US3731094A

US3731094A - Electron beam apparatus with means for generating a rotation-symmetrical magnetic field

Info

Publication number: US3731094A
Application number: US00173045A
Authority: US
Inventors: Poole J Le
Original assignee: US Philips Corp
Current assignee: US Philips Corp
Priority date: 1970-08-21
Filing date: 1971-08-19
Publication date: 1973-05-01
Anticipated expiration: 1990-05-01
Also published as: CH536026A; CA937340A; BE771537A; GB1367360A; FR2104603A5; DE2138892A1; DE2138892B2; NL7012387A; SE377001B

Abstract

An apparatus for generating an electrostatic space charge in a rotation-symmetrical magnetic field, in which an electron beam from an electron gun which is arranged laterally on the field space generates the space charge in the field space. The electron beam may be adjusted by mechanical adjustment of the gun and also by means of the potential of a conducting sleeve in the magnetic field. By means of this potential, it is also possible to adjust the ratio between the contributions to the space charge which are due to primary electrons and to secondary electrons produced by the primary electrons in a residual gas in the field space.

Description

o Elllllltlil'l Mates Pfilfilll 1 3,73l,94 Le Poole May l, W73

[54] ELECTRON BEAM AllARA'llJS WITH 2,233,264 2 1941 Marton ..250 49.5 o MEANS FQR GENERATHNG A 2,890,342 6/1959 Columbre ..250/49.5 A ROTATHON-SYMMETRKCAL MAGNET: FIELD FOREIGN PATENTS OR APPLICATIONS [75] Inventor: Jan Bart Le Poole, Delft, Nether- 578273 6/1946 GreatBmam 1 ds an Primary Examiner-James W. Lawrence Asslgnee: PhIIiPS Corporation! New Assistant Examiner-Harold A. Dixon v York, Attorney-Frank R. Trifari [22] Filed: Aug. 19, 1971 [57] ABSTRACT [21] Appl. No.: 173,045

An apparatus for generating an electrostatic space charge in a rotation-symmetrical magnetic field, in [30] Forms Apphcanon Pnomy Data which an electron beam from an electron gun which is Aug.21, 1970 Netherlands ..7012387 arranged laterally on the field space generates the space charge in the field space. The electron beam [52] U.S. Cl. ..250/49.5 D, 250/495 A may be adjusted by mechanical adjustment of the gun [51] Int. Cl ..H0lj 37/26 and also by means of the potential of a conducting [58] Field of Search ..250/49.5 A, 49.5 D Sleeve i the magnetic fi ]d By means f this m tial, it is also possible to adjust the ratio between the References cued contributions to the space charge which are due to UNITED STATES PATENTS primary electrons and to secondary electrons produced by the primary electrons in a residual gas In 3,209,147 9/1965 DuPouy et al. ..2SO/49.5 D the field space, 3,100,260 8/1963 Wilska ...250/49.5 D 2,452,919 11/1948 Gabor ..250/49.5 D 14 Claims, 5 Drawing Figures Patented May 1, 1973 4 Sheets-Sheet 1 mAAA B Fig. 1'

INVENTOR. JAN B. LE POOLE AGENT Patented May 1, 1973 3,731,094

4 Sheets-Sheet 2 J /ZZ J 4 Fig.3

INVENTOR. JAN 8. LE POOLE Patented May 1, 1973 3,131,094

4 ts-Sheet 3 INVENTOR. JAN B. LE POOLE Patented May 1, 1973 4 Sheets-Sheet 4 INVENTOR. LE POOLE JAN B.

ELECTRON BEAM APPARATUS WITII MEANS I OlR GENERATING A ROTATION-SYMMETRICAL MAGNETIC FIELD The invention relates to an electron beam apparatus comprising an electron source for generating an electron beam and means for generating a rotation-symmetrical magnetic field.

An electron beam apparatus of this kind is known from the British Pat. Specification No. 578,273, in which it serves to reduce the spherical aberration of electromagnetic lenses. In a rotation-symmetrical electromagnetic lens field electrons emitted by an electron source form an electrostatic space charge which exerts a compensating effect on the spherical aberration of an electron beam which is axially injected into the lens field. The electron source used for this purpose consists of an annular cathode, which is coaxially provided near an axial extremity of the electromagnetic lens field.

