US3786268A

US3786268A - Electron gun device of field emission type

Info

Publication number: US3786268A
Application number: US00243215A
Authority: US
Inventors: S Nomura
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1971-04-12
Filing date: 1972-04-12
Publication date: 1974-01-15
Anticipated expiration: 1991-01-15
Also published as: DE2217660A1; JPS5323663B1; NL155979B; NL7204863A

Abstract

In an electron gun device of the field emission type, there is provided a heating power source for supplying a pulsating current to a filament of hair pin shape provided with a cathode tip of needle shape so as to intermittently heat said cathode tip, thereby maintaining a stable electron beam emitted from said cathode tip by means of an electric field applied between said cathode tip and an anode.

Description

United States Patent [191 Nomura ELECTRON GUN DEVICE OF FIELD EMISSION TYPE [4 1 Jan. 15,1974

OTHER PUBLICATIONS Electron Gun Using a Field Emission Source," A. V.

[75] Inventor: Setsuo Nomura, Katsutai Japan Crewe, Review of Scientific Instruments, Vol. 39 No. [73] Assignee: Hitachi, Ltd., Tokyo, Japan APrll 68 P- [22] Filed: 1972 Primary Examiner-James W. Lawrence [21] Appl, No.: 243,215 Assistant Examiner-C. E. Church At!0rneyPaul M. Craig, Jr. et a1.

[3 )1 Foreign Application Priority Data ABSTRACT Apr, 12, 197] Japan 46-22338 In an electron gun device of the field emission type. 52 1 US. Cl. 250/306, 250/310 there is Provided a heating POWer Source for Supplying [51 Int. Cl. n01 j 37/26 a pulsating Current to a filament 9f hair P Shape P [581 Field of Search 250/495 R, 49.5 A, vided with a cathode tip of needle shape 89 as to inter- 5 5 PE, 419 SE mittently heat said cathode tip, thereby maintaining a stable electron beam emitted from said cathode tip by 5 References Cited means of an electric field applied between said cath- UNITED STATES PATENTS ode and 3,646,344 2/1972 Flows 250 495 PE 11 Claims, 11 Drawing Figur s F 1 i6 4 HIGH VOLTA- 5 1 GE SOURCE 1 l G) 7 l I I 2 K VOLTAGE I V tie JOURCE 1 11 1" i l I l I i 3 I l.\ i I N PATENTEU 3.786.268

SHEET 1 [1F 3 CATHODE I 8- HEATING PWR SOURCE I I W O {6 g 4 1 HIGHVOLTA- 5- I GE SOURCE 2 i ,2 if? g i VOLTAGE L j JOuRcE r v 5 5; 3 F. a d 2 Q LU I 5 LL] OPERATION TIME T (min) Lg FIG. 3 i

0: H P- B 6 J LL] 1 I 5'0 4'0 5'0 TIME T(min) PAIENTEDJIIN I 5 I974 SHEET 2 (IF 3 FIG. 4

I. 3 2 232 53% zomhmd SEE IO l5 TIME T (min) FIG.

HEATING CURRENT OF A CATHODE FIG. 60

HEATING CURRENT IIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIII OF A CATHODE EMITTED ELECTROIN BEAM FIG. 70

HEATING CURRENT OF A CATHODE FIG. 7b

W EMITTED ELECTRON I BEAM PATENTEUJAHI5I974 3.786.268

SHEET 3 BF 3 FIG. i A C 8... 7. i

DIRECT PULSE CURRENT OSCIL-- SOURC E LATOR POWER SOURCE J HEATING 9 POWER sOuRCE ELECTRON VICE SCANNING ie OSCI LLOSCOPE SECONDARY 1 ELECTRON I5 DETECTOR SCANNING SIGNAL 5 ELECTRON GUN DEVICE OF FIELD EMISSION TYPE BACKGROUND OF THE INVENTION This invention relates to an electron gun device of the field emission type which is suitable for use as an electron source in an electron microscope and the like, and more particularly to stabilizing means for stabilizing the electron beam emitted from said electron gun.

As is well known, an electron gun device of the field emission type is so'constructed as to emit electrons from a cathode tip of needle shape disposed in a vacuum by applying a strong electric field for electron emission to said cathode tip. Recently, since the electron beam density obtained from such an electron gun device is remarkably higher than that of the thermal electron emission type, it has become important for use as an electron gun device for electron optical apparatus, such as an electron microscope and the like.

