US3681638A - Storage tube comprising electro-luminescent phosphor and cadmium sulfide field sustained conducting target - Google Patents

Abstract

This invention provides a high resolution direct view storage tube which, in addition to having superior resolution capabilities, also requires less complex circuitry than standard type direct view storage tubes. More particularly, a meshless storage tube is disclosed which makes use of a meshless multilayered thin film structure as a combined image storage and display medium. The principle components of this structure include a thin film electronic storage medium, an opaque antifeedback layer, and an electroluminescent image display layer. In operation, a bias voltage is maintained across the entire structure, and a high resolution image is initially impressed upon the storage medium by means of a scanning electron beam. The scanning beam creates local conductivity modulations within the storage film which correspond to the input image, and these modulations, in turn, alter local field configurations across the electroluminescent layer, thus creating a visible output image. After initial scanning, the conductivity modulations in the storage layer are maintained for an extended period of time (several tens of seconds) provided that an applied electric field continues to be maintained. This phenomenon is hereinafter referred to as ''''field sustained conductivity.'''' Removal or reversal of the applied electrical field restores the storage element to its initial insulating condition, and the screen is thereby erased.

Description

United States Patent Bleha, Jr. et al.

[ 1 Aug. 1, 1972 [54] STORAGE TUBE COMPRISING ELECTRO-LUMINESCENT PHOSPHOR AND CADMIUM SULFIDE FIELD SUSTAINED CONDUCTING TARGET [72] Inventors: William P. Bleha,Jr., Santa Monica; Ronald F. Scholl, Malibu, both of Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: June 18, 1971 21 App]. No.: 154,383

52 us. c1. ..313/68 D,3l3/92R [51] Int. Cl ..H0lj 31/58, l'IOlj 31/10, l-lOlj 29/45 [58] Field of Search ..313/68 D [56] References Cited Primary ExaminerRobert Segal Attorney-W. l-l. MacAllister, Jr. et al.

[57] ABSTRACT This invention provides a high resolution direct view More particularly, a meshless storage tube is disclosed which makes use of a meshless multi-layered thin film structure as a combined image storage and display medium. The principle components of this structure include a thin film electronic storage medium, an opaque antifeedback layer, and an electroluminescent image display layer. In operation, a bias voltage is maintained across the entire structure, and a high resolution image is initially impressed upon the storage medium by means of a scanning electron beam. The scanning beam creates local conductivity modulations within the storage film which correspond to the input image, and these modulations, in turn, alter local field configurations across the electroluminescent layer, thus creating a visible output image. After initial scanning, the conductivity modulations in the storage layer are maintained for an extended period of time (several tens of seconds) provided that an applied electric field continues to be maintained. This phenomenon is hereinafter referred to as field sustained conductivity. Removal or reversal of the applied electrical field restores the storage element to its initial insulating condition, and the screen is thereby erased.

3 Clains, 6 Drawing Figures PATENTED 1 1 I972 3 6 81, 6 3 8 sum 1 0F 5 William P. Bleho,Jr., Ronald F. Scholl,

INVENTORS.

ATTORNEY.

PATENTEDAuc 1 i972 Device Current mA SHEET '4 OF 5 IOO ililii] I llllii iliiili] iiiilii] Induced current Sustained current 7 Sample Erase current I t l l- 1 Device Area 03 cm Gold -Si|icon Monoxide Composite Contact IO 7 2O 3O 40 50 Device Voltage Fig. 5.

PATENTEDmc I I972 3.681.638

sum 5 or 5 Sample thickness I25 Test oreo=.3cm Applied voltage 40V lnduced current Erase current Sustained current 0 IO 20 3O 4O 5O 6O 7O 8O 90 I00 I20 Time,$ec.

Fig.6.

STORAGE TUBE COMPRISING ELECTRO- LUMINESCENT PI-IOSPHOR AND CADMIUM SULFIDE FIELD SUSTAINED CONDUCTING TARGET BACKGROUND OF THE INVENTION The present invention represents an improvement in the apparatus and method described in US. Pat. No. 3,344,300 entitled, Field Sustained Conductivity Devices With CdS Barrier Layer. In this patent, a method of making a field sustained conductivity device is taught, comprising the steps of disposing a layer of cadmium sulfide in contact with an aluminum electrode member, and forming a barrier region in said layer of cadmium sulfide by heating thealuminum electrode member and the layer of cadmium sulfide at a temperature of from 200 to 400 C. at least 2 hours in a sulfur-containing atmosphere. A principal disadvantage of devices made by this method is that they can sustain a potential difference of only a few volts usually less than volts, and operate at relatively low field-sustained current levels (tens to hundreds of micro-amps).

