US3236686A

US3236686A - Storage electrode

Info

Publication number: US3236686A
Application number: US163739A
Authority: US
Inventors: Donald L Schaefer
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1962-01-02
Filing date: 1962-01-02
Publication date: 1966-02-22
Anticipated expiration: 1983-02-22
Also published as: GB987780A; DE1421871A1

Description

Feb. 22, 1966 D. SCHAEFER 3,236,686

STORAGE ELECTRODE Filed Jan. 2, 1962 FIG.|.

FIG.3. [a E ..../30 K as INVENTOR;

DONALD L. S

HAEFER, B X MF m7 HIS A ORNEY.

United States Patent 3,236,686 STORAGE ELECTRODE Donald L. Schaefer, Carnillus, N.Y., assignor to General Electric Company, a corporation of New York Filed Jan. 2, 1962, Ser. No. 163,739 12 Claims. (Cl. 117-222) The present invention relates to improvement in image orthicons and more particularly to an improved electric charge storage target for producing a point by point electric charge pattern corresponding to a visual image or other information to be converted to electrical signals by scanning the target electrode with an electron beam.

As is well known to those skilled in the art, in the operation of a camera tube of the image orthicon type light falling on the photocathode of the tube produces emission of electrons which are accelerated axially toward and impinge upon a transversely extending charge storage target of thin sheet or membrane-like form. The primary electrons arriving at the charge storage target produce emission of secondary electrons which are collected by a closely spaced confronting foraminate or mesh-like elec tron collector electrode so as to leave a charge pattern on the storage target which is representative of the optical image supplied to the photocathode of the tube. The stored information is read out as modulation of the return current of an electron beam which scans the charge storage target and neutralizes the charge thereon. To provide charge neutralizaiton by the target scanning electron beam sufiicient to avoid after-images or stickiness of the target, the time constant of the charge transfer mechanism must be compatible with the target scanning rate, and hence the resistivity of the target in the through-dimension or neutralization current direction thereof is an important target characteristic, and must be within desired limits, such limits being in the range to 10 ohm centimeters for television scan applications. Another important characteristic of the target, which in large measure determines the sensitivity of such a tube, is the secondary emission ratio of the target responsive to arrival of the primary electrons from the photocathode. Hence a target material of high secondary electron emissivity is desirable for high sensitivity. The signal to noise ratio of the tube is also importantly affected by the spacing of the target and secondary electron collector electrode, a very close spacing of the two, such as 0.5 to 2 mils, being desirable for acceptably high signal to noise ratios. Thus mechanical strength and rigidity of the target, and resistance to vibrational movement such as to change or vary the target-to-collector electrode spacing during tube operation, is also an important consideration in respect to such targets.

One of the problems with prior art image orthicons wherein the charge storage target has consisted of a thin sheet or membrane of glass having acceptable mechanical strength and rigidity, is that target electrodes of this type have been subject to a substantial increase in resistivity after an extended period of use for example of the order of several hundred hours. This increase in resistivity is apparently due to the fact that the electric conduction in such targets is ionic, and causes a gradual formation of a barrier layer by movement of the mobile ions, usually sodium ions, which provide the electric charge conduction from one face of the glass membrance to the other. This phenomenon, which has been called burn in, causes an after-image to be retained on the storage target for a period many times the target frame-scanning rate, with the result that the image of a given scene lingers on in superimposed fashion in later scenes.

In US. Patent No. 2,922,907, assigned to the same assignee as the present invention, there is described a ice charge storage target consisting of a thin layer of finegrained homogeneous polycrystalline magnesium oxide which conducts electronically and hence avoids the difficulties involed with ionic conduction, and which also has a desirably high secondary electron emission ratio. One of the problems in the use of such a target, however, is that of supporting a target of desired thinness in membrane-like fashion and with sutficient tautness and freedom from excessive movement relative to the electron collector electrode when subjected to vibration.

In the copending commonly assigned US. patent application Serial No. 131,185 of John D. MacKenzie and S. P. Mitoft filed August 14, 1961, and now abandoned and superseded by application Serial No. 213,960, filed as a continuation-in-part thereof August 1, 1962, there is described a charge storage target made of a semiconducting borate glass which is electronically conducting and has a resistivity which can be made to fall in the desired range for television scanning rates. That glass target consists of boron oxide (B 0 compounded with a secondary component which must be an alkaline earth metal oxide or a mixture of alkaline earth metal oxides, and a third component consisting of a multivalent metal oxide or a mixture of multivalent metal oxides. Although that improved glass target substantially eliminates the burn in or stickiness difliculty experienced with the ionically conducting glass targets of the prior art, the secondary emission properties and hence sensitivity of such improved glass target are not all that are desired.

