US3317771A - Photo-emissive electron device - Google Patents

Description

May 2, 1967 P. D. WILLIAMS PHOTOEMISSIVE ELECTRON DEVICE Filed Oct. 31. 1963 INVENTOR. PAUL D. WILLIAMS Wig/ ad, FM 7 ATTORNEYS United States Eatent ()fi 3,317,? Patented May 2, 1957 3,317,771 PHOTO-EMISSIVE ELECTRON DEVICE Paul D. Williams, Portola Valley, Califi, assignor, by mesne assignments, to Varian Associates, a corporation of California Filed Oct. 31, 1963, Ser. No. 320,314 Claims. (Cl. 31394) This invention relates to electron emissive devices and more particularly to such devices in Which electron emission is caused by light striking a photo-emissive surface.

The main object of this invention is to provide improved structures for making use of the well known phenomenon of photo emission.

A more specific object of the invention is to provide an electron tube in which the cathode is a photo emitter and in which the interelectrode spacing is closer than has heretofore been possible. The maximum frequency at which an electron tube can operate is limited by the spacing between electrodes; the closer the interelectrode spacing, the higher the frequency. In conventional electron tubes employing thermally emissive cathodes, the high operating temperature limits the closeness of electrode spacing because of thermal distortion of parts. In conventional photo emissive tubes, substantial electrode spacing is required in order to provide a path for light to reach the photo emissive cathode surface. Modern requirements for electron tube operation at increasingly higher frequencies makes it necessary to find a solution to the problem of close electrode spacings.

Another and releated object of the invention is to provide photo emissive tubes in which it is possible for given size electrodes to be enclosed in a smaller tube envelope than was heretofore possible with conventional photo emissive tubes. In contrast to the invention construction, conventional photo emissive tubes require relatively large light-admitting envelopes to contain given size electrodes. Since photo emissive tubes are light-operated, the modern advances in the field of light transmission, such as the development of lasers, makes it increasingly necessary to provide improved light-operated tubes.

An additional object of the invention is to provide a photo emissive tube in which the activating light can pass through two opposite Walls of the tube envelope.

An additional object of the invention is to provide an improved method for making close spaced electrodes particularly for photo emissive electron tubes.

By way of brief description, the objects of the invention are achieved according to one embodiment by a photo emissive structure in which the anode is made of transparent material, such as glass or quartz, having a transparent electrically conductive surface, the structure being so arranged that the light which activates the cathode passes through the anode. According to another embodiment of the invention, the cathode is made of transparent material through which the activating light passes. In a further embodiment, both the anode and cathode are transparent. The described construction makes it possible for the electrodes to be extremely close spaced. In other words, light-admitting space is not required between the electrodes. Instead the light passes through one or more of the end electrodes. In some embodiments one or more of the transparent electrodes serve as walls of the tube envelope to decrease the overall envelope size.

These and other object and features of advantage will be apparent to those skilled in the art from the following detailed description wherein reference is made to the accompanying drawings in which:

FIGURE 1 is a cross-sectional view on the center line of an electron emissive device according to the invention in the form of an extremely close spaced diode;

FIGURE 2 is a cross-sectional view on reduced scale showing the anode of FIGURE 1 being processed in a bell jar to form spacers between the anode and cathode;

FIGURE 3 is an exploded perspective view of the anode and mask from FIGURE 2 on a larger scale;

FIGURE 4 is a perspective view on enlarged scale of the anode after being processed as in the apparatus of FIGURE 2.

Referring to the drawings in more detail, FIGURE 1 shows an extremely close spaced diode 10. The diode comprises a photo emissive cathode 11 and a transparent electron receiving electrode or anode 12. The anode comprises a quartz or glass disk 13, preferably having its lower surface 14 and its upper surface 15 made optically flat. The upper surface 15 is coated with a transparent electrically conductive coating 16 which extends around the periphery of disk 13 and forms an annular terminal ring on the bottom surface 14. Transparent conductive coating processes are Well known in the art, and any such suitable process can be employed to apply coating 16; for example, the process disclosed in British Patent 632,256, dated Nov. 18, 1949.

The cathode 11 is preferably formed in identical manner as described for the anode 12, including an optically fiat quartz or glass disk 26 and an electrically conductive transparent coating 21. In addition, the cathode has a conventional photo emissive coating 22 on the conductive coating 21. The spacing between the cathode and anode is provided by a plurality of dielectric spacer disks 23 as will be hereinafter described in more detail. The thickness of coatings 16, 21, 22 and 23 are shown many times greater than actual in order to be visible.

