Nanotechnology – Nalin Kumar, Chenggang Xie, Applied Nanotech Holdings Inc

Search Patent for Flat panel triode structure using flat field emission cathode

Abstract for Flat panel triode structure using flat field emission cathode

Flat panel displays of field emission types with a triode structure are disclosed. They can be used to display visual information. Display includes a plurality light-emitting and field-emission anodes. Each anode emits light in response to emission from the respective cathodes. Each cathode includes a layer made of low-work function material with a relatively flat emission area. This includes a plurality distributed electron emission sites. A grid assembly is placed between the anodes. This allows for control of emission levels from the anodes from the respective cathodes. The preferred embodiment of this invention uses an amorphic, diamond-film layer as the low-work function material. The grid assembly contains a conductive material that is deposited between the plurality and cathodes as well as over the interstices between them. The conductive layer has apertures, with the cathodes aligned and the same size.

Background for Flat panel triode structure using flat field emission cathode

“Field emission computer monitors, in general, are not new. Displays that include a number of field emission cathodes as well as corresponding anodes have been around for years. The anodes emit light when electron bombardment is received from the cathodes. It is important to understand the nature of field emission before you start a discussion about such displays.

Field emission refers to a phenomenon where an electric field near the surface or emission material narrows the width a potential barrier. This allows for a quantum tunnelling effect, in which electrons pass through the potential barrier and are released from the material.

The field strength needed to trigger the emission of electrons from a material’s surface depends on its “work function”. Many materials have a positive function, and therefore require an intense electric field to cause field emission. However, some materials have a lower, or even negative work function, and therefore do not require intense fields to cause emission. These materials can be deposited on a conductor as a thin film. This creates a cathode that has a low threshold voltage to emit electrons.

“In prior art devices it was possible to increase field emission of electrons through providing for a cathode geo which focused electron emission at one, relatively sharp point at the tip of a conical Cathode (called micro-tip cathode). These micro-tip cathodes have been used for many years in triode emission displays.

“For example, U.S. Pat. No. No. 4,857 799 was issued to Spindt and al. on August 15, 1989. It is directed at a matrix-addressed flatpanel display using field emission cathodes. The cathodes are integrated into the display backing structure and activate the corresponding cathodoluminescent zones on a faceplate. In the preferred embodiment, the face plate is placed 40 microns away from the cathode arrangement. A vacuum is also provided between the plate (and cathodes) in this space. The spacing is maintained by spacers that look like legs interspersed between the pixels. Electrical connections to the bases of the cathodes can be diffused sections through a backing structure. Spindt et al. Spindt et al. employ a number of micro-tip field emission capacitors in a matrix arrangement with the tips aligned with apertures in the extraction grid above the cathodes. The display described by Spindt and al. can be made with an anode added to the extraction grid. This is a three-decade display.

Micro-tips are difficult to make because they have intricate geometries. If the micro-tips do not have the same geometry throughout the display, there will be variations in the emission from tip to tip, which can lead to uneven illumination. These micro-tip displays can be expensive because of the tight tolerances involved in manufacturing.

U.S. Pat. No. No. 5,038,070 was issued to Bardai and al. on Aug. 6, 1991. It describes a triode display. It discloses a plurality field emitters in hollow pointed cones or pyramids made by molding. A plurality of field emitters extends from an electrically conductive surface. In order to make an electrical connection with the conductive layers, an electrically conductive mesh must be adhered to the opposite surface of the conductive. This is done by high-temperature brazing. The mesh is strong and has good thermal conductivity. To form a field emitting triode array, or similar, additional elements like a gate or anode structure can be added to the conductive layer.

“The field emitter structure described in Bardai and al. is that emitter cones must be photolithographically grown, which is a very complex and expensive procedure.”

“Yet another tride micro-tip structure is shown in Recent Developments at Microtips display at LETI”, published in Technical Digest of IVMC Nagahama 1991. R. Meyer describes a microtip display with two key features. (1) Cold electron emission by field effect of large matrix arrays of micro-guns (or microtips) and (2) low voltage cathodoluminescence, which is a few hundred volts. Meyer again uses micro-tip cathodes, which have the same disadvantages as those mentioned above.

“Another patent to Spindt and al., U.S. Pat. No. No. 5,015,912, published on May 14, 1991, describes a matrix-addressed flat-panel display that uses micro-tip cathodes. Spindt et al. Spindt et al. discloses a grid structure that can be used in conjunction with microtip cathodes.