However, in a conventional electron gun device of the field emission type, the emitted electron beam density depends to a great extent upon the surface condition of said cathode tip. Since the surface of said cathode tip has a tendency to become contaminated due to absorption of residual gas in vacuum during use and also is destroyed by means of ion bombardment, the emitted electron beam density from said cathode tip varies with time; and thus, the duration during which a stable electron beam density can be maintained is very short.

For example, when a field emission of electrons is effected in vacuum by using a clean cathode tip, firstly, its surface is contaminated due to the residual gas in vacuum, and thus, the emission of electrons therefrom becomes unsteady, thereby causing a rapid decrease in the electron beam density. After a while, this decrease in beam density begins to slowdown, and a comparatively stable electron beam density is obtained during a short time. Then, as the surface of said cathode tip is destroyed by means of the ion bombardment thereon,

a strong electric field becomes to be locally applied to the projecting portions thereof formed due to said ion bombardment and the electron beam density therefrom gradually increases, until finally, a very large electron beam is emitted. When such a condition occurs, the cathode tip is destroyed and the electron emission therefrom stops due to the large Joule heat and the strong electric field. In order to prevent occurrence of such phenomena, hitherto, when a stable electron beam is no longer obtained, operation of the electron gun device has been stopped by cutting off application of the electric field for electron emission thereto, and during this time, reconstruction (cleaning and smoothing) of the surface of said cathode tip has been attained by heating it to a high temperature (about 2,000C.). However, the duration during which it is possible to obtain a stable electron beam even after reconstruction is not only short, but operation of the electron gun device also must be stopped during said reconstruction.

In order to prevent such contamination of the cathode surface and to maintain a smooth surface thereon,

another conventional method has been used wherein the cathode tip is heated to a high temperature of about 1,500C. in operation. However, since a high electric field is applied to the surface of said cathodetip according to said method the so-called build-up phenomena locally occurs thereon with the result that the emitted electron beam gradually increases and thus, after all, the stable electron beam is not obtained.

Moreover, in order to prevent occurrence of said build-up phenomena, there is proposed still another conventional method wherein the electric field for producing electron emission is intermittently applied to said cathode tip while it is continuously heated, but in this case, since electrons are emitted therefromonly at the time when said electric field is applied thereto, there is the disadvantage that only an intermittent electron beam is obtained.

Thus, since it is difficult to obtain a stable electron beam in the conventional electron gun device of the field emission type for a long time, attempts have presently been made to maintain the duration of comparatively stable electron beam generation as long as possibe by making the degree of vacuum at the periphery of said cathode tip as high as possible, for example, 10 Torr. However, it is not easy to keep the degree of vacuum within the limits of 10 Torr. Further, regardless of how high the degree of vacuum is made, the duration of comparatively stable electron beam generation becomes only a little longer and the complete stability of the electron beam over a long period of time is not assured.

SUMMARY OF THE INVENTION The prime object of the invention is to provide a new and original electron gun device of field emission type wherein the emitted electron beam can be stably maintained for a long time.

Another object of the invention is to provide an electron gun device of the type described capable of being adjusted and controlled for stabilizing the electron beam without interrupting the operation of said electron gun device.

A further object of the invention is to provide an electron gun device of the type described which is economical to manufacture and simple and easy to stabilize during use.

In order to realize the above objects, the invention is characterized in that the cathode tip is intermittently heated under the condition wherein the electric field for electron emission is applied to its surface, thereby maintaining an extremely stable electron beam emitted during the intermittent heating of said cathode tip.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1, 8 and 9 are schematic diagrams showing various embodiments of electron gun devices of the field emission type according to the invention;

FIG. 2 is a curve showing the successive variation of the emitted electron beam in a conventional electron gun device operated without heating the cathode tip;

FIG. 3 is a curve showing the successive variation of the emitted electron beam in a conventional electron gun device when the field emission is interrupted and reconstruction due to heating of the cathode tip is done;

FIG. 4 is a curve showing the successive variation of the emitted electron beam for the case where intermittent heating of the cathode tip is performed in accordance with the present invention;

FIG. 5 is a diagram to explain the intermittent heating of the cathode tip in accordance with the invention;

FIGS. 6a and 6b are diagrams to explain the variation of the electron beam at the time when the intermittent heating of the cathode tip is interrupted; and

