US3452241A - Electron gun suitable for electron microscope - Google Patents

Description

United States Patent US. Cl. 315-45 Claims ABSTRACT OF THE DISCLOSURE To prevent positive ions from hitting the filamentary cathode in an electron beam gun and damaging it, the electron beam is hollowed out and an electrode is positioned in the hollow to stop the flow of the positive ions back to the cathode.

This invention relates to an electron beam producing means, for example, an electron gun, which produces a straight electron beam and includes an ion trap along the axis of the beam. The beam producing means provided by this invention is particularly suitable for use in an electron microscope but is not limited to that use.

As an electron beam producing means or gun operates to produce a high voltage electron beam, undesirable ions are also being produced which are accelerated by the same field that accelerates the electrons of the beam but in the opposite direction. These ions hit the cathode comprising the source of electrons for the beam and may damage or eventually destroy it. A known method of preventing the ions from hitting the cathode is to use a bent beam. However, use of such a beam in an electron microscope, for example, causes asymmetrical deflection defocusing of the beam which degrades the electron microscope image produced by such a beam. Also, when a beam is accelerated, as by accelerating electrodes, the voltage applied to the accelerating electrodes has a lens effect on the beam, whereby a change in accelerating voltage requires a change in focusing of the beam.

It is an object of this invention to provide an improved straight electron beam producing apparatus including an in-line or axial ion trap.

It is another object of this invention to provide an electron gun including such anion trap.

It is still another object of this invention to provide an electron beam source for a linear electron accelerator in which defocusing of the beam by changes of acceleration voltage is minimized.

In accordance wit-h this invention, lens means is pro vided for hollowing out an electron beam in a symmetrical manner with respect to the axis of the beam and for returning the electrons of the beam to their beam form, to produce an electron free space in the middle of the beam. An ion trap is positioned in the electron free space in the beam. Other lens means are provided for further focusing the beam and means are provided for accelerating the electrons thereof, the other lens means and the accelerating means including an element in common. While the electron beams so provided can be used for many purposes for which an electron beam gun is suitable, such as the beam gun of a cathode ray tube, it can also be used in a ice high voltage beam microscope with further beam accelerating means. When so used, the defocusing of the beam resulting from changes in voltage of the further acceleration means is minimized by the construction of the electron gun.

The invention may be better understood upon reading the following specification in connection with the accompanying drawing in which the sole figure is a fragmentary section of an electron microscope taken through a vertical axis thereof.

Turning to the figure, a filamentary cathode 10 which is heated and held at a high negative accelerating potential with respect to ground by suitable means, not shown, acts as a source of electrons for the beam 12. A grid cap 14 which is held at a bias voltage such as 45 volts with respect to the filamentary cathode 10' surrounds the cathode 10. The grid cap 14 is symmetrical about its central vertical axis as viewed in the figure. The central portion of the cap 14 is in the form of a plate 16 having a hole 18 therethrough for the passage of electrons from the cathode 10 and the outer portion of the grid cap 14 is formed to provide an outwardly and downwardly slanting conical wall 20. A further axially symmetrical electrode 22 which comprises an annular shaped disk 24 and a cylindrical portion 26 is positioned coaxially with the cap 14 and with the disk 24 below the cap 14, the cylindrical portion 26 surrounding the grid cap 14. The electrode 22 is maintained by suitable means, not shown, at the voltage of the cathode 10. An axially symmetrical electrode 28, which is maintained at about 6,000 volts positive with respect to the cathode 10 by suitable means, not shown, is also positioned coaxially with the cap 14. The electrode 28 comprises a hollow portion 30 which is frusto-conical both at its outer and inner surfaces, the tip of the portion 30 extending up into the conical portion 20 of the grid 14. The upper end of the portion 30 has a hole therethrough for passage of electrons. The lower part 32 of the conical portion 30 is cylindrical in internal shape. The conical portion 30 is joined to an annular disk-like portion 34 by an intermediate portion 36. The lower edge of the intermediate portion 36 is hollowed out as shown to provide clearance between the electrode 28 and an electrode 38 as will be more fully explained.

Due to the shape of the lower surface of the grid cap 14 and to the shapes of the electrodes 22 and 28 and due to the potentials applied thereto, the beam of electrons 12 exiting through the hole 18 will expand radially in a symmetrical manner as it goes down and will therefore take the conical shape shown, the cylindrical portion 32 cansing a speeding up of the radial expansion of the beams since the surface 32 does not recede from the beam as does the internal conical surface portion just above it. This expanding portion of the beam 12 acts as if it originated at the point 40, at the apex of the lines comprising the extensions in an upward direction of the envelope of the beam 12. Means are provided to hollow out the beam 12 and then to contract it in a radial direction. The beam hollowing out means includes the axially symmetrical electrode 38 which is maintained at the same voltage as the cathode 10 by suitable means, not shown. The electrode 38 includes a pointed portion 42, the point thereof extending upwardly, the portion 42 being supported by spider arms 3 44 from an annular disk 46 comprising another portion of the electrode 38.

