United States Patent [191 [11] 4,410,832 Smith et a1. [45] Oct. 18, 1983

[54] EBS DEVICE WITH COLD-CATHODE [56] References Cited U.S. PATENT DOCUMENTS [751 Inventors: Bernard Smith, Ocean; Stanley 2,537,388 1/1951 Wooldridge ...... 313/336

Dubuske, Lincroft, both of NJ. 2,982,147 5/1961 Redman ...... 313/309

3,803,510 4/1974 Nicol] ...... 313/336 3,891,887 6/1975 Barry et a1. 315/3 Assignee: The United States of America as [73] 3,916,255 10/1975 Crandall ...... 315/3 represented by the Secretary of the 4,008,412 2/ 1977 Yuito et a1. . 313/309 Army, Washington, DC. 4,160,188 7/1979 Butterwick . 313/366 4,178,531 12/1979 Alig ...... 313/309 [21] Appl. No.: 461,291 Primary Examiner-j-Sax?eld Chatmon Attorney, Agent, or Firm-—Robert P. Gibson; Jeremiah [22] Filed: Jan. 27, 1983 G. Murray; Michael C. Sachs [57] ABSTRACT Related US. Application Data Apparatus and method are described for fabricating a long life cold cathode electron beam semiconductor [63] Continuation of Ser. No. 216,588, Dec. 15, 1980. device (EBS). Fabrication is given of a vacuum tube structure capable of sustaining sufficiently high vacuum‘ [51] Int. C1.3 ...... H01J 23/16; H01J 29/96 over extended time to prevent poisoning of the cold [52] US. Cl...... 315/3; 313/ 302; cathode and steps are give for growth of a plural tip 313/336; 313/366 cold cathode structure. [53] Field of Search ...... 315/3; 313/309, 302, 313/336, 366, 367 4 Claims, 4 Drawing Figures

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SIGNAL IN US. Patent Oct. 18, 1983 4,410,832

FIG. 1 ) IOIX (I02 IO4W / Mlllll O ll)

