Electron emitter comprising metal oxide-metal contact interface and method for making the same

[57] ABSTRACT

The electron emitter herein disclosed comprises a tubular glass capillary substrate, on the interior of which is deposited an indium-doped tin oxide film. Gold ohmic contacts embrace the ends of the capillary, traversing the end edges and overlapping both the exterior of the capillary and the end margins of the film. A high-resistance region exists at the interface between at least one of the contacts and the oxide. It is formed by applying increased voltage across the contacts until there is a current increase independently of applied voltage, followed by a decrease in film conductivity.

I Claim, 6 Drawing Figures

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Patented May 16, 1972

3,663,857 .2 Sheets-Sheet 1

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JAMES E SPROUSE TRIPONITHURA A. RAJU

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Patented May 16, 1972

3,663,857

Sheets-Sheet 2

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COLLECTOR VOLTAGE (Volts)

CAPILLARY VOLTAGE (Volts)

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BY

INVENTORS.

ARTHUR M. SOELLNER JAMES E SPROUSE

TRIPONITHUR A. RAJU

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3,663,857

ELECTRON EMITTER COMPRISING METAL OXIDEMETAL CONTACT INTERFACE AND METHOD FOR MAKING THE SAME

BACKGROUND OF THE IN VENTION

1. Field of the Invention

The present invention is an electronic emitter. The background literature on the function of electronic emission is extensive, and some of the more common methods are now mentioned.

Thermionic emission of electronics from hot cathodes, either filamentary or indirectly heated, is a common part of the prior art and is well described in the literature, for example:

Herbert J. Reich, Theory and Applications of Electron Tubes (New York: McGraw-Hill, 1944), pp. 19-42;

Frederick E. Terman, Electronic and Radio Engineering, 4th Ed., (New York: McGraw-Hill, 1955), pp. 172-176;

Μ. I. T. Staff, Applied Electronics (New York: Wiley &amp; Sons Co., 1943), pp. 74-98;

V. K. Zworykin, Television, 2nd Ed., (New York: Wiley &amp; Sons Co., 1954), pp. 33-42.

Another common method for supplying electrons is photoemission, i.e. the emission of electrons from matter through excitation by light or other radiant energy, as described in Zworykin, op. cit., pp. 42-49, or the Μ. I. T. text pp. 101-109, for example.

The emission of ions in the disintegration of radioactive substances is utilized in glow- and arc-discharge tubes as indicated in Reich, op. cit., p. 11.

Another common method for supplying electrons is by secondary emission, certain substances giving off electrons when bombarded by impacting electrons, as described in Terman op. cit., pp. 176-178, the Μ. I. T. text, pp. 109-112, and 35 Zworykin op. cit., pp. 49—60. This phenomenon is utilized in electronic multipliers.

Field emission is produced by the action of intense electric fields at a surface, as described in the Μ. I. T. text, pp. 99-101.

2. Description of the Prior Art

In recent years, advances in transistor technology have caused increasing emphasis on semiconductor junctions as sources of electrons. Among the advantages of such sources are longevity and emitter efficiency, a minimum of energy being dissipated as heat.

The prior art is characterized by low electron-emission efficiency, which is defined as the ratio of the emission current to the total current flowing in an electric circuit. The electron emission efficiency of the emitter in accordance with the invention commonly attains values in excess of 50 percent and 50 an efficiency of 85 percent, with a collector current of 4 milliamperes was achieved in one instance. This compares to an efficiency of 2 percent to 5 percent for tunnel sources of the prior art.

A primary object of the invention is to provide a cold, solid 55 state source of electrons which is controllable and is rharar·terized by a very high electron emission efficiency.

Another object of the invention is to provide a source of electrons in which the thicknesses of certain thinly laid elements, described below, are not critical.

A further object of the invention is to provide a source of electrons which can be formed in a variety of geometric configurations.

Still a further object of the invention is to provide means for generating a highly collimated beam of electrons.

In THE practice of the invention, very high emitter efficiencies have been achieved. The emission is instantaneous. It is known that current from 10 to 200 microamperes can be obtained. The electron stream can be modulated at high frequencies. Power required is as low as 50 milliwatts.

