US3515553A

US3515553A - Photolithographic deposition of phosphors on faceplate of crt using spraying of photosensitive pva-phosphor suspension in plural layers

Info

Publication number: US3515553A
Application number: US248356A
Authority: US
Inventors: John W Tiley
Original assignee: Philco Ford Corp
Current assignee: Space Systems Loral LLC
Priority date: 1951-09-26
Filing date: 1951-09-26
Publication date: 1970-06-02
Anticipated expiration: 1987-06-02

Description

- J, W. TlLEY 3,515,553 PHOTQLITHOGRAPHIC DEPOSITION 0F PHOSPHORS ON FACEPLATE ITIVE PVA-PHO LAYERS SPHOR June 2, 1970 or CRT usme SPRAYING 0F PHOTOSENS SUSPENSION IN PLURAL Filed Sept. 26, 1951 INVENTOR. (JO/7W W. 7715) United States Patent 3,515,553 PHOTOLITHOGRAPHIC DEPOSITION OF PHOS- PHORS ON FACEPLATE OF CRT USING SPRAY- ING OF PHOTOSENSITIVE PVA-PHOSPHOR SUS- PENSION IN PLURAL LAYERS John W. Tiley, Hatboro, Pa., assignor, by mesne assignments, to Philco-Ford Corporation, Philadelphia, Pa., a corporation of Delaware Filed Sept. 26, 1951, Ser. No. 248,356 Int. Cl. G03c 5/00 US. Cl. 96--36.1 9 Claims ABSTRACT OF THE DISCLOSURE A method of depositing on the faceplate of a color television picture tube an array of discrete elements of a phosphor emissive of light of a given color, each element being precisely configured and positioned and having a uniform thickness great enough to emit substantial light upon energization thereof. Such array of phosphor elements is deposited by spraying a suspension consisting of a polyvinyl alcohol, an alkali-dichromate salt, and the phosphor in particulate form over the entire substrate in a plurality of successive layers, one after the other, using sufficient air such that the suspension reaches the substrate in a tacky condition. The photosensitive coating so formed is exposed to actinic radiation in those areas where phosphor elements are desired, thereby to render those areas insoluble in a solvent in which the unexposed portions of said coating are soluble, and is washed with such solvent to remove the unexposed portions of the coating.

To deposit on the screen additional arrays of phosphor elements emissive of light of respectively diiferent colors, the foregoing group of steps is repeated, in each instance with a suspension containing the particular phosphor to be deposited. After all the phosphor elements respectively emissive of different colored lights have been so deposited, the faceplate is baked to fix the exposed portions and remove the polyvinyl alcohol by pyrolysis.

The present invention relates to cathode-ray tubes and to methods for manufacturing the same. The invention is particularly applicable to cathode-ray tubes for producing color television images by means of a fluorescent screen structure adapted to produce light of three different primary colors, and the invention will be specifically described in this connection. It should be Well understood, however, that the invention is also applicable to cathode-ray tubes for other purposes wherein a fluorescent screen structure producing a plurality of individual responses is required.

Cathode-ray tubes for producing a color television image may be based on a number of operating principles. In one form, the cathode-ray tube may comprise a conventional beam generating and intensity controlling and accelerating system and a fluorescent screen assembly, the latter comprising stripes of fluorescent material adapted to produce light of three different primary colors. The stripes are arranged in laterally-displaced color triplets, each triplet comprising three phosphor stripes which respond to electron impingement to produce light of the desired primary colors. In accordance with one method of operating such a tube, the phosphor stripes are arranged vertically so that the normally horizontally scanning cathode-ray beam produces red, green and blue light successively. From a color television receiver there may then be supplied to the intensity control electrode of the cathode-ray tube, an appropriate signal the contemporaneous value of which represents the desired response of the particular phosphor stripe on which the electron beam of the tube is impinging.

Patented June 2, 1970 In another form, the cathode-ray tube may be constituted essentially as above described with the exception that the screen structure comprises a myriad of dots of phosphor materials adapted to produce the desired three primary colors upon impingement of the electron beam. The said dots are arranged in a given predetermined pattern both in the horizontal and vertical directions of the screen assembly so that the scanning beam, under the control of the video color signal, produces an image properly blended to reproduce the desired color of the image televised.

To achieve a desired degree of definition comparable to that now commonly available in so-called black-andwhite image reproducers, the image reproducing screen should contain a relatively large number of groups of the phosphor portions. In the case of the cathode-ray tube having a screen constituted by vertically arranged color triplets, the number of triplets should approximate the number of picture elements contained in one line scan of the reproduced image. Similarly, in the case of a screen structure made up of dots of the phosphor material, the number of groups of dots per scanning line should approximate the number of image elements per scanning line.

To achieve proper blending of the colors produced by each triplet, it is further necessary that phosphor stripes should be positioned relatively close together, be accurately positioned relative to each other and be uniformly distributed over the surface of the screen member. Furthermore, each of the stripes should be uniform in width and thickness and in the amount of phosphor material contained per unit length thereof. Similar considerations apply to screen structures in which the phosphor materials are arranged in dot groups.

