatism which together with the natural astigmatism of the lens produces a resultant astigmatism. Rotation of the insert about its axis has the effect of varying the resultant astigmatism, which exhibits a minimum value in a particular position of the insert. Axial displacement increases or decreases the effect of the insert and consequently permits variation of the degree of correction. It the defect for which correction is required is of thesame order of magnitude as the artificial one,'it can be compensated for in this manner. I
A cylindrical insert 5 by which this correction effect is ensured is shown in Fig. 2 viewed in the axial direction. The outer wall of this cylinder is truly circular but the inner wall thereof is elliptical. However, the centre of the elliptical inner cross-section coincides with the centre of the circular outer cross-section and this ensures symmetry with' respact to two planes passing through the axis at right angles to each other. Thus, the wall has a varying thickness which is at its minimum in one plane of symmetry and at its maximum in the other. Rotation of this cylinder through enables the resultant astigmatism to assume any desired value.
As an alternative, the edge of the end of the cylindrical insert nearer the pole face may be given a height varying along the circumference. This will generally work out as a length of the cylindrical insert varying along the circumference. Also in this case there must be symmetry with respect to two planes passing through the axis at right angles to each other. Embodiments of a cylindrical insert 5 formed into such a shape are shown in Figs. 3 and 4. The profile may be toothed as shown in Fig. 3, its developed surface being bounded by a broken line. It may alternatively be a curve or exhibit a form as shown in Fig. 4, the developed surface of which exhibits a curve, for example, a sinusoidal line. The elevations and depressions are identical in form and two cylinders as shown in Fig. 3 or 4 have interlocking profiled edges. This assists in an even variation of the astigmatism and consequently in easy adjustment.
The astigmatism of the lens may generally be looked upon as the combination of a fundamental departure from normal and one or more departures of higher order. To correct the latter, provision may be made of separately adjustable cylinders co-axial with the others. Since each of these components has a sinusoidal variation, this form is most suitable for the profile of the edge.
What is claimed is:
1. A magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a gap therebetween, said pole shoes having axially-aligned bores extending therethrough and communicating with said gap, and a hollow, ferromagnetic insert member rotatably mounted within the bore of one of the pole shoes and axially movable therewithin, said ferromagnetic insert having an end portion, communicating with said gap, that has a center of symmetry and is also symmetrical with respect .to orthogonal planes having the and having a center of symmetry at the bore axis and .having mirror symmetry with respect to each of orthogonal planes having the bore axis on their line of .inter .section, whereby rotation ofsaid insert member changes the field distribution in the gap and enables the lens astigmatism to be corrected.
3. A lens as set forth in claim 2 wherein said .ferromagnetic insert is also axially displaceable Within said bore.
4. A magnetic electron lens comprising apair of spaced apart ferromagnetic pole shoes defining a gap ttherebe- "tween, said pole shoes having axially-aligned bores extending therethrough and communicating with said gap, and a soft ferromagnetic substantially cylindrical hollow insert member rotatably mounted within the bore of one of the pole shoes, said insert having a.hollow end portion, communicating with said gap, whose outer and inner periphery have the shape of a circle and ellipse,
respectively, having a common center, said insert also .having mirror symmetry with respect'to each of orthogonal planes having the bore axis as their-line of intersection, whereby rotation of said insert member enables astigmatism of the lens to be corrected.
5. A magnetic electron lens comprising a pair of spaced apart ferromagnetic pole shoes defining a gap therebetween, said pole shoes having axially-aligned bores extending therethrough and communicating with said gap, and a ferromagnetic substantially cylindrical hollow insert member rotatably mounted within the bore of one of the pole shoes, said insert having a hollow end portion, communicating with said gap, with axially-projecting portions of different length, said end portion also having mirror symmetry with respect to each of orthogonal planes having the bore axis as their line of intersection, whereby rotation of said insert enables astigmatism of the lens to be corrected.
6. A lens as set forth in claim 5 wherein said end portion of said insert member has a toothed profile.
7. A lens as set forth in claim 5 wherein said end portion of said insert member is shaped in the form of a sinusoidal line.
8. A lens as set .forth .in claim 5 wherein said insert member is also axially displaceable along the bore,
References Cited in the file of this patent UNITED STATES PATENTS
US379716A 1952-09-16 1953-09-11 Variable magnetic electron lens Expired - Lifetime US2880324A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
NL314474X 1952-09-16