Accordingly one object of the present invention is to provide an improved charge storage target having the enhanced rigidity and mechanical strength of glass and yet having a sensitivity substantially in excess of that of glass targets of the prior art.

Another object is to provide an improved target of the character described which is readily manufacturable and whose electrical resistivity properties may be readily precisely controlled.

Another object is to provide a charge storage target of the character described which is particularly suitable for use in image orthicons and the like and which in such applications provides substantially prolonged life free of burn-in in comparison with prior art ionically conducting glass targets.

These and other objects of the present invention will be apparent from the following description and the accompanying drawing wherein:

FIGURE 1 is a view, partially broken away in axial section, of the image section of an image orthicon having a charge storage target constructed in accordance with the present invention.

FIGURE 2 is an enlarged fragmentary sectional View of a preliminary step in the manufacture of a charge sto arget according to the present invention; and

FIGURE 3 is a view similar to FIGURE 2 showing a later step in the fabrication of a target-collector electrode assembly according to the present invention.

Briefly the present invention is based upon my discovery that an improved charge storage target for image orthicons and the like can be provided by a composite construction consisting of a base layer of electronically conducting glass preferably of the type described in abovementioned copending application Serial No. 131,185, and a top layer overlying the base layer and consisting of homogeneous polycrystalline semi-conducting metal oxide having a secondary emission ratio greater than that of the base layer. Surprisingly, I have found that in spite of the apparently sharp discontinuity in material composition between the base layer of glass and the top layer of metal oxide, there is no corresponding discontinuity in electronic conduction characertistics, and

hence storage target of both optimized physical and electrical properties results.

Referring to the drawing, in FIGURE 1 there is shown the image section of an image orthicon tube having an improved charge storage target constructed in accordance with the present invention. Within the envelope 2 of the tube and on the front face or viewing window 4 thereof there is disposed a suitable photocathode 6. Responsive to an optical image falling on the photocathode 6 electrons are emitted and accelerated rearwardly and axially by the accelerating effect of internal electrodes 8, 10 and the collimating magnetic field of an external focusing magnet (not shown). The primary electrons from photocathode 6 fall on the forwardly facing surface of the transversely disposed membrane-like imperforate charge storage target 12. This in turn produces emission of secondary electrons from the target 12 which are collected by the adjacent mesh-like or foraminate electron collector electrode 14. This produces a pattern of charge stored on the forwardly facing surface 16 of the target 12, which by electric conduction through the target produces a corresponding pattern of charge on the rearwardly facing surface 18 of the target 12. The latter charge pattern is then read out by the neutralization action of a target scanning electron beam 20, generating a readout signal in the form of modulation of the current of the return portion 22 of the beam.

The improved target constructed in accordance with my invention is shown in greater detail in FIGURES 2 and 3 and consists of a base layer 30 of electronically conducting borate glass which is secured at its periphery to, and may be exclusively supported by, annular support 32. The base layer 30 extends across support 32 in drumhead fashion with suificient tautness desirably to avoid sags or wrinkles and to preclude excessive vibration or movement relative to mesh electrode 14 under conditions of tube vibration.

Directly superimposed on layer 30 and overlying it in direct contact therewith on the side facing the photocathode is a second layer 36 consisting of homogeneous polycrystalline semiconducting metal oxide having a secondary emission ratio higher than that of the glade layer 30. The layer 36 may consist, for example, of beryllium oxide or aluminum oxide, and a preferred material is magnesium oxide. The glass layer may have a thickness in the range of .05 mil to .03 mil, and the magnesium oxide layer 36 may have a thickness in the range of 100 to 1,000 angstroms.

