US 3809899 A
An electron-beam tube for a scanning electron microscope employs the use of a TF built-up field emission cathode which can be operated from preferably the 100 plane in a substantially continuous mode to provide a stable electron beam having high current density, high resolution, and very high electron optical brightness from a source of very small proportions. The tube, which comprises an evacuated envelope having chambers of different vacuums, is designed to facilitate either quick-change cathode replacement or attendance to the specimen with minimum loss of operating time since the vacuum of the entire tube need not be released. Also, the chamber containing the field emission cathode can be separable from the tube to allow replacement by a new preprocessed cathode in a pre-evacuated chamber. In this case, the mounting means includes a device for puncturing a seal in the cathode chamber to allow the electron beam to pass therethrough.
Description (OCR text may contain errors)
United States Patent [191 Baker et a1.
11] 3,809,899 May 7,1974
T. Considine, Portland; Herbert E. Litsjo, Portland, all of Greg.
 Assignee: Tektronix, Inc., Beaverton, Oreg.
 Filed: Aug. 17, 1972  Appl. No.: 281,375
 US. Cl 250/311, 250/396, 313/346  Int. Cl ..H0lj 37/26 OTHER PUBLICATIONS Electron Gun Using a Field Emmision Source, Creve, Review of Scientific Instruments Vol. 39 No. 4
April 1968 pp. 576583.
Primary Examiner--Archie R. Borchelt Assistant Examiner-C. E. Church Attorney, Agent, or Firm-Adrian J. La Rue 5 7] ABSTRACT An electron-beam tube for a scanning electron microscope employs the use of a TF built-up field emission cathode which can be operated] from preferably the 100 plane in a substantially continuous mode to provide a stable electron beam having high current density, high resolution, and very [high electron optical brightness from a source of very small proportions. The tube, which comprises an evacuated envelope having chambers of different vacuums, is designed to facilitate either quick-change cathode replacement or attendance to the specimen with minimum loss of operating time since the vacuum of the entire tube need not be released. Also, the chamber containing the field emission cathode can be separable from the tube to allow replacement by a new preprocessed cathode in a pre-evacuated chamber. In this case, the mounting means includes a device for puncturing a seal in the cathode chamber to allow the electron beam to pass therethrough.
22 Claims, 9 Drawing Figures PATENTEDIAY 119M 3.809.899
SHiEI'l UF 2 1 &
SHEET 2 BF 2 PATENTEDHAY 7 1914 ELECTRON-BEAM TUBE INCLUDING A THERMIONIC-FIELD EMISSION CATHODE FOR A SCANNING ELECTRON MICROSCOPE BACKGROUND OF THE INVENTION Previously, scanning electron microscopes largely employed heated-filament cathodes to produce an electron beam. These heated-filament cathodes, how ever, have the disadvantage of short life and large size, and they produce a beam of comparatively low electron-optical brightness. In such a cathode, electrons released thermally at different velocities from a large surface area result in a broad electron beam of relatively low average power and very low current density. To make the heated cathode practical for use in a scanning electron microscope, an elaborate and expensive lens system is required. Even then, the lens system is at best a compromise since not all of the major disadvantages can be overcome.
Many previous attempts have been made to obtain stable operation and a useful length of life of a field emission cathode. A serious disadvantage of these devices is that an extremely good vacuum environment is required to minimize damage by ion bombardment. Also, the effect of molecules landing on the emission area and lowering the work function must be minimized to reduce the possibility of locally producing such a high emission that a destructive vacuum arc occurs. One method of minimizing these effects is to heat the field emission cathode. At elevated temperatures, the atoms in the emitting area are more mobile and thus are less disturbed by ion impact. Correspondingly, the field emission cathode tip geometry is more easily preserved. At the same time, the dwell time on the surface of landing molecules is descreased, reducing the possibility of the generation of a vacuum are.
