Patent Publication Number: US-5841219-A

Title: Microminiature thermionic vacuum tube

Description:
This application is a continuation of U.S. application Ser. No. 08/547,670, filed Oct. 17, 1995, for MICROMINIATURE THERMIONIC VACUUM TUBE, which application is a continuation of U.S. application Ser. No. 08/126,075, filed Sep. 22, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to microminiature thermionic vacuum tube devices such as diodes, triodes, tetrodes, and the like constructed with solid-state semiconductor device fabrication techniques to have ultra-small (i.e., micron-scale) dimensions. 
     Vacuum tubes were developed around the turn of the century and immediately became widely used for electrical amplification, rectification, oscillation, modulation, and wave shaping in radio, television, radar, and in all types of electrical circuits. With the advent of the transistor in the 1950&#39;s and integrated circuit technology in the 1960&#39;s, the use of the vacuum tube began to decline, as circuits previously employing vacuum tubes were adapted to utilize solid-state transistors and like circuit components. The result is that today more and more circuits are utilizing solid-state semiconductor devices, with vacuum tubes remaining in use only in limited circumstances such as those involving high power, high frequency, or hazardous environmental applications. In these last mentioned applications, solid-state semiconductor devices generally cannot accommodate the high power, high frequency or severe environmental conditions. 
     There have been a number of attempts at fabricating vacuum tube devices using solid-state semiconductor device fabrication techniques. One such attempt resulted in a thermionic integrated circuit formed on the top side of a substrate, with cathode elements and corresponding grid elements being formed co-planarly on the substrate. The anodes for the respective cathode/grid pairs were fabricated on a separate substrate which was aligned with the first-mentioned substrate such that the cathode to anode spacing was on the order of one mm. With this structure, all the cathode elements were collectively heated via a macroscopic filament heater deposited on the back side of the substrate. This structure required, therefore, relatively high temperature operation and the need of substrate materials which had high electrical resistivity at elevated temperatures. Among the problems with this structure were inter-electrode electron leakage, electron leakage between adjacent devices, functional cathode life, etc. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a microminiature thermionic vacuum tube device which may be manufactured using solid-state semiconductor fabrication techniques to have ultra-small (i.e., micron-scale) dimensions. 
     It is also an object of the invention to provide such a device which may operate in generally harsh environments--high temperature, high radiation. 
     It is a further object of the invention to provide such a device which may be utilized in high electrical power and/or high frequency applications. 
     It is another object of the invention to provide such a device which is efficient and reliable in operation. 
     The above and other objects of the invention are realized in a specific illustrative embodiment of a microminiature thermionic vacuum tube device comprising an insulating or highly resistive substrate, electrically conductive materials disposed on the substrate to define and surround a first void extending from the substrate upwardly through the material, a cathode disposed on the material to bridge over the first void, for emitting electrons when heated, first electrically resistive material disposed on the electrically conductive material to surround the cathode and define a second void thereabove, an electrically conductive grid disposed on the electrically resistive material to project partially into the second void to define an opening in the grid above the cathode, for allowing the passage of electrons therethrough, second electrically resistive material disposed on the grid to define a third void above the opening in the grid, and an electrically conductive anode disposed on the second electrically resistive material over the third void to receive electrons emitted by the cathode and thereby produce an electrical current. The electrically conductive material is selectively heated to thereby heat the cathode and cause the emission of electrons; a positive voltage is applied to the anode to cause it to attract electrons and a voltage is selectively applied to the grid to control the magnitude of the flow of electrons through the opening in the grid, to thereby effect control of electrical current produced. 
     In accordance with one aspect of the invention, the first void is formed to extend downwardly into the substrate to form a column void below the cathode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will become apparent from the consideration of the following detailed description presented in connection with the accompanying drawings in which: 
     FIG. 1A is a side, cross-sectional view of a microminiature thermionic vacuum tube with a column void made in accordance with the principles of the present invention; 
     FIG. 1B is a perspective view of the thermionic vacuum tube shown in FIG. 1A; 
     FIG. 2A is a side, cross-sectional view of a microminiature thermionic vacuum tube with a trench or trough void made in accordance with the principles of the present invention; 
     FIG. 2B is a perspective view of the thermionic vacuum tube shown in FIG. 2A; 
     FIG. 3A is a side, cross-sectional view of another embodiment of a microminiature thermionic vacuum tube with a column void, also made in accordance with the principles of the present invention. 