It is an object of the invention to provide improved cathode-ray tubes having screen structures adapted to produce a plurality of responses and in which the response generating elements of the screen are accurately positioned on the surface of the screen structure.

A further object of the invention is to provide improved cathode-ray tubes for producing a color television image, which tubes comprise a screen structure having a multiplicity of phosphor portions adapted to produce light of three different primary colors, and in which the phosphor portions are of accurately determined size and are accurately positioned on the surface of the screen.

Another object of the invention is to provide novel methods for manufacturing cathode-ray tubes of the foregoing described type.

Still another object of the invention is to provide a novel method for making cathode-ray tube screens, which method is simple and economical and leads to screen structures having spaced phosphor portions positioned with great accuracy.

These and further objects of the invention will appear as the specification progresses.

In accordance with the invention, the foregoing objects are achieved in a cathode-ray tube comprising a beam intercepting screen member having discrete individual portions adapted to produce a plurality of individual responses upon impingement by the beam, by a novel method of photo-chemically depositing the screen portion which insures accuracy in the amounts of the material deposited and a precise positioning of the portions. More particularly, and for manufacturing a cathode-ray tube screen for producing a color television image and comprising laterally-displaced triplets of stripes of phosphor material adapted to produce light of three different primary colors, I provide on a screen base a photosensitive coating containing, in partial or total suspension or dispersion, a given one of the multiple phosphor materials to be deposited. By means of a mask of suitable configuration, or by means of an optical system producing 3 an equivalent optical image, the coating is exposed to light so that only selected portions of the coating are activated, i.e., only those portions of the coating corresponding to one of the three phosphor stripes of each of the color triplets to be formed. After suitable processing, the unactivated portions of the coating are removed and a second photosensitive coating containing, in partial or total suspension or dispersion, a second one of the phosphors to be deposited is then applied to the surface of the screen. The second coating is then optically activated through a mask, or optical system, producing a light pattern complementing the pattern of the exposure of the first coating and establishing the desired position of the stripes of the second phosphor material. After removing the unactivated portions of the second coating, a third photosensitive coating containing the third fluorescent material is applied to the screen surface and optical- 1y activated through a mask or optical system producing a light pattern complementing the patterns of the first and second formed phosphor stripes and establishing the position of the stripes of the third phosphor material. Upon removal of the unactivated portions of the third coating, the screen base and its superimposed phosphor stripes may be subjected to further processing to permanently fix the deposited phosphor stripes to the screen base.

In practice, and in the case of screen structures in which the phosphor stripes of each color are of similar configuration and dilfer only in their relative position on the screen surface, the same mask or optical system may be used, it being only necessary to displace the light image to appropriate new positions for each subsequent activation of the consecutively applied coatings on the screen base.

The process of the invention preferably further comprises additional novel steps hereinafter to be described which have been found to contribute significantly to the production of multiple phosphor screens having unusually desirable characteristics.

The invention will be described in greater detail with reference to the appended drawings forming part of the specification and in which:

FIG. 1 is a cross-sectional diagrammatic view of a cathode-ray tube in accordance with the invention;

FIG. 2 is a cross-sectional view, partly cut-away, of a portion of a screen structure made in accordance with the invention; and

FIG. 3 is a diagrammatic view showing one form of apparatus for carrying out the process of the invention.

Referring to the drawing, the cathode-ray tube shown in FIG. 1, comprises, within an evacuated envelope a conventional beam generating and accelerating electrode system comprising a cathode 12, a control electrode 14 for varying the intensity of the cathode-ray beam, a focusing electrode 16, and a beam accelerating electrode 18 which may consist of a conductive coating on the inner wall of the envelope and which terminates at a point spaced from the end face 20 of the tube in conformance with well-established practice. Suitable heating means (not shown) are provided for maintaining the cathode 12 at its operating temperature. The end face 20 of the tube is provided with a beam intercepting fluorescent screen structure, one form of which is shown in detail in FIG. 2. In the arrangement shown in FIG. 2, the fluorescent structure is formed directly on the faceplate 20. However, the structure may alternatively be formed on a suitable light transparent base which is independent of the faceplate 20 and may be spaced therefrom. In the arrangement shown, the end face 20, which in practice consists of glass having preferably substantially uniform transmission characteristics for the various colors of the visible spectrum, is provided with a plurality of groups of elongated parallelly arranged

stripes

24, 26 and 28 of phosphor material which, upon impingement by the cathode-ray beam, fiuoresce to produce light of the different primary colors. For example, the stripe 24 may consist of a phosphor which produces red light, the stripe 26 may consist of a phosphor which produces green light, and the stripe 28 may consist of a phosphor which produces blue light. Each of the groups of stripes may be termed a color triplet and, as will be noted, the sequence of the stripes is repeated in consecutive order over the area of the faceplate 20.