The cathode and anode are positioned in an envelope comprising a semi-cylindrical glass dome 25 which allows light from any suitable source to enter the envelope. A metal anode terminal ring 26 is bonded on one side to dome 25 and on the other side to a cylindrical glass side wall 27. The upper end of the envelope is formed by a metal sealing ring 28 bonded to wall 27, and a metal closure disk 29 having an upturned rim received Within the sealing ring 28. The members 28 and 29 are brazed or heliarc welded together as at 30. The members 28 and 29 serve as an external terminal for the cathode. The interior of the envelope is evacuated, either by means of conventional pinch-off tabulation (not shown) or by making the final seal in a vacuum chamber in which the tube was evacuated prior to making the seal.

The cathode and anode are held in place by metal coil springs 31 and 32. Spring 31 abuts the ring 26 and the annular portions of the conductive coating 16 on the bottom of the anode. Spring 32 abuts the closure disk 29 and the annular portion of the conductive coating 21 on the top of the cathode. Thus, the springs 31 and 32 serve as electrical leads for the anode and cathode, and also serve to position the anode and cathode in the envelope. The springs also serve to position the anode and cathode relative to each other by constantly forcing the anode and cathode toward each other so that the spacing therebetween is always exactly the thickness of the spacers 23. Assembly of the device can be easily accomplished by first preparing parts 25, 26 and 27 as a subassembly and then inserting parts 31, 12, 11, 32 and 29 in the order named.

Formation of the spacer dis-ks 23 is shown in FIGURES 24. FIGURE 2 shows a conventional glass bell jar 25 removably sealed on a metal base plate 36. The base plate is apertured to receive an exhaustv pipe 38 connected to a pump (not shown) for the purpose of evacuating the bell jar. A pair of metal support posts and conductors 39 are mounted in the base plate and insulated therefrom by dielectric sleeves 40. Each of the posts is bored to receive the ends of a heating wire 41, such as tungsten, and a thumb screw 42 in each post locks the Wire in place.

Wire 41 is coated, at least along its central portion, with a vaporizabie dielectric material 43, such as silicon dioxide.

The anode 12 is positioned on the base plate with the coated surface 16 upward. Then the anode is covered with a thin masking plate 46 preferably of metal and preferably having a downwardly extending rim 47 for automatically centering the plate on the anode. FIGURE 3 is an enlarged perspective view of plate 46 about to be lowered onto the anode which has been previously coated with the transparent conductive layer 16. The masking plate has at least three holes 48, or other suitably shaped type of aperture, through which vaporized silicon dioxide can pass to form spacers which will hold the anode and cathode disks apart without permitting the disks to tilt.

Deposition of silicon dioxide 43 is performed by first evacuating bell jar 35 and then passing current through wire 41 via posts 39 until the wire is hot enough to vaporize the silicon dioxide. The vaporized silicon dioxide passes through holes 48 in the masking plate and condenses on the anode. FIGURE 4 shows anode 12 after spacers 23 have been deposited by the described process.

The thickness of the deposition can be very accurately controlled by adjusting such variables as amount of silicon dioxide on wire 41, time of heating current, and distance between the anode and coating 43. Because vapor deposition deposits particles of atomic or molecular size, the thickness of spacers 12 can be made extremely small with great accuracy and reproducibility. For example, the spacers 23 can be made to have a thickness of one micron or even substantially less. Since the spacing between the anode and cathode is the same as the thickness of spacers 23, unprecedented minimum electrode spacing can be obtained. The spacing can even be made so close that tube need not be evacuated because the length of electron travel will be so short that the statistical likelihood of elec trons striking air molecules will no longer be a problem. The interelectrode spacing will not change perceptibly during operation for two reasons. One reason is that a photo emissive cathode operates at relatively low temperature compared to a conventional thermally emissive cathode. The other reason is that even if the light source or ambient thermal conditions do create substantial heat, silicon dioxide has an extremely low coeflicient of expansion, and because of the spring arrangements 31 and 32, the spacing is determined solely by the silicon dioxide spacers 23.

In order to operate the device of FIGURE 1, any conventional light source (not shown) is positioned outside the device so that the light therefrom will enter through the glass dome 25. The light passes through the transparent anode 13, through the transparent conductive coating 16, and strikes the photo emissive surface 22. The photo emissive surface reacts to the light energy and emits electrons which travel to the conductive surface 16, thus completing the circuit through the device. The electron emissive device 10 can be used in the same manner as a conventional photo emissive tube, and is particularly suitable for use as receiver for information transmitted by a laser in the form of a beam of light.