While only two spider arms 44 are shown, there may be as many spider arms (three for example) as is desired. The outer portion of the electrode 38 is turned up and surrounds the outer edges of the disk 34 forming part of the electrode 38. The inner edge of the disk 46 is formed into an upstanding portion 48 whose inner shape is cylindrical and whose outer shape is conical and slants outwardly in a downward direction. The lower portion of the disk 46 is formed to receive a portion of an electrode 52 as will be explained. Since the electrode 38 is at a high negative potential, that of the cathode 10, and since the electrode 28 is at a high positive potential with respect thereto, the portion 36 of the electrode 28 is cut away to prevent arcing between the electrodes 28 and 38. Also the turning up of the outer potrion of the electrode 38 to surround the electrode 28 is to prevent arcing between the electrode 28 and the microscope casing, not shown, of which the described electron gun may be a part. The shape and position of the pointed electrode portion 42, and the negative voltage applied to it, causes the electron beam to be hollowed out to provide a substantially electron free space within the beam 12 immediately surrounding the pointed portion 42, the space so formed being symmetrical with respect to the axis of the beam 12.

The means for causing the beam 12 to contract in a radial direction includes the electrodes 38, 50 and 52. The electrode 50, which is also axially symmetrical, includes an outer annular disk portion 54, a transitional portion 56, spider arms 58 and a central disk potrion 60. The transitional portion 56 has a conical upper surface and a more steeply conical lower surface and joins the lower annular portion 50 to the spider arms 58 which support the disk 60. While only two arms 58 are shown, there may be as many thereof (three for example), as may be desired.

The electrode 50 is maintained at the same high positive potential with respect to the cathode as the electrode 28 by suitable means, not shown, whereby the arms 58 are also highly positive, and whereby the electrons of the beam 12 are attracted thereby. To reduce attenuation of the beam 12 by the arms 58, the arms 58 are positioned in the shadow of the arms 44 which are at the same potential as the cathode.

The electrode 52, which is held at the potential of the cathode 10 by suitable means, not shown, comprises an annular disk 62 having an inner diameter equal to the inner diameter of the central upstanding portion 48 of the electrode 38 surrouding the beam 12 and an upstanding cylindrical portion 64 having the same inner diameter as the disk 62 and a much smaller outer diameter. The cylindrical portion 64 extends up into and fits into the electrode 38 and is electrically connected thereto, to form with the inner surface of the portion 48 a long equipotential cylindrical surface. To provide clearance for the spider arms 44 and 58, vertical slots 66 are cut in the upper end of the cylinder 64. While no insulation is required for the arms 44, the arms 58 and the portion 56 of the electrode 50 are insulated by vacuum from the cylinder 64. Due to the voltages applied to the electrodes 38, 50 and 52, the electrons are attracted inwardly by the positive voltage on the disk 60, and the electrons are also repelled inwardly by the negative voltage on the inside surface of the electrodes 38 and 52 to cause the beam 12 to consolidate and to cross over at the point 68 and to become a solid beam. The electrons of the beam 12 go downward from the point 68 so that they appear to originate at the point.

70 which is the point of intersection of straight lines tangent to the envelope of the beam. An axially symmetrical electrode 72 is positioned coaxially with the electrodes 28, 38, 50 and 52. This electrode 72, which is maintained at a positive potential of about 560 volts with respect to the cathode 10 by suitable means, not shown, comprises an outer annular disk 74 in an inner annular disk 76 connected by a transitional portion 78. The disk potrion 76 extends up into the lower part of the annular disk 62 to produce the necessary electrostatic field shape to cause the beam to appear to come from point 70. The hole in the disk 76 increases in diameter in a downward direction in a known manner. The beam 12 passes through the hole in the portion 76, and the beam emerging therefrom, due to the electrodes mentioned and the voltage applied thereto, looks as if it originated at the point 70 with the same angular divergence and envelope as if it originated from point 40. However, an ion that originates in the portion of the described gun below the ion impermeable disk 60 hits the disk 60 and is stopped thereby. The ion cannot hit and therefore does not injure the cathode 10, whereby the life of the cathode 10 is extended by the provision of the described ion trap thereof.