This set-up has the drawback that the rotation symmetry of the lens field for the image forming electrons is readily disturbed by the annular cathode and in particular by supply lines to the cathode. The resultant rotation-symmetry errors result in image defects for the image-forming electron beam.

The invention has for its object to eliminate this drawback and to this end an electron beam apparatus of the kind set forth according to the invention is characterized in that the electron source consists of an electron gun which is situated near an axial extremity of the rotation symmetrical magnetic field and at the side of the continuation of thesymmetry axis thereof, a principal direction of the electron gun being directed towards the magnetic field for the electron beam and crossing the magnetic field at an angle of no more than approximately In this arrangement the electron source can be situated such that it has no effect on the rotation symmetry of the magnetic field, while at the same time the control possibility for the electron beam is enhanced-The latter applies to the current intensity, to the energy with which and the angle at which the electrons are injected into the magnetic field.

For optimum compensation at minimum current in tensity of the electron beam, it is favorable to cause the electrons in the rotation-symmetrical magnetic field to traverse a path which is as long as possible. For this purpose, the angle of incidence and the velocity of the electrons are adapted to the magnetic field strength of the rotation-symmetrical magnetic field so that these electrons describe a helix of very small pitch therein.

According to a preferred embodiment of the invention, the space charge in the magnetic field is not formed exclusively, or even mainly, by electrons from the injected electron beam, but by secondary electrons which are produced by ionization of gas present in the magnetic field. Consequently, a gas pressure between approximately 10 and 10- Torr is to prevail in the field space. A cylindrical conductor which is coaxially arranged in the magnetic field and whose potential is adjustable may be of assistance for the purposeyboth in order to force the ions produced out of the field space and to co-determine the angle of incidence of the primary beam. The angle of incidence can also be controlled by means of a hinged arrangement of the gun. An advantage of the use of secondary electrons for the space charge is, besides the lower required primary current intensity for the beam to be injected, that the angle of incidence is less criticalfor optimum compensation.

The invention will now be described with reference to the accompanying diagrammatic drawing, in which:

FIG. I is a diagrammatic view of an electron beam apparatus according to the invention,

FIG. 2 is a diagrammatic view of an electron gun suitable for use as an electron source in an electron beam apparatus according to the invention;

FIG. 3 is a diagrammatic view of an electron microscope, one of the electromagnetic projector lenses of which forms part of an electron beam apparatus according to the invention,

I FIG. 4 is a diagrammatic view of a micro-analyzer one objective lens, constructed as a tube lens, of which forms part of an electron beam apparatus according to the invention, and

FIG. 5 is a diagrammatic view of an ionization manometer, an ionization measuring chamber of which as a rotation-symmetrical field space forms part of an electron beam apparatus according to the invention.

In a preferred embodiment of an electron beam apparatus according to the invention as shown in FIG. I, a rotation-symmetrical magnetic field is generated in a field space I by an electromagnetic coil 2 having windings 3, a ferromagnetic shielding 4 and an air-gap 5. This electromagnetic coil may be replaced by a permanent magnet or may be formed by pole shoes of an electro magnet. In the field space I an electron'beam 6 is shown, which is used, for example, for an electronoptical image in an electron microscope, a scanning micro-analyzer, an electron beam machining apparatus or another apparatus of this kind, where an electron beam having satisfactory projection properties is desired. Situated near an axial extremity of the coil 2 are an electron gun 7 having a cathode-9 provided with filaments 8, a control electrode I0 andv an anode 11. For focusing an electron beam emitted from the cathode 9 a magnetic lens 13 is mounted in the anode I I. In the rotation-symmetrical magnetic field the electron beam I2, if injected into the magnetic field under an angle of no more than approximately 30 and crossing the symmetry axis, will follow a helix 14. The pitch of this helix is determined by the intensity of the magnetic field and the energy of the injected electrons. In a preferred embodiment a sleeve I5 of non-ferromagnetic electrically conducting material is situated around the field space I. When this sleeve 15 is at a positive potential, it will push away the ions produced by the electron beam and a negative space charge can build up in the magnetic field. The direction in which the electron beam 12 is injected can be adjusted by varying the preferably positive potential of the sleeve 15. An energy for the electrons from the electron beam 12 corresponding to approximately 500 to 1,000 eV produces a maximum active section area for ionizing a gas in the space passed by the beam.