/

SIGNAL

FIG. 3

SIGNAL OUT

FIG. 4 GRID _--> INSULATION GR ID————> INSULATION

C O N D U C T. O R 4,410,832 1 2 regard to matters of heat expansion, propensity to EBS DEVICE WITH COLD-CATHODE degass or poison, and best emission qualities, among other matters considered. Moreover, those select mate The invention described herein may be made for or rials which alone were found most suitable for use as by the Government for governmental purposes without low threshold cold cathode emitter material have been the payment of royalties thereon or therefor. identi?ed and employed. A fabrication technique for This application is a continuation of application Ser. growth of metal semiconductor cold cathode tips which No. 216,588, ?led Dec. 15, 1980 . involves growth of tips with accompanying grid struc ture for the EBS embodiment has also been developed BACKGROUND OF THE INVENTION and employed. ' Production of an EBS Device (Electron Beam Semi conductor Device) with cold cathode instead of con OBJECTS OF THE INVENTION ventional hot cathode has hitherto been considered Accordingly, one object of this invention is to pro impractical. It had been experienced that a cold cathode vide an BBS ampli?er device with cold cathode versus v in such conditions would rapidly be poisoned by its hot cathoe emitter. . environment. This was due to inability to maintain suf? Another object of this invention is the provision of ciently high to very high vacuua (from 10-7 Torr to fabrication techniques which would enable a suf? 10*8 Torr) within the vacuum tube. This vacuum prob ciently high vacuum to be realized in an operating lem is due to degassing of the interior components of the sealed-off vacuum device. ‘ device, and the very walls of the vacuum envelope itself 20 in such very high vaccua. Once poisoned by such atmo LIST OF FIGURES sphere, the energy required for sustained emission of cold cathode, given by the emission equation, increases FIG. 1 illustrates a conventional hot cathode EBS drastically; before long the cold cathode is entirely device; FIG. 2 illustrates a cold cathode EBS accoding to this 7 inoperational. The life span of such a device would be invention; quite short as a result so as to render it impractical. The FIG. 3 illustrates a multi~tip cold cathode EBS ac conventional hot cathode EBS device itself only has a cording to this invention; and maximum life span of some 20,000 hours. Avoiding the FIG. 4 illustrates fabrication of cold cathode tips in necessity of attaching appendage pumps to maintain a vacuum and numerous other inherent failure mecha relation to this invention. nisms would also be greatly desirable. To date, how DETAILED DESCRIPTION OF THE ever, the hot cathode type EBS ampli?er has been the INVENTION ' only real choice known. Development of a cold-cathode EBS device would FIG. 1 illustrates a conventional hot cathode EBS be of obvious great advantage. The life-span of such a device as point of reference. As is known 'to those solid-state device would theoretically be unlimited; a skilled in the art, electrons from the hot cathode source cold cathode device would operate at room tempera 101 bombard the target EBIT device 108. The present ture, which would eliminate the requirement of a ?la commercially available EBIT (Electron Beam Injected ment power supply. A cold cathode device would be Target) devices individually provide ampli?cation of as lighter weight, making it thereby ideally suited for air 40 much as 2,000 fold energy for every electron which borne and space applications. bombards it at sufficient energy greater than threshold, In solving the vacuum problem, the appendage approximately 6000 volts. A signal to be ampli?ed may pumps needed for the conventional hot cathode device be fed to Anode 101, for instance, modulated by a ?rst would be eliminated as well. This last simpli?cation grid 102 in conventional triode or tetrode fashion with would even further lower the costs of production and additional grids for ampli?cation purposes. The grids operation which reduction would be possible because of have the function, amongst others, of directing and the above cited advantages. modulating beams inroute to the EBIT target. The conventional hot cathode device in FIG. 1 has BRIEF DESCRIPTION OF THE INVENTION many signi?cant disadvantages contributing to its rela The cold cathode EBS device is fabricated through tively short life span and undue complexity. Most signif solution of several problem factors which have plagued icant in short life span is the eventual poisoning of the fabrication and application of such devices until now. EBIT device by increasingly poisoned atmosphere J udicious choice of materials to be employed within the within the envelope; such poisoning being traceable to vacuum tube and careful preprocessing of all materials at least the following factors. First, getters convention' before assembly have signi?cantly minimized the effect 55 ally used in such vacuum devices when activated, ex of the above mentioned undesirable factors. Considered plode and evaporate in that rare?ed vacuum environ most important in this regard is identi?cation of getters ment. As known in the art, a getter is an amount of for use within the vacuum envelope, which getters will porous material used to absorb and collect stray gas not degass in a rare?ed vacuum as is common, thereby molecules, and are necessary in preserving vacuums in fouling the environment and especially the cathode such devices. Unfortunately, when the conventional emitter. Next in importance is a careful choice of mate getters are evaporated, the structural elements near the rials which by their nature are least likely to degass in getter are coated with deposits, such as barium. These vacuum, yet which meet the other important standards contaminants degrade a device such as the EBIT, and in regard to performance under heat, expansion rates, adversely affect the life of the hot cathode EBS device. degree of fragility and costs for instance. A further In the case of the cold cathode device, these contami important step of the fabrication technique to be de nants, additionally, would immediately degrade the scribed in detail below is the chemical cleaning and cold cathode inaddition to the EBIT, further limiting pre?ring of most of the materials before assembly to the life of an EBS device and making its construction remove volatile surface matter. These techniques fur with a cold cathode unfeasible. As will be