US2075377A - Means and method of forming discrete areas - Google Patents

Description

March 30, 1937. R. H. VARIAN 2,075,377

MEANS AND METHOD OF FORMING DISCRETE AREAS I I 1 I I "5a! a w 5 i1 3 I i 1 I POWDER SURFACE.

PHOTOELECTR/C LAYER.

v H x POWDER LAYER. 'INVENT R,

' CONDUCTOR. V AT ()RNEYS.

Patented Mar. 30, 1931 Russell H. Varian, San Francisco, Calif., essignor to Farnsworth Television Incorporated, a corporation of California Application March 13, 1935, Serial No. 10,883

Claims.

My invention relates to a means and method of forming discrete conductive areas, and more particularly to a means and method of forminga mosaic photoelectric surface.

5 Among the objects of my invention are: to

provide a means and method for forming a surface having a plurality of discrete, closely adjacent conductive islands; to provide a mosaic photoelectric emitter; to provide a mosaic surface having a plurality of discrete conductive areas upon which photoelectric material may be deposited; to provide a means and method for preparing a silver surface for the deposition of photoelectric material thereon; to provide a means and method of condensing a vaporizable material as a discontinuous surface; and to provide a means and method for regulating the sensitivity of a photoemissive. substance during the activation thereof when deposited in discrete areas.

Other objects of my invention will beapparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of the inventionherein described, as various forms may be adopted within the scope of the claims.

Referring to the figures which illustrate several steps in the formation of a preferred photoelectric cathode:

Figure 1 is a plan view of a preferred type ofv photoelectric cathode, showing the distribution of the treated surfaces thereof.

Figure 2 illustrates a step in the formation of a mosaic silver surface upon a preferred form of cathode.

Figure 3 is a longitudinal sectional view of a television dissector tube similar to that described in the application of Philo T. Farnsworth, Serial No. 668,066, filed April 26, i933, for an Image dissector, wherein the mosaic photoelectric cathode of my invention is mounted.

Figure 4 is a sectional view through a photoelectric cathode, on which a mosaic photoelectric layer has been formed on a loosely deposited, finely divided, insulating powder. a

In many instances, one example of which is the cathode of the dissector tube disclosed in the above identified application, it is desirable to make a photoelectric emitter comprising a plu-,

rality of closely adjacent discrete emitting islands. A number of methods are known in the art whereby such mosaic emitters can be pro-. duced, one of which includes the step of evaporating silver through a fine screen onto an insulating surface. It is quite common to project the 55 silver vapor from a point source above the screen 1 a substantially point source.

.form the conductive islands by evaporating, silever, is not ideal. The most important disadvantages of the screen method are, "using a mica sheet for an example of the insulator upon which the deposit is made:

l. A mono-molecular layer of alkali metal on the mica surface between the islands may easily I cause a leak between the islands.

2. The regular arrangement of the islands, when combined with a moving beam or other scansion arrangement of definite aperture size, can easily produce interference patterns similar to those seen when looking through two wire screens.

3. The spaces between the islands are necessarily large, due to the size of the screen material, and the islands themselves are relatively large, due to the practical inability to make a screen of sumcientlyfine mesh to produce the number of islands usually desired.

There is another method of making a mosaic surface wherein active material is evaporated directly onto an insulating surface, the identity of the islands relying on a tendency of the deposited material to group itself in minute balls on the surface. This method, however, is uncertain in its action and :the method ishardly applicable to commercial production, it being very difficult; to form uniform distribution over relatively large areas. i

Broadly stated, I have found thata mosaic photoelectric surface having small islands closeshadows when the surface is exposed to metal vapor travelling in relatively straight lines from I may prefer to ver onto such an indented surface, and-thereafter condense alkali metal or similar photoelectric material thereon. I also prefer to oxidize the silver surface before the depositionof the photoelectricmaterial thereon, and l have found that a better emitter is eventuailyobtained if the oxidation is performed by oxygen ions reaching the surface 'by diffusion rather than with high velocities imparted thereto by excitation of the mosaic itself. a.