A preferred embodiment of an electron gun for such an electron beam apparatus includes, besides the already mentioned filaments 8, cathode 9, control electrode 10, anode Ill and lens 13, means for a mechanical adjustment possibility for the electron beam 12. For this purpose, as is shown in FIG. 2, for example between the control electrode and the anode deformable rings 16 are provided, and around the anode a deformable ring 17 is provided, so that, for example by externally operable adjusting screws, not shown, the gun can be realigned after mounting.

FIG. 3 shows a preferred embodiment of an electron beam apparatus which forms part of an electron microscope. In the very diagrammatically shown electron microscope are situated an electron gun 18 having a cathode 19, a control electrode 20 and an anode 21, a condenser lens 22, a main lens formed by the electromagnetic coil 2, a projector lens 23 and a target screen 24. In this embodiment the electron gun 7 is mounted between the main lens 2 and the projector lens 23. The diagrammatic representation of an electron microscope serves merely for the purpose of illustration. In each electron microscope accommodating an electromagnetic or permanent-magnetic lens, an electron beam apparatus according to the invention can be incorporated. Moreover, it is not necessary that the electron beam 12 be injected into the main lens. According to the invention it is equally possible to provide one of the condenser lenses or a projector lens or whatever rotation-symmetrical magnetic lens with a space-charge cloud.

FIG. 4 shows another embodiment of an electron beam apparatus according to the invention. The means for generating a rotation-symmetrical magnetic field are formed therein by an objective lens 25 of the type such as is described, for example, in US. Pat. No. 3,394,254, which can be used, for example, in a scanning micro-analyzer. An elongated magnetic coil 26 which is characteristic of this type of lens and which is enclosed in a cooling body 27 forms a magnetic field which is particularly suitable for using a space charge according to the invention so as to compensate for spherical aberration.

Measurements have shown that the spherical aberration coefficient of a magnetic lens may readily be reduced by a factor of by using a space charge according to the invention. The resolving power is determined by the fourth root of the spherical aberration coefficient, so that the gain, though noticeable, is not appreciable. The free operating distance is determined by the focal distance of the last lens and this can be chosen a factor of 5 larger in said example while maintaining proper focussing. It was found possible to achieve a focus size of l um with a normal beam, apertures of one-thirtieth radian and a free working distance having a length of 50 mm.

In compensating for the spherical aberration it is of minor importance how the space charge is produced. In this context two different processes may be considered. Either the electron beam itself, owing to the small pitch of its helical path remains in the magnetic field so long that a significant space charge is built up or the electrons from the electron beam ionize a residual gas to such an extent that a significant space charge is built up by the secondary electrons produced in this ionization. In practice, both processes will occur simultaneously. A space charge which is formed exclusively by the electron beam itself most clearly approximates the ideal space charge distribution for optimum compensation. The build-up thereof, however, is comparatively critical as regards the direction of injection and energy of the electrons, and a comparatively large beam current is required. If the space charge is mainly built up by secondary electrons as a result of ionization, said electrons being kept in the lens space by the composite electrical field of the sleeve 15 and the rotation-symmetrical field, it will be less critical as regards the angle of injection and the energy of the electron beam, and a smaller beam current will be sufficient. For example, it was found that at a residual gas pressure of approximately 10* Torr a beam current of l u A produces a space charge offering sufficient compensation. A desired ratio between the contributions of each of the processes to the total space charge can be selected in particular by varying the potential ofthe sleeve 15.

When the electrostatic and electromagnetic fields in the field space are properly chosen, the secondary electrons produces by ionization of a residual gas can escape from the field only by colliding with gas molecules. Hence, the production of secondary electrons and the probability of their escape are proportional to the gas pressure. As a result, the build up of a space charge cloud of secondary electrons is not very critical for the pressure in the field space.

On the other hand, when the electrons are not detained by an electrostatic potential this production of gas ions and electrons in an electron beam apparatus according to the invention, can be used for sensitive gas pressure measurements. FIG. 5 shows a preferred embodiment which is suitable for this purpose. An airtight envelope 30 comprises, as in known ionization manometers, an aperture 31 for connection to a space in which the gas pressure is to be measured, a collector 32 and an anode 33. Around the envelope, means are provided for generating the rotation-symmetrical mag netic field, which means in the embodiment shown take the form of an electromagnetic coil 2, while an electron gun 7, which in this case may be provided with a threeelectrode electrostatic lens 34 for collimating the electron beam 12, extends into the envelope 30.