As above pointed out, the number of color triplets formed on the image producing face of the cathode-ray tube should at least approximate the number of picture elements contained in one line scan of the image. In a typical case, for a cathode-ray tube producing a color image having a definition comparable to that now commonly available in so-called black-and-white television receivers, there may be approximately 350 color triplets arranged on the faceplate 20. For a picture area having a 20" diagonal, and thus having a 16" base line, such as would be accommodated by a 20" cathode-ray tube, each of the color triplets is restricted to a maximum Width of approximately .045" and accordingly, each of the stripes of a triplet is limited to a maximum width of approximately .015". In practice, the individual stripes of fluorescent material may be spaced by a small amount from the adjacent stripes so that the maximum permissible width of the stripes is less than the above-noted value, and may approach .005 to .010. For cathode-ray tubes of smaller size having the same degree of image definition, the width of the stripes must be proportionally less.

In accordance with the invention, phosphor stripes having a size of the order of magnitude above noted and which are of uniform density and are precisely positioned on the surface of the faceplate 20 are achieved by a photochemical deposition process which in its preferred form embraces the following steps:

As a preliminary to the deposition of the

fluorescent stripes

24, 26, and 28, the inner surface of the tube face 20 is cleaned to thoroughly remove all organic and inorganic impurities from the surface. This cleaning may be effected by vigorous scrubbing with soap and water followed by successive rinses in alcohol and distilled water. Preferably, the surface of the tube face is cleaned chemically, for example, with a solution of an alkali phosphate. More specifically, the surface of the faceplate may be cleaned by immersion, for 5 to 10 minutes, in an aqueous solution containing approximately 10% by weight of trisodium phosphate and maintained relatively hot, i.e., at approximately F. The solution should be agitated to ensure thorough cleaning.

Preferably, and without an intervening rinse, the faceplate so treated is immediately immersed in an acid solutionspecifically a 1 to 10% solution of a mixture of hydrochloric acid and nitric acid. This abrupt immersion into the acid solution, I have found, brings about beneficial effects in the cleaning of the glass surface, in that, apparently, the reaction between the acid and the alkali hosphate remaining on the glass surface causes the formation of minute gas bubbles which disrupt any impurity films remaining on the glass surface after the alkali cleaning step. These cleaning steps may be repeated one or more times individually or in combination depending on the initial condition of the surface of the faceplate. The surface of the faceplate is then rinsed one or more times in ethyl alcohol and subsequently in distilled water until all traces of the cleaning reagents are removed from the glass surface. The faceplate is then dried, for example by means of a warm filtered air blast.

In some instances, it may be desirable to take further steps ensuring a clean surface, in which case the last alcohol rinse may be followed by an immersion in a dilute (24%) solution of hydrofluoric acid, the immersion being only sufiiciently long to form a barely perceptible etch on the glass surface.

The glass plates so processed is now ready for the treatment to deposit thereon the stripes of the various fluorescent materials. For this purpose, a stock solution is initially prepared consisting of 25 grams of polyvinyl alcohol in 600 cc. of distilled water. Various polyvinyl alcohols of different average viscosity values, and hence of different degree of polymerization, may be used. However, I have found that satisfactory results are obtained with a polyvinyl alcohol of medium viscosity such as prepared by Du Pont under the trade name Elvanol 52-22.

The polyvinyl alcohol is added to the distilled water and, with continuous stirring at for example 120 F., is completely dissolved in about 23 hours. After filtering the solution, 200 cc. of ethyl alcohol and 25 cc. of a solution of ammonium or potassium dichromate containing 22 grams of the salt per 100 cc. of water are added. This solution is photosensitive and should be kept under subdued light.

The stock solution so formed serves as a base solution for each of three coating suspensions containing the respective fluorescent materials to be deposited. Each of the coating suspensions may consist of:

22 /2 cc. of the above-described stock solution.

32 cc. of ethyl alcohol.

22 grams of the appropriate phosphor material having a particle size passing through a 200 mesh per inch stainless steel screen. Phosphor materials are, of course, in-- soluble in the photosensitive material.

For forming stripes of blue fluorescent material on the surface of the faceplate 20, a coating suspension as above defined and containing blue fluorescent material is applied to the cleaned surface of the faceplate under conditions of subdued or red light. I have found that a coating of the required uniformity is more readily produced if the coating suspension is sprayed onto the faceplate surface as a plurality of thin layers, the spraying of each layer being effected with a sufficiently large amount of air so that the coating material reaches the surface of the faceplate in a semi-dry or tacky condition. Under these conditions the formation of liquid pools of the coating material leading to non-uniformities of the end product, is avoided. Successive thin layers are applied until a coating of such thickness that it contains the desired amount of phosphor material per unit area thereof is obtained. In practice the thickness of the coating is made of the order of .025 to .050 inch and contains approximately .5 to 6 mg. of phosphor material per cm. Such a coating is substantially opaque.