Spindt and al. It provides its matrix-addressing scheme entirely within its cathode assembly. Each cathode has a number of electron emitting tips that project upwardly from it toward a face structure. To control and generate electron emission from the tips, an electrically conductive gate is placed adjacent to them. This arrangement is perpendicular the base stripes and contains apertures through the which electrons may be emitted from the tips. To produce emission from selected cathodes, the extraction electrode must be used in conjunction with the individual cathodes. Because the extraction field required to produce electron emission is greater than 50 Mv/m, micro-tip cathodes made of silicon, molybdenum, or tungsten require a grid-cathode arrangement. The grid must be within 1 micrometer of the micro-tip cathodes. This tight tolerance requires that gate electrodes are produced using optical lithographic techniques on an electric insulating layer. This electrically separates each pixel’s gates from the common base. This type of photolithography can be expensive and time-consuming due to the high accuracy required to create such displays. Spindt et al. also teaches how to extract grids. It was designed specifically to work with micro-tip cathodes and not other geometries.

Spindt and colleagues have identified two main problems with the device. 1) Formation of micro-tip cathodes; 2) Formation and alignment of extraction electrodes relative to the cathodes. Spindt and colleagues have described the structure. It is difficult to construct large-area displays because of its complexity.

“The prior art is directed to microtip cathodes, despite their enormous manufacturing difficulties. They are advantageously used in conjunction with an extraction grid in the triode (three terminals) structure.

“In a triode structure (three terminals) pixel structure, the electron extraction grid structure is interspersed among the corresponding anode and cathode pairs. The grid provides an additional control parameter that gives rise to several benefits in the case of triode displays. The grid can be independently controlled to produce independent cathode anode and cathode electric fields. This allows for electron emission to be achieved by applying a low voltage control voltage to the cathode grid field. However, the grid-anode voltage may be extremely high (several hundred-to several thousand volts) which will result in a higher power efficiency. Because electrons can pass through the anode-phosphor material at a higher potential, and thus be excited by electrons with a greater kinetic energie, this is possible. The second is that voltages can be applied selectively to excite grid-anode pairs (on the order 40 volts), which allows for more traditional electronics in drive circuitry. The lower electric field between the anode and grid (on the order 1-5 volts per millimeter) reduces the dielectric requirements for spacer material to separate cathode from anode assemblies. In order to improve control over electron extraction and emission, the prior art extraction grid structures were designed in conjunction with micro-tip Cathodes.

“In Ser. No. No. 07/851 701, filed Mar. 16, 1992, which was filed on Mar. The Ser. No. 07/851 701 describes a cathode with a relatively flat emission surface. In its preferred embodiment, the cathode employs an emission material with a low effective work function. The material is placed over a conductive layer to form a plurality emission sites that can emit electrons in the presence a low intensity electric field.

Flat cathodes are less costly and more difficult to make in large quantities because they lack the micro-tip geometry. There were many benefits to the flat cathode design. This application is commony assigned with the invention.

“The discovery of amorphic diamond is a relatively recent advancement in materials science. In Thin-Film Diamond (published in the Texas Journal of Science vol. ), the structure and characteristics of an amorphic, are detailed. 41, no. 41, no. 4, 1989, by C. Collins et., whose entire text is incorporated by reference. Collins et al. Collins et al. describe a method for producing an amorphic, diamond-like film using a laser deposition process. Amorphic diamond is composed of a variety of micro-crystallites. Each one has a unique structure that depends on how the film was prepared. It is not clear how these micro-crystallites form and what their properties are.

“Diamond has negative electron affinity in (111) direction. The n-type of diamond therefore has a negative work function. This means that only a low electric field is needed to distort any potential barriers on the diamond’s surface. Diamond is an excellent material to use with field emission cathodes. The prior art actually used diamond films as an emission surface for micro-tip cathodes. The prior art did not recognize that amorphic, which has physical characteristics which are significantly different from other forms, is a good emission material. Ser. No. No. 07/851 701, which was later abandoned, was the first publication to describe the use of amorphic diamond film as an emission material. The preferred embodiment of the invention was actually amorphic diamond film used with a flat cathode to create a completely different field emission cathode design. Amorphic diamond film contains micro-crystallites that can be used as electron emission sites. This depends on their structure. Amorphic diamond microcrystallites are distributed on a flat surface for cathode emission. Some of these will act as localized electron emission spots.

