US2914690A

US2914690A - Electron-emitting surfaces and methods of making them

Info

Publication number: US2914690A
Application number: US551028A
Authority: US
Inventors: Sommer Alfred Hermann
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1955-12-05
Filing date: 1955-12-05
Publication date: 1959-11-24
Anticipated expiration: 1976-11-24
Also published as: DE1269253B; FR1168953A; NL212695A; BE553132A; NL96983C; GB842894A

Description

Nov. 24, 1959 A. H. SOMMER 2,914,690

ELEcTRoN-EMITTING suRFAcEs AND METHODS oF MAKING THEM Filed Dec. 5, 1955 7001./ .225k 3T: l, ,2. .L

2.3i 42 'I f INVENTOR. j 4LP/F5@ H. S04/MM Fg' .BY

WMZ@ i A State Patentor Y photocathode iilm.

ELcTRoN-EMITTlNG SURFACES AND METHODS or MAKING THEM Alfred `Hermann Sommer, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware y i Application December 5, 195'5, VSerial No. 551,028

9 Claims. (Cl. 313-65) This invention relates to electron discharge devices having -iilms and to the art of making them, and, particularly, to electron-emitting iilms for use Yin phototubes, photomultiplier tubes, camera tubes for television and the like.

Electron-emitting films emit electrons when bombarded by visible or invisible radiation, charged particles, electrons or the like. Such films may be made by properly depositing and reacting aplurality of chemical substances on a support member of glass or metal or the like. One type of electron-emissive film emits electrons when bombarded by visible or invisible radiation and is known as a photocathode.

- In general, a good photocathode film has high .and uniy form sensitivity, that is, the iilm emits the same large posite lsurface. Thus, the requirement yis imposed that the lm1be thin enough to be penetrated and excited by the incident light. in order to replace emitted electrons, the cathode iilm is electrically connected to a t tsource of electrons and another requirement is that the cathode iilm be sufficiently thick to have the requisite conductivity to achieve electron replacement. An optimum photocathode film Vis of uniform thickness so that electron emission and electron replacement are achieved uniformly overthe entire area thereof. However, present methods of preparation do not provide such uniform `In the method of making a photocathode in accordance with this invention, a suitable support plate of glass, or the like, isiirst provided with `a thin film of potassium and then an electron-ernitting surface, such as a photo- Vcathode surface, is formed thereon. The electron-emitting surface may be an alkali-activated antimony system or the like. The thin potassium film promotes the formation of a uniform and sensitive-photocathode which vis comparatively easily reproduced. In `the drawing,

Fig, 3 is a sectional view along therline 3-3 in Fig. 2; and,

Fig. 4 is an enlarged' sectional view of a portion of the face plate and photocathode in the tube shown in Fig. l.

yFor purposes `of illustration, the following description relates .to a television camera pickup tube which utilizes an alkali-antimony photoemissive surface or photocathode made according to the principles of the invention. Referring to Fig. l, a television camera Itube 1,0, for example, ofthe image orthicon type, includes an `envelope having a face plate y14 on the inner surface of which 'a photoemissive surface or photocathode 16 is provided 2,914,690 Patented Nov..`24, 195,9

ICC

according to the principles of the invention. voltages are shown in Figure 1 for purposes of illustrating the operation of the tube.A These voltage values are not limiting. In operation of the tube of Fig. 1, a scene which is to be televised is optically focused upon the face plate 14 and on the photocathode 16. Photo'electrons are released from the photocathode'with a pattern of distribution corresponding to the `pattern of light coming from the scene to be televised. The photoelectrons are focused upon a thin glass target 18 by an electromagnetic coil 20 which surrounds the envelope 12 and provides a uniform magnetic field coaxial therewith. 'I'he photoelectrons are also accelerated toward the glass target by

ring electrodes

22 and 23 so that they strike the glass target 18 with sufficient energy to produce secondary emission therefrom. A ne mesh screen 24 is positioned closely adjacent to the surface of the target 20 and collects the secondary electrons. The-loss of electrons from the surface of the glass target leaves a positive charge pattern on the glass target surface corresponding in distribution in intensity to the optical scene focused on the photocathode 16. Since the glass target is very thin, a corresponding pattern of potentials is. present on the surface away from the photocathode.