After a coating of the desired thickness is formed, it is dried thoroughly in the dark at a relatively cool temperature i.e., by means of an air blast at a temperature of somewhat less than 70 F. Drying for to minutes under these conditions has been found to be sufficient.

The continuous coating so formed is then photochemically treated to form spaced stripe portions of the fluorescent material on the surface of the faceplate 20. For this purpose the envelope 10 is mounted in a jig assembly and the deposited coating is exposed to a suitable light source through an optical system or mask producing a light image the configuration of which conforms to the desired pattern of the blue phosphor material. More particularly, and as shown in FIG. 3, the envelope 10. the faceplate of which has been coated as above described, is positioned in a jig assembly 50-52 which supports the envelope in fixed relationship to a light source 54. Interposed between the envelope 10 and the light source 54 and preferably arranged against the outer surface of the faceplate 20 is a mask 56 formed with transparent or translucent stripes the number per inch of which is equal to the number per inch of the blue phosphor stripes to be formed on the cathode-ray tube face and the Width of which is equal to and preferably less than the desired width of the phosphor stripes to be formed. The mask 56 may be formed in any well-known manner. For example, the mask may consist of a glass plate having an opaque coating which is ruled in a manner well known to the art of making diffraction gratings to provide the desired accurately spaced transparent stripes. Alternatively, the mask may consist of a photographic plate carrying the image of a ruled tracing, the latter being made preferably to a larger scale so that any errors in the tracing are reduced in the photocopying thereof. The light source 54 provides actinic radiation to harden locally the coating on the faceplate; it should approximate a point source, and for this purpose an electric arc lamp with closely spaced electrodes and without the usual refiector or condensing lens has been found satisfactory.

The coated faceplate so positioned is exposed to the light source 54 for a period determined by the photosensitivity of the coating, the desired density of the stripes to be formed, the intensity of the light source and the distance between the faceplate and the light source. When using an arc lamp with a A" are operating at 510 amperes and spaced approximately 3' from the faceplate, an exposure between 30 seconds and 10 minutes has been found to be sufficient. This wide range of exposure times has been found to be largely due to variations of the photosensitivity of different coating suspensions presumably made under the same conditions. Because of this, the required exposure should be determined beforehand by means of suitable test strips of the coating suspension. Furthermore, it has been found that the particles of fluorescent material suspended in the coating tend to produce a certain amount of scattering of the light impinging on the coating and thereby broaden the effective width of the transparent stripes of the mask. Accordingly, the possibility of such light scattering should be taken into account in the design of the mask, i.e., by making the transparent stripes thereof narrower than the desired width of the phosphor stripes to be formed.

The exposure of the coating to light produces localized reactions between the polyvinyl alcohol gel and the alkali dichromate salt contained therein, bringing about a localized hardening of the exposed areas and making these areas relatively insoluble in water. The exact photochemical reaction which makes the exposed portions of the coating relatively insoluble in Water is not fully known to me. However, it appears that the dichromate salt is at least partially decomposed by the impinging light energy and in this condition reacts with or catalyzes the polyvinyl alcohol gel either polymerizing the same or otherwise changing its molecular structure.

As a next step in the process of the invention, the unexposed portions of the phosphor-containing coating are dissolved by a water wash leaving the hardened exposed stripe portions to remain on the surface of the faceplate. A particularly effective treatment for this purpose, I have found, is to initially dampen the coating uniformly throughout its area using a fine mist spray of water so limited in quantity as to prevent the formation of liquid pools on the surface of the coating. Thereafter, a dissolving water bath may be applied using relatively large amounts of water to relatively rapidly dissolve the unexposed portions of the coating. I have found, that by initially dampening the coating as above described, streaks and other irregularities in the processed coating are avoided. Furthermore, the initial dampening of the coating, which permits the subsequent use of a large volume dissolving bath without harming the coating, makes it possible to utilize such a dissolving bath to slowly dissolve the relatively hard and less soluble exposed portions of the coating to thereby control the density of the deposited stripes with great precision, thereby permitting a greater latitude in the exposure time.

The striped coating of hardened gel carrying the fluorescent material suspended therein and remaining after the unexposed portions of the coating are dissolved, is thoroughly dried in a warm air blast and thereafter baked at a relatively low temperature, i.e., at about 300 F., for approximately 30 minutes to permanently set the stripes. Alternatively, the hardened phosphorbearing stripes may be permanently set by treatment with a dilute borate solution, such as a 1-5% solution of boric acid, followed by thorough rinsing in distilled water.

