US3535575A - Storage target having storage dielectric of significantly greater thickness than spacing between elements of grid disposed upon it - Google Patents

Description

Oct. 20, 1970 HERMAN 3,535,575

STORAGE TARGET HAVING STORAGE DIELECTRIC 0E SIGNIFIOANTLY GREATER THICKNESS THAN sEAcING BETWEEN ELEMENTs OF GRID DISPOSED UPON IT Filed D60. 6, 1967 Fig. 2.

Elvin E. Hermon,

\ INVENTOR.

Emma

ATTORNEY.

US. Cl. 3l368 3 Claims ABSTRACT 0F Til-IE DISCLQSURE A single-gun converter tube having a storage target including a dielectric member of high electrical resistivity and having a conductive grid on the surface thereof which faces a scanning electron gun for writing information on the dielectric member by a charging action which takes place transversely to the axis of the electron beam.

This invention relates to storage tubes, and particularly to the type utilizing a storage target whereby electrical signs may be stored for some predetermined period and subsequently read-out. More particularly, the present invention relates to storage tubes of the scan conversion type whereby electrical signals representing one pattern or of one frequency domain can be converted into electrical signals representing a different pattern or signals of a different frequency domain.

Scan conversion storage tubes are useful, for example, where it is desired to present a radar-type display at te evision scan frequencies. However, the radar signals are derived by a different and an extensively slow scanning frequency in comparison with television scanning frequencies. For example, the well-known P.P.I. radar scan may have a cycle time of about ten seconds in comparison with television-type scans which may have a cycle time of of a second. It is also extremely useful to be able to store electrical signals representing some predetermined information and to repeatedly read these stored signals to provide a continuous picture.

It will therefore be appreciated that a suitable electronic or electrical conversion system must employ some means for storing electrical signals (e.g., the slower scan signals) while permitting these signals to be read out as often as desired and/or at a different scan rate or in a different coordinate system. Storage tubes capable of performing such a function have been provided heretobefore but have been characterized by some rather undesirable features. In general, many of the storage tubes of the prior art useful for these purposes (usually referred to as scan-converters) have utilized a storage target comprising a perforated conducting screen having a coating of secondary emissive dielectric material on one side thereof. In one type of scan-converter tube, this target is disposed between a pair of opposed electron guns. One gun, called the writing gun, scans the coated side of the target at one scan frequency, for example, to thereby and thereon establish a stored charge pattern by secondary emission, representative of the input signals; the other gun, called the reading gun, scans the uncoated side of the target at a different scan frequency and, in effect, penetrates the storage target and is modulated in accordance with the stored charge pattern. The varying electron current of the reading beam thus obtained may be utilized to provide an electrical output signal corresponding to the charge pattern and hence of the original input signals. Such a tube is shown and described in US. Pat. No. 2,547,638 to B. C. Gardner. A related type of scan-converter tube which achieves the same func- Patented Get. 20, 1970 tion employs only one electron gun and a solid metallic plate having a secondary electron emissive coating thereon with a mesh electrode in contact therewith on the side facing the electron gun. This tube is the so-called barrier grid storage tube and is shown and described in US. Pat. No. 2,538,836 to A. S. Jensen. In this tube the input signals to be stored are applied to the metallic plate during bombardment of the dielectric by the scanning electron beam.

A single-ended scan-converter tube which utilizes a single electron gun for performing both writing and reading function is disclosed in Re. Pat. No. 24,776 to R. C. Hergenrother. In this tube the target comprises a g ass substrate covered with a layer of calcium tungstate with a mesh electrode adjacent thereto but spaced therefrom.