The electron beam is accelerated and further focused by the electrode 72 in conjunction with the electrodes 80 and 82, whereby the electrode 72 acts both as an element of the coaxial ion trap described as well as an element of a beam accelerator. The axially symmetrical element 80 comprises an outer annular disk portion 84 and a centrally positioned upstanding hollow portion 86 having a conical upper surface and a cylindrical inner surface. The electrode 80 also includes a downwardly extending hollow portion 88 having a cylindrical inner surface 90 and a cylindrical outer surface 92 adjacent to the disk 84 and an outer conical surface 94 which decreases in diameter in a downward direction joining the cylindrical surfaces 92 and 90. The diameter of the surface 90 is greater than the inside diameter of the portion 86. An apertured disk 96 closes the top of the cylindrical surface 90. The aperture in the disk 96 may be biconical as shown. The disk 96 is thinner than the disk 84 and the lower surfaces of the disks 84 and 96 are in the same plane. The upper end of the portion 86 of the electrode 80 extends into the transitional portion 78 of the electrode 72. The voltage applied to the electrode 80 by suitable means, not shown, may be 660 volts positive.

The electrode 82, which is axially symmetrical, has a positive voltage of 560 volts applied thereto by suitable means, not shown. This electrode 82 comprises an outer Iannular disk portion 98 having a turned down rim 100. The electrode 82 also includes a central annular disk portion 102 having a hole therethrough for the passage of electrons which has a biconical shape comprising a shorter frusto-conical portion, whose diameter decreases in a downward direction, and a longer frusto-conical portion whose diameter increases in a downward direction, the two smaller diameter portions of the frustums being joined. The shape of the aperture through the disk portion 102 aids in maintaining focus of the beam 12. The electrode 82 also includes a transitional portion 104 having an upper conical surface conforming in angle with the surface 94 of the electrode 80 and spaced therefrom, the conical surface 94 extending into the transitional portion 104. The outer surface of the transitional portion 104 is cylindrical. The portion 106 of the electrode 82, which, as shown, may be made up as a separate part from the portions 98, and 104 thereof for the purpose of ease of manufacture, fits over the lower end of the cylindrical outer surface of the transitional portion 104. The portion 106 has a conical inner surface 108 whose diameter decreases in a downward direction. An apertured plate 110 closes the lower part of the lower portion 106. The aperture in the plate 110 may have a wedge shaped inner edge as shown. X deflecting plates 112 and Y deflecting plates 114, only one of the Y plates being shown, are provided in the chamber within the conical surface 108 and between the apertured disks 102 and 110. By adjustment of potentials provided by suitable means, not shown, applied to the deflection plates X and Y, the direction of the beam 12 emerging below the disk 10 may be controlled.

The device so far described may be used as a complete electron gun requiring no additional acceleration and including the described ion trap. However, the described gun may also be used as a source of a beam of accelerated electrons subject to further acceleration as in an electron microscope. When so used, a further acceleration electrode 120 may be provided. This electrode 120 comprises a disk aperture for passage of the electron beam 12 there through, to which high positive voltage in the order of 6.6 to 33 kv. (kilovolts) may [be applied. This accelerating electrode 120 may be one of a plurality (up to 15) of similar electrodes positioned along the beam 12 below the electrode 120 to which successively higher positive voltages are applied, the highest of which may be as high as 500 kv. when the voltage on the electrode 120 is 33 kv. The voltage on the accelerating electrodes, of which only the electrode 120 is shown, while accelerating the beams 12 passing therethrough also produces an electron lens action which results in a defocusing of the beam 12. Using the described electron gun with accelerating electrodes such as electrode 120, only the voltage on the grid cap 14 need be changed to retain focusing of the beam 12, as the voltage of the accelerating electrodes such as electrode 120 is changed. This is due to the fact that the accelerating electrode 120 and any subsequent accelerating electrodes such as electrode 120 (not shown) constitute a weak electrostatic aperture lens having a very long focal length. The electrodes 72, 80 and 82, on the other hand, constitute a much stronger electrostatic lens having a much shorter focal length. Because of the proximity of this strong lens comprising the electrodes 72, 80 and 82 to the electrode 120 and to the subsequent accelerating electrodes (not shown), the strong lens 72, 80 and 82 is immersed in the focal length of the weak lens comprising the electrode 120 and subsequent electrodes (not shown). In this situation a small focusing change in the stronger lens 72, 80 and 82 can overcome a focusing change due to a great voltage change in the weaker lens. The small focusing change in the strong lens 72, 80 and 82 is provided by varying the voltage in the grid cap 14, without varying the voltages in the electrodes 70, 80 and 82. Since change of the voltage on the grid cap 14 changes the position of the point 70, where the electrons of the beam 12 appear to originate, the focusing effect on the beam of electrons of the lens 72, 80 and 82 is changed.