FIGS. 7a and 7b are diagrams to explain the variation of the electron beam in the case where the heating temperature of the cathode tip is excessively high.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an electron gun device of the field emission type according to the invention which is suitable for use in an electron microscope and the like. In the figure, the electron gun structure includes a cathode tip 1 of needle shape for electron emission and usually made of tungsten, an anode 2 for forming an electric field for producing electron emission (a first anode), an accelerating anode 3 (a second anode), a filament 4 for heating the cathode tip 1, a vacuum container 5, a high voltage source 6 connected between the cathode and the anode for forming an electric field for electron emission, a voltage source 7 connected between the respective anodes for accelerating the emitted electrons, and a cathode heating power source 8 connected between the terminals of the filament 4 for supplying a pulsating current to heat the filament 4 intermittently.

In operating the above device, firstly, the vacuum container is evacuated to obtain a high degree of vacuum of about Torr therein and a voltage of about 1 to 5 KV is applied between the cathode l and the first anode 2. Also, a voltage of about to 100 KV is applied between the first anode 2 and the second anode 3. At this time, it is not intended to heat the cathode tip 1.

Then, a strong electric field for electron emission is formed adjacent to the top portion of the cathode tip 1 and thereby an electron beam is emitted therefrom. This electron beam passes through an opening disposed at the center of the first anode 2 and further is accelerated toward the second anode 3. Therefore, an electron beam of high energy and density is emitted through the opening of the second anode 3, which beam is utilized in a well known manner.

As aforementioned, since the quality of the emitted electron beam greatly depends upon the surface condition of the cathode tip in such an electron gun device of the field emission type, hitherto, it has been very difficult to obtain a stable electron beam for a long period of time.

FIG. 2 shows an example of the successive variation of the emitted electron beam in a conventional electron gun device. In this electron gun device, the cathode tip is made sufficiently clean and smooth by flashing the surface thereof and then field emission there-from is effected in a high vacuum of about 10" Torr without heating the cathode tip. At this time, as is apparent from FIG. 2, the intensity of the emitted electron beam gradually decreases during the time period A.

Next, during the time period B, a comparatively stable electron beam is obtained; but thereafter, in contrast to this stable period, during the time period C, the electron beam gradually increases. This is a phenomena produced for the following reasons. First of all, in

the top portion of the cathode tip, electron emission becomes difficult due to the progression of contamination of its surface by reason of the absorption of gas; but thereafter, contrary to this, emission becomes easy due to gradual destruction of its surface by means of ion bombardment, etc.

In this way, in the conventional device, it is difficult to obtain a sufficiently stable electron beam for a long time, and the comparatively stable electron beam is obtained within only the extremely limited time duration B shown in FIG. 2. This time duration B is about 5 to 10 minutes with a degree of vacuum of 2 X 10" Torr and under the condition where the intensity ofthe electron beam is minimum.

Owing to the above-described phenomena, hitherto. when the time duration B is over, electron emission is interrupted and reconstruction of the surface of the cathode tip, including cleaning and smoothing thereof, is performed.

FIG. 3 shows the variation of the emitted electron beam in the case where application of the electric field for electron emission is stopped so as to interrupt elec tron emission for one minute during every heating period of about 10 minutes while reconstruction of the cathode surface is effected by heating the cathode tip 1 to about 2,000C. As is apparent from the figure, the emitted electron beam varies similarly to the above case after reconstruction of the cathode surface is com pleted and thus also the time duration for obtaining a comparatively stable electron beam is a short time of about 5 to 7 minutes. Further, in this case, since the cathode tip is heated to a high temperature, the cathode tip itself is in danger of modification.

Still further, as aforementioned, according to said conventional method, the cathode tip is steadily heated under the condition wherein the electric field for electron emission is steadily applied to its surface; but in this case, when the heating temperature of the cathode tip is higher than 1,000C., there is the danger of modification of the structure thereof. On the other hand, if said heating temperature is less than C, the desired effect of reconstruction of the cathode surface is hardly obtained. When the cathode tip is steadily heated within the temperature range of 150 to l,000C., the cathode tip is not modified and this is effective to some extent in producing emanation of the absorbed gas from the cathode tip and the desired smoothing thereof; however, steady heating within such temperature range produces a movement in the absorbed gas remaining on the cathode surface so that the locally emitted electron beam is not sufficiently stable.