The last-mentioned surface of the glass target is scanned by an electron beam formed'by an electron gun 26 located at the oppositeL end of the tube from the photocathode. The electron beam is aligned by oppositely disposed coils 25 and 25', and is focused on the target 18 by the coil 20. The beam is scanned over the surface of the Vtarget by deflection fields produced by coils formed into a yoke 27, and is well known in the art. As the electrons of the beam approach the target 20 they are decelera'ted to substantially zero velocity by means of a decelerating ring electrode 28. Some ofthe electrons from the cathode ray beam are deposited on the positive areasl of the target surface in an amount to neutralize the positive charges on the opposite surface of the target. These deposited .electrons drive each positive target area to gun cathode potential so that the remainder of the beam electrons are reflected back along the `tube envelope toward the electron gun 26 by the neutralized target area. The reflected electrons are collected and amplified in a multiplier section `30 to form video signals in the output circuit of the tube (not shown).

by means of a strip of silver 32 (Fig. 2) painted on the inner wall of the envelope` 12 in contact with the photo- Y cathode 16. A spring contact lead 34 contacts the silver stripand is connected, as by welding, to a pin 36 which extends `through the wall of the envelope. and to which electrical contact may ,be made. Similarly, the accelerating

electrodes

22 and 23 are welded to wire leads 37 and 38, respectively, which extend through the wall of the envelope.

A heater filament 40 of tungsten or the like, for ,use in evaporating material during the making of the photocathode 16, is disposed between the mesh screen 24 and the electrode 23 as shown in Fig. 3. One end of the filament is secured to a lead 42 which passes insulatingly' extends through the wall'of the envelope. The `other end of the filament 40 is secured to the electrode 23 by a lead 44. Thus, electrical connection to the filament is provided through the external pins 38 .and 43. Three' other

chemical carriers

45, 46 and 47 (Fig. 3), each vin the form of different elongated metal channels, as shown, or in any other convenient form, are also provided within the envelope 12. In one convenient arrangement, one end of each

carrier

45,46 and 47 is secured to the ,outer surface of accelerating electrode 23 and the other end-of each carrier is secured to wire leads 48, 49 and'50, re-

3 v v spectively,' .which extend through the Wall of the envelope.' Thus, external electrical connections are provided for passing current through each of the chemical carriers. 45, 46 and 47. n

Inf assembling' the tube,the materials to be evaporated are prepared as pellets, powders or the like and are secured to or deposited in their variousV carriers or supportv members. Specilically, a plurality of pellets or beads 52 comprising high grade commercial antimony with only traces `of iron, sulfur, arsenic and lead permitted are secured, lat spaced intervals, on the filament 40. A mixture of one'part by weight of potassium chromate, one part by weight of` aluminum, Aand 8 parts by weight oftungsten is placed in the carrier 45. A mixture ofone part -by Weight lof sodium chromate, one part by Weight of aluminum and 8 parts by weight of tungsten isfplaced the carrier 46. A mixture of one part by 'weight of cesium chromate `and 3 parts by weight of siliconY is placed in the carrier 47. The percentages of the Various components `of the latter three mixtures disposed in the

carriers

45, 46 yand 47 are not critical and may be varied within Wide ranges, as desired.

After the various tube components are assembled in the envelope 12, the tube is mounted on an exhaust pump by means of an exhaust tubulation (not shown) and then baked yat a temperature in the range of 375 to 400 C. for about one hour to remove occluded gases from the envelope and from the metal parts.

` The photocathode iilm 16 is prepared in the following manner: First, the tube is heated, for example in an oven, to a temperature in the range of 140 C. to 170 C. With the tube thus heated, a thin lm 54 (Fig. 4) of potassium is evaporated onto the inner surface of the face plate 14 of the envelope 12. This evaporation is effected by passing current from a suitable power source (not' shown) through the

leads

38 and 43 and heating the carrier 45 and thereby evaporating potassium metal released by the reaction of the potassium chromate, aluminum and tungsten. As the potassium is deposited on the face plate the photoelectron emission therefrom is measured and when the emission reaches a peak value, the evaporation of potassium is discontinued.

This electron emission from the potassium ilm may be measured by connecting the photocathode by its lead 36 andthe electrode 23 by its lead 38 in series with an ammeterk56 and then directing light from a source 58 ontothe face plate. The tube is kept at an elevated temperature as described above to render the glass of the face plate conductive so that the foregoing measurement of the potassium film may be made during the evaporation of the potassium. The potassium lm thus formed'is invisible and is believed toI -be in the form of a monatomic layer. Next, a film 60 of antimony is deposited over the potassium lm 54 on the face plate 14. This operation may be performed with the tube at room temperature. In order'to evaporate the antimony from the pellets 52 a Vheating current is passed through the

leads

38 and 43 to heat the filament 40 to the vaporization temperature for the antimony metal. For reasons which are not fully understood, the presence of the potassium iilm on the face plate promotes the formation of a thinner, more uniform antimony film than would be formed on the glass face plate alone. The deposition of the antimony may be continued until the light transmission from the source 58 through the face plate is in the range of 50 percent to 95 percent of the light passed prior to the formation of the film. The light transmission through the face platemay be measured by means of a photocell in the manner disclosed in U.S. Patent No. 2,676,282 of I. I. Polkosky. The light transmission prior to the deposition of the tilm may be arbitrarily assumed to be 100. To form' one type of photocathode, the antimony iilm '60 is then activated with three alkali metals, potassium,

Y 4 l Y sodium, and cesium. First, the tube is again heated to a temperature in the range 'of 140 C. to 170 C. in order to control the amount of potassium deposited on the lm 60. Potassium is then evaporated onto the antimony tilm from the material remaining in the carrier 45 by passing heating cun'ent therethrough by Way of the

pins

38 and 48. The evaporation of the potassium is continued until the photo-emission reaches a peak value as indicated by ammeter 56. Evaporation of potassium is then discontinued and the temperature of the tube is raised to about 220 C. and the excess potassium metal is pumped out.