It was found that to provide useful levels of current power from the heated emitter, a high electric field is required (see US. Pat. No. 2,916,668, W. P. Dyke et al.). However, the continuous application of a high electric field to a heated emitter was found to be not practicable, because it resulted in deformation of the tip, known in the literature of the art as build-up," leading to instability and electrical breakdown. For these reasons, pulsed operation of these emitters has been extensively employed, however, application of the high electric field is usually restricted to relatively short periods of time, after which it is necessary to reform the tip geometry by heating, or flashing.
Other experiments lead to the discovery that a narrow beam of high current density can be produced from the crystalline plane of Miller indices 1, 0, 0, i.e., 100 plane, of a material having a cubic crystalline structure, such as tungsten or molybdenum. However, it was found that operation was unstable, and that continued operation usually resulted in destruction of the tip through vacuum arc (see the article Activation Energy for the Surface Migration of Tungsten in the Presence of a High-Electric field," by P. C. Bettler and F. M. Charbonnier Physical Review, Vol. 119, No. 1, July 1, I960). Later, stable operation was achieved by adding to the emitter tip a layer of a second element having a low work function, such as zirconium (see US. Pat. No. 3,374,386, F. M. Charbonnier et al.). Even then, the electron beam current level was restricted, and the cathode life limited.
Another pertinent article, Angular Confinement of Field Electron and Ion Emission, (Journal of Applied Physics, Vol. 40, No. 12, November 1969) by L. W. Swanson and L. C. Crouser, further discusses the attributes of a zirconium-coated plane TF built-up tungsten emitter. In the conclusion, however, it is pointed out that to get substantial current levels from duller emitters, an ultrahigh vacuum and ultraclean electron collection surfaces are required.
Another'serious problem of scanning electron micro scopes using field emission cathodes is that it is frequently necessary to obtain access to the interior of the electron-beam tube, for example, when replacing a defective cathode or when attending to the specimen table inside the device. This access to the tube requires a release of the vacuum, accompanied by a long wait to reestablish the vacuum when microscope operation was desired. One method of minimizing this disadvantage was the development of a two-chamber tube envelope, which allowed the cathode and specimen chambers to be maintained at different pressures, either of which could be released while maintaining the other.
SUMMARY OF THE INVENTION According to the present invention, an improved electron-beam tube has been developed for use in scanning electron microscopes. One of the major features of this tube is the use ofa thermal-field built-up" field emission cathode, from which substantially continuous and stable operation in a relaxed vacuum atmosphere has finally been achieved. This cathode combines the hitherto unattainable attributes of high current density, high-power level, stability, small source size, and long cathode life. For example, greater than 200 microamperes of current in a solid half-angle of 10 can be drawn from a cathode having an operating life in excess of 1,000 hours. Furthermore, this cathode allows the use of a vacuum which is one or more orders of magnitude less than that required for field emission cathodes in present use, permitting the use of a smaller and less costly vacuum pump. This facilitates the use of a preprocessed cathode together with an integral ion pump in an evacuated tubular envelope as a unitary structure. The correct amount of heat and high continuous electric field are applied in the proper sequence to form the desired buildup. The built-up area emits electrons more freely than any other area of the crystalline structure. These electrons which are emitted from the relatively small single crystallographic plane area which is normal to the axis of the cathode, thus producing an extremely high-brightness electron beam. Further, such performance is reproducible, facilitating repeated day-to-day operation in, for example, a scanning electron microscope.
Another feature of this electron-beam tube is that the evacuated envelope comprises interconnecting chambers, one of which is maintained at the necessary low pressure and contains the cathode, and another of which can be maintained at a higher pressure and contains the specimen table. These chambers can be separable. A small aperture provides communication be tween the chambers. This aperture is of a size that allows the pressure difference between chambers to be maintained while allowing the electron beam to pass therethrough. To provide a quick-change replacement of the cathode, a new cathode together with an integral ion pump mounted in a chamber pre-evacuated to the correct pressure replaces the chamber containing the old cathode. After the new cathode chamber is mounted to the other part of the tube, a hole is punched through a metallic diaphragm in the cathode chamber to allow the electron beam to pass from the cathode to the specimen chamber. The punching mechanism is included in the mounting apparatus, and the puncture operation can be performed under vacuum without any loss of vacuum.