The glass of layer 30, as described and claimed in copending application Serial No. 131,185, is a borate glass comprising a so-called glass network former of boron oxide and from to 40 mole percent of the alkaline earth metal oxide or oxides on the basis of the B 0 employed, to which basic borate glass is added at least 15 mole percent of a multi-valent metal oxide or oxides such as for example chromium, iron, antimony, vanadium, titanium, nickel, cobalt, magnesium, molybdenum, tungsten, and arsenic. This borate glass has a volume electrical resistivity in the range of 10 to 10 ohm-centimeters desirable for through-conduction of electric charge with the proper time constant for use in image orthicons or other target applications where the target is scanned at television raster scan rates. The secondary constituent of alkaline earth metal oxide or oxides serves as a so-called glass network modifier as distinguished from the so-called glass network former. This glass layer 30 is thus composed of a network former primary constituent in the form of boron oxide B 0 and a network modifier secondary constituent such as calcium oxide, barium oxide, strontium oxide, magnesium oxide, or a mixture of two or more of these oxides, and tertiary constituent which is a multivalent metal oxide or oxides. Where magnesium oxide is employed as the secondary constituent and it is necessary to production of a homogeneous body to overcome the immiscibility of MgO in B 0 an additional component such as aluminum oxide or potassium oxide may be employed as part of the secondary constituent of the composition. This miscibility-promoting additive may be suitably employed in an amount ranging from about 1 mole percent to 5 mole percent of the ultimate glass composition and it may be incorporated in the raw mixture oxides preparatory to the heating step, or alternatively it may be added when the MgO and E 0 or CaO and B 0 are in the liquid state. The third constituent which is a compounding or doping agent should be present to an amount of at least 15 mole percent but not in excess of mole percent, calculated on the basis of the basic borate glass product e.g. CaO-n(B O and therefore the doped semiconducting glass layer 30 consists of not more than mole percent of CaO-n(B O or equivalent new borate glass.

A preferred form of the glass for substrate layer 30 is doped CaO.2B O and may be made as follows: Baker reagent grade CaO, V 0 and Fe O are mixed together with Pacific Coast Borax pure B 0 and the mixture is fused in an open platinum crucible at 1250 C. to form doped CaO.2B O The V O Fe O is used in amounts of 32 mole percent and 10 mole percent respectively, on the CaO.2B O basis. In other Words the actual amounts of these several constituents of the glass product are 55 mole percent CaO.2B O 30 mole percent V 0 and 15 mole percent Fe O This glass product is then allowed to cool to about room temperature and mechanically homogenized by grinding to about a 10 mesh size. The resulting material is then heated to about 1000 C. for about one hour in a platinum crucible, and is thereafter ready for blowing.

In the manufacture of the composite target a bubble of the fresh borate glass of layer 30 is first blown, and a portion of sufficient size is cut out of the solidified bubble. The selected portion of the bubble is heated sutficiently so that it grows soft and sags or may be pressed flat and secured to the annular support 32. A thin layer of the parent metal of the desired homogeneous semiconducting oxide 36, such as magnesium, is then vapordeposited on the glass 30 and heated in an oxidizing atmosphere, for example by baking in air at a temperature of about 400 C. for several hours, sufficiently to convert it to the oxide 36. The glass 30 does not fill the interstices between the individual grains of the crystals of the oxide layer 36, and hence does not interfere with the desired electronic conduction substantially straight through the grain boundaries of the layer 36.

Thus there has been shown and described a composite charge storage target which combines the best mechanical strength features of prior art glass targets, yet has substantially improved sensitivity of the order of twice that obtainable with prior art glass targets.

Surprisingly and in spite of the sharp discontinuity in material composition between the secondarily emissive oxide layer 36 and the base or support glass layer 30, there has been found to be no corresponding electrical boundary or discontinuity in electronic conduction characteristics, and hence the charge pattern formed on the photocathode-facing surface 16 of layer 36 is readily transferred to the readout beam side of layer 30 with the desired time constant for neutralization readout by the scanning electron beam.

Moreover conduction through the entire thickness of the composite target has been determined to be electronic, thus insuring prolonged operating life without the mass movement characteristic of ionic conduction which in-- duces stickiness and burn in.

Thus the composite target of the present invention provides long operating life with excellent sensitivity, and excellent resistance to vibrational movement or microphonics.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than the illus;

trative embodiments heretofore described. Accordingly, it is to be understood that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A charge storage electrode for a picture signal generating tube or the like comprising a membrane including a base layer of a glass consisting essentially of a first constituent of boron oxide B a second constituent of an alkaline earth metal oxide in an amount of from 20 to 40 mole percent on the basis of the B 0 content, and a third constituent of a multivalent metal oxide in an amount of from 15 to 80 mole percent calculated on the basis of the first two constituents, and a very thin coating on said base layer of a homogeneous polycrystalline semiconducting metal oxide having a secondary electron emission ratio higher than that of said base layer.

2. A charge storage electrode as defined in claim 1 wherein said glass contains in substantially uniform distribution from 15 to 80 mole percent of vanadium pentoxide and has the nominal composition CaO.nB O V 0 3. A storage electrode as defined in claim 1 wherein said glass contains in substantially uniform distribution vanadium pentoxide and magnitite and has the nominal composition CaO.nB O about 55 mole percent, V O about 30 mole percent, Fe O about 12 mole percent.