Although the specific arrangement described for FIG- URE l is preferred, the dielectric spacers 23 can be deposited on the cathode 11 instead of on the anode 12. Similarly, it should be understood that the cathode and anode unit can be inverted from the position in FIGURE 1. In the inverted arrangement the light does not reach the photo emissive surface 22. by passing through the transparent anode. Instead the light reaches the photo emissive surface by passing through the transparent cathode. Regardless of whether the cathode and anode are arranged as shown in FIGURE 1 or inverted, it should 4. be understood that the upper electrode in FIGURE 1 need not be made transparent and can be made of metal. If the upper electrode is made of metal, it will be coated directly with the photoemissive material 22 when used as the cathode and need not be coated at all when used as the anode.

Although preferred embodiments of the present invention are shown and described herein, it is to be understood that modifications may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. An electron discharge device comprising an envelope having a cylindrical dielectric side wall, an electrically conductive closure member attached to one end of said cylinder, an electrically conductive terminal ring attached to the other end of said cylinder, a transparent closure member attached to said terminal ring, a transparent anode disk in said cylinder, said disk having a transparent electrically conductive coating on its surface facing away from said transparent closure member, means forming an electrical connection between said transparent conductive coating and said terminal ring, a cathode disk between said anode disk and said conductive closure member, said cathode disk having a photoemissive surface facing said anode, dielectric spacing material between and abutting said disks, and means forming an electrical connection between said photo emissive surface and said closure member.

2. An electron discharge device as claimed in claim 1 in which said means forming an electrical connection for said anode comprises a coil spring between said anode disk and said terminal ring.

3. An electron discharge device as claimed in claim 1 in which said means forming an electrical connection for said cathode comprises a coil spring between said cathode disk and said conductive closure member.

4. An electron discharge device as claimed in claim 3 in which said means forming an electrical connection for said anode comprises a coil spring between said anode disk and said terminal ring.

5. An electron discharge device comprising an anode disk and a cathode disk having major surfaces with electron active portions in close spaced equidistant juxtaposition, the spacing between said electron active portions being of the order of microns, dielectric spacing material between and abutting said major surfaces of said disks at points removed from said electron active portions, said major surface of said cathode disk facing said anode disk having a photo emissive coating thereon, said major surface of said anode disk facing said cathode disk being conductive, one of said disks being transparent, means forming an electrical connection to said photo emissive coating on said cathode disk, and means forming an electrical connection to said conductive surface of said anode disk.

References Cited by the Examiner UNITED STATES PATENTS 1,965,849 7/1934 McIlvaine 313--102 X 2,451,400 10/ 1948 McIlvaine 3 l3-102 X 2,490,740 12/1949 Nicoll 313-94 X 2,876,374 3/1959 Riggen 313102 2,894,167 7/1959 Day 313102 X 3,093,507 6/1963 Lander et al 117-212 X 3,116,427 12/1963 Giaever 30788.5 3,214,629 10/1965 Apker 313346 JAMES W. LAWRENCE, Primary Examiner.

P. C. DEMEO, Assistant Examiner.

Claims (1)

1. AN ELECTRON DISCHARGE DEVICE COMPRISING AN ENVELOPE HAVING A CYLINDRICAL DIELECTRIC SIDE WALL, AN ELECTRICALLY CONDUCTIVE CLOSURE MEMBER ATTACHED TO ONE END OF SAID CYLINDER, AN ELECTRICALLY CONDUCTIVE TERMINAL RING ATTACHED TO THE OTHER END OF SAID CYLINDER, A TRANSPARENT CLOSURE MEMBER ATTACHED TO SAID TERMINAL RING, A TRANSPARENA ANODE DISK IN SAID CYLINDER, SAID DISK HAVING A TRANSPARENT ELECTRICALLY CONDUCTIVE COATING ON ITS SURFACE FACING AWAY FROM SAID TRANSPARENT CLOSURE MEMBER, MEANS FORMING AN ELECTRICAL CONNECTION BETWEEN SAID TRANSPARENT CONDUCTIVE COATING AND SAID TERMINAL RING, A CATHODE DISK BETWEEN SAID ANODE DISK AND SAID CONDUCTIVE CLOSURE MEMBER, SAID CATHODE DISK HAVING A PHOTOEMISSIVE SURFACE FACING SAID ANODE, DIELECTRIC SPACING MATERIAL BETWEEN AND ABUTTING SAID DISKS, AND MEANS FORMING AN ELECTRICAL CONNECTION BETWEEN SAID PHOTO EMISSIVE SURFACE AND SAID CLOSURE MEMBER.

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