US3777209A

US3777209A - Non-thermionic electron emissive tube comprising a ceramic heater substrate

Info

Publication number: US3777209A
Application number: US00254259A
Authority: US
Inventors: A Mcdonie; R Faulkner; J Rhoads
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1972-05-17
Filing date: 1972-05-17
Publication date: 1973-12-04
Anticipated expiration: 1990-12-04
Also published as: JPS4950863A; GB1425194A; CA998732A; DE2325136B2; FR2184980B1; FR2184980A1; DE2325136C3; NL7306812A; DE2325136A1

Abstract

A non-thermionic electron emissive tube of the type comprising an evacuated envelope, an electron emissive cathode assembly in the envelope, and a collector anode for electrons emitted from the emissive layer. The cathode assembly comprises a thin ceramic substrate. On one face of the substrate is a non-thermionic cathode. On the opposite surface is a heater pattern of resistive metallizing.

Description

United States Patent 11 1 McDonie et al.

[ Dec. 4, 1973 NON-THERMIONIC ELECTRON EMISSIVE TUBE COMPRISING A CERAMIC HEATER SUBSTRATE [75] Inventors: Arthur Frederick McDonie; Richard Dale Faulkner; James Lee Rhoads, all of Lancaster, Pa.

[73] Assignee: RCA Corporation, New York, N.Y.

22 Filed: May 17, 1972 1211 Appl. No.: 254,259

3,066,236 11/1962 Sandbank 313 304 x 3,330,991 7/1967 Lavine ct al. 313/346 R x FOREIGN PATENTS OR APPLICATIONS 1,432,317 5/1965 France 313/337 281,761 3/1966 Australia 313 270 Primary Examiner--David Schonberg Assistant Examiner-Paul A. Sacher AztomeyGlenn H. Bruestle and Donald S. Cohen [57] ABSTRACT A non-thermionic electron emissive tube of the type comprising an evacuated envelope, an electron emis sive cathode assembly in the envelope, and a collector anode for electrons emitted from the emissive layer. The cathode assembly comprises a thin ceramic substrate. On one face of the substrate is a nonthermionic cathode. On the opposite surface is a heater pattern of resistive metallizing.

8 Claims, 4 Drawing Figures BACKGROUND OF THE INVENTION The invention relates to non-thermionic electron emissive tubes.

Non-thermionic electron emissive tubes generally contain an electron emitting surface, or cathode which need not be heated for operation. The cathode commonly is a layer of semiconductor material, the surface of which is cesiated by application thereon of a workfunction-reducing layer of cesium or cesium and oxygen.

For cathodes of the type in which the semiconductor is silicon or a compound or alloy of the elements of Groups IIIA and VA or IIB and VIA of the Periodic Chart of the Elements, the process for applying the work-function-reducing layer includes heating the cathode layer to a relatively high temperature, on the order of 400C to 600C, after the cathode layer has been mounted in the tube, but before the work function reducing layer is applied thereon.-Such cathodes, and activation procedures for them, are described for instance in U.S. Pat.'No. 3,630,587 issued to Garbe on 28 Dec. I971, U.S. Pat. No. 3,632,442 issued to Turnbull on 04 Jan. 1972, and U.S. Pat.'No. 3,644,770 issued to Bell on 22 Feb. 1972. As indicated in Garbe et al. and Turnbull, the heating of the semiconductor material of the cathode at elevated temperatures, approximating 300C., prepares the surface of the semiconductor by removing impurities from its surface prior to applying the work function reducing layer thereon.

One present means for providingthe'heating is by a resistance heated filament wire situated in closeproximity to the back side of a metal substrate on which the semiconductor layer is disposed. When the filament is heated,'radient energy therefrom heats the metal substrate and the cathode layer. Another present means for the heating is the use of a focussed external intense light source, such as a high intensity quartz lamp in conjunction with a parabolic mirror. The light is focussed through the transparent tube envelope directly onto the cathode material. Both of these approaches utilize transfer of heat by radiant energy, and result in substantial losses of heat to other structures of the tube. For example, in photomultiplier tubes, some radiant energy given off by the filament can pass by the cathode substrate as stray radiation and heat nearby dynodes so that they are damaged. Also, some of the radiant energy is reflected from the metal substrate of the cathode to other components. Similarly, the focussed light from the external light source is reflected on passing through the tube envelope, and further reflected from the cathode layer itself to other internal tube components. When the dynodes having an antimony layer, for later cesiation to cesium antimonide, are heated during the heating of the cathode, the antimony evaporates from the surface onto other portions of the tube envelope, thus degrading the quality of the tube.

Another difficulty with present heating means is that the heating is often nonuniform. This results in nonuniformities in the characteristics of the activated cathode.