According to the invention it is alternatively possible to generate an ion beam in a rotation-symmetrical field by means of a spiralling electron beam. For this purpose an electron beam apparatus according to the invention is to be provided merely with means for separating the ions from the primary and/or secondary electrons and with means for collimating and deflecting the ion beam. All this can be realized by known means so that in a comparatively small space having a small electron current a comparatively large ion production is obtainable.

What is claimed is:

1. An electron beam apparatus comprising an electron gun for generating an electron beam and means for generating a rotation-symmetrical magnetic field, said electron gun being arranged near an axial end of the rotation-symmetrical magnetic field and at the side of a continuation of the symmetry axis thereof, a principal direction for the electron beam of said electron gun being directed towards the magnetic field and crossing the axis of the magnetic field under an angle of at the most approximately 30 2. An electron beam apparatus as claimed in claim 1, including means for adjusting the electron beam to be generated by the electron gun when it enters the rotation-symmetrical magnetic field.

3. An electron beam apparatus as claimed in claim 2, wherein the means for adjusting the electron beam consist of a cylindrical electrical conductor which is coaxially arranged in the rotation-symmetrical magnetic field. 7

4. An electron beam apparatus as claimed in claim 3, wherein during operation the electrons of the electron beam enter the rotation-symmetrical magnetic field with an energy of between 100 and 1,000eV.

5. An electron beam apparatus as claimed in claim 4, wherein during operation the electrically conducting cylinder is kept at a positive potential.

6. An electron beam apparatus as claimed in claim 1, wherein the means for generating a rotation-symmetrical magnetic field comprise an electromagnetic lens for focusing a beam of image-forming electrons which is axially injected into the magnetic field.

7. An electron microscope provided with an electron beam apparatus as claimed in claim 6, wherein the electromagnetic lens is an electromagnetic imageforming lens of the electron microscope.

8. A scanning electron microscope provided with an electron beam apparatus as claimed in claim 6, wherein the electromagnetic lens is an imaging lens of the scanning electron microscope which is situated nearest to the location of an object to be scanned.

9. An electron beam machining apparatus provided with an electron beam apparatus as claimed in claim 6, wherein the electromagnetic lens is a convergence lens of the electron beam machining apparatus.

10. A scanning micro-analyzer provided with an electron beam apparatus as claimed in claim 6, wherein the magnetic lens is an electromagnetic lenses of the scanning micro-analyzer.

11. An electron beam apparatus as claimed in claim 1 wherein during operation a gas pressure of between 10" and 10" Torr prevails at the area of the rotationsymmetrical magnetic field.

12. An ionization manometer provided with an ionization measuring space and an electron beam apparatus as claimed in claim 1 wherein the rotation-symmetrical magnetic field of the electron beam apparatus extends at least partially into the ionization-measuring space of the ionization manometer.

13. An electron beam apparatus as claimed in claim 1 wherein the rotation-symmetrical magnetic field space forms part of an ionization space for producing gas ions by means of the electron beam.

14. An electron beam apparatus as claimed in claim 13, characterized in that a pressure of between 10 and 10'' Torr prevails in the ionization space.

Claims

1. An electron beam apparatus comprising an electron gun for generating an electron beam and means for generating a rotationsymmetrical magnetic field, said electron gun being arranged near an axial end of the rotation-symmetrical magnetic field and at the side of a continuation of the symmetry axis thereof, a principal direction for the electron beam of said electron gun being directed towards the magnetic field and crossing the axis of the magnetic field under an angle of at the most approximately 30 *.

2. An electron beam apparatus as claimed in claim 1, including means for adjusting the electron beam to be generated by the electron gun when it enters the rotation-symmetrical magnetic field.

3. An electron beam apparatus as claimed in claim 2, wherein the means for adjusting the electron beam consist of a cylindrical electrical conductor which is coaxially arranged in the rotation-symmetrical magnetic field.

7. An electron microscope provided with an electron beam apparatus as claimed in claim 6, wherein the electromagnetic lens is an electromagnetic image-forming lens of the electron microscope.

11. An electron beam apparatus as claimed in claim 1 wherein during operation a gas pressure of between 10 4 and 10 10 Torr prevails at the area of the rotation-symmetrical magnetic field.

14. An electron beam apparatus as claimed in claim 13, characterized in that a pressure of between 10 and 10 4 Torr prevails in the ionization space.