In US. Pat. No. 2,728,020, L. Pensak discloses a double-ended scan-converter tube which, like the aforementioned Gardner patent, utilizes two cathode ray guns with the storage target disposed therebetween, with one gun being used for writing and the other for reading. In one embodiment Pensak discloses a target comprising a thin continuous dielectric layer of mica having a fine conductive grid or screen on one side of the mica sheet. In another embodiment Pensak discloses a thin continuous dielectric layer of glass disposed on an electrically conductive support mesh. According to Pensaks patent the tube, in either embodiment, is operated by scanning the dielectric side of his target with an electron beam to write or store information thereon. Reading is accomplished by scanning the mesh side of the target with a second electron beam. While not expressly described by Pensak in his patent the charge-storing mechanism is now known to be due to a capacitative effect in the thickness dimension of the dielectric between the surface thereof struck by the writing beam and the mesh conductor or wire on the opposite side of the dielectric. Reading is accomplished by means of the influence of the electric fields resulting from this capacitycharging effect on the reading beam scanning the mesh side of the target, the electric fields being able to exert such influence through the interstices in the mesh. There is no teaching or suggestion in Pensaks patent that both reading and writing could be accomplished by a single electron gun scanning the proper side of the target. One of the disadvantages of Pensaks double-ended scanconverter tube as disclosed is the inability to erase stored charges from the reading gun side or with the reading beam. This is because the discrete charge-storing mechanisms in Pensaks target are opposite the mesh wires which prevents the reading beam from being able to act on these charge-storing mechanisms. In other words, the reading beam cannot see or reach the dielectric underlying the mesh structure. Hence, the reading beam is unable to effect, as for erasure, the stored charges. Thus, Pensaks patent discloses a scan-converter tube taught as only being useful with two electron guns having a storage target disposed between the guns.

In the co-pending application of NY J. Koda and L. S.- Yaggy, S.N. 567,348, filed July 15, 1966, and assigned to the instant assignee, scan-converter tubes of either the double-ended or single-ended type are disclosed employing a metallic mesh support member on one surface of which is disposed a continuous dielectric film. Koda and Yaggy avoid the problem of not being able to erase the target from one side thereof with a single electron gun by utilizing a target having metal diffused through portions of the dielectric from the mesh support so as to define a conductive grid in the dielectric film on the surface opposite the surface thereof in contact with the mesh support. This target structure in effect forces the charging mechanism to occur in the dielectric between the mesh or grid wires and thus insures visibility of the same dielectric areas by both the reading and writing beams which, prior to their invention, was prevented due to the intervening mesh structure as in the tube of the aforementioned patent to Pensak. The target of Koda and Yaggy also prevents the writing beam electrons from creating electrical interference by being picked up in the output collector on the reading side of the target. In other words, the target of Koda and Yaggy serves to provide complete read-write isolation.

With respect to the scan-converter tubes of the prior art utilizing perforated storage target structures, the operation depends upon the transmission of electrons through the interstices of the target to achieve modulation of such transmission by the charges stored on the target surrounding the interstices. Among the shortcomings of such tubes is the fragility of the mesh member. Since the mesh member in these tubes must be at the focus of the electron beam, the mesh must be a very fine pitch. The mesh member must also be relatively transparent so that near-zero potential charges on the dielectric will allow the maximum number of electrons to pass through the interstices in the reading process. Hence, such mesh members must be quite thin and are thus extremely fragile. The mesh member in one currently available tube has a thousand pitch mesh or 25-micron spacing between the mesh wires or strips, each mesh strip being 5 microns Wide and only about 1.5 microns thick.

Another difficulty with such typical target meshes is their susceptibility to vibration. Although of low inertia, these meshes tend to vibrate with rather large excursions when subjected to shock and vibration. In some cases they have been known to rupture due to the stresses incurred. These meshes have also been found susceptible to shock excitation and subsequent mechanical ringing when sudden changes in electrical stresses induced by modes switching are incurred.

Still a third difficulty with target meshes lies in the fabrication process itself. Typically, these meshes are made by selectively plating or vacuum depositing metal onto a substrate which is then peeled off as a mesh. Because of such processing, the mesh apertures are often non-uniform; the mesh itself is stretched and requires extreme care in the handling and processing involved in order to be useful in a cathode ray tube.

With the exception of the storage tube of the aforementioned co-pending application of Koda and Yaggy, the storage targets have one common characteristic: the dielectric is usually a thin coating and when scanned by an electron beam, dielectric charging takes place in the thickness dimension of the dielectric coating, that is, along the axis of the electron beam transmission. This feature gives rise to relatively high capacitance in the target since the dielectric coating is, as stated, usually very thin. To some extent, this intrinsically limits the writing speed in such storage tubes.

It is therefore an object of the present invention to provide an improved scan-converter storage tube.

Another object of the invention is to provide an improved storage target for storage tubes of the scan converter type.

Yet another object of the invention is to provide an improved and rugged storage target for scan-converter storage tubes.

