US3808498A

US3808498A - Electron beam generating source

Info

Publication number: US3808498A
Application number: US00246895A
Authority: US
Inventors: M Fujisawa
Original assignee: Jeol Ltd
Current assignee: Jeol Ltd
Priority date: 1971-11-17
Filing date: 1972-04-24
Publication date: 1974-04-30
Anticipated expiration: 1991-04-30
Also published as: JPS5247660B2; JPS4856366A

Abstract

This specification discloses an electron beam generating source having a control electrode comprising electrically conductive and nonconductive or slightly conductive layers, said nonconductive or slightly conductive layer facing an anode. The surface of said conductive layer in contact with said nonconductive or slightly conductive layer faces a cathode, thereby preventing electron emission from said contact surface and so by eliminating or almost entirely eliminating micro discharge.

Description

United States Patent [1 1 g Fujisawa Apr. 30, 1974 ELECTRON BEAM GENERATING SOURCE Prima Examiner-John Kominski I t 1 M1 F T k J [75] men now 0 yo apan Attorney, Agent, or FirmWebb, Burden, Robinson & [73] Assignee: Nihon Denshi Kabushiki Kaisha, W bb Akisima-shi, Japan I [22] Filed: Apr. 24, 1972 21 Appl. No.: 246,895 ABSTRACT This specification discloses an electron beam generat- [30] Forelgn Apphcanon Pnomy Data ing source having a control electrode comprising elec May 12, 1971 Japan 46-92063 trically conductive nonconductive or li h l ductive layers said nonconductive or slightly conduc- [52] US. Cl 313/353, 250/31 1, 313/83, five layer facing an anode The Surface f Said conduc 3 13/355 tive layer in contact with said nonconductive or [51] Int. Cl. H01] 19/38 slightly conductive layer faces a Cathode, thereby [58] new of Search 313/353 venting electron emission from said contact surface 313/85 82; 250/495 311 and so by eliminating or almost entirely eliminating micro discharge. [56] References Cited UNITED STATES PATENTS 19 C'aims, 10 Drawing Figures 3,021,446 2/1962 Ekkers et al 313/353 7 PATENTEDAPRSO-IUH SHEET 3 BF 4 HIGH VQLTAGE SOURCE 1 ELECTRON BEAM GENERATING SOURCE This invention relates to an electron beam generating source capable of generating an electron beam having an energy stability of an extremely high order.

In a high resolution electron beam apparatus, high stability of .the accelerating voltage is most essential, particularly in the case of a high resolution type electron microscope where a stability in the order of is required in order to avoid any loss in resolution. In an attempt to satisfy this requirement, electron beam generating sources in present use employ a feedback circuit which operates to reduce fluctuations in the accelerating voltage, thereby improving the stability. The principle of operation is as follows: A signal, proportional to the fluctuation of the output voltage of the high voltage generating device, is detected and fed into a differential amplifier where it is compared with a reference voltage. The resultant differential signal is fed back to the high voltage generating device in order to control the output voltage fluctuation. The disadvantage of this circuit, however, lies in its inability to eliminate the high frequency component present in the electric micro discharge occurring between the gun electrodes due to the transitory nature (a few micro seconds) of the discharge. As a result, the required high degree of stability is unobtainable. For example, assuming the output impedance of the high voltage generating device to equal Z, the micro discharge current to equal 1, and the voltage subject to fluctuation by the micro discharge to equal B, then E=ZI High voltage generated by a power source is divided and applied to each electrode in order to accelerate the electrons emitted from the filament. With this type of tube, however, due to the plurality of electrodes, interelectrode micro discharge readily occurs and, since the high voltage is fluctuated by said discharge, the fluctuation of the accelerated electron beam inevitably increases. .Further, since the electrons generated by the discharge are also accelerated by the electrodes, they accumulate considerable energy and, unless suitable corrective measures are taken, serious damage to vital parts of the microscope becomes a real possibility.

In the existing accelerating tube, to avoid the possibility of damage as above described, the micro discharge is controlled by, conditioning the electrodes.