This change of potential on the grid cap 14 is from about 45 volts to about 60 volts, or about 33%, as the voltage on the electrode 120 is changed from about 6.6 kv. to about 33 kv. or about 500%. Therefore, the electron gun described minimizes the defocusing of the beam thereof upon further acceleration of the beam and reduces the changes in potential of the grid cap 14 necessary to track, that is to keep the beam focused as the acceleration voltage on the electrodes, such as electrode 120, is changed.

While the several electrodes (except the cathode which may be made of tungsten) may be made of any solid nonmagnetic conductor, preferably they may be made of stainless steel.

Many modifications of the above-described invention will occur to a person skilled in the art. For example, the voltage mentioned and the details of the shapes of the electrodes are exemplary only.- The above description is therefore to be considered as illustrative and not in a limiting sense.

I claim: 1. In combination: lens means for expanding an electron beam in all radial directions from the axis of said ibeam along a portion of its length and for hollowing out an expanded portion of said beam to provide an electron free space,

means impervious to the passage of ions in said space to stop ions from moving along said beam and through said electron free space,

means for contracting the radial diameter of said beam after the expansion of said beam,

said means for expanding the diameter and for hollowing out the expanded portion of said beam comprising a conical electrode surrounding said beam, and

a pointed electrode positioned along the axis of said beam.

2. The invention as described in claim 1 and including:

an electrode for focusing said contracted beam to cause the electrons of said beam to travel in the same direction and at the same speed.

3. The invention as described in claim 1 and including:

an electrode for focusing said contracted beam to cause the electrons of said beam to travel in the same direction and at the same speed, and

an electron accelerating means, said acceleration means,

including said focusing electrode.

4. An electron beam gun comprising:

a cathode having an electron emitting portion positioned along an axis,

a grid cap having a circular aperture surrounding and coaxial with said axis,

an axially symmetrical hollow frustro-conical electrode coaxially positioned with respect to said axis, the small end of said electrode being adjacent said grid p,

an axially symmetrical pointed electrode positioned coaxially with respect to said axis at a point beyond said conical electrode from said grip cap, the pointed portion of said pointed electrode extending towards said conical electrode,

an ion impermeable electrode positioned along said axis [beyond said pointed electrode in a direction away from said cap, and

an axially symmetrical hollow cylindrical electrode positioned coaxially with said axis and surrounding said pointed electrode and said ion impermeable electrode.

5. The invention as claimed in claim 4 in which said cylindrical electrode extends beyond said ion impermeable electrode in the direction away from said grid cap.

6. The invention as claimed in claim 4 in which said cylindrical electrode extends beyond said ion impermeable electrode in the direction away from said grid cap, and

an additional axially symmetrical focusing electrode positioned coaxially with said axis and within said cylindrical electrode.

7. The invention as claimed in claim 4 in which said cylindrical electrode extends beyond said ion impermeable electrode in the direction away from said grid cap,

an additional axially symmetrical focusing electrode positioned coaxially with said axis and within said cylindrical electrode,

said gun also including first and second axially symmetrical accelerating electrodes coaxially positioned with respect to said axis and beyond said focusing electrode,

said second accelerating electrode being more remote from said grid cap than said first accelerating electrode.

8. The invention as claimed in claim 4 and including means to apply positive potential to said conical and said ion impermeable electrodes with respect to said grid cap and cathode potential to said cylindrical electrode and pointed electrode whereby the electron stream produced by said cathode is hollowed out to produce an electron free space along said axis, said ion impermeable electrode being within said electron free space.

9. The invention as claimed in claim 7 including means to apply high positive voltage to said conical and said ion impermeable electrodes with respect to said grid cap and cathode potential to said cylindrical electrode and pointed electrode whereby the electron stream produced by said cathode is hollowed out to produce an electron free space along said axis,

7 8 said ion impermeable electrode being within said elec- References Cited tron free space, and further including UNITED STATES PATENTS means to apply low positive voltage to said focusing l t d d to the second one of said accelerating 1617360 11/1952 De Gler 313 82 electrodes and an intermediate positive voltage to 5 2661436 12/1953 Van Ormer 313*82 said first one of said accelerating electrodes. 2888605 5/1959 Brewer 315 '15 10. The invention as claimed in claim 9 and including 3,201,631 8/1965 Gale et 250-495 3,307,065 2/1967 Arnaud et al. 315-3l X a third axially symmetrical accelerating electrode positioned coaxlally with said axis and beyond said second RODNEY D. BENNETT JR" Primary Examiner.

accelerating electrode in a direction away from said grid 10 cap, and M. F. HUBLER, Assistant Examiner.

means to apply a positive voltage of the order of the voltage applied to said conical electrode to said third US. Cl. X.R.

accelerating electrode. 31382

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