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US (1) US2880324A (en)
BE (1) BE522811A (en)
CH (1) CH314474A (en)
DE (1) DE1006984B (en)
FR (1) FR1083345A (en)
GB (1) GB738121A (en)
NL (2) NL85499C (en)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3300681A (en) * 1965-10-22 1967-01-24 Gen Electric Adjustable magnet for a magnetron
US3787696A (en) * 1972-03-15 1974-01-22 Etec Corp Scanning electron microscope electron-optical column construction
US4798953A (en) * 1986-04-15 1989-01-17 Thomson-Csf Electronic beam device for projecting an image of an object on a sample

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2295403A (en) * 1940-11-15 1942-09-08 Rca Corp Apertured electron lens and method of alignment
US2356535A (en) * 1940-08-31 1944-08-22 Ruska Ernst Electronic lens
US2369782A (en) * 1943-04-01 1945-02-20 Rca Corp Electron lens system
US2490308A (en) * 1944-09-30 1949-12-06 Emi Ltd Electron lens system
US2567674A (en) * 1949-11-08 1951-09-11 Rca Corp Velocity modulated electron discharge device
US2587942A (en) * 1949-12-27 1952-03-04 Leitz Ernst Gmbh Electronic optical correction mechanism for magnetic lenses
US2624022A (en) * 1944-05-31 1952-12-30 Siemens Ag Device for regulating the focal length of magnetostatic pole shoe lenses
US2679018A (en) * 1950-06-30 1954-05-18 Rca Corp Magnetic electron lens pole piece

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE904095C (en) * 1940-06-01 1954-02-15 Siemens Ag Electron lens serving as an objective for an electron microscope
DE898217C (en) * 1943-01-27 1953-11-30 Siemens Ag Arrangement for changing the enlargement achievable with a magnetic electron lens

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2356535A (en) * 1940-08-31 1944-08-22 Ruska Ernst Electronic lens
US2295403A (en) * 1940-11-15 1942-09-08 Rca Corp Apertured electron lens and method of alignment
US2369782A (en) * 1943-04-01 1945-02-20 Rca Corp Electron lens system
US2624022A (en) * 1944-05-31 1952-12-30 Siemens Ag Device for regulating the focal length of magnetostatic pole shoe lenses
US2490308A (en) * 1944-09-30 1949-12-06 Emi Ltd Electron lens system
US2567674A (en) * 1949-11-08 1951-09-11 Rca Corp Velocity modulated electron discharge device
US2587942A (en) * 1949-12-27 1952-03-04 Leitz Ernst Gmbh Electronic optical correction mechanism for magnetic lenses
US2679018A (en) * 1950-06-30 1954-05-18 Rca Corp Magnetic electron lens pole piece

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3300681A (en) * 1965-10-22 1967-01-24 Gen Electric Adjustable magnet for a magnetron
US3787696A (en) * 1972-03-15 1974-01-22 Etec Corp Scanning electron microscope electron-optical column construction
US4798953A (en) * 1986-04-15 1989-01-17 Thomson-Csf Electronic beam device for projecting an image of an object on a sample

Also Published As

Publication number Publication date
FR1083345A (en) 1955-01-07
DE1006984B (en) 1957-04-25
CH314474A (en) 1956-06-15
NL172506B (en)
NL85499C (en)
GB738121A (en) 1955-10-05
BE522811A (en)

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