The emitter in accordance with the invention provides a very compact electron gun and has numerous applications to mass spectrometry, gas analyzers, cathode ray tubes, ultra high vacuum pressure gauges, multipliers, and plasma neutralizers, for example.

For a better understanding of the invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following description of the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the process by which the oxide film is placed in the capillary incorporated in a preferred embodiment of the invention;

HG. 2 is a perspective view, partly in section, showing a preferred embodiment of electron emitters in accordance with the invention;

FIG. 3 is a perspective view of a preferred embodiment of the invention in association with the circuit elements by which it is activated, as the final step in the process of forming an electron emitter;

FIGS. 4 and 5 are sets of performance curves, with voltage as abscissae and current as ordinates, on frameworks of Cartesian coordinates, and these are used as an aid in explaining the invention; and

FIG. 6 is a perspective view of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION AND THE PROCESS OF MAKING IT

A preferred embodiment of the invention is in capillary form (FIG. 2). It comprises a substrate 11 of glass tubing characterized by a high temperature softening point. Suitable parameters were found to be as follows:

Length Outer Diameter Inner Diameter centimeters 6.35 millimeters .75 to 1.25 millimeters

The hollow cylindrical laboratory-glass tubing 11 is prepared for incorporation in the finished product by cleaning in hot chromic acid, rinsing in distilled water, oven drying, and finally cooling to ambient temperature.

The invention is not limited to the use of a glass substrate. The function of the substrate is to serve as an insulating medium, so that any suitable insulating material, particularly a ceramic material, may be used in lieu of glass.

The first step in the preparation of the invention, assuming readiness of the substrate, is the depositing of a thin film of indium-doped tin oxide on the substrate. This is accomplished by evaporation. The substrate 11 is fitted to the output end of a delivery tube 12, the connection being sealed by an asbestos seal 13, and the elements 11,12 and 13 are inserted into a furnace 14. Oxygen is employed as the carrier gas and oxidizing agent. Oxygen is supplied by a suitable source IS, regulated by a flow meter 16, and passed through the delivery tube 12 and the substrate 11 at the rate of 600 milliliters per minute. The furnace heats the substrate to a temperature of 700° C. Upon the attainment of these conditions the solution containing the desired components is injected into the oxygen path by a syringe 17. The solution, comprising approximately 0.1 to 1.0 milliliter of a 10 percent hydrochloric solution containing 0.2 M SnCl, · 5H ₂0 and 0.1 M InCl₃, was injected into the delivery tube by a hypodermic syringe.

After the injection of this solution the heating of the substrate 11 is maintained for 2 minutes to assure complete depositing of the indium-doped tin oxide film. Then the capillary is removed from the oven and allowed to cool to atmospheric temperature.

The doping with P-type semiconductor material, such as indium, is employed in order to obtain higher resistance and to enhance the conditions for electron multiplication. In the practice of the invention, pure tin oxide, tin oxide doped with antimony, and tin oxide doped with indium have been found to be satisfactory.

After the depositing of the doped oxide film on the substrate, ohmic contacts are provided by evaporating gold onto the ends of the filmed substrate. That is to say, the film is al75

3,663,857 ready along the interior of the substrate and gold ohmic contacts are superimposed on end margins of the oxide. Oxide film is indicated at 18. The gold contacts 19 and 20 are substantially alike. Each contact extends over the oxide film at the end margin of the interior, as shown at 21, then traverses the end edge of the unit as shown at 22, and finally embraces the end margin of the outer periphery as shown at 23. The gold films 19 and 20 are deposited by vacuum evaporation. The areas on which gold is to be evaporated are bounded and defined by metal foil and plugs are inserted into the interior of the unit in order to confine the depositing of gold to the end margins and to prevent the deposit thereof throughout the length of the interior. A suitable axial length of each gold contact is 7 millimeters. The thickness of the gold contacts is not critical.

In lieu of gold any equivalent contact metal may be employed, preferably the relatively inert metals in Group I B of the periodic table of elements, such as copper or silver, i.e., a metal having good electrical conductivity. Aluminum and tin are also satisfactory.