It is therefore one object of the present invention to provide a method whereby stable field emission current may be continuously as well as intermittently drawn from the comparatively dull field emitter tip which has been altered to emit along a preferential axis.
It is another object of the present invention to provide an improved field emission cathode which combines advantages not previously attainable in a single field emission source, such as a continuous or pulsed electron beam of small cross section, high current density, and relatively high current power level, while at the same time exhibiting stability, repeatable performance, long cathode life, and operation in a relaxed vacuum environment.
It is a further object of the present invention to provide an improved scanning electron microscope, employing a built-up" thermal field emission cathode.
It is an additional object of the present invention to provide a scanning electron microscope that is smaller and less costly than previous systems.
It is still another object of the present invention to provide a scanning electron microscope comprising a plurality of chambers which can be operated at different pressures and which can be seaprable.
It is yet another object of the present invention to permit maintenance of the cathode or specimen table without releasing the vacuum of the entire tube.
It is still a further object of the present invention to facilitate quick-change cathode replacement by installing a pre-evacuated cathode chamber and punching a hole in a sealing diaphragm member under vacuum to allow the electron beam to pass.
It is still an additional further object of the present invention to provide a unitary gun structure including a cathode and a vacuum pump mounted in a tubular envelope which can be sealed, baked, and preevacuated in the manufacturing process to facilitate immediate use when installed in a scanning electron microscope.
It is still another additional object of the present invention to provide a cathode which can be preprocessed, or built up, prior to installation in a scanning electron microscope.
The subject matter which we regard as our invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings.
DRAWINGS FIG. 1 is a schematic of one embodiment of a 100 plane TF built-up field emission cathode according to the present invention;
FIG. 2 shows a perspective of the major planes of a body centered cubic crystalline structure, represented by the Miller indices, when the structure is oriented for emission from the plane;
FIGS. 30, 3b, and 3c show a schematic of these crystalline planes as the electron emission is transformed, or flipped from the 310 planes to the 100 plane;
FIG. 4 is a schematic view of one embodiment of an electron-beam tube showing features in accordance with the present invention;
FIG. 5 is an enlarged sectional view of a portion of an electron-beam tube showing a preferred embodiment of interconnection of separable chambers, while FIG. 5A is an isometric view showing the cam engaged with the punching member; and
FIG. 6 is an enlarged sectional view of a portion of an electron-beam tube showing a second embodiment of interconnection of separable chambers.
DETAILED DESCRIPTION Referring to FIG. 1, in one embodiment of the cathode of the present invention, an electron emitting portion 1, hereinafter referred to as a field emitter tip, is attached to a support filament 10, forming a cathode. The field emitter tip 1 may be of monocrystalline tungsten, molybdenum, or similar material, constructed so that the plane of the body-centered cubic crystal lattice structure having Miller indices 100 is oriented at tip 2, which is comparatively dull, i.e., radius in the order of one thousand to a few thousand Angstroms as compared to about 100 to 1,000 Angstroms for previous cathodes. The support filament 10 may be formed of a tungsten wire bent into a U-shape and the last 0.100 inch of wire nearest the field emitter tip 1 is etched to a suitable diameter to permit heating to a high temperature by passing an electric current through the wire. This current is supplied from a low-voltage AC current supply 18 via an isolation transformer 5. Transformer 5 must be capable of withstanding high voltage differences between the primary and secondary windings. A negative high-voltage DC supply 20 capable of supplying several hundred microamperes of direct current is connected to the cathode. Anode 8 is commonly, but not necessarily, grounded through an ammeter 9, which is used to measure the amount of emission current.
