US2858463A

US2858463A - Storage screen for direct-viewing storage tube

Info

Publication number: US2858463A
Application number: US519384A
Authority: US
Inventors: Nobuo J Koda; Sidney T Smith
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1955-07-01
Filing date: 1955-07-01
Publication date: 1958-10-28
Anticipated expiration: 1975-10-28
Also published as: BE548321A; DE1035280B; CH338532A; GB813854A; FR1154131A

Description

Oct. 28, 1958 N. J. KODA 11m. 2,358,463

' STORAGE SCREEN FOR DIRECT-VIEWING STORAGE TUBE Filed July 1, 1955 2 Sheets-Sheet 1 3 LLA in Oct. 28, 1958 'N. J. KODA EI'AL 2,858,

STORAGE SCREEN FOR DIRECT-VIEWING STORAGE TUBE 7 Filed July 1; 1955 2 Sheets-Sheet 2 hwy/w): f/m/zy 7144/74, .ZZZJQ 4 0500 42' 4 001,

Unitfid tates Pa nt STORAGE SCREEN FOR DIRECT-VIEWING STORAGE TUBE Nobuo J. Koda, Culver City, and Sidney T. Smith, Pacific Palisades, Calif., assigno'rs to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application July 1, 1955, Serial No. 519,384

3 Claims. (Cl. 313-68) viewing half-tone storage device disclosed in a copending application, now Patent No. 2,790,929 entitled Direct-Viewing Half-Tone Storage Device, issued to E. E. Herman and G. F. Smith on April 30, 1957, and assigned to the same 'assignee as the present invention. In the Smith et al. patent, a direct-viewing half-tone storage device is disclosed which incorporates a storage screen comprising an electroformed nickel screen having patches of dielectric material of uniform thickness distributed uniformly over approximately 50% of the exposed surface on one side of the meshes thereof. In the operation of a tube of this type, a charge pattern is produced on the storage screen which, in turn, controls the flow of flood electrons to a viewing screen to produce a visual presentation of the charge pattern.

It is highly desirable that individual interstices or holes in the storage screen have uniform control over the flow 0f flood electrons over the entire storage screen. The advantage of this is that a storage element of information on the storage screen can be of the same order of magnitude as an individual hole. On the other hand, if individual holes in the storage screen have random c ntrol with an average near the desired value, a storage element of information on the storage screen must contain approximately at least 10 individual holes or control elements to provide a true representation of the desired average control. Also, in conjunction with the above, it is desirable that the individual interstices of storage screen have a uniform extended cut-off characteristic. As disclosed'in the Herman et a1. patent, an.

extended cut-oif characteristic was achieved by covering only a portion of the meshes of the screen with dielectric material. This extended cut-ofi characteristic, however, was not uniform for each interstice of the storage screen as is the storage screen of the presentcase.

In accordance with the present invention, a process is disclosed for fabricating a storage screen wherein the dielectric material is disposed symmetrically about each interstice which may be circular. or square. When this condition exists, each interstice of the screen exerts the same control over the flow of flood electrons. In the preferred embodiment of this screen, a thin layer of dielectric material is disposed ,on the central portion of each mesh of an electroformed screen. An advantage of having the layer of dielectric material disposed on the meshes of the conductive screen in this manner is that the erasure speed is increased, i. e., the storage surface can be charged in a negative direction more efliciently. To erase stored images or to provide a negative background for good contrast the dielectric material is charged storage screens.

negatively with low velocity electrons having a secondary emission ratio less than unity. The electric field from the viewing screen penetrates the interstices of the storage screen and influences the electric field immediately adjacent the layer of dielectric storage material. This is particularly true for highly transparent, coarse During the erasure process this field from the 'viewing screen causes some of the-low-velocity flood electrons to be deflected away from the storage surface, to fall into the interstices, and to ultimately strike the viewing screen. This action results in a loss of erasure electrons and contributes to the objectional bright flash on the viewing screen during erasure. In the case of the present invention the layer of dielectric material, due to its central location, is shielded by a maximum amount from the influence of the viewing screen field. This would eliminate the need for lowering the potential applied to the viewing screen during erasing intervals.