Still another object of the invention is to provide an improved storage target for scan-converter storage tubes whereby target dielectric charging occurs transverse to the axis of the scanning electron beam.

Yet another object of the invention is to provide an improved, ruggedized storage target for scan-converter storage tubes in which dielectric charging occurs transverse to the axis of the scanning electron beam.

These and other objects and advantages of the invention are realized by providing a storage tube having a unique dielectric storage target. The storage target comprises a 4 self-supporting solid dielectric block or substrate of quartz, for example, characterized by having high electrical resistivity. As compared with the dielectric layers and coatings of prior art storage tubes, the dielectric member of the present invention is relatively thick. On one surface of this substrate a metallic grid is formed so that the dielectric surface is exposed between the metal strips forming the grid. The surface of the target on which the grid is formed is disposed in a tube having a single electron gun so that this grid surface faces the scanning electron gun.

The invention will be described in greater detail by reference to the drawings in which:

FIG. 1 is a perspective view, partly in section, of a portion of a storage target according to the invention; and

FIG. 2 is an elevational view, partly in section, of a single ended or single gun scan converter tube according to the invention.

Referring to FIG. 1, a portion of a storage target 2 according to the invention is shown as comprising a dielectric substrate member 4 having thereon and in physical contact therewith an electrically conductive grid 6. The substrate member 4 may be quartz, for example, and of any thickness as determined by the mechanical properties desired. Typically the substrate may be /8 of an inch in thickness. Suitable substrate materials are mica and glass or other high resistivity dielectric materials. Typipically, the volume resistivity of fused silica (quartz) is about 10 ohm-cm. Which makes it eminently satisfactory for the purposes of the present invention. The grid 6 may be of gold, for example, and about 10 A. thick. Other suitable materials for the grid are molybdenum and nickel. While the target of the invention will be described herein as a mesh of intersecting electrically conductive lines or strips, it is to be understood that an array of parallel conductive strips may also be used. Hence, the term grid as used herein and in the appended claims is intended to include an electrode member in the form of a mesh of intersecting conductive lines as well as an array of parallel conductive lines.

There are a number of techniques available for fabricating such a target assembly. Thus, the conventional photoresist process may be employed to form a grid pattern in photoresist polymer material in accordance with optical exposure from a photographic master. After development of the photoresist to form a desired pattern consisting of an exposed grid-like format, metal may be vapor-deposited onto the exposed areas of the substrate member 4 through the openings in the photoresist pattern. The final step would be the removal of the polymer film as by conventional photoresist film solvents. Alternatively, the metal to form the grid may be vapor-deposited initially over the entire surface of the substrate member 4 after which a photoresist pattern consisting of an array of lines of photoresist material is formed so as to permit removal of the metal as desired at exposed areas between the photoresist lines to leave a grid of metal in situ on the substrate memher. The metal may be removed by chemical etching or by ion beaming sputtering. Still another method for laying down and forming the metal strips or grid in a precise fashion is that of nucleation image formation and development as disclosed in a co-pending application, S.N. 582,079 filed Sept. 26, 1966, by A. F. and E. E. Kaspaul, assigned to the instant assignee. In this method, the substrate member 4 is scanned with a focussed electron beam in accordance with the grid pattern to be formed. The surface of the substrate member is thereby sensitized and upon exposure to the vapors of the metal to be used for forming the grid, atoms of the metal are deposited and grow, or nucleate, only to those portions of the substrate surface which have been so sensitized. For a more detailed explanation of this process, reference may be had to the aforementioned co-pending application.

Referring now to FlG. 2, a cathode ray storage tube is shown comprising a tubular envelope 7 of electrically insulating material such as glass, for example, in one end of which is disposed a conventional electron gun 5 which is employed for performing both reading and writing functions on a time-shared basis. Also shown is a collimating lens system 8, '8, and 8" for collimating the beam formed by the electron gun 5. Deflection means or plates 14 and 14' are also provided for the gun 5 in order to orthogonally deflect the electron beam produced by the gun 5. Magnetic deflection means such as electromagnetic coils may also be used in place of the electrostatic deflection plate system shown. All these components and their operation and functions are Well understood in the art of electron tube optics, and further detailed description thereof is not deemed necessary herein. Near the end of the tubular envelope 7 and remote from the end thereof containing the electron gun 5, a target member 2, as shown in FIG. 1 and described previously, is disposed with the grid surface thereof facing the electron gun 5.