The tube is then held at a temperature in the range of 180 C. to 220 C. Yand the antimony film is treated `with a second alkali metal, sodium, which is evaporated onto the coated face plate by heating land reacting the material in the carrier 46. The requisite heating may be achieved by passing current through the leads 38 and 49. The evaporation of sodium Vis also continued until peak photoemission is achieved and the excess is then pumped out of the tube.

Finally, the tube temperature is brought into the range of 130 C. to 160 C. and cesium is evaporated onto the coated face plate from the carrier 47 until electron emission from the photocathode reaches a peak value and the excess cesium is pumped out. Thus, the photocathode 16 is essentially complete and the tube is cooledand processed further as required.

Under some circumstances, it may be desirable to evaporate additional antimony onto the photocathode, as it is formed, after each activation with an alkali metal'.

In the foregoing process, the order in which the sodium and potassium are evaporated onto the antimony coated face plate may be reversed. In addition, the principles of the invention may also be employed with secondary electron-emissive coatings and with other photocathode systems, for example, antimony-potassiumrubidium, antimony-rubidium-potassium, antimony-potassium-sodium, antimony-sodium-potassium, antimony-potassium lithium or the like. In each case, the selected photocathode system is formed on a face plate coated with a iilm of potassium.

The method of the invention provides electron emitting surfaces which exhibit sensitivities over their areas which do not vary by more than about 3 percent to 7 percent. For example, `one photocathode showed sensitivities varying Vover the whole surface in the range of l70 to 175 microamperes per lumen of incident light. On the other hand, photocathodes made according to prior art practices have sensitivities over their areas which may vary up to percent. For example, one such photocathode had sensitivities in the range of 20 to 110 microamperes per lumen.

Thus, the method of the invention provides electronemitting surfaces which are reproducible and which have high and uniform sensitivity.

What is claimed is:

l. An electron-emissive electrode comprising a supporting base, a ilm of potassium on said base, a film of antimony on said potassium film, said antimony lm including reaction products of antimony with a plurality of alkali metals.

2. An electron-emissive electrode comprising a supporting base, a iilm of potassium on said base, a film of antimony on said potassium film, saidantimony film including reaction products of antimony with sodium and potassium.

3. An electron-emissive electrode comprising a supporting base, a film of potassium on said base, a lm of antimony on said potassium film, said antimony tilm including reaction products of antimony with sodium, potassium and cesium.

4. An electron-emissive electrode comprising a supporting base, a film of potassium on said base, a film of antimony on said potassium film, said lm including small amounts of a plurality of alkali metals.

y5. An electron-emissive electrode comprising a supporting base, a ilm of potassium on said base, a lm of antimony on said potassium lm,rsaid lm including small amounts of sodium, potassium and cesium.

6. An electron-emissive electrode comprising a supporting glass base, a ilm of potassium on said base, a lm of antimony on said potassium film, said film including small amounts of potassium and lithium.

7. An electron-emissive electrode comprising a supporting glass base, a iilm of potassium on said base, a lm of antimony on said potassium film, said lilm including small amounts of at least two alkali metals.

8. An electron discharge device including an electronemitting portion, said portion comprising a supporting base, a film of potassium on said base, a lm of antimony on said potassium lm, said antimony lm including reaction products of antimony with a plurality of alkali metals.

9. A camera tube comprising an envelope, a face plate at one portion of said envelope, an electron-emissive surface on said face plate and including a film of potassium on said face plate, and a lm of antimony on said film of potassium, said lm of antimony including reaction products of antimony with a plurality of alkali metals.

References Cited in the file of this patent UNITED STATES PATENTS 2,206,372 Sommer July 2, 1940 2,391,280 Teal Dec. 18, 1945 2,603,757 Sheldon July 15, 1952 2,654,048 McGee Sept. 29, 1953 2,747,133 Weimer May 22, 1956 2,770,561 Sommer Nov. 13, 1956

Claims

1. AN ELECTRON-EMISSIVE ELECTRODE COMPRISING A SUPPORTING BASE, A FILM OF POTASSIUM ON SAID BASE, A FILM OF ANTIMONY ON SAID POTASSIUM FILM, SAID ANTIMONY FILM INCLUDING REACTION PRODUCTS OF ANTIMONY WITH A PLURALITY OF ALKALI METALS.