The prior art is primarily focused on triode flat panel displays that are based on microtip cathodes made of silicon, molybdenum and tungsten or similar materials. It has been difficult to find a matrix-addressable flatpanel display that is 1) simple to design and 2) inexpensive to make. It also uses a triode (three terminals) pixel structure, which employs a cathode with a flat emission surface that contains a plurality distributed localized electron emission spots.

“The prior art also fails to address the problem that a grid structure can be used in conjunction with flat cathodes.”

“The present invention builds on the idea that amorphic diamond films can be deposited on relatively flat field emission cathodes. It provides a triode display structure using a novel extraction grid to locate the cathodes and cause emission from them.”

The present invention is a flat panel display that uses a luminescent-phosphor similar to the one used in CRTs. It also has a thin profile. The flat panel display of the invention comprises (1) a plurality light-emitting cathodes and anodes that emit light in response to emission from the respective cathodes. Each cathode includes a layer of low-work function material with a relatively flat emission area that contains a plurality distributed localized electron emission spots. (2) an interspersed grid assembly between the corresponding anodes. These apertures have diameters equal to those of the corresponding corresponding cathodes.

The flat panel display uses a tride (three terminals) pixel structure. Each grid strip and every cathode strip can be individually addressed by the grid and cathode voltage driver. The matrix-addressable display uses grid and cathode assembly arranged in perpendicular relationships. A pixel is the intersection of a grid strip or a cathode strips. This means that each pixel in the display can be individually illuminated.

The grid strips have a unique construction that allows them to work with flat cathodes. The grid strips are made up of a substrate of SiO2, on which is deposited a layer of conductive material, preferable of a metal. The conductive layer is etched to create apertures, which correspond to specific cathode/anode pairs. The edges of the apertures are located substantially higher than the edges of the corresponding cathodes.

“The cathode assembly comprises a plurality of flat cathodes are, in the preferred embodiment of the present invention, photolithographically patterned either (1) through the apertures in the grid or (2) in alignment with the apertures in the grid. Each cathode is composed of a conductive layer that has been deposited on a substrate, and a resistive layer that has been deposited on top. The resistive layer is then covered with a thin film with a low effective work function. The resistive layer acts as an electrical shield between different subdivisions of the cathode strip.

Anode assemblies consist of a conductive material, such as indium-tinoxid in the preferred embodiment, deposited on a substrate with low-energy phosphor (such a zinc oxide in this preferred embodiment), and then deposited on top of the conductive layer. To provide a color display, an alternative embodiment of this invention allows for the addition of a number of red, green, and blue phosphors to the conductive layer.

The anode and cathode assemblies will be joined with a peripheral glass frit seal to a printed circuit board. Spacers made of glass fibers, glass balls, or a fixed spacer using typical deposition technology ensure proper spacing between them. The assemblies are hermetically sealed. A vacuum is drawn through an exhaust tube into the space between the cathode and anode assemblies. These structures have well-known systems for maintaining vacuums. A device known as a “getter” is used to collect the gases in the vacuum.

Flexible connectors are used to access the individual rows and columns in grid strips and cathode stripes. They can be accessed externally using typical semiconductor mounting technology. These connectors attach to grid and cathode driver so that each pixel can be addressed.

An individual pixel is lit when the electrical potential difference between the portions of a cathode or grid strip that correspond to it is sufficient to extract electrons out of the emission material coating the cathode. This causes emission of electrons through the control grid to the anode. The electrons traveling to the anode strike the low-energy phosphor material and produce light.

“The gap between the grid and cathode in a triode display is approximately 1 micrometer. To cause emission, the spacing between the cathode and grid is only 40 volts. There are commercially available devices that can switch 40 volts. These voltage drivers are also known as grid drivers or cathode drivers. When the driver voltage is applied to the corresponding grid strip or cathode strips, a pixel will be addressed and illuminated. This causes the emission of electrons from the area adjacent to the grid strip. Because the threshold potential between grid and cathode is not reached, electrons cannot be emitted from a specific pixel area if the driver voltage is only applied to the appropriate cathode strip.

“The present invention allows for the implementation of grey scale mode display by controlling voltage supply to control grid. This modulates emission of electrons from cathode to anode and, in turn, modifies photon emission of phosphor material deposited at the anode.