Parenthetically, the initial resistances as measured between contacts 19 and 20, prior to activation of the tin oxide film, i.e., prior to building up the high-resistance region discussed below, indicated an ohmic circuit. Values of 4 to 20 megohms were representative. After activation, the resistance measurements went up to substantially infinity and 900 megohms, respectively. The film thicknesses employed varied from 0.1 to 10 microns. Parenthetically, the gold contacts can be deposited by liquid expansion as well as by vacuum evaporation.

Reference is now made to FIG. 3 which shows suitable circuit arrangements for activation of the FIG. 2 embodiment, i.e., the production of the high-resistance region.

The capillary is placed within a glass container 24, as, for example, one 12 inches long and 4 inches in diameter. The chamber is evacuated to a pressure of 3 X 10'^e Torr. The vacuum is provided by vacuum pump 34. A metallic collector 33 of disc-like configuration, approximately 0.75 inches in diameter, is mounted approximately 0.25 inches in front ofthe end face of the unit.

Electrical lead-out connections 25,26 and 27 are made with ohmic contacts 19 and 20 and collector 33, respectively, and brought out to the exterior of the tube 24. It will be understood of course that these are provided prior to the evacuation of the tube. In circuit with collector 33 and lead 27 are placed a current measuring device 28 and a source of direct current voltage 29, so poled as to impose a positive potential on the collector. The meter 28 measures collector current. In circuit with lead 26 and contact 20 are a current measuring device 30 and a source 31 of direct current voltage, so poled as to render contact 20 positive, i.e., the anode end. Total current is measured by a current measuring device 32 in circuit between ground and lead 25 from contact 19, i.e., the emitter or cathode end.

The collector 33 is preferably made of an alloy high in nickel content, containing iron, chromium and a trace of carbon.

The film deposition techniques utilized have the advantage that while vacuum techniques are preferred, the depositions can be performed in air rather than in a vacuum for both the gold and the tin oxide films. Furthermore, it is not necessary to regulate the thickness of either the gold or tin oxide films since it was found that the emission does not critically depend on these factors. The activating process consists in applying a voltage across the metal-tin oxide films until a narrow high-resistance region forms. Once this occurs, electron emission is observed. Although generally performed in a vacuum, this activating or forming process is also accomplished successfully in air in a similar manner. Following the formation ofthe highresistance region the completed capillary device can be stored in air for future use.

The high-resistance region is formed in the following manner. A voltage is applied across the metal contacts 19 and ' by variable voltage source 31 and slowly increased. The film current through meter 30 initially increases in a slightly nonlinear manner until a value of anode voltage 20 is reached where film current in 30 continues to increase independent of anode voltage. This is accompanied by an increase in film temperature, generally up to 300° C. or more. An abrupt decrease of film current follows, often accompanied by arcing. Values of film current, which, prior to the transition, are generally between 1 and 100 microamperes, measure between 0.1 and 2 microamperes after the transition, indicating a decrease in film conductivity. A decrease in the film temperature follows the transition. Values of anode voltage at which this transition occurs vary from 100 to 1,500 volts, dependent on the initial resistance of the tin oxide film.

Following the transition, electron emission was always observed from a narrow high-resistance region, generally of a few microns width, which formed in the tin oxide film adjacent to the positive gold film 20 and completely underlying the gold. The resistance of this region is usually greater than 50 megohms while the resistance of the remaining oxide film is not materially changed. Concomitant with the electron emission is a faint violet luminescence accompanied by random scintillations in the high-resistance region of the tin oxide film.

The activating voltage at which this occurred was approximately 1,250 volts. When a potential comparable to that on the anode end 20 of the capillary is placed on the electron collector 33 by source 29, the microammeter 28 in the electron collector indicates that an electron current is flowing through it. This phenomenon occurs for either end used as the anode when the initial resistance is of the 4 megohm order.