The process for fabricating a storage screen of this type comprises first evaporating a dielectric material over one side of an electroformed screen without depositing any dielectric material within the interstices thereof. This may be accomplished, for example, by the use of a material known as photo resist in a manner hereinafter explained. After the dielectric material has been evaporated on the screen, the meshes of the screen are then increased to the desired width by plating additional metal on the exposed conductive portions of the surfaces thereof.

It is therefore an object of the invention to provide a process for fabricating an improved storage screen adaptable for use in a direct-viewing storage device.

Another object of the invention is to provide a storage screen capable of being more efliciently erased and having uniform control characteristics in each interstice of its entire area.

Still another object of the present invention is to provide a process for fabricating a storage screen having a thin layer of dielectric material disposed symmetrically over only a portion of the exposed surface on one side of the meshes of an electroformed screen.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, Will be better understood from the following description considered in connection with the accompanying drawings in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Figs. 1 and 2 are plan and cross-sectional views, respectively, of an enlarged portion of the storage screen of the present invention;

Figs. 3 and 4 are plan and cross-sectional views, respectively, of an enlarged portion of an electroformed screen; and Figs. 5 to 10 illustrate steps in the process of fabricating the storage screen shown in Figs. 1 and 2.

Referring now to the drawings, Figs. 1 and 2 show plan and cross-sectional views, respectively, of an embodiment of the storage screen of the present invention. More particularly, the storage screen comprises a metallic screen 10 having of the order of 250 meshes per inch, a thickness of 0.001 inch, and a transparency of approximately 40%. Along the central portion of the meshes of screen 10 is disposed a layer 12 of dielectric material such as, for example, magnesium fluoride. Thus, layer 12 has the configuration of a dielectric screen that is disposed in register with the metallic screen 10. The width of the meshes forming the dielectric screen, however, is approximately only one-half the width of the meshes of screen 10. The thickness of the layer 12 of Patented -oer. 28,1958

dielectric, material is not critical and may, for example, be from 1 to micronsthick.

In accordance with the present invention, the storage screen described above is fabricated by commencing with an electroform'ed nickelscreen14 ('Fig. 2) having the requisite number of meshes per inch and a thickness of the order of 0.0006 inch; The width, of the meshes of screen 14, however, is made to be substantially equal to that of the dielectric screen, formed, by the layer12. A plan and a cross-sectional view of an enlargedv portion of the nickel screen14= are shown in Figs. 3 and 4, respectively. The nickel screen 14 is stretched taut in a jig 16 as shown in Fig. 6.

The first principal step in fabricating the storage screen of the present invention, is to dispose the layer" 12 of dielectric material on one side of the meshes of the nickel screen 14 without depositing any dielectric material on the inner surface of the interstices thereof. Conventional evaporation techniques have been found to be unsatisfactory as some molecules of the dielectric material are deposited within the interstices of the nickel screen 14 which result in non-uniformities in the finished storage screen. This step, however, may be accomplished by first coating the entire surface of the nickel screen 14, as shown in Fig. 5, with a layer 18 of photo resist in the absence of light. Photo resist may comprise, for example, a photosensitive lacquer or gelatin that is characterized by the fact that it is hardened by exposure to light. Prior to this, it maybe dissolved in a liquid such as water or alcohol, depending on the type of photo resist employed. Also, the photo resist that is hardened by exposure to light may be removed by raising it to an appropriate temperature determined by its composition or by dissolving it in an appropriate chemical solution.

The portions of the layer 18 of photo resist on one side of the nickel screen 14 and on the inner surface of the interstices thereof are exposed to light thereby hardening these portions. This exposure may be made by projecting light towards one side of the coated screen 14 at a slight angle with the normal thereto in the manner shown in Fig. 6. To accomplish this, several sources of light may be employed or alternatively, the jig 16 in which the coated screen 14 is mounted may be rotated while being exposed so that light will be projected over the entire inner periphery of each interstice of the coated screen 14. The portions of the layer 18 of photo resist exposed to the light will be hardened. These portions are indicated by theshaded areas 20 of Fig. 7. The unexposed portions of layer 18 are indicated by the dotted areas 22 in the same figure.