By the foregoing process there is formed on the inner surface of the faceplate hardened and set spaced stripes of the polyvinyl alcohol gel containing the blue phosphor material in suspended form. The tube face is now processed to form stripes of a second of the fluorescent materials, i.e., the green fluorescent material, at appropriate positions in the spaces between the blue phosphor stripes. For this purpose a green phosphor photosensitive coating suspension is applied to the surface of the faceplate 20 in the manner above described for forming the blue phosphor stripes. More particularly, the faceplate 20, carrying the spaced blue phosphor stripes, is spray coated under aerated conditions with the green phosphor photosensitive coating suspension, forming on the exposed portions of the faceplate and over the previously formed blue phosphor stripes a multiple layer coating, each of the layers of which is deposited in a semi-dry or tacky condition so that the formation of liquid pools of the coating material is avoided. After suitably drying the so deposited second coating, for example by means of an air blast as previously described, the cathode-ray tube envelope is repositioned within the jig assembly shown in FIG. 3 with the blue stripes thereof arranged vertically and the so formed second coating is exposed to light from the source 54 through the mask 56. The same mask previously used may be adapted for the exposure leading to the green phosphor stripes, by suitably horizontally adjusting the position thereof in the direction of the double-headed arrow a distance equal to onethird of the distance between similar portions of two adjacent blue stripes, so that the transparent stripes thereof are arranged at the desired positions between the stripes of blue phosphor material previously deposited.

The required exposure of the green-phosphor-containing photosensitive coating is governed by the same factors above described in connection with the exposure leading to formation of the blue phosphor stripes, and in general, for a green phosphor coating suspension as above specifically described, the exposure will be of the order of seconds to 10 minutes.

Following the exposure of the green-phosphor-containing coating, it is subjected to a water bath which preferentially dissolves the unexposed portions thereof contained on the surface of the faceplate and over the blue phosphor stripes. To prevent streaking and other irregularities, the water bath is preferably preceded by a fine mist water spray whereby the coating is uniformly dampened without the formation of liquid pools. Since the unexposed portions of the coating are readily dissolvable in Water, these portions will be rapidly removed from the coating leaving the photochemically hardened exposed portions. The hardened portions are, nevertheless, soluble to some degree in the water bath, the degree of solubility being a function to some extent, of the temperature of the water bath. At room temperature, there will be a slow dissolution of the hardened portions and, in the preferred embodiment of the invention, the water bath is continued beyond the time necessary to remove all of the unexposed portions, to thereby control the density of the second formed green phosphor stripes to the desired value relative to the density of the first formed blue phosphor stripes. To allow for a suitable range of adjustment of the density of the green phosphor stripes, it may be desirable to overexpose the green phosphor photosensitive coating to a slight extent.

The second formed stripes 'may then be set by means of a drying and baking treatment at about 300 F. for about 30 minutes, or by treating with a borate solution such as a 1-5 solution of boric acid. In this latter instance, the faceplate is subsequently thoroughly rinsed in distilled water to remove all traces of borate solution.

Red phosphor stripes are deposited on the faceplate 20 in similar fashion to the deposition of the blue and green phosphor stripes. More particularly, a photosensitive coating solution having the composition above specifically described, and containing red phosphor particles in suspension, is applied in uniformly thin layers to the desired thickness over the surface of the faceplate by means of a fine spray using abundant air so that each deposited layer of the coating is semi-dry or tacky as it forms on the surface and the formation of liquid pools is avoided. It will be noted that this latter coating will be superimposed in part on the previously deposited stripes and contacts the glass surface of faceplate 20 only over those portions thereof left bare by the blue and green phosphor stripes. The so deposited third coating is thereafter dried by means of an air blast, and the cathode-ray tube envelope is again positioned in the jig assembly 50-52 with the blue and green stripes thereof arranged vertically and with the faceplate in close contact with the mask 56. The mask '56 is then adjusted horizontally so that light falling through the transparent stripes of the mask falls on the deposited coating midway between the spaced adjacent pairs of the blue and green phosphor stripes.

After exposing the third deposited coating to the light from the source 54 for a sufficient interval as set forth above in connection with the exposure of the greenphosphor-containing coating, the coating is uniformly dampened using a fine mist spray of water. Thereafter, the unexposed portions of the coating are preferentially dissolved by means of a water bath as previously described, the water bath 'being continued if desired, to more slowly remove portions of the hardened exposed coating to thereby control the density of the red phosphor stripes to accurately match the densities of the blue and green phosphor stripes. The so formed red-phosphor-containing stripes may then be set by a baking treatment at 300 F. or by a treatment with a borate solution as previously described.

By means of the process so far described, there is formed, on the inner surface of the faceplate 20, hardened polyvinyl alcohol gel stripes of the blue, green and red phosphors arranged in consecutive order across the surface of the faceplate 20. The structure so formed is then treated to decompose and remove the gel and permanently bond the phosphors to the surface of the faceplate. This may be accomplished by baking the faceplate at a temperature of the order of 850 F. or higher to a limit established by the softening temperature of the glass of the faceplate and/or the decomposition temperature of the phosphor material. At a temperature of 850 F. a baking treatment for approximately two hours has been found to decompose and remove the material constituting the gel to a satisfactory degree.

In some instances, it may be desirable to enhance the bond between the phosphor particles and the faceplate and for this purpose a suitable binder may be added to the deposited phosphor stripes, for example, by immersing the coated faceplate in a 1-5% solution by weight of potassium or sodium silicate, prior to the baking treatment.