In the formation of any photoelectric surface it has hitherto been quite customary to sensitize the alkali metal, for example, by creating a glow discharge within the envelope and to check the emission of the cathode during the sensitization process. With an emitter comprising a large number of discrete emitting particles, it is a complicated and dimcult process to do this checking by measuring the emssion of any or all of the islands. I therefore prefer to provide, adjacent the mosaic surface, and preferably in the same plane therewith, an extended smooth surface upon which continuous photoelectric material is deposited. It is relatively easy to check the emission from this continuous surface, and I have found when the emission from the continuous surface is at its maximum that the emission from the photoelectric islands isalso at its maximum. A simple method of controlling the sensitivity of 2 the islands is thereby attained.

The roughened indented surface, having the proper overhangs, may be formed in a number of ways, as the physical characteristics only of the surface are the important factors, the par- 2 ticular means by which such a surface is obtained being of less consequence. I may deeply scratch and thereby roughen a relatively soft material, such as mica, to form the surface, or I may deposit upon a relatively smooth insulating surface a layer of finely divided insulating powder, preferably one of such fineness that it has adherent properties. I have found that finely divided and powdered quartz is a satisfactory. material to employ, but other refractory materials will be equally satisfactory, the requirements for them being principally that the caesium be not able to combine with them, and that they may be deposited in such a way as to form overhangs.

Regardless, however, of how the surface is obtained, I have found that when vaporizable metals, such as silver or the alkali metals, are caused to condense upon such an indented surface, preferably from a point source, the metal atoms arriving in straight lines, that every point on the roughened surface will be covered with the condensed metal which is not shaded by higher portions of the surface. The islands are insulated, one from the other, because many of the points of contact, between grains when powder is used are on surfaces which are either parallel to the direction travelled by the metal ions or are under projecting portions of one or groups of grains.

Referring to the drawing for a more detailed discussion and description of my invention, I prefor to start the foundation of my cathode with a surface as shown in plan in Figure 1. A mica disc I is preferably provided with a mask, not shown, having a square opening therein. This mask is placed over the mica sheet and powdered quartz is deposited on the surface by being blown,

usually in suspension, from a spray gun, for example, until a square powdered surface is deposited on the plate, as indicated by the peripheral outline 2. I then mount this mica sheet on a conductive cathode plate 4, the plate being of slightly larger diameter than the mica sheet, and the sheet being held thereon by clips 5 bearing on the edge of the mica. The cathode plate with its accompanying mica sheet carrying the powder layer 2. is mounted on risers 6 fastened in turn to a clamp I, secured around a stem 9. A cathode connection lead lli passes through the stem and connects to one of the risers. I then prefer to mount this stem in a processing envelope ll and position the cathode close to one end of the blank. On the opposite end of the blank I insert a supporting stem l2, carrying a silver chamber M on a chamber lead Iii sealed through the supporting stem. The chamber I I is hollow and is provided with an aperture l6 directed toward the cathode. assembly. Within this chamber I prefer to place a quantity of pure silver I1.

, I then exhaust the processing envelope and heat the silver chamber, preferably by means of an eddy current coil l9 positioned outside the envelope and surrounding it, to a temperature sufficiently high to vaporize the silver II. The

of the cathode surface. Of course at the edges there will be a slight angle, but this angle is unimportant as long as it is not too great.

During condensation on the powder layer,

shadowing will take place with a result as shown in Figure 4. The silver will be condensed upon those portions of the powder layer which present a surface normal to the oncoming silver, but will not deposit under the overhangs nor upon the sides which are parallel to the line of travel of the incoming vapor. The result is the formation of small and numerous isolated conductive silver areas on the powder layer.

It will, of course, be obvious that upon-that portion of the mica surface which is not covered with powder, that silver will be deposited in a continuous layer, forming an area about the mosaic layer which is continuous and conductive.- As the clips 5 of the cathode extend over onto the clean surface, the continuous silver layer will make contact thereto. The lead I!) therefore makes direct connection to the continuous surface.

In experiments performed to. determine the continuity of silver surfaces deposited on a clean surface and on a powdered surface, it was found that under identical conditions where the thickness of the silver deposit was the same, that the plain silver surface on the mica, without any powder had a resistance, over a given length, of three ohms. Using the same length of path for measurement, it was found that the silver which had been deposited upon powder had a resistance, in every direction, greater than 5 x 10" ohms, thus proving that the islands are completely discontinuous, allowing practically no leakage therebetween.