Two methods of conditioning are used. One method is to use the high voltage power source for both electron beam acceleration and conditioning and the other method is to use a separate power source for conditioning. In the former method, since the voltage'necessary for conditioning the electrodes is about 1.2 to 1.3 times that of the maximum voltage for accelerating the electron beam, it is necessary to increase the capacity of this dual purpose" source which involves various technical problems associated with very high voltage application. In the latter method, the provision of a separate power source, eliminates the problem of high voltage application but, even so, in both methods, an electrical circuit is necessary with the result that the structure of the accelerating tube becomes more complex and hence, more expensive. Furthermore, each time the vacuum is broken, for example, after filament exchange, conditioning is necessary, a factor which is both troublesome and time-consuming.

The micro discharge that it is desirous to eliminate is caused by the multiple exchange of ions between two electrodes (for example, the Wehnelt electrode and the anode) attributable to the emitted electrons and the source of the discharge is the electron emission from the field emission sources distributed on the surface of the electrode facing the anode.

In order, therefore, to achieve the essential objects of this invention and thereby ensure an electron beam stability of the required high order and also to do away with the necessity of conditioning the electrodes which, as previously stated, involves the provision of an electrical circuit, added complexities and expenses, etc., it is necessary to prevent the electrons from being emitted from the control electrode or accelerating electrodes. For example, in order to attain this object, one embodiment of this invention incorporates a control electrode such as a Wehnelt electrode, or in the case of a multi-stage tube, a plurality of electrodes, comprising one layer of electrical conductive material and a second layer of electrically nonconductive or slightly conductive material. The layer of conducting material is arranged to face the cathode or the field side having a potential lower than that of the electrode and the layer of nonconducting or slightly conducting material is arranged so as to face the anode ,or the field side having a potential higher than that of the plurality of electrodes. High voltage is applied to the conducting layer. Since the surface of the conducting layer in contact with, the nonconductive or only slightly conductive layer faces the anode or the field side having a potential higher than the potential of the conducting layer itself,

electron emission from said contact surface is prevented with the result that micro discharge from the control electrode or inter-electrode micro discharge in the case of the multi-stage tube, is almost entirely eliminated.

One object of this invention is to provide an electron beam generating source capable of generating an electron beam having a stability of an extremely high order.

Another object of this invention is to provide a charged particle accelerating tube in which electrode conditioning is unnecessary.

Various other objects and advantages of this invention will become apparent by reading the following detailed description in conjunction with the accompanying drawings in which:

FIG. 1 shows one embodiment of this invention in which the control electrode comprises an electrically conductive base coated with a thin layer of electrically nonconductive or slightly conductive material;

FIGS. 2, 3 and 4 showmodified embodiments of the embodiment shown in FIG. 1;

FIG. 5 shows a second embodiment of this invention in which the control electrode comprises an electrically nonconductive material and a thin layer of slightly conductive material;

FIG. 6 shows a third embodiment of this invention in which the control electrode comprises an electrically nonconductive or slightly conductive base coated on the inside with a thin layer of electrically conductive material;

FIG. 7 shows a modified embodiment of the embodiment shown in FIG. 6;

FIG. 8 shows one embodiment of this invention in which the accelerating electrodes comprise an electrically conductive base and a thin layer of nonconductive or slightly conductive material; and,

FIGS. 9 and 10 show two more embodiments of this invention in which the respective accelerating electrode comprises an electrically nonconductive or slightly conductive base and a thin layer of electrically conductive material.

Referring to FIG. 1, an electron gun chamber 1 is positioned on top of a column 2 of an electron microscope. An insulator 3 is filled on the interior with insulating pitch 4. A high voltage cable 5 is set in the pitch 4. One end of the lead wires 6, 7 and 8 are connected to a filament heating source 9 and a DC. high voltage source 10, their opposite ends being connected to three rods ll, 12 and 13 respectively. A filament 14 is connected to

rods

11 and 12, to which an alternating current generated by the filament heating source 9 is applied via lead wires 6 and 7 and

rods

11 and 12. A Wehnelt electrode 15 comprising an electrically conductive base 16 coated with a thin layer 17 of electrically nonconductive material is threadably secured to an electrically conductive member 18 fixed to the insulator 3. The member 18 is connected to the rod 13 and the base 16, thereby enabling negative high voltage generated by the DC. high voltage source to be applied to the Wehnelt base 16 via the lead wire 8, the rod 13 and the member 18. The surface of the member 18 is coated with a thin layer 19 of electrically nonconductive material.