By means of the foregoing described process there is formed, on the inner surface of the faceplate 20, stripes of the blue, green and red phosphors, which stripes are accurately positioned on the surface of the faceplate, are uniformly spaced throughout the area of the faceplate, are of uniform density and width throughout their length, and are sharply defined. Furthermore, the stripes of a given phosphor are of uniform size and characteristic and have the desired density relative to the stripes of the other phosphors.

In certain circuit applications, it is desirable to provide an indexing signal suitable as a control quantity insuring a synchronous relationship between the moment that the cathode-ray beam impinges on a phosphor stripe of a given primary color and the moment that the video signal applied to the tube has a value indicative of the intensity to which the said phosphor stripe is to be energized by the beam. Such an indexing signal may be produced by means of indexing stripes arranged on the screen of the cathoderay tube and adapted to produce a signal indication in a suitable output circuit of the tube each time the electron beam impinges on one of the indexing stripes as it scans the phosphor stripes. To achieve the desired synchronous relationship, it is necessary that the indexing stripes be positioned on the screen in a geometric relationship exactly indicative of the geometric distribution of the phosphor stripes. The faceplate 20 shown in FIG. 2 is provided with such indexing stripes, which have been indicated by the numeral 30. These indexing stripes are generally arranged in a geometrical configuration indicative of the geometry of the phosphor stripes. In the specific arrangement shown the stripes 30 are each arranged on a consecutive one of the phosphor stripes of a given primary color i.e., on consecutive stripes 26. It should be well understood, however, that the index stripes may be arranged in other configurations, such as in a configuration in which the index stripe covers only part of the underlying phosphor stripe or may even cover two or more of the phosphor stripes. The indexing stripes 30 gen erally consist of a material adapted to produce a response detectably different from the response produced by the remainder of the screen structure upon impingement of the beam. For example the stripes may consist of a material having a secondary-emissive ratio detectably different from that of the materials of the remainder of the screen structure and for this purpose the stripes 30 may consist of gold or of other high atomic number metal such as platinum or tungsten, or may consist of an oxide such as magnesium oxide. These materials are inorganic and are not affected by the final baking above described.

The stripes 30 may be formed precisely in position on a screen structure by means of a modification process of the invention as follows:

Prior to the baking treatment above described and following the formation of the phosphor containing stripes of hardened gel, the inner surface of the faceplate 20 is provided with a clear photosensitive coating of polyvinyl alcohol. Such a coating may be formed by spraying with a solution consisting of 7 parts of the stock solution above described diluted with 3 parts of ethyl alcohol. The coating is preferably formed as the series of thin layers each spray-deposited using abundant air so that the coating material reaches the surface of the phosphor stripe coated faceplate in a semi-dry or tacky condition. After drying the coating by means of a relatively cool air blast, a coating of magnesium oxide is formed thereon by spraying a suspension of approximately 75' grams of finely divided magnesium oxide in 500 cc. of ethyl alcohol. In practice it has been found to be unnecessary to initially mix the magnesium oxide and the photosensitive coating material because the subsequently sprayed oxide suspension becomes imbedded into the underlying coating. Furthermore, it has been found that the magnesium oxide has relatively high adhesive properties and the removal thereof from those portions of the envelope surface which are not to be coated is facilitated by the underlying clear coating when the material is applied separately as above described. When the magnesium oxide is applied as a suspension in the photosensitive coating, I prefer to initially apply an underlying clear coating for the above noted reason. The coating deposited in this modification of the invention may be relatively thin. For example, the clear photosensitive coating may be of the order of .001 to .005" thick and magnesium oxide applied in an amount only sufiicient to uniformly cover the surface of the underlying coating.

The continuous dual coating so formed is then photochemically treated to form the spaced magnesium oxide index stripes. To do so, the envelope 10 is repositioned in the assembly jig 5052 with the mask 56 so arranged that the transparent stripes thereof are in line with the desired phosphor stripes 26 over which the index stripes 30 are to be formed. The dual coating may then be exposed to the light source 54 through the underlying, but nevertheless somewhat translucent, phosphor stripe whereby the dual coating is selectively hardened by the photochemical reaction occurring therein. An exposure of the order of magnitude above described in connection with the formation of the phosphor stripes has been found to be sufiicient.

The coating is then preferentially dissolved by a wash in a water bath, the wash being preferably preceded by uniformly dampening the coating by means of a fine mist water spray as previously described.