     FIG. 3B is a side, cross-sectional view of the embodiment of FIG. 3A, but with a trench or trough void; 
     FIG. 4A is a side, cross-sectional view of the present invention similar to FIG. 1A, but having a plurality of column voids; 
     FIG. 4B is a side, cross-sectional view of the present invention similar to FIG. 2A, but having a plurality of trench or trough voids; 
     FIG. 5A is a side, cross-sectional view of the present invention similar to FIG. 3A, but having a plurality of column voids; and 
     FIG. 5B is a side, cross-sectional view of the present invention similar to FIG. 3B, but having a plurality of trench or trough voids. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1A, there is shown a side, cross-sectional view of one embodiment of a microminiature vacuum tube which may be fabricated using solid-state semiconductor fabrication techniques, such as thin film deposition, sputtering, etc. The device includes a substrate 4 which may be made of a single crystal, a polycrystalline material, a amorphous material, or other high resistivity semiconductor substrate material. For example, the substrate 4 might illustratively be made of polycrystalline silicon, amorphous silicon, silicon and gallium arsenide semiconductor substrates or the like. 
     Deposited on the substrate 4 are the component parts of the microminiature vacuum tube device, with these parts being shown greatly enlarged and out of scale to better illustrate the structure. A low resistance metal 8, such as gold, aluminum, intermetallic or the like, is deposited on the substrate 4 about a void 12. Deposited or formed over the void 12 and partially over the low resistance metal 8 is an element 16 which will serve as the cathode filament of the vacuum tube device. The cathode filament 16 is placed in contact with the low resistance metal 8 since it is via this layer that the cathode filament will be stimulated to emit electrons. As will be described later, this will be carried out by heating the cathode filament to cause it to thermionically emit the electrons. Disposition of the cathode filament 16 over the void 12 serves to reduce the thermal load and stress which might otherwise be imposed on the vacuum tube device during operation. In effect, the void 12 serves to localize the cathode filament heating element 16 to contain the heat therein. Advantageously, the cathode filament 16 is made of a refractory metal such as molybdenum, platinum, titanium, tungsten, or the like. These materials have a relatively low coefficient of expansion which, because of the small distances which will be present between the component parts of the vacuum tube device, are desirable to minimize the possibility of the component parts thermally expanding or growing to ultimately touch. The latter event, of course, would disable the vacuum tube device. 
     A resistive material 20 is deposited on the low resistance metal 8 and formed to define a void 24 which surrounds the cathode filament 16. The resistive material 20 might illustratively be ceramic, silicon dioxide or the like. 
     Deposited on the resistive material 20 is an electrically conductive grid layer 28, a portion of which 30 projects into the void 24 to define an opening 32 positioned directly above the cathode filament 16. The grid layer 28 might illustratively be made of tungsten, gold, tantalum or the like. The grid layer 28, and in particular the projections 30, serve as a conventional grid in a triode vacuum tube structure. 
     Deposited on the grid layer 28 is another layer of resistive material 34, formed to define a void 36 which is above the opening 32 in the grid layer 28, as shown in FIG. 1A. The resistive material 34 may be the same as the resistive material of layer 20. 
     Deposited on the resistive layer 34 to bridge over the void 36 is an electrically conductive anode 40. The electrically conductive material 40 may be the same as the electrically conductive material of layer 28. As can be seen, the anode 40 is positioned vertically above the void 36, the opening 32 in the grid layer 28, the void 24, and the cathode filament 16. This provides a vertically oriented, solid-state thermionic, triode vacuum tube device which is immune to high temperatures and harsh environments such as those with high radiation. 