The field emission electron source of FIG. 1 is operated in a relaxed vacuum environment, while 1 X 10 torr is preferred, operation is possible in a vacuum as poor as l X 10 torr, as follows. First, the tip 2 is flashed at a temperature above approximately 2,300K using the AC power source. Flashing cleans the tip of contaminants. Then the temperature of the field emitter tip 1 is set to a temperature slightly below the temperature at which thermionic emission starts, about l,900K. A negative DC high voltage is then applied to the field emitter tip 1 from supply 20. This voltage is increased until an emission current of from 1 to 100 microamperes, depending on the radius of the tip 2, is registered on the ammeter 9. At this point, the tip 2 is usually emitting electrons along the axes perpendicular to the crystallographic 310 planes. Refer to FIGS. 2 and 30. It should be noted that sometimes other modes of emissio'n may occur. As the high voltage is slowly increasedfan electrostatic field gradient is produced at the emitter tip 2, resulting in build-up as the crystalline structure begins to alter its shape. Electron emission at this point can be seen in FIG. 3b. Then, when the electrostatic field gradient is of sufficient magnitude, usually of the order of a few tens of megavolts per centimeter, the crystalline shape at tip 2 is altered to a point where electron emission has flipped to the 100 plane, as shown in FIG. 3c. This transformation, or fiipping" process will be accompanied by concurrent increase in total emission current. Stable, continuous emission of at least 200 microamperes in a solid half-angle of along the axis perpendicular to the 100 plane can be maintained at this stage by maintaining the cathode temperature and electrostatic field gradient at the finally stated values.
While it is desired to operate the cathode in a continuous manner for a substantial period of time, it is possible to turn the high-density beam current off and then back on again at a later time with reproducible results, thus eliciting an intermittent or pulsed mode of operation. One method of achieving such intermittent operation is the following:
If a turn-off procedure of first reducing the temperature to ambient, then turning off the high voltage is used, the electron beam will immediately be emitted along the preferred crystallographic axis on the next turn-on cycle where the high voltage is turned on first, then the cathode is heated by the application of a low voltage hereto. It can be seen, then, that a pulsed mode of operation can be achieved by proper manipulation of temperature and voltage, for example, gating the high voltage on slightly before application of heater current and then off after the cathode has cooled to a sufficient level.
While the foregoing description specifically details the operation of a 100 plane TF built-up cathode, significant results have been achieved with a built-up thermal-field emission cathode wherein the preferred crystallographic axis is normal to the plane described by Miller indices 3, I, 0. Therefore, it should be pointed out that stable and substantially continuous operation can be drawn from a 310 plane TF built-up cathode as well.
Referring now to FIG. 4, in one embodiment of a scanning electron microscope according to the present invention, a built-up thermal-field emission cathode 10 is mounted in a chamber 12 of an evacuated tubular envelope 14. Tubular envelope 14 can be constructed of glass, ceramic, or similar material as well as metal. The cathode chamber 12 is maintained at a high vacuum, i.e., 1.0 X 10 torr or higher by a high-vacuum pump 16 which is mounted preferably inside the cathode chamber. A low voltage AC supply 18 and a highvoltage DC supply 20 are provided to operate the cathode 10 in the thermal'field mode as discussed previously.
A second chamber 24, which corresponds to the specimen chamber in a scanning electron microscope and contains beam-deflecting elements as well as a specimen table (not shown), is maintained at a lower vacuum, typically 1 X 10 or I X 10 torr, by vacuum pump 26. Cathode chamber 12 and specimen chamber 24 are separated by a wall 26, which has an aperture 28 axially aligned with the emitter tip of cathode 10 through which the electron beam emitted by cathode 10 can pass. The size of aperture 28 is chosen so that high-vacuum pump 16 can maintain the pressure of cathode chamber 12 against the leakage through aperture 28 from the lower vacuum of specimen chamber 24. Typically, an aperture having a hole diameter 0.020 inch is sufficient to maintain the pressure difference.
Magnetic lens 30, which could be replaced with an electrostatic lens, focuses the electron beam to a very small cross-sectional area. A scanning electron microscope would of course require an electron-beam deflection system and other features not shown in FIG. 4, but they are not germaine to this invention and need not be described in detail.