The process of the invention has been specifically illustrated in its use with a polyvinyl alcohol as manufactured by E. I. du Pont de Nemours and Co. under the trade name Elvanol 52-22. This is a material of medium viscosity as measured by a 4% aqueous solution thereof and is partially hydrolyzed to the extent of approximately 88%. While a medium viscosity material is preferred, higher or lower viscosity materials may also be used for the purposes of the invention. High viscosity materials, however, tend to polymerize more rapidly whereby a precise control of the amount of phosphor material deposited is less readily obtained, whereas low viscosity materials tend unduly to lengthen the exposure time required to achieve the desired degree of polymerization. The hydrolysis range of the material selected is also subject to a wide latitude; however, it is pointed out that materials hydrolyzed to a small extent, i.e., below about and materials which are substantially completely hydrolyzed, are more water resistant, thereby making these materials more dilficult to dissolve in water baths following the exposures. Another polyvinyl alcohol suitable for the invention is that also manufactured by the Du Pont Co. under the trade name Elvanol 54-22, which material is a medium viscosity material and is hydrolyzed to the extent of about 91%.

The synthetic organic gels, including those above specifically described, are preferred for the process of the invention because of their uniformity and purity. However, natural organic gels free of contaminating impurities may also be used. Thus animal gelatines, animal glues, egg albumen and the various natural gums, such as gum acacia, which are rendered water insoluble when exposed to light in the presence of a dichromate or other photosensitive catalyzing reagent, are also suitable for the purposes of the invention.

Any of the commonly available phosphor materials having the desired color responses may be used, such as the silicates, phosphates and tungstates. Some of the sulphide types of phosphors may be poisoned by the dichromate and accordingly, this possibility should be considered in the use of these latter type materials. The aforementioned phosphors are inorganic and are not affected by the final baking which removes the gel.

Ethyl alcohol has been described as the diluent in the various solutions set forth because of its relative freedom from toxic effects on the personnel during the spraying operations and because it prevents the formation of air inclusions during the spraying of the layer. It is apparent that other diluents which exhibit an anti-foam characteristic either inherently or by means of a suitable anti- -foaming additive and which may be rapidly evaporated during spraying, may be used. For example, methyl alcohol, may also be used if suitable health precautions are taken.

While I have described my invention by means of specific examples and in specific embodiments, I do not Wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What I claim is:

1. In the manufacture of a cathode ray tube having a screen structure comprising a light transparent base and phosphors fixed thereto which are emissive of light in 11 responsive to electron bombardment, a method of apply ing said phosphors comprising the following steps:

spraying a photosensitive suspension on said base to form a first photosensitive layer over said base; spraying a photosensitive suspension on said first photosensitive layer to form a second photosensitive layer over said first layer; each of said suspensions having a solubility inversely proportional to exposure thereof to actinic radiation and each comprising a polyvinyl alcohol, a photosensitive material comprising an alkali-dichro mate salt capable of varying the solubility of said gel upon exposure to actinic radiation, and inorganic phosphor particles which (1) are insoluble in said gel and said emulsion, and (2.) are emissive of light of one color in response to electron bombardment;

each of said suspensions being sprayed with sufficient air to cause eachof them to reach said base in a tacky condition;

exposing to actinic radiation those regions of said layers in which said phosphor particles are to be retained, so as to render said exposed regions relatively insoluble in a solvent capable of dissolving the unexposed regions of said layers;

washing said layers with said solvent to dissolve and remove the unexposed regions thereof; and

baking the resultant screen structure to remove said organic gel without affecting said inorganic phosphor particles. 2. The method of claim 1 comprising, subsequent to said washing step and prior to said baking step, the further steps of spraying a photosensitive suspension on said base to form a third photosensitive layer on said base and the exposed regions of said first and second layers;

spraying a photosensitive suspension on said third layer to form a fourth photosensitive layer over said third layer;

each of the last two named suspensions having a solubility inversely proportional to exposure thereof to actinic radiation and comprising a polyvinyl alcohol, a photosensitive material comprising an alkali-dichromate salt capable of varying the solubility of said gel upon exposure to actinic radiation, and inorganic phosphor particles which (1) are insoluble in said gel and said emulsion, and (2) are emissive of light of a second color in response to electron bombardment;

each of said last two named suspensions being sprayed with sufiicient air to cause each of them to reach said base in a tacky condition;

exposing to actinic radiation those regions of said third and fourth photosensitive layers in which said phosphor particles emissive of light of said second color are to be retained, so as to render said exposed regions of said third and fourth layers relatively insoluble in a solvent capable of dissolving the unexposed regions of said third and fourth layers, said exposed regions of said third and fourth layers being offset from said exposed regions of said first and second layers; and

washing said third and fourth layers with said solvent to dissolve and remove the unexposed regions thereto, 3. The method of claim 2 comprising, subsequent to said washing of said third and fourth layers and prior to said baking step, the further steps of:

spraying a Suspension on said base to form a fifth photosensitive layer on said :base and the exposed regions of said first, second, third, and fourth layers;

spraying a photosensitive suspension on said fifth layer to form a sixth photosensitive layer over said fifth layer;