     The device of FIG. 1A would be operated in essentially the same fashion as that of a conventional vacuum tube including a source of thermal energy 44 coupled to the low resistance metal layer 8 for providing heat to heat the cathode filament 16 and cause it to emit electrons. The thermal source of energy 44 might illustratively simply be a voltage source for supplying a current to the low resistance metal layer 8 to flow through the cathode filament 16, causing it to heat and emit electrons. Coupled to the grid layer 28 is a control voltage source 48 for selectively applying a voltage to the grid layer to control the flow of electrons through the opening 32 of the grid layer, from the cathode filament 16. Of course, controlling the flow of electrons through the opening 32 effectively controls the electrons reaching the anode 40 which, by reason of a positive anode voltage source 52, attracts and receives the electrons to develop a desired electrical current. Such operation of the microminiature vacuum tube device of FIG. 1A is well-known. 
     Because thin film deposition may be used in constructing the microminiature vacuum tube device of FIG. 1A, micron size dimensions may be achieved. For example, the spacing between the cathode filament 16 and anode 40 may be fabricated to be from between two to fifty microns but preferably would be between about two to five microns. Similarly, the spacing between the cathode filament 16 and the opening 32 in the grid layer 28 would be between about one to three microns, and the spacing between the opening 32 and the anode 40 would be between about one to three microns. Because of the small dimensions, the device of FIG. 1A can operate at frequencies in the terahertz range and yet not suffer from velocity saturation effects that generally limit the upper frequency range of operation of other solid-state and semiconductor devices. 
     Although a single microminiature vacuum tube device is shown in FIG. 1A, it is apparent that a plurality of such devices could be formed on the substrate 4 with each individual device insulated and separated from one another by gaps or voids or high temperature insulator material 42 (see FIGS. 4A, 4B, 5A and 5B such as ceramic, silicon dioxide, sapphire, or the like, which would also be deposited on the substrate 46, surrounding each device. 
     FIG. 1B provides a perspective view of the device of FIG. 1A, which more clearly illustrates the column void 12 over which the cathode filament 16 is placed. 
     FIG. 2A is similar in structure to FIG. 1A with the exception that instead of a void in the shape of a column 12, the void is now in the shape of a trench or trough 10. Otherwise, the vacuum tube is constructed in the same manner as the device described in FIG. 1A. 
     FIG. 2B provides a perspective view of the device of FIG. 2A, which more clearly illustrates the trench or trough void 10 over which the cathode filament 16 is placed. 
     FIG. 3A shows an alternative embodiment of a microminiature vacuum tube made in accordance with the present invention. The FIG. 3A device is also a cross-sectional view, and shows a construction very similar to the FIG. 1A device except that the layer of low resistance metal 8 is thinner than that of the FIG. 1A device, the substrate 4 is thicker and includes a column void 12 formed in the substrate 4 directly below the cathode filament 16. The purpose of the column void 12 is to localize and isolate the cathode filament 16 to reduce the thermal load and stress which might otherwise occur on the other components of the device. The other components and structure of the device of FIG. 3A are similar to those of FIG. 1. 
     FIG. 3B is a device with the same structure as in FIG. 3A, but with trench or trough voids 10 instead of the column voids. 
     FIG. 4A illustrates a plurality of vacuum tube devices made in accordance with the invention as illustrated in FIGS. 1A and 1B with column voids 12, but having an insulative material 42 separating the individual devices. 
     FIG. 4B illustrates a plurality of vacuum tube devices made in accordance with the invention as described in FIGS. 2A and 2B with trench or trough voids 10, but having an insulative material 42 separating the individual devices. 
     FIG. 5A illustrates a plurality of vacuum tube devices made in accordance with the invention as described in FIG. 3A with column voids, but having an insulative material 42 separating the individual devices. 
     FIG. 5B illustrates a plurality of vacuum tube devices made in accordance with the invention as described in FIG. 3B with trench or trough voids 10, but having an insulative material 42 separating the individual devices. 
     It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention and the appended claims are intended to cover such modifications and arrangements.