In the above arrangement, since the lead wire 8 is connected to the lead wire 7 through a bias resistance 20, a bias voltage is applied between the filament 14 and the Wehnelt electrode which controls the elec trons emitted from the filament. Moreover, since the surface of the Wehnelt electrode facing the anode 29, is nonconductive, the field emission sources distributed on said surface disappear and micro discharge between the Wehnelt electrode and the anode is almost entirely eliminated. Consequently, the fluctuation of the accelerated electron beam energy is very small.

In one embodiment, the inventor constructed a Wehnelt electrode with a stainless steel base covered by a thin layer of silicon oxide. A voltage of 100 kV was applied between the Wehnelt electrode and the anode and no micro discharge whatsoever was recorded.

In the above embodiment the Wehnelt electrode comprises an electrically conductive base and a nonconductive outer layer. It is possible, however, to use a thin layer of slightly conductive material instead of the nonconductive layer. By so doing, the potential gradient of the Wehnelt electrode surface becomes very small and the pockets of high field intensity disappear. As a result, micro discharge from the electrode is almost entirely eliminated. Further, since the charge which builds up on the layer surface flows into the electrically conductive base, electron beam deflection due to said charge is prevented. It was confirmed by experiment that if the product of 0(Q'cm) and 8(,u.) of the layer (0' is the resistivity and 5 is the thickness of the layer) is in the range 10 -10 the stability of the accelerating voltage is extremely high. If the product of (r and 8 of the layer is allowed to exceed 10", however, a slight discharge from the layer surface and a certain amount of beam deflection occurs, due to the charge which builds up on the layer surface. Again, if the product of 0' and 8 is less than 10 the accelerating voltage tends to fluctuate slightly, due to an increase in the movement of the electrons between the base and the layer.

In FIG. 2 only part of the electrically conductive base is coated with nonconductive material. That is to say, the side of the base is left uncovered, since discharge does not occur from this portion due to the fact that the equipotential line is almost parallel and the grounded wall of the microscope is not immediately adjacent. The portion of the base immediately adjacent to the opening 21 is also left uncovered, in order to prevent the electron beam from being affected by the charge which builds up on the layer surface. An advantage of this arrangement is that before exchanging the filament the base connected to the high voltage source can easily be discharged by means of a grounding rod 22 as shown in the drawing.

In FIG. 3 thenonconductive layer in the vicinity of the electrode aperture through which the electron beam passes is coated with an electrically conductive layer 23 in order to prevent the beam from being affected by the buildup of charge on the surface of the nonconductive layer.

In FIG. 4 an electrically conductive piece 24 connected to the base 16 is used to achieve the same result as that obtained with the electrically conductive layer 23 in FIG. 3.

Now referring to the embodiment of FIG. 5, the Wehnelt electrode 15 comprises an electrically conductive base 16, a thin layer 25 of electrically nonconductive material and a thin layer 26 of slightly conductive material. The layer 25 controls the electron field emission from the base surface and the layer 26 reduces the potential gradient of the Wehnelt electrode surface. Thus, micro discharge from the Wehnelt electrode is almost entirely eliminated. Further, the charge which builds up on the layer surface flows into the electrically conductive base.

Referring to the embodiment of FIG. 6, the Wehnelt electrode 15 comprises an electrically nonconductive or slightly conductive base 30 such as glass coated on the inside surface with a thin layer of electrically conductive material 31. The layer 31 is electrically connected to the member 18 by an electrically conductive leaf spring 32 so that high voltage is applied between the layer 31 and the anode 29. Micro discharge from the layer is prevented by the nonconductive or slightly conductive base.

In FIG. 7 the part of the Wehnelt electrode on the side of the aperture through which the electron beam passes is formed of electrically conductive material 34 in order to prevent a charge from building up on the base and to regulate the field in the vicinity of the apex of the cathode 14.

FIG. 8 shows an embodiment of an electron beam accelerating tube according to this invention. In the figure the accelerating electrodes 40 separated by insulators 43 comprise an electrically conductive base 41 and a thin layer of nonconductive or slightly conductive material 42. A high negative voltage generated by a high voltage source44 is divided and applied to each base 41 so that the electrons emitted from a cathode 45 are accelerated by the electrodes 40 and an anode 46 which is grounded. Micro discharge from each electrode base 41 is almost eliminated without the necessity of conditioning, since each base surface facing the anode is coated with nonconductive or slightly conductive material.