“The grid is supported with a layer dielectric material. Anisotropically etched, the dielectric material is removed from between the cathode (and its corresponding aperture). The result is a multitude of mushroom-shaped structures made of dielectric material that support the grid layer. Alternativly, the dielectric layer may be isotropically etched so that the mushroom-shaped structures can be etched away. This will leave the grid suspended locally. This creates an air-bridge structure.

The present invention has many advantages, including low power consumption, high brightness, and low cost. The cathode assembly according to the invention is simpler and cheaper to make, as sophisticated photolithography isn’t required to create the flat cathode arrangement or grid assembly.

“The above has provided a broad overview of the technical features and advantages of the present invention so that you can better understand the detailed description of this invention. The claims of the invention contain additional features and benefits. These will be described in detail here. The inventive concept and its specific embodiment can be used to modify or design other structures. This is a benefit that should be recognized by all who are skilled in the art. Those skilled in the art should realize that similar constructions to the invention do not violate the spirit or scope of the invention, as defined in the appended claims.

“Turning to FIG. 1. This is a top view showing the joined cathode-extraction grid assemblies of this invention. In FIG. 2. The grid structure 101 is made up of electrically isolated and individually addressable sections. These strips are connected in a perpendicular fashion to cathode strip, which together form a cathode architecture 101. Anode strips not shown are parallel to the cathode stripes. This orthogonal arrangement provides a vertically and horizontally-addressable structure that forms the basis of a flat panel display. External connectors 220 allow for electrical access to the grid structure 101 and cathode structures 102. The preferred embodiment of this invention allows for the separation of the grid and cathode strips by using a dielectric layer.

“Turning to FIG. 2. This is a side-view of a pixel’ 100 on a triode flat panel display according to the present invention. Each cathode strip 101 of the cathode architecture 101 of FIG. 1. consists of a substrate 110 and a conductive 150 layer. The resistive 160 layer 160 is used to protect the flat cathodes 170. Each flat cathode 170 is spaced apart, which results in isolation maintained by resistive layer 160. Anode assembly 104 is composed of a substrate 120 made from glass, and a conductive 130 made from indium-tinoxide (ITO). A low energy phosphor 140 such as zinc oxide (ZnO) is also included. If you require a color display, red, green, and blue phosphors may be used in place of the ZnO. A plurality of dielectric spacingrs 190 separate the anode 104 from the grid structure. These spacers maintain the desired separation distance between the anode 104 and grid structure.

The grid structure 102 is found between the anode assembly (104) and cathode strips (103). Through the grid structure 102, electrons are accelerated towards the conductive layer 130. This causes the low-energy phosphor 140 to emit light. The grid structure 102 is separated under the cathode strip 103 by a spacer180. This is, in the preferred embodiment, a layer dielectric material, preferably SiO2. Apertures will be etched through both the grid structure 102 and the SiO2 in order to create a channel that runs from the cathodes through the appropriate apertures in each grid structure to the anodes.

“The pixel 100 can be illuminated when sufficient driver voltage is applied between conductive layer 150 and grid structure 102 that corresponds to that particular Pixel 100. Combining the two driver voltages with the constant DC supply, the constant DC voltage provides sufficient threshold potential between sections of the grid (both of FIG. 1) is associated with the pixel 100. The threshold potential causes electron emission from flat cathodes 170.

Referring to FIG. “Referring to FIG. 10, a biasing circuit has been shown to drive a display pixel with an operating voltage 300 volts using voltage drivers 1000, 1001. One strip 1002 from grid structure 102 can be seen. You can address the pixel (intersection between grid strip 1002 & cathode strips 1003) by either cathode or grid strip 1003, which are orthogonally to one another. Cathode 1003 can be addressed with 25 volt driver 1001 while grid strip 1002 can be addressed using 25 volt drivers 1000. These both float on a 250-volt DC power supply. The DC power supply output voltage is set to slightly lower than the threshold voltage of each pixel. Example: A display pixel that has a threshold voltage 300 volts. It is the 250 volt DC power supply that is used.

“Turning Now to FIG. 3. This is a partial side-view of FIG. 3’s joined extraction grid and cathode assemblies. 2, taken along Section III–III of FIG. 1. FIG. Spacers 180 are used to maintain the correct distance between the grid structure (102) and the substrate (103). The spacers 300 should be a layer dielectric material. The grid structure 102 has a plurality 310 apertures, which can be aligned or not with the corresponding cathodes (not illustrated).