Emission is verified by maintaining the capillary voltages at 20 constant at values of 500, 750, 1,000, and 1,250 volts and varying the collector voltage at 33 from 0 volts to values somewhat higher than the respective capillary voltage. See FIG. 4. Emission is also verified by maintaining the collector voltage at 33 constant at values of 500, 750, 1,000, and 1,250 volts and varying the capillary voltage at 20 from 0 volts to values somewhat higher than the respective collector voltages See FIG. 5. ⁶

With reference to the curves of FIGS. 4 and 5, it can be seen that the greater the magnitude of voltage placed across the capillary, the greater the amount of electron emission current which is measured at the collector. For each value of capillary voltage used, the collector electron current saturates at a point where the collector voltage is approximately equal to the capillary voltage. There is a range of collector voltage for each fixed capillary voltage where the collector current reverses direction.

The high-resistance region generally forms at the interface between the oxide film and the positive contact 20. As indicated, it can be formed adjacent the contact 19. The range of negative collector currents indicated in FIG. 4 is caused by secondary emission from the collector, picked up at 20.

A collimated electron beam is obtained at the collector 33, the diameter of this collimated beam approximating that of the inside diameter of the capillary. Since tin oxide films have a relatively high secondary emission coefficient, these electrons are predominately secondary electrons produced along the inside diameter of the oxide film within the capillary.

It is of interest to note that an alternating current signal, up to high frequencies, can be used to modulate the electronic emission from the end ofthe capillary.

The curves of FIGS. 4 and 5 relate to a specific embodiment of the invention which was made and tested. In this embodiment the activating voltage, as stated above, had a value of at>proximately 1,250.

The activation process is accompanied by measurable increases in pressure but at the conclusion of the formation process the pressure drops substantially to normal. Additionally, the temperature drops to a point slightly in excess of ambient temperature.

3,663,857

DETAILED DESCRIPTION OF A SECOND EMBODIMENT OF THE INVENTION

In another embodiment of the invention the substrate is a glass shde 36 on which is deposited an indium-doped tin oxide turn 37. Superimposed on the oxide 37 are spaced metallic contacts 39 and 40, which correspond to contacts 19 and 20 ofthe preferred embodiment. As indicated by the dashed line 38, the collector is a metallic element which is simply superimposed over the slide, as shown in FIG. 6. The slide is a glass microscopic slide drawn in exaggerated scale for purposes of clarity, having the following dimensions:

Length 3 inches

Width 1 inch

Thickness . 1 to .3 millimeters

The films are deposited on the slide in substantially the same manner as has been described with reference to the FIG. ² embodiment. The depositing temperature is approximately 600 C. The gold contacts may optionally be underlaid by a relatively thin layer of gold of somewhat larger area, which relatively thin layer of gold is in immediate contact with the metal oxide. That is to say, extremely thin layers of gold may be evaporated onto the tin oxide and then two heavier layers 39 and 40 applied for ohmic contacts.

The collector 38 is a metallic element similar to collector 33 and is mounted approximately 0.75 inch above the substrate 36. Electrical connections are made in a manner similar to FIG. 3.

Application of a continuously increasing voltage between contacts 39 and 40 of the FIG. 6 embodiment results first in a linear volt-ampere characteristic, followed by a non-linear characteristic, a very rapid current increase, arcing, and finally an instantaneous drop, as described above. The indium ^d°P®^{d ύη oxide fllm} « one embodiment had a film resistance i ,²~ “^egohms’ ^{the act}>^vating voltage was approximately 1,200 volts.

Emission is confirmed by maintaining the slide voltages constant at 500, 750, 1,000 and 1,250 volts and varying the collector voltage from zero volts to values somewhat higher than the slide voltages (the slide voltage being that between the contacts 39 and 40). The emission was confirmed because collector current is observed.

The reference numeral 41 indicates the high-resistance region which forms in the metal oxide film, under contact 40, for example. It will be understood that this high-resistance region is like that formed under contact 20 in the FIG. 2 embodiment.

Emission is predominately in a direction parallel to the substrate 36, in the case of the FIG. 6 embodiment. This was confirmed by using a magnet to produce a deflecting and focusing action. In the case of the FIG. 2 embodiment emission is predominately in the axial direction. While the theory of action of both the FIGS. 2 and 6 embodiments need not be disclosed herein, the evidence is strong that the emission is not thermal and the possibility of tunneling action is indicated.