SUMMARY OF THE INVENTION In the novel tube, a cathode assembly is provided comprising a thin ceramic substrate on which a cathode 2 is disposed. A heater pattern of metallizing is provided on the substrate in direct thermally conducting contact with the cathode.

' With the novel structure the cathode is heated by thermal conduction rather than by radiation. The direct thermal contact provides much more efficient heat transfer between the heater and the cathode. Therefore, the heater need not be heated to temperatures so high as to result in damaging stray radiation to other tube components. Also, the heating of the cathode layer is uniform. After activation of the cathode, the heater pattern becomes a passive structure, since the non-thermionic cathode need not be heated for operation.

BRIEF DESCRIPTIONOF THE DRAWINGS FIG. 1 is a side view of a photomultiplier tube in accordance with the preferred embodiment of the invention.

FIG. 2 is a sectional view, on an enlarged scale, of the tube of FIG. 1 through a plane transversely through the tube as shown by the section line 22=in FIG. 1.

FIG. 3 isa plan view of one face of the cathode substrate of the tube of FIGS. 1 and 2.

FIG. 4 is a plan view of the opposite face of the substrate of FIG. 3.

' PREFERRED EMBODIMENT OF THEINVENTION In the preferred embodiment of thein vention, a photomultiplier tube 10 shown in FIGS. 1 and 2, includes a photocathode assembly 12 in accordance with the invention. The tube 10 includes a glass envelope 14, a number of cesium-antimonide-coated electron multiplying dynodes 16, a number of field electrodes 18, and an anode 20 for collecting multiplied electrons which travel from one dynode 16 to another generally along the path indicated by the dashed lines 22. Also included in the envelope l4, but'not shown in the drawings, are sources of cesium and oxygen for activating the photocathode 12 structure.

The photocathode assembly 12 is shown in more detail in FIGS. 3 and 4. Referring now to FIG. 3, the assembly 12 comprises a thin rectangular wafer 24 ofaluminum oxide (A1 0 about 1 cm (centimeter) wide, 3.5 cm long, and 0.5 mm thick. One surface of the wafer is provided with a rectangular pad 26 of molybdenum metallizing about 25 pm thick, which is applied by screen printing and firing a molybdenum-steatite metallizing ink commonly used for metallizing ceramics. A lead portion 28 of the pad 26 extends to a contact-fastening hole 30 at the base end of the wafer 24. Over the pad 26 is a thin photocathodelayer 31 of vapor-phase-grown polycrystalline gallium arsenide phosphide between about 5pm and 30 um thick containing about percent gallium arsenide. Details of vaporphase-growth are described, for instance, in U.S. Pat. No. 3,218,205 issued 16 Nov. 1965 to Ruehrwein.

Referring now to FIG. 4, the opposite face of the wafer 24 is provided with a zig-zag pattern of molybdenum about 25 pm thick to form a resistance heater strip 32 in contact with the wafer 24. Each end of the nearly entirely across the width of the wafer 24. These

apertures

36, 38 provide heat dams to improve the uniformity of heating by minimizing end heat losses. Also, to further improve the uniformity of the heating, the heater strip 32 is narrowed somewhat at

portions

40, 42 near the

heat dams

36, 38. The purpose of this is to increase the heat output from the heater in these regions to compensate for heat losses which occur at the top and bottom ends despite the heat dams.

Owing to the described arrangement of the cathode assembly 12, heating of the photocathode layer 26 is by direct thermal conduction. Since there is relatively little heat loss to other tube components such as the dynodes 16, undesirable evaporation of antimony from the dynodes 16 is avoided.

Electrical leads 44 of high temperature spring metal are connected to the wafer 24 through the

holes

30, 34 in the base of the wafer 24 after it is mounted in the tube, as shown in FIG. 1.

GENERAL CONSIDERATIONS The invention has utility in various types of electron emissive tubes utilizing non-thermionic cathodes, and in which it is desirable to avoid unnecessary heating of other internal components of the tube when activating the cathode. Such non-thermionic cathodes are generally cesiated. The cathode can bea forward-biasedjunction emitter cathode, photocathode, or secondary emitter. The semiconductor can be any semiconductor suitable for a cathode layer. It may be, for instance, silicon or a compound or alloy from Groups lIlA and VA or IIB and VIA of the Periodic Chart of the Elements.

The thickness of the ceramic wafer material is preferably great enough to result in a relatively uniform temperature on the cathode side. If the substrate wafer is too thick, however, there may be spalling of the ceramic due to thermal stress. The pattern of the heater element can be any of various patterns which result in a relatively uniform heat output over the surface. It is desirable, however, where the substrate wafer iselongated, as in the preferred embodiment, to provide for additional heat input to the substrate wafer near the ends of the substrate.