“Turning to FIG. 4. This is a partial view of the emitter array, without supporting pillars prior to cathode deposit. The emitter array consists of the substrate, cathode-conductive layer and resistive layers. All are illustrated and detailed with regard to FIG. 1. A SiO2 dielectric coating 400 is placed over the substrate. This provides a base for the extraction gate conductive layer (102). FIG. 4, layer 102 has already been deposited on layer 400 and apertures photolithographically etched therein. FIG. FIG. 4. is a cross-section. The apertures are indicated as spaces in layer 102. After the apertures are etched, the SiO2 layers is isotropically removed from the part of layer 102 that is between the dielectriclayer 400. The plurality of gate apertures that correspond to a specific pixel are close-knit in the region of the pixels, so isotropic etching the SiO2 layer creates an air-bridge structure in which the layer 102 is suspended locally over the pixel without any support from the pillars. In the preferred embodiment, a particular pixel may have a number of gate apertures and cathodes. However, the layer 102 is supported by the layer 400 on all sides, as illustrated in FIG. 4. Notably, however, the SiO2 isotropic etch results in layer 102 being slightly etched back from the edges and apertures. This important feature is a key feature of the present invention. We will discuss it in detail in FIG. 5.”

“Turning to FIG. 5 shows a partial side-view of an emitter array that has no supporting pillars following cathode deposit. The cathodes 500 are shown having been deposited through the apertures as well as on the resistive layers. It is important that the cathodes do not exceed the apertures of the grid structure. The cathodes are located entirely below the apertures. This is an important feature of the invention. The grid creates an electric field around a cathode that is uniformly distributed over its surface. The surface is able to emit electrons. Because electrons are not able to strike the anode but only the cathodes, they do not tend to be emitted directly below the grid. Because electrons that fall to the anode will not waste power, this results in a greater display efficiency.

“Turning to FIG. 6 shows a partial side-view of an emitter array and supporting pillars prior to cathode deposit. After apertures have been etched into the grid layer 101, the SiO2 dielectric layers 400 beneath is anisotropically heated until all of the SiO2 has been etched from below the apertures. Between each aperture, there are a multitude of mushroom-shaped columns 600.

“Turning to FIG. 7 shows a partial side-view of an emitter array and supporting pillars following cathode deposit. Important to notice that the cathodes 700 measure the same width as the apertures in grid layer. Important to also note is that 600 pillars are etched slightly back from the grid layer’s edges. As in FIG. 5, it is important to recall that the cathodes deposited are of the same diameter as the apertures in grid layer. Recall, as in FIG. 5, that the cathodes are the same size as the apertures. It is extremely undesirable for the dielectric layer to touch cathodes 700 directly. This creates a “triple junction” of SiO2 space, cathode and cathode. Otherwise electrons emitted by the cathodes700 have a tendency climb the walls the dielectric layers, creating a low resistance path that inhibits emission of electrons to the anode. Display inefficiency is the result, just as in the above case. This phenomenon can be minimized by providing a dielectric coating etched back from apertures and so removed at a small distance to the cathode.

The preferred method for producing the invention is to deposit the cathodes 700 through apertures in the grid-conductive layer using the grid-conductive layer as a mask. Alternative methods to those shown in FIGS. Alternative to the one shown in FIGS. 4-7, the cathodes may be formed on top of the cathode-conductive layer before deposition of the grid conductor layer and the dielectric layer. This alternative method has one drawback: the cathodes must be aligned with the grid conductive layers apertures. Inefficiency or inoperability could result from misalignment.

“Turning Now to FIG. 8 is an ineffective grid structure. The structure is generally known as 801. It consists of a cathode substrate 802. On this layer, is deposited a cathode conductor layer 803 along with strips of cathode emission material layers 804. To form individual cathode-anode pair, a dielectric layer 805 of conductive material is placed on the material layer 804. Next, a grid layer 806 of conductive material will be deposited on top of the material layer 805. The grid layer 806 is formed in the same strips as the dielectric layers 805 and has corresponding apertures. An anode assembly 807 containing a phosphor 809 is placed above grid layer 806. It is held at a controlled distance to grid layer 806 by a number of fibrous dielectric spacesrs 808.

“The structure 801 can be used with flat cathodes but it does have some disadvantages. The first is that the electric field between the grid layers 806 and 806 strips is higher than that which exists under the grid layer 806. This results in many emitter electrons being directed to the grid layer 806 instead of the anode 807 as previously stated. These electrons do not strike a phosphor so the energy they produce is lost.