SUMMARY OF THE INVENTION advantages in the ease and safety of fabrication, and

the economic advantage of greatly improved reproducibility of the multiple device components. Performance superiority is demonstrated by a better than order-of-magnitude increase in display highlight brightness, increased storage times, and a marked improvement in the stability of all device characteristics. These improvements resulted primarily from the modification of fabrication processes and the use of a new type of rectifying negative electrode. The secondary results of these changes are that device fabrication is presently less complex, presents less of a safety hazard (H 8 and other sulfur-bearing process gases are no longer required), and the improved reproducibility of each fabrication step has led to significantly higher yields.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic drawing of a storage tube including a cadmium sulfide thin film field sustained conductivity storage target;

FIG. 2 illustrates apparatus used to carry out thedeposition of the cadmium sulfide film step in fabricating the device of FIG. 1;

FIG. 3 illustrates apparatus used to carry out the post deposition thermal processing step in fabricating the device of FIG. 1; a

FIG. 4 illustrates apparatus used to carry out the coevaporation of the opaque insulating layer and the composite electrode in the device of FIG. 1; and

FIGS. 5 and 6 illustrate voltage and time versus current characteristics of a representative device according to FIG. 1.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and to FIG. 1 in particular, a meshless storage tube using an improved field sustained conducting target is comprising an evacuated envelope 10 having a large bulbous portion 12 and a small diameter neck portion 14. An electron gun 16 is disposed in one end of the neck portion 14 and includes a cathode 18, an intensity modulating grid 20, and a beam-forming section 22. Any conventional electron gun may be utilized in the tube of the present invention and further detailed description-thereof is not deemed necessary herein. Disposed between the electron gun l6 and the bulbous portion 12 and around the neck portion 14 of the tube is an electromagnetic deflection yoke 24. It will be understood that the electron beam produced by the electron gun 16 is directed down the neck portion 14 of the tube and through the electromagnetic fields established by the deflection yoke 24 whereby the electron beam may be deflected vertically and horizontally with respect to the axis of the tube 10. While an electromagnetic deflection system has been shown, it is also possible to utilize an electrostatic deflection system in order to make the electron beam follow a predetermined path.

The end of the large bulb portion 12 which is opposite the neck section 14 is provided with an optically transparent faceplate portion 26. Disposed on the inner surface of the faceplate 26 is a target structure 30 comprising an optically transparent, electrically conductive electrode member 31 in the form of a thin film or layer of metal such as gold, for example, which is designated the bottom electrode. Such transparent conductive electrodes are well known and may be formed of other materials than metals. Thus it is possible to utilize a thin film of tin oxide, commercially known as NESA, for this purpose. Disposed on the transparent electrode layer 31 is a layer 32 of phosphor material of the type capable of having its light output modulated or of luminescing when subjected to a dc electric field. A typically suitable phosphor material of this electroluminescent type may be ZnSzCu, Cl, or Mn. A more detailed description of electroluminescent phosphors suitable for use in the tube of the present invention and their preparation is found in an article by P. Goldberg and J. W. Nickerson in the Journal of Applied Physics (1963), Vol. 34, at page 1,601. A thin opaque insulating layer 33 is disposed over the electroluminescent layer 32.

A thin film or layer 34 of field sustained conductive material is disposed over the opaque insulating layer 33. A two-layer electrode film or combined layer 36, which may be designated the top electrode, is superposed on the field sustained conductivity layer 34 and connections to external voltage sources 40 and 42 are made through the wall of the tube 10 so as to permit the establishment of a predetermined electric field across the field sustained conductivity layer 34 as well as across the electroluminescent phosphor layer 32. The top electrode 36 usually includes a composite film 43 of two materials which have diverse conducting properties immediately adjacent the thin film 34 of cadmium sulfide and a metal or other conductive film overlayer 44. Special three-layer or single layer contacts are sometimes used. Voltage sources 40, 42 apply a potential of either polarity across electrodes 31 36.

- these conductivity changes, to integrate successive excitations and to return to the low conductivity state as a result of amomentary reversal or removal of an electric field applied across the semiconductor film 34 by the top and bottom electrodes 31, 36 and voltage sources 40, 42. I e

The fabrication of the meshless image storage and display screen according to the invention may be classified into five basic processing steps: (1) deposition of the bottom electrode 31, electroluminescent layer 32 and opaque layer 33 onto the faceplate 26; (2) thermal processing of the electroluminescent layer 32, (3)

evaporation of the cadmium sulfide thin film 34 onto the opaque layer 33; (4) thermal processing of the cadmium sulfide thin film 34, and (5) deposition of the top electrode 36 on the cadmium sulfide thin film 34 emerging from step 3. Following this outline, the fabrication of a meshless storage tube having an improved field sustained conductivity storage target will now be described. It will be understood that many variations in the process are available and that dimensions and shape are exemplary only.