The cathode chamber can be made separable from the specimen chamber to allow quick-change replacement of the cathode, i.e., if the existing cathode needs to be replaced due to failure or improper operation, the entire chamber containing the cathode, defining an electron-beam tube, can be'removed from the associated second chamber containing a target structure and replaced with a new pre-evacuated and sealed chamber or electron-beam tube containing a preprocessed cathode. Such a replaceable cathode, or electron source available as an off-the-shelf electronsource unit, could be used in any application in addition to a scanning electron microscope, for example, in a cathode-ray tube wherein the target structure is a fluorescent viewing screen. The unitary structure can also include an ion pump to maintain the required vacuum environment during use. Additionally, the second chamber containing the target structure can also be made available as an off-the-shelf replacement unit. A quick-change mounting means permits the desired unit to be readily replaced in a short period of time and be operating with no significant loss of operating time. Re ferring to FIG. 5, a preferred embodiment for mechanically joining the chambers is shown. The pre-evacuated cathode chamber 12 includes a mounting flange 30 and a thin metallic sealing diaphragm 32. Also included are the cathode 10 and the wall 26 with aperture 28. A plug 34 containing a hollow needle-like punch 36 is placed into an opening of flange 30 adjacent the metallic sealing diaphragm 32. The cathode chamber assembly is then mounted along with sealing O-rings or gaskets 40 and 42 to a mating flange 44 on the specimen chamber 24. The mating flanges 30 and 44 are joined securely together by a clamp 46. After the specimen chamber 24 is pumped to the desired vacuum, actuating cam 50 is pushed in until it is aligned with the punch 36. Then, cam 50 is rotated, its lobe forcing the punch 36 to puncture the metallic sealing diaphragm 32 (see FIG. 5a) without loss of vacuum. After punch 36 has punctured the diaphragm 32, it is held in place by a nipple portion near its conical end. The electron beam can then pass from the cathode 10, through aperture 28, and then through the hollow punch 36 along its path to the specimen chamber.
FIG. 6 shows an alternate embodiment for mechanically joining the cathode and specimen chambers. Parts corresponding to those shown in FIGS. 4 and 5 are identified by the prime symbol. A pre-evacuated cathode chamber 12' includes a mounting base 30 and a metallic sealing diaphragm 32'. Also included are the cathode l0 and the wall 26 with the aperture 28. A hollow, needle-like punch 36 is placed into an opening in the mounting socket 44 of the specimen chamber 24. A collar 46' is slipped over the mounting base 30' and a split retaining ring 38 is fitted into a machined groove in the base 30. The cathode chamber assembly is then mounted along with O-rings or gaskets 40' and 42' to a mating socket 44' on the specimen chamber 24'. The collar 46 is rotated, engaging the threads on the lip of socket 44. As the specimen chamber 24 is can then pass through the hollow punch 36 as de-- scribed for the preferred embodiment.
While we have shown and described the preferred embodiments of our invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from our invention in its broader aspects.
1. The method of producing stable electron emission from a thermal field emission cathode having a cubic crystalline structure and including a comparatively dull tip having a radius of about 1,000 to 3,000 Angstroms, such method comprising:
continuously heating said cathode to a temperature slightly less than the temperature at which substantial thermionic electron emission occurs but at which atoms of said cathode are capable of movement therealong; and
continuously at the emitter tip of said cathode an electrostatic field gradient of a magnitude sufficient to cause the atoms to migrate towards the tip so as to build up an area extending outwardly therefrom and to alter the geometry of said tip so that electron emission occurs from predetermined crystallographic plane means of said cathode thereby producing stable and continuous electron emission therefrom.
2. The method according to claim 1 wherein the cathode is metal having a cubic crystalline structure defining tungsten.
3. The method according to claim 2 wherein said crystalline structure of said metal is oriented to allow emission from preferential plane means.
4. The method according to claim 3 wherein said preferential plane means is defined by Miller indices 1, 0, 0.
5. The method according to claim 3 wherein said preferential plane means is defined by Miller indices 3, l, 0.
6. The method according to claim 1 wherein said cathode is operable in a relaxed vacuum environment, said environment being as relaxes as 1.0 X 10 torr.
7. The method according to claim 1 wherein said emitter tip is comparatively dull and having a radius of about 1,000 to 3,000 Angstroms.