Also in the tube envelope 2 is a decelerator electrode 16 positioned between the electron gun 5 and the target member 2. The electrode member 16 is disposed adjacent the storage target member 2 and is spaced about of an inch therefrom, for example. Such an electrode may comprise a conductive screen having as high a transparency to electrons as possible. Typically the electron transparency or transmission value of this electrode may be 40-50%.

Before proceeding with a description of the operation of a tube employing the storage target of the invention, the target charging process will be explained. With reference to FIG. 1, the charging process during the writing mode is transverse, meaning that the electron beam charges the dielectric area between the metal strips as shown in approximate equivalent circuit form in FIG. 1. As the regions between metal strips charge transversely, there is also an incidental and lesser voltage gradient in the depth dimension of the dielectric because the dielectric is quite thick relative to the space between the metal conducting strips of the grid. Thus, at every depth plane some transverse charging occurs but to a progressively lesser extent due to capacitance division in the depth plane.

ERASURE To prepare the target for writing information, any previously inadvertent or stored charges should be erased. This is accomplished simply by raising the mesh or grid well above the first secondary emission crossover point for arriving primary electrons. By capacity coupling, the dielectric surface is also elevated above the first crossover point. Hence, the impinging electrons knock out secondary electrons from the dielectric surface and thereby drive the potential of this surface to the collector potential (equilibrium). The grid 6 on the dielectric member itself serves as the primary collector for the escaping electrons. In effect, the dielectric surface is written uniformly to saturation.

PRIMING In order to write electrical information into the tube of the invention the storage target must be at a uniform negative potential relative to the surface grid 6. The process of setting this negative potential prior to writing is called priming. Using the cathode of the electron gun 5 as a zero voltage reference point, a potential of about 750 volts positive is maintained on the decelerator mesh 16. The grid 6 of the storage target is maintained at a potential several volts (e.g.,) volts) above zero. Initially the dielectric elements between the metal strips forming the surface grid 6 are at approximately the same potential as the grid itself. Under these conditions, primary electrons from the gun 5 will land on the dielectric, driving all exposed elements thereof down to cathode potential since the arriving electrons are at a velocity which is below that velocity at which the secondary emission ratio of the dielectric material is unity (called the first cross-over point). It will be understood that the dielectric material has a secondary emission ratio in response to electron beam bombardment which ratio is greater than unity for beam velocities above a predetermined level and which ratio is less than unity for beam velocities below such predetermined level.

WRITING To write information on the storage target, the storage grid 6 is elevated to a potential which is well above the first cross-over point (e.g., about 350 volts positive) taking with it by capacitance coupling, the dielectric elements by a corresponding amount. The signal to be recorded may be applied by intensity modulating the writing electron beam. Alternatively, the beam may be width-modulated or deflection-speed-modulated in accordance with the electrical signals to be stored. Thus, according to the signal level, and by modulating the electron beam, secondary electron emission will occur from the exposed dielectric surface in accordance with electron beam scansion thereof, resulting in positive charging of the dielectric elements toward the potential of the grid wires or strips surrounding each exposed dielectric element.

READING Upon completion of the writing process and preparatory to reading out stored signals, the potential of the storage grid 6 is shifted downward to a potential (e.g., +4 volts) which is below the first cross-over point of the secondary emission curve. This results, by capacity coupling, in shifting all dielectric elements downward by a corresponding amount. Specifically, the grid is shifted downward in potential to a value such that all dielectric elements arrive ultimately at a potential slightly below that of the cathode potential, ranging downward from this potential depending inversely upon the amount of charge recorded during the writing period. At the same time the surface grid is at a potential somewhat above zero (cathode potential; e.g., about +4 volts).

To read the stored information or signals, the target is scanned with the electron beam from the gun 5. As the electron beam encounters the various transverse negative charges of the dielectric elements lying between metal strips of the surface grid 6, these charges serve to control by electron mirror action the relative number of electrons allowed to land on the metal strips versus those repelled back to the decelerator electrode 16 where they are collected. Reading is thus accomplished by selective control of the number of electrons repelled back to the decelerator electrode 16 versus the number allowed to land on the metal strips of the surface grid 6. Output signals can therefore be derived from either the target grid itself or from the decelerator mesh 16 as illustrated in FIG. 2. Signal output from the decelerator electrode would however have a DC factor due to the interception of electrons directly as they pass from the cathode to the target.