In the electron beam accelerating tube shown in FIG. 9 the accelerating electrode 40 comprises an electrically nonconductive or slightly conductive base 50 and a thin layer of electrically conductive material 51 facing the low potential side. The divided high voltage is applied to each thin layer. Micro discharge from the thin layer is eliminated by the nonconductive or slightly conductive base again without the necessity of conditioning. In this case, it is desirable to coat each base surface in the vicinity of the aperture through which the electron beam passes with a thin layer of electrically conductive material in order to prevent acharge from building up on the surface.

In the electron beam accelerating tube shown in FIG. the accelerating electrodes 40 comprise a flower pot type insulating base 53 and a thin layer of electrically conductive material 54 facing the low potential side. The divided high negative .voltage is applied to each layer. The advantage of the embodiment is that the tube is very easily assembled by mounting the bases on top of each other.

Having thus described the invention with the detail and particularity as required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims:

I. An electron beam generating source comprising:

an electron emitter,

an anode for accelerating electrons emitted from said emitter, and

a control electrode for controlling said electrons emitted from said emitter comprising an electrically conductive layer only and an electrically nonconductive layer, said nonconductive layer facing the anode side.

2. An electron beam generating source according to claim 1 wherein said control electrode comprises an electrically conductive base coated with a thin layer of nonconductive material.

3. An electron beam generating source according to claim 2 wherein the surface of said base facing said anode is partially coated with said nonconductive thin layer, the surface of said base in the vicinity of the aperture through which the electron beam passes being exposed. Y

4. An electron beam generating source according to claim 1 wherein said control electrode comprises a nonconductive base coated with a thin layer of electrically conductive material.

5 An electron beam generating source comprising:

an electron emitter,

an anode for accelerating electrons emitted from said emitter, and

a control electrode for controlling said electrons emitted from said emitter comprising an electrically conductive layer and a slightly conductive 6 layer, said slightly conductive layer only facing the anode side.

6. An electron beam generating source according to claim 5 wherein said control electrode comprises an electrically conductive base coated with a thin layer of slightly conductive material.

7. An electron beam generating source according to claim 6 wherein the product 0(0 cm) and 6(p.) of said thin layer is in the range 10 to 10 ,11 being the resistivity and 6 being the thickness of the layer.

8. An electron beam generating source according to claim 5 wherein said control electrode comprises a slightly conductive base coated with a thin layer of electrically conductive material.

9. An electron beam generating source comprising:

an electron emitter,

an anode for accelerating electrons emitted from said emitter, and

a control electrode for controlling said electrons emitted from said emitter comprising an electrically conductive base, one layer of nonconductive material and one layer of slightly conductive material, said base facing the emitter side.

10. A charged particle accelerating tube comprising:

a charged particle source, and

a plurality of accelerating electrodes for accelerating the charged particles emitted from said source, each electrode comprising an electrically conductive layer and nonconductive layer, said nonconductive layer facing the field side having a potential higher than that of said electrode.

1l. A charged particle accelerating tube according to claim 10 wherein said accelerating electrodes comprise an electrically conductive base coated with a thin layer of nonconductive material.

12. A charged particle accelerating tube according to claim 10 wherein said accelerating electrodes comprise a nonconductive base coated with a thin layer of electrically. conductive material.

13. A charged particle accelerating tube according to claim 10 wherein said accelerating electrodes comprise a flower pot type non-conductive base, the surface of said base facing said charged particle source being coated with a thin layer of electrically conductive material.

14. A charged particle accelerating tube comprising:

a charged particle source, and a plurality of accelerating electrodes for accelerating the charged particles emitted from said source, each electrode comprising an electrically conductive layer and a slightly conductive layer, said slightly conductive layer facing the field side having a potential higher than that of said electrode.

15. A charged particle accelerating tube according to claim 14 wherein said accelerating electrodes comprise an electrically conductive base coated with a thin layer of slightly conductive material.

16. A charged particle accelerating tube according to claim 14 wherein said accelerating electrodes comprise a slightly conductive base coated with a thin layer of electrically conductive material.

17. A charged particle accelerating tube according to claim 14 wherein said accelerating electrodes comprise a flower pot type slightly conductive base, the surface of said base facing said charged particle source being coated with a thin layer of electrically conductive material.

18. In an electron beam generating source comprising an emitter and control and accelerating electrodes which may be associated with a power supply for creating an accelerating electric field, the improvement comprising at least one electrode comprising an electrically conductive layer and an electrically nonconductive to slightly conductive layer, said nonconductive to slightly conductive layer only facing the field side having a potential higher than said electrode.