pointed out ther take into account selection of proper materials in later, those getters which will not explode under these 4,410,832 3 4 operating conditions have been identi?ed for use in such degass so as to poison the cathode tips and adjacent cold cathode applications. A next factor which causes structural elements. One type material used for Saes poisoning of the atmosphere is degassing of the interior Getters is identi?ed as zirconium, a good gettering ma walls of the vacuum tube envelope itself. This is a dis terial. tinct probability in the very high vacuums used here, Maintaining the spacing between grids and cathode is l0—-7 to 10-8 Torr, e.g. All elements within the vac of course crucial to beam optics. For this reason, in uum envelope are candidates for degassing and must be selecting materials different heat expansion rated mate examined, such as the cathode grid structure and the rials and different materials in general are to be avoided. EBIT at high enough temperatures and vacuum. Even Ideally, all the structural elements should have the same the insulation on the ?lament heater wires in hot cath expansion rates and thermal characteristics to maintain ode devices had been found to crack, explode or degass proper beam optics. in the conventional heat and vacuum environment, In regard to choice of materials for construction of leading to voltage breadkown due to leakage paths from the cold cathode EBS device the following factors are the ?lament to the hot cathode. In the vacuum of this to be considered amongst others. First, materials must cold cathode invention, a vacuum as high as 10- 8 Torr be selected of those known to be least likely to degass. is successfully achieved, and degassing effects success This is true even though all materials will additionally fully overcome. be pre?red to remove volatile surface matter therefrom. Noteworthy in FIG. 1 is the added complexity of For the interior walls, such materials as the high purity providing appendage pumps represented there by tubes ceramics, Al2O3-99.5%, Lucalox 99.99, or pure sap at 104 and 105. In the conventional device the pumps 20 phire Al2O3-l00% might be chosen for resistance to are continually operated in an attempt to sustain the degassing. An all ceramic envelope is used in one em vacuum suf?ciently rare, this despite the use of added bodiment. Another factor however, is that the material getters. In the structure of this invention represented in must not be fragile for breakage purposes in general, FIG. 2 for instance, these pumps are dispensed with. and for rapid expansion in particular, so that glass, for The complexity of ?lament wires and accompanying 25 instance, is not advisable. Another factor is having ma transformer circuitry for the heater voltages of the terial capable of withstanding temperatures in the pre cathode of the conventional EBS, and other failure ?ring technique of this invention; the envelope is pre mechanisms inherent are also eliminated with the cold ?red to some 1000", as in a RF vacuum furnance for cathode type device of this invention. instance. In this connection, it is well to mention that This invention is shown pictorially in FIG. 3 and the EBIT or the cathode cannot withstand more than schematically in FIG. 2. As mentioned, cold cathode 250° C., e.g., without destruction so that it is is not (201) replaces the conventional hot cathode emitter of mounted in the envelope during this pre?ring. By way the conventional design. The fabrication of this semi of contrast, pre?ring the study envelope which might conductor component shown pictorially in FIG. 3, will be ceramic for instance, tends to make it cleaner for later be discussed in greater detail. To be noted here is high vacuum device application. A primary consider that the cold cathode 201 and grid, or grids as desired ation in selection of materials is that they be as poor of (202,203), are fabricated together as one piece using a gas absorber, adsorber, or desorber as is possible to solid state technology. obtain. In this regard, molybdenum, nickel and tungsten The target EBIT device 204, is reversed biased in the are good materials for the grids, cathodes, and wiring usual way (as is ) the grid 202, the accelerating anode 40 within. However, other considerations might affect 203 is forward biased approximately 10 KV. A signal making this choice. Tungsten is too hard and brittle, for ampli?cation purposes is applied at the ?rst grid, ideally the material must be easily machined, shaped though it might even be applied at the cathode with and formed into the desired shape and not break easily. good result as has been discovered. The presence and As mentioned earlier, to maintain proper spacing be number of grids is entirely optional; the invention may 45 tween components it is wise to choose materials function with several grids and an accelerating anode throughout with similar expansion and physical proper though in practice a combination of one grid and accel ties. erating anode has been found to be the most versatile. The grid material, naturally, would have to be a good The signi?cant function of the grid amongst other func high vacuum material which would be as passive as tions, is to direct as well as to modulate the emitted 50 possible, neither absorbing or desorbing gasses well. particle beam down a center path toward the EBIT Titanium would be a poor choice in this regard because target. The function of the anode is to accelerate the its getters (absorbs) too well. beam. The cathode here might be a ?eld emitter as Cathode emitter materials are best selected at a low known, and the invention would be similarly operated. threshold voltage characteristic of 200 volts. Some of In this invention, getters produced by the Saes Getter the materials chosen are HOZ-W, Si OZ-MO, or SnOZ, C0. of Milano, Italy have been identi?ed and selected which have been determined to be able to meet all the for use, represented pictorially at 205. These type get above quali?cations, withstand the high electrical ?elds ters do not explode in a vacuum or upon activation as for this application without arcing due to thermally do the standard getters. As is known, all getters must be generated causes, outgassing, evaporation or leakage, heated to an activation temperature in some fashion; 60 and useable for such low thresholds (a Tungsten ?eld here it is done by passage of electrical current at 206. emitter by contrast, would require some 10,000 volts Two