4. A target electrode for a picture signal generating tube or the like comprising a membrane including a base layer of a glass consisting essentially of a first constituent of boron oxide B 0 a second constituent of an alkaline earth metal oxide in an amount of from 20 to 40 mole percent on the basis of the B 0 content, and a third constituent of a multivalent metal oxide in an amount of from 15 to 80 mole percent calculated on the basis of the first two constituents, and a coating on said base layer of polycrystalline semiconducting oxide of at least one metal from the group consisting of magnesium, aluminum and beryllium.

5. An electric charge storage electrode of laminar construction comprising a base layer from .05 to 0.3 mil thick of an electronically conducting glass consisting of boron oxide B 0 and from 20 to 40 mole percent of an alkaline earth metal oxide or oxides, said glass being doped with from 15 to 80 mole percent of a multivalent metal oxide, and a top layer of a homogeneous polycrystalline metal oxide having a thickness of from 100 to several thousand angstroms and having a secondary electron emission ratio higher than that of said glass, said polycrystalline metal oxide including at least one metal taken from the group consisting of magnesium, aluminum and beryllium.

6. A charge storage electrode as defined in claim 5 wherein said top layer is magnesium oxide.

7. A target electrode for an image orthicon tube or the like comprising a thin membrane-like electronically conducting oxide glass plate adapted to be secured at its periphery to an annular support, said plate being electronically conducting and having a resistivity at room temperature of the order of to 10 ohm centimeters, and a coating on one face of said glass plate, said coating comprising polycrystalline semiconducting oxide of at least one metal from the group consisting of magnesium, aluminum and beryllium, said coating having a secondary electron emission ratio higher than that of said glass.

8. A target electrode as defined in claim 7 wherein said glass plate has a thickness of from .05 mil to 0.3 mil and said coating has a thickness of from 100 to several thousand angstroms.

9. A storage electrode comprising an annular support having a dimension comparable to that of a target electrode assembly, a sheet of electronically conducting borate glass having a room temperature volume electrical resistivity of the order of 10 to 10 ohm-centimeters extending across said annular support and supported at its marginal edge thereby, and a layer of homogeneous polycrystalline magnesium oxide overlying one side of said glass sheet to a thickness in the range of 100 to several thousand angstroms.

10. A two-sided target electrode for a camera tube in which a charge pattern corresponding point by point with the image to be converted into an electrical signal is established on one side of said target electrode and in which an electron beam is deflected over the other side of said target electrode to deposit electrons thereon as a function of the charge pattern on said one side, said target electrode comprising an imperforate membrane of a substantially water-insoluble electronically conducting borate glass having a room temperature volume electrical resistivity of the order of 10 ohm-centimeters, and a layer of homogeneous polycrystalline magnesium oxide having a thickness in the order of 100 to several thousand angstroms overlying said one side of said glass membrane in direct contact therewith.

11. The target as defined in claim 10 wherein said borate glass contains at least 15 mole percent of a multivalent metal oxide.

12. A two-sided target electrode for a camera tube in which a charge pattern corresponding point by point with the image to be converted into an electrical signal is established on one side of said target electrode and in which an electron beam is deflected over the other side of said target electrode to deposit electrons thereon as a function of the charge pattern on said one side, said target electrode comprising an imperforate base layer of a glass consisting essentially of a first constituent of boron oxide B 0 a second constituent of an alkali earth metal oxide in an amount of from 20 to 40 mole percent on the basis of the B 0 content, and a third constituent of a multivalent metal oxide in an amount of from 15 to mole percent calculated on the basis of the first two constituents, and a coating on said base layer of polycrystalline semiconducting oxide of a metal from the group consisting of magnesium, aluminum and "beryllium.

References Cited by the Examiner UNITED STATES PATENTS 2,620,287 12/1952 Bramley 117-210 2,922,906 1/1960 Day et al. 3l3-103 3,069,578 12/1962 Hares et a1. 31368 RICHARD D. NEVIUS, Primary Examiner.

Claims

1. A CHARGE STORAGE ELECTRODE FOR A PICTURE SIGNAL GENERATING TUBE OR THE LIKE COMPRISING A MEMBRANE INCLUDING A BASE LAYER OF A GLASS CONSISTING ESSENTIALLY OF A FIRST CONSTITUENT OF BORON OXIDE, B2O3, A SECOND CONSTITUENT OF AN ALKALINE EARTH METAL OXIDE IN AN AMOUNT OF FROM 20 TO 40 MOLE PERCENT ON THE BASIS OF THE B2O3 CONTENT, AND A THIRD CONSTITUENT OF A MULTIVALENT METAL OXIDE IN AN