each of the latter two photosensitive suspensions having a solubility inversely proportional to exposure thereof 12 to actinic radiation and comprising a polyvinyl alcohol, a photosensitive material comprising an alkalidichromate salt capable of varying the solubility of said gel upon exposure to actinic radiation, and inorganic phosphor particles Which 1) are insoluble in said gel and said emulsion, and (2) are emissive of light of a third color in response to electron bombardment; each of said latter suspensions being sprayed with sufficient air to cause each of them to reach said base in a tacky condition; exposing to actinic radiation those regions of said fifth and sixth photosensitive layers in which said phosphor particles emissive of light of said third color are to be retained, so as to render said exposed regions of said fifth and sixth layers relatively insoluble in a solvent capable of dissolving the unexposed regions of said fifth and sixth layers, said exposed regions of said fifth and sixth layers being offset from said exposed regions of said first and second layers and said third and fourth layers; and washing said fifth and six layers with said solvent to dissolve and remove the unexposed regions thereof. 4. The method of claim 1 wherein each of said photosensitive suspensions is Water soluble and wherein said solvent employed for each layer is water.

5. The method of claim 1 wherein said alkali-dichromate is ammonium dichromate.

6. The method of claim 1 wherein said alkali-dichromate salt is potassium dichromate.

7. In the manufacture of a cathode ray tube having a screen structure comprising a light transparent base and phosphors fixed thereto which are emissive of light of different colors in response to electron bombardment, a method of applying said phosphors comprising the following steps:

depositing a first plurality of photosensitive layers on said base, the first layer in said first plurality being deposited by spraying over said base a first photosensitive suspension, each subsequent layer in said first plurality being deposited by spraying said first photosensitive suspension over the next previous layer; said first photosensitive suspension having a solubility inversely proportional to exposure thereof to actinic radiation, and comprising a polyvinyl alcohol, a photosensitive material comprising an alkali-dichromate salt capable of varying the solubility of said gel, and inorganic phosphor particles which (1) are insoluble in said gel and said emulsion, and (2) are emissive of light of one color in response to electron bombardment; each of said first photosensitive suspensions being sprayed with sufficient air to cause it to reach said base in a tacky condition; exposing to said actinic radiation the regions of said first plurality of layers in which said phosphor particles are to be retained, so as to render said regions relatively insoluble in a solvent capable of dissolving unexposed regions of said layers; washing said first plurality of layers with said solvent to dissolve and remove the unexposed regions thereof; depositing a second plurality of photosensitive layers on said base, the first layer in said second plurality being formed by spraying a second photosensitive suspension over said base and said exposed portions of said layers of said first plurality, each subsequent layer in said second plurality being deposited by spraying said second photosensitive suspension over the next previous layer; said second photosensitive suspension having a solubility inversely proportional to exposure thereof to said actinic radiation, and comprising a polyvinyl alcohol, a photosensitive material comprising an alkalidichromate salt capable of varying the solubility of the gel, and inorganic phosphor particles which (1) are insoluble in said gel and in said material and (2) are emissive of light of a second color in response to electron bombardment;

each of said second photosensitive suspensions being sprayed with sufiicient air to cause it to reach said base in a tacky condition;

exposing to said actinic radiation the regions of said second plurality of layers in which said phosphor particles are to be retained, so as to render said exposed regions of said second plurality of layers relatively insoluble in a solvent capable of dissolving the unexposed regions of said second plurality of layers;

washing said second plurality of layers with said solvent to dissolve and remove the unexposed regions thereof;

depositing a third plurality of photosensitive layers on said base, the first layer in said third plurality being formed by spraying a third photosensitive suspension over said base and the unexposed portions of said first and said second plurality of layers, each subsequent layer in said third plurality being deposited by spraying said third photosensitive suspension over the next previous layer;

said third photosensitive suspension having a solubility inversely proportional to exposure thereof to said actinic radiation and comprising a polyvinyl alcohol, a photosensitive material comprising an alkali-dichromate salt capable of varying the solubility of said gel, and inorganic phosphor particles which (1) are insoluble in said gel and said material, and (2) are emissive of light of a third color in response to electron bombardment;

each of said third photosensitive suspensions being sprayed with suflicient air to cause it to reach said base in a tacky condition;

exposing to light the regions of said third plurality of chromate salt is ammonium dichromate.

9. The method of claim 7 wherein said alkali-dichromate salt is potassium dichromate.

References Cited UNITED STATES PATENTS 2,216,735 10/1940 Carothers 96-68 2,367,420 1/ 1945 Mullen 96-68 XR 2,417,713 3/1947 Stein 96-36 2,653,871 9/1953 Marsh 96-93 XR 2,675,315 4/1954 Staehle et al. 96-93 XR 3,406,068 10/1968 Law 96-3651 2,540,635 2/1951 Steier 95-55 2,472,128 6/1949 Staehle 95-55 OTHER REFERENCES Wall: History of Three-Color Photography, Ameri- 30 can Photographic Publishing Co., 1925, pp. 456-457,

NORMAN G. TORCHIN, Primary Examiner C. L. BOWERS, 111., Assistant Examiner US. Cl. X.R.