It has been confirmed as a result of experiments with various methods for stabilizing the electron beam that an extremely stable emitted electron beam can be obtained by intermittently heating the cathode tip under the condition wherein the electric field for electron emission is continuously applied to the cathode surface during this intermittent heating. This is believed to result from the fact that the absorbed gas on the cathode surface is removed by the instantaneous heating of the surface and at the same time disarray of the atomic arrangement of the cathode surface produced by means of the ion bombardment, etc., is eliminated. Then, the cathode temperature decreases at once with elimination of the heating, thereby preventing movement of the absorbed gases and ions thereon.

FIG. 4 shows the example of this stabilized phenomena. Firstly, when field emission is effected in a vacuum of about 2 X l0 Torr by using the cathode tip immediately after flashing and without heating it, as aforementioned, the intensity of the emitted electron beam first decreases and then increases again. Next, at point e of FIg. 4, when intermittent heating of the cathode tip (a heating period of seconds, a heating timewidth of 0.1 second, a heating temperature of about 500C.) is again started under the condition wherein the electric field for electron emission is continuously applied to the cathode surface, during the time period B", the electron beam becomes extremely stable.

In the above case, if the intermittent heating of the cathode tip is suddenly abandoned, the current intensity again increases, as shown by the dotted line at f in FIG. 4, but after this, if intermittent heating thereof is reestablished again, the value of the electron beam returns to an original value, thereby stabilizing it again.

FIG. 5 shows the waveform of a filament heating current pulse supplied from the power source 8 to the filament 4 so as to intermittently heat the cathode tip. In the same figure, r, is the heating period (the repetition period of said pulse) and T is the heating time width (the time widthof said pulse).

The heating period 1, is determined in accordance with thedegree of vacuum in the device and the condition of use thereof, but usually may be about 10 seconds. Further, the heating time width 1', depends upon the heating temperature of the cathode and heating condition therein, but usually maybe less than 1 second. It is desirable to set the heating time width T at 0.1 to 0.2 seconds and the heating period 1, at l to seconds.

Still further, the amplitudes of the respective heating currents depend upon the heating temperature of the cathode and heating condition therein, but it is desirable to select it in such a manner that the heating temperature of the cathode is within the temperature range of 150 to 1,000C. If said heating temperature-is less than 150C, the desired reconstruction of thecathode surface is hardly obtained. On the contrary, if the heating temperature is higher than l,00OC., since the cathode surface reaches proper condition immediately after flashing during every heating period, the initial reduction of the emitted electron beam due to absorption of the residual; gas in vacuurnafter heating becomes larger and variation of the emitted electron beam is larger.

FIG. 7 shows the variation of the electron beam in a case where the heating temperature of the cathode tip is set to about 1,500C. As shown in FIG. 7b, the emitted electron beam greatly varies as a result of the intermittent heating of the cathode.

FIG. 6 shows the variation of the electron beam in a case where the intermittent heating of the cathode is periodically interrupted. As shown in FIG. 6a, if the heating is effected during a period t and is interrupted during a period of t,, the emitted electron beam is stably maintained during the period 1,, as shown in FIG. 6b, but gradually increases during the period However, the electron beam returns to an original value again upon reapplication of the heating and becomes stable.

Such intermittent heating of the cathode is effected by applying a pulsating current, as shown in FIG. 7a, from the heating power source 8 to the filament 4 in the arrangement of FIG. 1. This power source 8 is constructed in a well known manner; for example, a pulse oscillator utilizing controlled charge and discharge of a condenser may be used as the power source 8.

Moreover, the intermittent heating of the cathode tip may be started simultaneously with the starting operation of the electron gun device, or it may be selectively begun upon detection that the emitted electron beam has begun to be unstable. FIG. 8 shows an exemplary heating power source constructed according to the latter operation.

In FIG. 8, the power source 8 includes a pulse oscillator 9, a direct current source 10, and a switch 11. As is apparent from the above description, according to the invention, the emitted electron beam can be stably maintained for a long time by means of a very simple manipulation for merely intermittently heating the cathode tip as electron emission is continued.

Therefore, remarkable effects can be obtained by using an electron gun device of the field emission type according to the invention as an electron gun device suitable for an electron beam device requiring an electron beam which has a high electron beam density and is stable for a long time.