19. In an electron microscope or the like having an evacuated chamber, an electron beam generating source and an electron beam optical system for focusing and directing the beam upon a specimen all within said chamber, the improvement comprising said electron beam generating source comprising a cathode for emitting an electron beam, electrodes for accelerating and controlling the electron beam, said cathode and electrodes being functionally associated with a suitable power supply to create an accelerating electric field, and at least one electrode comprising an electrically conductive layer and an electrically nonconductive to slightly conductive layer, said nonconductive to slightly conductive layer only facing the field side having an electric potential higher than said electrode such that micro discharge from said electrode is reduced and yet potential gradients that would efi'ect the electron beam are not developed on the surface of said electrode.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION P n 3,808l498 Dated April 30. 1974 In n flSQ Minoru Fuiisawa It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the first page of the patent on the line listing the Foreign Application Priority Data delete -May 12, 1971 Japan 46-9Z063--,

and insert the following:

-May12, 1971 Japan 46-31815 July 7,1971 Japan 46-50074 November 17,1971 7 Japan 46-92063 7 December 20, 1971 Japan 46-103364--.

In the Claims:

Claim 1 Column 5 Line 43 delete -only-.

Claim 1 Column 5 Line 44 --layer facing-- should read -1ayer only facing.

Signed and sealed this 10th day of September 1974.

(SEAL) Attest:

MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-IOSO (10-69) uscoMM-Dc scan-pas V 11.5. GOVERNMENY PRINTING OFFICE i959 0-356-33L

Claims

1. An electron beam generating source comprising: an electron emitter, an anode for accelerating electrons emitted from said emitter, and a control electrode for controlling said electrons emitted from said emitter comprising an electrically conductive layer only and an electrically nonconductive layer, said nonconductive layer facing the anode side.

3. An electron beam generating source according to claim 2 wherein the surface of said base facing said anode is partially coated with said nonconductive thin layer, the surface of said base in the vicinity of the aperture through which the electron beam passes being exposed.

5. An electron beam generating source comprising: an electron emitter, an anode for accelerating electrons emitted from said emitter, and a control electrode for controlling said electrons emitted from said emitter comprising an electrically conductive layer and a slightly conductive layer, said slightly conductive layer only facing the anode side.

7. An electron beam generating source according to claim 6 wherein the product sigma ( Omega cm) and delta ( Mu ) of said thin layer is in the range 108 to 1012, sigma being the resistivity and delta being the thickness of the layer.

9. An electron beam generating source comprising: an electron emitter, an anode for accelerating electrons emitted from said emitter, and a control electrode for controlling said electrons emitted from said emitter comprising an electrically conductive base, one layer of nonconductive material and one layer of slightly conductive material, said base facing the emitter side.

10. A charged particle accelerating tube comprising: a charged particle source, and a plurality of accelerating electrodes for accelerating the charged particles emitted from said source, each electrode comprising an electrically conductive layer and nonconductive layer, said nonconductive layer facing the field side having a potential higher than that of said electrode.

11. A charged particle accelerating tube according to claim 10 wherein said accelerating electrodes comprise an electrically conductive base coated with a thin layer of nonconductive material.

12. A charged particle accelerating tube according to claim 10 wherein said accelerating electrodes comprise a nonconductive base coated with a thin layer of electrically conductive material.

14. A charged particle accelerating tube comprising: a charged particle source, and a plurality of accelerating electrodes for accelerating the charged particles emitted from said source, each electrode comprising an electrically conductive layer and a slightly conductive layer, said slightly conductive layer facing the field side having a potential higher than that of said electrode.

19. In an electron microscope or the like having an evacuated chamber, an electron beam generating source and an electron beam optical system for focusing and directing the beam upon a specimen all within said chamber, the improvement comprising said electron beam generating source comprising a cathode for emitting an electron beam, electrodes for accelerating and controlling the electron beam, said cathode and electrodes being functionally associated with a suitable power supply to create an accelerating electric field, and at least one electrode comprising an electrically conductive layer and an electrically nonconductive to slightly conductive layer, said nonconductive to slightly conductive layer only facing the field side having an electric potential higher than said electrode such that micro discharge from said electrode is reduced and yet potential gradients that would effect the electron beam are not developed on the surface of said electrode.