8. The method of stably and substantially continuously operating a thermal-field cathode having a comparatively dull tip with the tip having a radius of about 1,000 to 3,000 Angstroms in a vacuum environment as relaxed as 1.0 X 10' torr, such method comprising:
applying a substantially continuous low voltage to said cathode and heating said cathode to a temper ature slightly less than the temperature at which substantial thermionic electron emission occurs but at which atoms of said cathode are capable of movement therealong; and
applying a substantially continuous high voltage potential to said cathode and providing an electrostatic field gradient of a magnitude sufficient to cause the atoms to migrate towards the tip so as to build up an area extending outwardly therefrom and to alter the geometry of the tip so that electron emission switches from a plurality of axes of said cathode to a preferred axis thereof from which said electron emission is both substantial and stable.
9. The method according to claim 8 wherein said cathode is a tungsten cathode.
10. A stable field emission electron source comprising:
a thermal-field metallic cathode having a comparatively dull emitter tip with said tip having a radius of about 1,000 to 3,000 Angstroms;
means for heating said cathode substantially continuously to a temperature at which substantial thermionic electron emission occurs but at which atoms of said cathode are capable of movement therealong; and
means for producing at said emitter tip of said cathode a substantially continuous electrostatic field gradient of a magnitude sufficient to cause the cathode atoms to migrate towards said emitter tip so as to build up an area extending outwardly therefrom and to alter the geometry of said emitter tip'so that electron emission switches from a plurality of axes of said cathode to a preferred axis thereof.
11. The stable field emission electron source according to claim 10 wherein said means for heating said cathode comprises a resistive wire adjacent said emitter tip through which current from a low-voltage AC supply is passed, and said means for producing said electrostatic field gradient comprises a negative high voltage applied from a DC supply to the cathode of said field emission source.
12. The stable field emission electron source according to claim 11 wherein said cathode is a tungsten cathode.
13. The stable field emission electron source according to claim 11 wherein said cathode is operable in a relaxed vacuum environment, said environment being as relaxed as 1.0 X 10 torr.
14. An electron beam tube comprising:
an evacuated tubular envelope having a first chamber and a second chamber, said first chamber being maintainable at a lower pressure than said second chamber;
interconnecting means for providing communication between said first and second chambers;
field emission electron source means having a comparatively dull emitter tip with a radius of about 1,000 to 3,000 Angstroms disposed axially in said first chamber adjacent said interconnection means and in alignment therewith;
means connected to said field emission electron source means to provide stable and substantially continuous operation thereof including a lowvoltage power supply means for substantially continuously heating said electron source means to a temperature slightly less than the temperature atsource means an electrostatic field gradient of a magnitude sufficient to cause the atoms to migrate to said emitter tip so as to build up an area on said tip extending outwardly therefrom and to alter the geometry of said emitter tip and for increasing emission current in the form of an electron beam along a preferential axis of said electron soruce means: and
focusing means disposed in said second chamber for focusing said electron beam.
15. The tube according to claim 14 wherein said interconnecting means includes an aperture adjacent said field emission electron source means, such aperture being of a size sufficient to allow said electron beam to pass therethrough while maintaining a pressure difference thereacross.
16. The tube according to claim 14 wherein said field emission source means is a tungsten cathode.
17. The tube according to claim 14 wherein said interconnecting means includes means to enable said first chamber and said second chamber to be separable and rejoinable.
18. The tube according to claim 17 wherein said first chamber includes a cathode defining said electron source means and an ion pump to form a unitary sealed structure.
19. The tube according to claim 18 wherein said first chamber is sealed and pre-evacuated prior to being joined with said second chamber 20. The tube according to claim 19 wherein said cathode is preprocessed to the desired operating condition prior to being joined with said second chamber.
21. The tube according to claim 20 wherein said interconnecting means includes means for mechanically joining said chambers and means for puncturing a sealing diaphragm member in said first chamber to provide communication between said first chamber and said second chamber.
22. The tube according to claim 21 wherein said puncturing means includes a hollow punching member disposed axially with said electron source for the purpose of allowing the electron beam to pass therethrough.