PREPARATION OF FACEPLATE 26 AND BOTTOM ELECTRODE 31 .The first step in device fabrication is to prepare the support faceplate 26 and deposit thereon the so-called bottom electrode 31. In general, a variety of transparent conducting materials can be deposited to serve as the bottom electrode. It is required, however, that the inner surface of faceplate 26 bewell cleaned by an appropriate technique prior to the electrode deposition. A SnO,Sb electrode on the glass faceplate 26 is a preferred material since it is highly transparent. When a non-commercial vacuum deposited bottom-electrode 31 is freshly prepared in the laboratory, its surface requires no cleaning or other treatment in preparation for the cadmium sulfide deposition. The electroluminescent layer 32 and thin opaque insulating layer 33 are deposited over the bottom electrode 31 by standard evaporation or co-evaporation techniques. The electroluminescent layer 32 is then heat-treated to achieve film activation.

DEPOSITION OF CdS FILM 34 The third step in the device fabrication is the vacuum deposition of the CdS film 34. The deposition may be done in the bell jar of a conventional high vacuum system in the pressure range of l x to 1 X 10" Torr. The pressure, however, does not appear to be critical. A cross-section drawing of the instrumentation is shown in FIG. 2. The vacuum is enclosed by a baseplate 50 and a glass bell jar 52.'The electrode faceplate 26 is held by a stainless steel substrate holder 53 and heated'b'y quartz lamps 54. A removable shutter 55 shields the faceplate 26 until deposition on it is to be commenced. The thickness of the CdS film is directly and continuously monitored on the faceplate by the use of optical interference. This is accomplished with the use of a laser 56 and detector 57 positioned outside the bell jar 52.

The electronic grade CdS powder, in the form of a pressed cylindrical pellet 58, is evaporated from a formed tantalum boat 60 which is resistively heated by current passing through buss bars 62 and current feedthroughs 64. The boat 60 is designed such that as the CdS evaporates, the pressed pellet 58 settles down into the boat. This gives an efficient thermal evapora tion over the long period of time required for the deposition of the CdS films. The evaporation rate is controlled by controlling the current to the tantalum boat. The .current is set so that 2.5;; of CdS as monitored by optical interference is deposited onthe electroded faceplate 26 in 1 hour. Typical thicknesses of CdS films are 5 12.5 p. so that deposition times of 2-5 hours are required. Successful results have been obtained with thicknesses from 2-15 p. and evaporation rates from 0.5 to 10.0 p/hr. To avoid the heating of the various elements in the deposition chamber by radiation from the tantalum boat 60,'a water cooled plate 65 is positioned beneath the boat 60 and extending to the diameter of a cylindrical stainless steel deposition chamber 66 disposed thereabout. The water-cooling is i used to maintain the temperature of the wall of chamber 66 as measured by a thermocouple 67 below 60 C. This low temperature, as compared with the faceplate 26 temperature of 130. C., as measured by a thermocouple 68, is necessary to obtain the desired characteristics in the films. It should be noted, however, that the important fact is that the chamber 66 and baseplate 50 are maintained at a lower temperature than the faceplate 26 and the methods of achieving this can be determined by one skilled in the art. Also, the temperatures given can be changed to vary the conductivity and current-voltage characteristics of the CdS films. A range of substrate temperatures from 100 to 200 C. and chamber wall temperatures from 40 to 90 C. have been used to make CdS films of the given characteristics.

PosT DEPOSITION THERMAL PROCESSING The fourth step in device fabrication is the postdeposition thermal processing of the device as it emerges from step 3. The preferred process under the present invention can be seen by reference to FIG. 3. A controllable furnace 70 is providedwith a quartz tube 72 of suitable diameter. A gas inlet tube 74 introduces gas which is preheated while passing through the core of the furnace. The gas exits through short exit tube 75. The temperature (for monitoring and control) near the center of the tube and also near the center of the hot zone is provided by a thermocouple 76 sheathed in a quartz tube 77. The electroded faceplate 26 with film 34 is placed in the tube near the center of the hot zone. A controllable flow of gas from a gas cylinder 78 is provided by pressure regulator 79 and fiowmeter 80.

It should be recognized that other configurations obvious to those skilled in the art, can be used. In operation the following procedure is followed. First the faceplate 26 is inserted in the tube 72 and the tube is flushed out with the gas from cylinder 78. Typically Argon is used, but other non-sulfur atmospheres have been used successfully, including nitrogen and air.