It will also be apparent that the size of the' conductive islands, and to some extent their spacing, may be controlled by varying the size, shape, and manner of deposition. If small powder grains are deposited in a dense, relatively smooth surfaced layer, the islands will be very small and close together. If, however, the same powder is laid down in a very rough manner with large groups of grains loosely attached to each other, the islands will be relatively large. If the powder grains. are made large and loosely deposited, theislands will be large and the distance between them will also be relatively large. It is therefore possible to make mosaic metal layers having practically any island dimensions desired.

I have found that the mosaics formed by the means and method described above are, in their average size, well below the size of the smallest elementary scanning area desired. I have also found that they do not tend to run together into linear groups, and it is of course obvious from the way in which the islands are created that they form no pattern, being indiscriminate in their relation one to another. While I have no direct evidence that the islands have a uniform average size, I do know, when the cathode is used in a television dissector, such as will be later described, that the detail pmduced in the image, as reconstructed in a receiver, definitely shows, by the uniform detail, that there is no tendency for the islands to run together to form linear streams or consolidated masses which are of sufficient size to cause any blurring in' the image therein.

Returning again to the cathode having a silver surface, as formed in the processing envelope II, I next prefer to remove the silvered cathode from the processing envelope, preferably by a separation, as indicated by the dotted line 20 in Figure 2. I then prefer to mount the silvered cathode in a television dissector tube envelope 2|, the glass sealing together as indicated by a ring seal 22. u

The other end of the dissector tube. envelope is provided with an anode assembly 24. This particular tube has been fully described in the above identified application, and as the particular form of dissector tube in which the cathode is used is no part of the instant invention, it will not be necessary to go into the function of this anode assembly in this application. Further processing of the cathode is carried out in the tube shown in Figure 3. The tube envelope 2| is connected to a pump through a stop-cock 25, and the exhaust tube 28 is provided with a connection to a source of alkali metal vapor, preferably a bulb 21, containing 45 caesium 29, and adapted to be heated by a gas flame or other source 30. The tube is preferably may alsobe oxidized directly by heat or by high frequency applied to the cathode plate behind the mica, but I have found that in every case the electrodeless ring discharge gives the best surface. Why this should be I am not atthis time prepared to state, but it would appear that 65 the oxidization by means of an oxygen ion having little or no impact velocity, gives a better surface than when heat is .used or when the ions are directed into the surface with substantial impact velocity imparted by electrode poten , The bulb :1 is then heated to introduce caesium vapor into the envelope, where it comblues with the silver oxide to form a photoelectric surface on both the discrete areas and on the continuous silver area.

It is not necessary that the caesium vapor be directed onto the surface from a point source, as it appears that the caesium vapor deposits on and combineswith the silver oxide without the need for direction, but does not deposit between the islands nor on the walls to any great extent.

During the deposition of the caesium, or during later activation if any additional processing is desired, the degree of sensitivity of the device may be constantly checked by illuminating the cathode, utilizing the direct connection In to the continuous area of the cathode for a measurement of the emission therefrom, the anode assembly being used as a collector. When the point of maximum sensitivity has been reached the exhaust tube 26 is sealed and the completedenvelope removed from the exhaust system. The tube is then in the condition to be utilized as a television dissector tube, as described in the above identified application.

It will be apparent to those skilled in the art that the same means and method as above described may be utilized to form a mosaic photoelectric surface in apparatus other than that shown. Such uses will be apparent to those skilled in the art, and I do notwish to be limited by the single embodiment of the device as described in conjunction with a television dissector tube-the latter use being merely a convenient combination for purposes of illustration and description.

I claim:

1. The method of forming electrically discrete areas of a vaporizable conducting material on a surface which comprises depositing a layer ofclosely adjacent insulating particles on said sur-' face, condensing said material on portions of said particles, and regulating the average size of said areas by controlling the average size of the parthe following layers in order: a conducting layer,

a homogeneous insulating layer, a layer of finely ground insulating powder, a layer of silver de-' posited in discrete areas on said powder layer. and photoelectric material deposited on said discrete areas.

5.'A photoelectric mosaic cathode comprising the following layers in order: a conducting layer, a homogeneous insulating layer, a layer of adherent particles of insulating powder of heterogeneous sizes, a layer of photosensitive material deposited in discrete areas on said powder layer, said conductive layer being of at least as great an area as the, area covered by the photoelectric material, and underlying said material.

RUSSELL H. VARIAN.

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