Specifically, in a case where the electron gun device of the invention is used as the electron gun device of a scanning type electron microscope, as shown in FIG. 9, it is possible to observe a bright and stable specimen image without completely disturbing the image by effecting the intermittent heating synchronously with electron beam scanning on a specimen 15, for example, within the time t required for blanking the fly-back line of the imageon an oscilloscope 17.

In FIG. 9, the arrangement includes an electron gun device 12 of the invention, a focusing lens 13, a deflector 14, a secondary electron detector 16, and a scanning signal source 18.

What is claimed is:

l. An'electron gun device of the field emission type comprising: a filament of hair pin shape; a cathode tip of needle shape secured to said filament; at least one anode; means for continuously applying an electric voltage between said cathode tip and said anode so as to produce an electric field therebetween which causes electrons to be emitted from said. cathode tip toward said anode; and heating power source means for supplying a pulsating current to said filament so as to intermittently heat said cathode tip while the electric field between said cathode tip and said anode is continuously applied, whereby an electron beam emitted from said cathode tip can be stably maintained for a long time.

2. An electron gun device of the field emission type according to claim 1 wherein the time width of said pulsating current is less than 1 second and the heating temperature of said cathode tip is within a temperature range between C. and 1,000C.

3. An electron gun device of the field emission type according to claim 1 wherein said heating power source means includes a pulse oscillator for generating said pulsating current.

4. An electron gun device of the field emission type according to claim 3 wherein said heating power source means further includes a direct current source, and switching means for switching from the connection between said filament and said direct current source to the connection between said filament and said pulse oscillator and vice versa.

5. An electron gun device of field emission type according to claim 2 wherein the repetition period of said pulsating current is within an extent of 1 to 20 seconds.

6. In a scanning type electron microscope including an electron gun device of field emission type; focusing lens means for focusing the electron beam; deflector means for scanning the electron beam on a specimen; secondary electron detector means for detecting secondary electrons; an oscilloscope connected to said detector means so that the intensity of an electron beam therein is modulated by the output of said detector means and having a deflector for scanning said electron beam therein; scanning signal source means for supplying scanning signals to the deflector means and the deflector in said oscilloscope; said electron gun device comprising: a filament of hair pin shape; a cathode tip of needle shape secured to said filament; at least one anode; means for applying an electric voltage between said cathode tip and said anode so as to produce an electric field therebetween which causes electrons to be emitted from said cathode tip toward said anode; and heating power source means for supplying a pulsating current to said filament so as to intermittently heat said cathode tip, whereby an electron beam emitted from said cathode tip can be stably maintained for a long time; and means for controlling said heating power source means to supply said pulsating current synchronously with said scanning to said filament.

7. An electron gun device of field emission type according to claim 6 wherein the pulsive current supplying means includes means for causing the time width of said pulsating current to be within a time required for eliminating a fly-back line of an image on the oscilloscope.

8. An electron gun device of the field emission type according to claim 7 wherein the time'width of said pulsating current is less than 1 second and the heating temperature of said cathode tip is within a temperature range between C. and 1,000C.

9. An electron gun device of the field emission type according to claim 8 wherein said heating power source means includes a pulse oscillator for generating said pulsating current.

10. An electron gun device of the field emission type according to claim 9 wherein said heating power source means further includes a direct current source, and switching means for switching from the connection between said filament and said direct current source to the connection between said filament and said pulse oscillator and vice versa.

11. An electron gun device of field emission type according to claim 8 wherein the repetition period of said pulsating current is within an extent of 1 to 20 seconds.

Claims

1. An electron gun device of the field emission type comprising: a filament of hair pin shape; a cathode tip of needle shape secured to said filament; at least one anode; means for continuously applying an electric voltage between said cathode tip and said anode so as to produce an electric field therebetween which causes electrons to be emitted from said cathode tip toward said anode; and heating power source means for supplying a pulsating current to said filament so as to intermittently heat said cathode tip while the electric field between said cathode tip and said anode is continuously applied, whereby an electron beam emitted from said cathode tip can be stably maintained for a long time.

2. An electron gun device of the field emission type according to claim 1 wherein the time width of said pulsating current is less than 1 second and the heating temperature of said cathode tip is within a temperature range between 150*C. and 1,000*C.

8. An electron gun device of the field emission type according to claim 7 wherein the time width of said pulsating current is less than 1 second and the heating temperature of said cathode tip is within a temperature range between 150*C. and 1,000*C.