Then the flow of Argon is typically reduced to CFH (at standard temperature and pressure) and the furnace turned on. Flow rates from 0.l CFI-I to CFH have been used with success. The oven is brought to the desired temperature, typically 500 C., and kept at that temperature for the desired time, typically one minute. Temperatures from 385 to 525 C. and times from 1 to 60 minutes have been successfully used. The particular time and temperature used depends on the thickness of the CdS film 34, the substrate material, and the type of gas used. Also the device characteristics, for a given thickness of CdS film, substrate material, and gas, can be altered by changing the temperature and time. After the desired time has elapsed, the quartz tube 72 is physically removed from the furnace 70 and allowed to cool in 20 minutes to 70 C., at which point the faceplate 26 is removed.

While this rapid cooling produces superior results, devices exhibiting the desired characteristics can also be obtained by leaving the quartz tube 72 in the furnace 70 and turning off .the power to the furnace. Under these circumstances, the faceplate 26 cools down about a factor of 10 more slowly.

DEPOSITION OF TOP ELECTRODE 36 I The final step in device fabrication is to apply the top electrode 36 to the device as it emerges from step 3. In the present device the top electrode'36 is usually made up of two layers rather than a single layer. The first layer 43 applied of the double layered electrode 36, FIG. 1, is a composite film of two materials which have diverse conducting properties (metal/dielectric, metal/semiconductor, semiconductor/dielectric, etc.) and the second layer 44, FIG. 1, is a simple metal film overlayer. Negative contact is made to the device via the metal overlayer 44.

The preferred type of composite film 43 has been a mixed coevaporated layer of gold and silicon monoxide. This film is prepared in a vacuum chamber 82 such as shown in FIG. 4. In practice the Au is evaporated using an electron beam evaporator 83 and the rate of Au evaporation is measured and controlled by a rate monitor 84. Similarly, the SiO is evaporated from a Drumheller source 85 and the rate of SiO evaporation is measured and controlled by rate monitor 86. Although the evaporations take place simultaneously, an optical shield 87 prevents each rate monitor from sensing any of the evaporant from the other source. This shield 87 does, however, allow the evaporant streams to mix in region 88 of the chamber 85. It is in this region 88 that composite film deposition occurs. The faceplate 26 as it emerges from step 3 is placed in a rotating substrate holder 90 shielded by a shutter 92 and the chamber 82 is pumper to approximately 10' Torr. The rates of the individual evaporants are then set to a predetermined level (which controls their relative composition in the deposited film), the shutter 92 is opened, and the film is deposited for a fixed time at the present rates to yield the desired thickness. Typical films are on the order of 2,500 A. thick and contain a few percent Au, but other compositions and thicknesses may also be used. Over this composite film 43 a continuous conducting electrode 44 is then deposited to complete the device. Negative contact is made to the device via the metal overlayer 44 of the top electrode 36. The preferred type of composite film has been a mixed coevaporated layer of gold and silicon monoxide. However, other metals have been successfully substituted for gold, such as aluminum, silver, platinum and tin and other dielectrics have been substituted for silicon monoxide, such as, for example, magnesium oxide. Semiconductor materials such as germanium have also been substituted for the metallic element in the composite film with good results. In addition to the coevaporation technique for obtaining the composite film, three other techniques have also been used with good success. One technique is to precipitate a monolayer of metal particles on the surface 'of the cadmium sulfide thin film 34 and then apply an overlayer of a dielectric such as silicon monoxide or an overlayer of a semiconductor such as cadmium telluride. Another technique is to first evaporate a very thin discontinuous metal film onto the cadmium sulfide surface followed by an overlayer of dielectric. Each of these techniques require a final overlayer of conducting metal.

The above contacting techniques have the common feature that the film surface immediately adjacent to the cadmium sulfide film in all cases consists of islands or patches of a material of one conductivity type (for example, metal) surrounded by regions of a material of a diverse conductivity type (for example, dielectric). It is this common unique feature which, when combined with the cadmium sulfide film as prepared above, gives rise to the enhanced sustained conductivity effects found in the field sustained conductivity device of the present invention.