electrical wire prongs are mounted in the tubes’ threshold). It is noted that certain types of cold cath walls where getter material may be mounted and a odes are available from various commercial sources, current may be passed from outside the tube to activate ready made. this material. One activated, the getter functions inde 65 The getter material would have to be, as mentioned pendently as a vacuum “sponge” ?guratively speaking, above, an excellent gas absorber. Typically, a getter is and helps maintain the vacuum within. As mentioned zirconium-aluminum alloy, preactivated to a certain earlier, these getters will not explode, evaporate, or temperature in the vacuum, 700° C. for instance, until 4,410,832 5 6 activated it may not absorb. As mentioned earlier Saes cause of the well-de?ned liquid-solid interface associ Co. getters have been found not to explode in a vacuum ated with the inherent steep temperature gradients. and are additionally considered a good getter choice. The UO2-W composites from which emitter struc The cathode-grid structure is shown pictorially in tures were formed are fabricated by ?rst dry mixing the FIG. 3 as 301. It is made of tin oxide; e.g., in one em desired proportions of high-purity oxide and metal bodiment. ' powders. This mixture is pressed into a cylinder of As shown in FIG. 4, the cathode types and grids are desired diameter and length and sintered inside an in formed physically in a deposition process providing ductively heated furnace by sequential heating to 1500“ - alternate layers of conductor and insulator materials C. in an N; and/or CO/COZ atmosphere. and using various etchings steps. The full process, in Unidirectional solidi?cation is achieved by moving one embodiment, is given further below. the heating zone of rods through the furnace hot zone at While the cathode has been shown as some 50 tips in 2-4 cm/h. By this method emitter arrays of better than a circular arrangement, this is entirely variable. They 3000 tips per cm2 have been fabricated. might be arranged in a symmetrical geometrical pattern > After the UOz-W composite trip array is fabricated, . with EBIT strips symmetrical corresponding to the 5 the wafer is polished to a 1 pm ?nish. The polished pattern of the cathode tips. Numerous modi?cations to surface is then acid etched and after etching, the tung the cathode, grid and target geometrical shape might be sten ?bers are exposed. Next, an insulator (SiOg or A1 made within the scope of the invention. In an article 203) is vapor deposited parallel to the tungsten pins’ axis entitled “Physical Properties of Thin-Film Field Emis to a thickness of 1-3 pm. Deposition of the insulator is sion Cathodes with Molybdenum Cones”, in the Jour 20 followed by deposition of a coating of moly or some nal of Applied Physics, December 1976, aspects of cold other suitable metal to act as the extractor or grid. cathode fabrication technology are discussed on page While the invention has been described in relation to 5250 there and the authors describe some of the break a particular embodiment it should be recognized by down conditions above mentioned. The suggestion for those skilled in the art that numerous modi?cations and ' avoiding this is by a more stringent vacuum, and provi 25 substitutions may be made within the scope of the in sion of a separate ?lament, but not by a choice of struc vention. tural materials as suggested by this invention, UOz-W . What is claimed is: and SiOg-W particularly not mentioned there. . 1. An electronic tube device comprising: Certain patents to Spindt et al., U.S. Pat. Nos. van EBIT electron beam injected target means; 3,755,705; 3,789,471 and 3,812,599, and a patent to a cold cathode emitting means comprising tungsten Smith, U.S. Pat. No. 4,149,308, describe cold cathode uranium oxide material, for bombarding said target structures and fabrication techniques. In them, the au means with electrons; . thors describe a method of growing a semiconductor grid means comprising molybdenum material, for array of cones to be used as cold cathodes. Their modulating said electrons including means for di method and structure, however, ends with fabricating 35 recting said electrons toward said target means the cathode and not with constructing of a cold cathode further, including means for electrically biasing vacuum tube as in this invention. ' said modulating and directing means; In a patent to Cook et al., U.S. Pat. No. 4,123,798 sealed tube means comprising sapphire ceramic mate there is described a memory device consisting of a cold rial for completely enclosing said target means, cathode used to bombard a target memory in vacuum. emitting means, and means for modulating and Cook et al. describe one of the problems of cold cath directing, in a vacuum of 10-7 to 10-3 Torr, fur odes, i.e., their rapid failure in these situations, and ther including a getter means speci?cally used for ' discuss the necessity of perfecting ?eld emission or maintaining said vacuum; and negative-electron af?nity type of cathodes for extended means for conducting input electrical signals into, and life. Cook et al. do not show the aspects of this inven 45 output electrical signals from, the said electronic tion; i.e., an EBIT target bombarded by a cold cathode tube device, whereby ampli?cation of said'input source for ampli?catin. Cook et al. did not disclose the electrical signals may be accomplished by use of cold cathode of this invention which would have had the said electronic tube device. the extended life mentioned. The only suggest the need 2. The device of claim 1 wherein the said emitting for making such advances, but do not solve them'as 50 means comprises material from the group of: done in this invention. SiOg-Mo, or Sn 0;. 3. The device of claim 1 wherein the said means for GROWTH OF UOz-W FIELD EMITTERS modulating and means for directing comprises material According to methods published by the Georgia from the group of: molybdenum, nickel, or tungsten. . Tech Staff, composites consisting of an oxide matrix 4. The device of claim 1 wherein the said tube means containing millions of less than l-um diameter metal for enclosing comprises pre?red material from the ?bers per cmz, may be grown from near eutectic com group of: A1 99.5, Lucalox 99.99, Molybdenum, or Ko positions using a direct rf-heating internal ?oating zone V3.1‘. technique. Very uniform composites are produced be # i t i i

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