Among the advantages of a scan-converter tube employing the novel target structure of the invention is an intrinsic high writing speed since the charging mechanism is transverse to the beam axis where the capacitance is relatively small compared with that of the storage targets of the prior art where the capacitances utilized are in the depth dimension and the dielectric is relatively thin. Another advantage of the storage target structure of the invention is the relative ruggedness thereof since there is no suspended metal mesh. An additional advantage is that the target structure permits the use of fabrication techniques such as nucleation for depositing the conductive grid strips to obtain fairly uniform line widths. Such uniform line widths reduces the spatial noise output of the tube generated by the random variations in the width of the metal strips employed in the conventional target structures of the prior art. A further advantage obtained through the novel storage target structure of the invention is the circumvention of fabrication difficulties associated with conventional mesh targets which require the mesh to be peeled from a surface and subsequently attached to a mounting ring with the attendant problems of nonuniform stretching and damage due to handling.

In summary, therefore, a tube utilizing the target structure of the invention exhibits a high dynamic range by virtue of the target mesh uniformity its low spatial noise output, mechanical ruggedness, high writing speed, and low susceptibility to target burning from the electron beam since the dielectric block serves as a good heat conductor of energy imparted by the landing electron beam in the writing and erasing processes. Targets of the invention also exhibit a minimum of blemishes since the metal grid is fabricated, in effect, as part of the target assembly itself.

Finally, for a given spacing between the strips forming the surface grid, the charge range on the dielectric required to modulate the output over a given percentage of its dynamic range is considerably greater than that of a commensurately spaced dielectric-coated mesh. It is believed that by virtue of this lower target m a tube employing the target of the invention will be less subject to shading due to imperfect reading beam collimation.

In comparison with prior art scan-converter tubes, the tube of the present invention is the only scan-converter tube which utilizes a single electron gun for both writing and reading in conjunction with a target consisting of a rugged self-supporting dielectric member having a surface grid thereon facing the electron gun. Prior art scan-converters employed either 2 electron guns (a la the afore mentioned Gardner patent) with a thin mesh support structure or a single gun with either a barrier grid target (a la the Jensen patent) or a dielectric target with a spaced mesh electrode (a la the Hergenrother patent). The target of the Pensak patent, while somewhat similar to the instant target, is taught as only being useful in a two-gun scanconverter and is incapable of being erased by the reading gun or from the reading gun side because of the capacitive charging effect in the thickness dimension of his target dielectric and the shadowing of the charging mechanisms on the reading beam side of the dielectric by his mesh structure.

What is claimed is:

1. An electronic tube for storing electrical input signals and providing electrical output signals corresponding thereto comprising:

a self-supporting member of dielectric material of high electrical resistivity and of predetermined thickness; and

a grid electrode member formed of electrically conductive elements having a pitch at least on the order of one thousand mesh disposed on one side of said dielectric member with the spacing between said conductive elements being small with respect to said predetermined thickness;

an electron beam producing means for storing information on said storage target and for reading said information stored therein, said electron beam pro ducing means facing and being adapted to scan the side of said storage target on which said grid electrode member is disposed; and

a decelerator electrode member disposed between said beam producing means and said storage target.

2. The invention according to claim 1 wherein said dielectric material is quartz.

3. The invention according to claim 1 wherein said self-supporting member is of quartz about /a of an inch thick, and said grid electrode member is a metal selected from the group consisting of gold, molybdenum and nickel and having a thickness of about 10 A.

References Cited UNITED STATES PATENTS 2,23 8,381 4/1941 Batchelor 3 l366 2,324,505 7/1943 Iarns et a1 3l366 X 2,706,264 4/1955 Anderson 313-68 X 3,259,791 7/1966 Jensen et al. 313-68 X 2,922,906 1/1960 Day et al. 3,243,642 3/1966 Gebel 31368 X FOREIGN PATENTS 878,958 6/1953 Germany.

ROBERT SEGAL, Primary Examiner US. Cl. X.R. 3l512

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