Various metals can be used for the heater pattern and for the metallizing on the emitter cathode face. Refractory metals, such as molybdenum and tungsten are preferably used where the cathode layer must be grown on the substrate wafer directly by vapor deposition. This is due to the severe conditions in the growth furnace for Ill-V compound vapor phase deposition. During growth of the cathode layer, the substrate wafer is exposed to highly reactive gases at temperatures on the order of 600C to 1,000C. Molybdenum and tungsten are the only metals in general use which can withstand such conditions and which are compatible with Ill-V compounds to the extent needed for growing a layer of sufficiently regular crystallinity for efficient performance of the cathode layer. Where the cathode layer is itself sufficiently conductive to operate without a metallizing pad under it, the cathode layer may be deposited directly on the ceramic.

Although the substrate wafer of the preferred embodiment is relatively opaque aluminum oxide, other oxide ceramics such as sapphire and spine] may be used instead. The choice of ceramic is not critical. The ceramic must be suitable for the vacuum conditions required in the finished tube. The ceramic is preferably suitable for metallizing with the molybdenum or tungsten, capable of withstanding the temperatures necessary for forming the cathode layer and for activating the layer after incorporation of the structure into a tube envelope. High alumina ceramics are especially suitable.

The heater pattern may be on the same side of the ceramic substrate wafer as the cathode, and may be separated from the cathode layer by an interposed electrically insulating layer such as silicon dioxide or be in direct physical contact with the cathode layer. Primarily, the heater pattern is in direct thermally conductive contact with the cathode layer. Direct thermally conductive contact means that the heat transfer from the heater to the cathode layer is primarily by thermal conduction, either by direct physical contact, or through intermediate thermally conducting material.

After the heater pattern is used for the cesiation processing of the cathode, it becomes a passive structure which is not used for operation of the tube, as the cesiated cathode is not heated for obtaining electron emission.

We claim:

1. An electron emissive tube of the type comprising:

an evacuated envelope;

a non-thermionic, electron emissive cathode, having a photocathode layer and a work function reducing material on said photocathode layer, in the envelope, and

a collector anode for electrons emitted from the electron emissive cathode,

wherein the improvement comprises:

a ceramic substrate on one surface of which the cathode is disposed, and a heater pattern of metallizing on the substrate in direct thermally conducting contact with the cathode.

2. The tube defined inclaim 1 wherein the heater pattern of metallizing is on the surface of the substrate opposite the cathode.

3. The tube defined in claim 1 wherein the emissive cathode comprises:

-a metal layer on the one surface of the substrate;

a layer of semiconductor on the metal layer, and

a work-function-reducing layer comprising at least one member of the group consisting of cesium, oxygen, and fluorine on the surface of the semiconductor layer.

4. The tube defined in claim 3 wherein the metal layer consists substantially of a metal from the group consisting of molybdenum and tungsten.

5. The tube defined in claim 4 wherein the semiconductor comprises at least one element from Groups lIIA, VA, 11B, and VIA of the Periodic Chart of the Elements.

6. The tube defined in claim 1 wherein the heater pattern metallizing consists substantially of a metal from the group consisting of molybdenum and tungsten.

7. An electron emissive tube in accordance with claim 1, characterized in that the ceramic substrate has a pair of spaced apertures therethrough and said heater pattern of metallizing extends over the ceramic substrate between-the apertures.

8. An electron emissive tube in accordance with claim 7, characterized in that the heater pattern of metallization comprises an electrically resistive metal film of substantially uniform width extending in a zig-zag pattern between said apertures, said metal film having a portion of narrower width respectively adjacent to each of said apertures.

I t 4' t

Claims

1. An electron emissive tube of the type comprising: an evacuated envelope; a non-thermionic, electron emissive cathode, having a photocathode layer and a work function reducing material on said photocathode layer, in the envelope, and a collector anode for electrons emitted from the electron emissive cathode, wherein the improvement comprises: a ceramic substrate on one surface of which the cathode is disposed, and a heater pattern of metallizing on the substrate in direct thermally conducting contact with the cathode.

2. The tube defined in claim 1 wherein the heater pattern of metallizing is on the surface of the substrate opposite the cathode.

3. The tube defined in claim 1 wherein the emissive cathode comprises: a metal layer on the one surface of the substrate; a layer of semiconductor on the metal layer, and a work-function-reducing layer comprising at least one member of the group consisting of cesium, oxygen, and fluorine on the surface of the semiconductor layer.

5. The tube defined in claim 4 wherein the semiconductor comprises at least one element from Groups IIIA, VA, IIB, and VIA of the Periodic Chart of the Elements.

7. An electron emissive tube in accordance with claim 1, characterized in that the ceramic substrate has a pair of spaced apertures therethrough and said heater pattern of metallizing extends over the ceramic substrate between the apertures.