After exposure to the light, the layer 18 of photo resist is then developed, i. e., the coated screen 14 is immersed in a liquid capable of dissolving the unhardened portions. Thus, the dotted areas 22 of layer 18 are washed away leaving only the hardened portions 20 as shown in Fig. 8. After this step of the process, only one side of the meshes of the nickel screen 14 remains uncoated. The thin layer 12 of dielectric material such as, for example, magnesium fluoride, may now be evaporated on the uncoated side of the meshes of the nickel screen 14. When magnesium fluoride is used, it is desirable to heat the screen 14 to a temperature of the order of 150 to 300 C. in order to get a good bond between the magnesium fluoride and the nickel. In that the photo resist layer 18 has a finite thickness, some dielectric material will be deposited on the exposed areas of the remaining hardened portions 20 as shown in Fig. 9;

After evaporation of the layer 12 of dielectric material, the nickel screen 14 is placed in an appropriate medium and heated to a temperature necessary to burn the hardened portion 20 of the photo resist layer 18 off of the screen 14 or in an appropriate chemical solution to dissolve the hardened photo resist away. The actual composition of the medium or chemical solution and the necessary temperature will depend on the type of photo resist employed in the process. As mentioned above, a portion of the layer 12 of dielectric material will have been disposed on top of the hardened portion 20 of photo resist layer 18 which has now been burned away. This portion of the layer 12 will thus be left unsupported. Thence, in View of the fact that the layer 12 of dielectric material is only from 1 to 10 microns thick, the unsupported portions thereof may be broken away by agitating the screen 14 in Water or other liquid, or by blowing air over and through the screen 14. After this step has been completed, the nickel screen 14 has the layer 12 of dielectric material disposed on only one side of the meshes thereof as shown in Fig. 10.

A coating 24 of metal (see- Figs. 1 and 2) is now electroplated on the nickel screen 14 to increase the area of conductive surfacesurrounding the dielectric. The extent or amount of this plating generally represents a compromise between the percentage of conductive surface desired relative to. the area of the layer 12 of dielectric material and the ultimate transparency of the storage screen. In the event that it is desired to plate copper on the exposed portions of nickel screen 14, the nickelv screen 14 supported by the jig-16 is immersed in a saturated solution of electrolyte such as, for example, copper sulfate with the layer 12 facing and parallel to an anode. A small direct-current voltage of the order of 1 volt is developed between the anode and the nickel screen 14. For a screen of the order of 5 inches in diameter, one

' volt will cause a current of-the order of 3 amperes to flow. During the plating operation, the nickel screen 14 is periodically rinsed with water and rotated in the plane parallel to the anode. The electrolyte may also be agitated or stirred to improve the uniformity of the plating. The plating operation is continued until the desired increase in the width of the meshes of the nickel screen 14. has been achieved.

What is claimed is:

1. In a direct-viewing storage device, a storage screen comprising a conductive screen, and a continuous layer of dielectric material disposed about and spaced substantially uniformly from the periphery of each interstice of said screen on one side of the meshes thereof.

2. In a direct-viewing storage device, a storage screen comprising an elcctroformed metallic screen, and a continuous layer of dielectric material having secondary electron emission characteristics disposed about and substantially uniformly spaced from the periphery of each interstice of said screen on one side of the meshes thereof, the thickness of said layer being small relative to the thickness of said screen.

3. In a direct-viewing storage device, the storage screen as defined in claim 2 wherein the area of said continuous layer of dielectric material is substantially equal to the portion of the area of said meshes remaining exposed on said one side.

References Cited in the file of this patent UNITED STATES PATENTS 2,156,435 Schroter et al. May 2, 1939 2,287,415 Bwinett June 23, 1942- 2,539,422 Larson Jan. 30, 1951 2,682,501 Teal June 29, 1954 2,685,660 Norgaard Aug. 3, 1954 2,754,449 Farnsworth July 10, 1956