DEVICE CHARACTERISTICS The storage characteristics of the device can be seen with reference to FIGS. 5 and 6. In FIG. 5 is shown a current-voltage characteristic of the device with the polarity of applied dc voltage as shown in FIG. 1. This is with the bottom electrode 31 biased positively with respect to the top electrode 36. In FIG. 5 the induced current is the current flowing through the device for a fixed voltage when an electron beam is incident on it. The sustained current 102 is the current flowing through the device for a fixed voltage, 5 sec. after the electron beam is removed. The erase current 103 is the current that flows through the device 5 sec. after the fixed voltage has been momentarily reduced to zero or made negative. It has been observed that the voltage can be momentarily reduced to zero for as little as 10 milliseconds and still retain this erase current 103 characteristic. It should be noted that these characteristics show a much higher level voltage operation and also higher sustained current 102 than in contemporary devices. In FIG. 6 is shown the behavior of the current through the device as a function of time. A fixed voltage of 40V is across the device. At time t 0 sec. the induced current 105 caused by an incident electron beam is indicated. After I 0 sec. the electron beam is removed and the sustained current level 106 (with 40V still across the device) is shown. At time t 28 sec. the voltage across the device is changed to zero and no current flows. At time 6 35 sec. the voltage is changed back to 40V and the erase current 107 is shown until 6 sec. It should be noted that the erase current 107 remains a smaller fraction of the sustained 7 current 106 for a longer time than in contemporary devices.

It should also be noted that the characteristics in FIGS. and 6 are typical of a particular processing schedule of the device. ,By varying parameters such as thickness of the CdS film 6, the time and temperature of post deposition thermal processing, and the type of (0.3 cm area; 5 sec. after removal of electron beam Erase current through device: 0.00001 1 mA (0.3

cm area; 5 see. after momentary removal of voltage across device) r In operation, when the electron beam from the electron gun 16 modulated in accordance with information representative signals, scans the target structure 30, the

field sustained conductivity layer 34 becomes conductive in a point-to-point fashion to a degree dependent upon the modulation ofthe electron beam. The electric field is thereby increased across the electroluminescent layer 32 in a similar point-topoint fashion causing this phosphor layer to luminesce and produce a visual presentation corresponding to the information to be displayed. Because the conductivity of the layer 34 is sustained even after bombardment thereof by the scanning electron beam has ceased, the electroluminescent phosphor layer remains excited and continues to luminesce at average brightnesses of l 4 ft. L. Stored displays may be erased by depending upon the decay of conductivity in the field sustained conductivity layer 34, the time of which may range from seconds to minutes. It is also possible to restore the barded state-at any time by momentarily reversing the electric field applied thereacross. The opaque insulatv ing layer 33 operates to avoid any optical feedback between the field sustained conducitivity layer 34 and i the'electroluminescent layer 32. The opaque layer 33 may comprise a thin film of germanium or a cermet of germanium and silicon monoxide, for example, and effectively prevents photons produced by the electroluminescent layer 32 from feeding back or otherwise reaching the field sustained conductivity layer 34 and adversely affecting its conductivity. What is claimed is:

1. A direct-viewing electronic storage display device stora ela er 'ncl di 1 l. a layer 0 ca mi u m sulfide disposed on said opaque layer, said layer of cadmium sulfide being heated at a temperature of from 385 to 525C. for a period of from 1 minute to 1 hour in a non-sulfur-containing atmosphere followed by immediate cooling in said atmosphere; an electrode member in the form of a composite layer of two materials which have diverse conducting properties disposed on said heat treated layer of cadmium sulfide, one of said materials being selected from a group consisting of gold, aluminum, silver, platinum, tin and germanium,

and the other of said materials being selected from the group consisting of silicon monoxide and magnesium oxide; and

3. a conductive film disposed over said composite I layer; e. and an electron gun disposed in said container, for

forming an electron beam of elemental cross-secv tional area and adapted to scan said composite storage layer. 2. The invention according to claim 1 wherein said non-sulfur-containing atmosphere in which said layer of cadmium sulfide is heated is a gas selected from a group of gases consisting essentially of argon, nitrogen and air.- g

3. The invention according to claim 1 wherein said layer of cadmium sulfide is from 2 to 15 microns thick.

* l I I

Claims (4)

1. 2. an electrode member in the form of a composite layer of two materials which have diverse conducting properties disposed on said heat treated layer of cadmium sulfide, one of said materials being selected from a group consisting of gold, aluminum, silver, platinum, tin and germanium, and the other of said materials being selected from the group consisting of silicon monoxide and magnesium oxide; and
2. 2. The invention according to claim 1 wherein said non-sulfur-containing atmosphere in which said layer of cadmium sulfide is heated is a gas selected from a group of gases consisting essentially of argon, nitrogen and air.
3. 3. The invention according to claim 1 wherein said layer of cadmium sulfide is from 2 to 15 microns thick.
4. 3. a conductive film disposed over said composite layer; e. and an electron gun disposed in said container for forming an electron beam of elemental cross-sectional area and adapted to scan said composite storage layer.

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