“Second: The ratio of the electric fields at and within the apertures of the grid layers 806 strips is dependent upon the dimensions of the grid layer806 apertures, and the thickness of dielectric layer 805. Display operation should be as good as possible. The aperture diameter and thickness of the dielectric layers 805 should not differ from one another. The preferred embodiment of this invention has the apertures measuring approximately 1 to 20 millimeters in diameter.

Third, because the emission layer 804 extends completely across the aperture, excess emission is generated from parts of the emission material proximate to the dielectric material (at “triple junction \”).). The emission layer 804 emits different amounts of emission. It is stronger at the edges of cathodes. This causes leakage currents to form along the dielectric layers 805, which causes the emission layer 804 to shorten across the dielectric layers 805, thereby preventing or completely disabling the operation of the pixels. The structure 801 therefore is defective.

“The main difference between FIG. 8 and the preferred structures shown in FIGS. The difference between FIGS. 5 and 7 is the fact that the emission layer 804 has a uniform layer with triple junctions, while individual cathodes can be seen in FIGS. 5, 7 and 8, respectively, the cathodes have been either deposited through the gate apertures (or previously placed in alignment with them). The cathodes are located directly beneath the apertures. They do not extend below the gate conductors. This is a disadvantage that has been described previously and is illustrated in FIG. 8.”

“Furthermore in the case FIG. “Furthermore, in the case of FIG. 7, wherein the individual cathodes have mushroom-shaped SiO2 Dielectric Supports, these supports are separated from each other so that triple junctions can be eliminated and surface current leakage is reduced. The emitters are not able to extend beyond the grid layer’s aperture and do not contact the dielectric layer. This minimizes the possibility of leakage currents. Instead, the cathodes are separate units that are deposited on the conductive layer.

“Turning Now to FIG. 9 shows a perspective view showing the cathode-extraction grid assembly with an interfacing dielectric layer. The substrate 901 is shown, upon which is deposited the conductive layer 902, described previously. As shown, the conductive layer 902 can be deposited in strips. The dielectric layer 903 will be deposited as a blanket over the conductive layers 902 and parts of the substrate 901. The control grid layer 904 will be deposited next on the dielectric 903. It is composed of strips that are perpendicular to the conductive layers 902 strips. Each strip has a plurality apertures that correspond to those in the dielectric 903. The dielectric layer 903 contains a plurality of apertures 906, which correspond to the cathodes that were created or are to be created in conductive layer 902. The grid layer 904 ends in a plurality end conductors 905, which can be coupled with drive circuitry. This allows the grid layer 904 and the conductive layer 902. FIG. 9 The anode layer as well as the fibrous spacing material are not shown. However, they would be located over the grid layer 904 if shown.”

The above description reveals that the present invention is unique in providing a flat panel display consisting of (1) a plurality light-emitting and field-emission anodes, each anode emitting light in response emission from the respective cathodes. Each cathode also includes a layer made of low-work function material with a relatively flat emission area. (2) A grid assembly interspersed between each anode and cathode to control the emission levels to the anodes of the thereby controlling the anodes.

“Even though the advantages of the present invention have been explained in detail, it is important to understand that many modifications, substitutions, and alterations may be made without departing from its spirit and scope as described by the attached claims.”

Summary for Flat panel triode structure using flat field emission cathode

“Field emission computer monitors, in general, are not new. Displays that include a number of field emission cathodes as well as corresponding anodes have been around for years. The anodes emit light when electron bombardment is received from the cathodes. It is important to understand the nature of field emission before you start a discussion about such displays.

Field emission refers to a phenomenon where an electric field near the surface or emission material narrows the width a potential barrier. This allows for a quantum tunnelling effect, in which electrons pass through the potential barrier and are released from the material.

The field strength needed to trigger the emission of electrons from a material’s surface depends on its “work function”. Many materials have a positive function, and therefore require an intense electric field to cause field emission. However, some materials have a lower, or even negative work function, and therefore do not require intense fields to cause emission. These materials can be deposited on a conductor as a thin film. This creates a cathode that has a low threshold voltage to emit electrons.

“In prior art devices it was possible to increase field emission of electrons through providing for a cathode geo which focused electron emission at one, relatively sharp point at the tip of a conical Cathode (called micro-tip cathode). These micro-tip cathodes have been used for many years in triode emission displays.

“For example, U.S. Pat. No. No. 4,857 799 was issued to Spindt and al. on August 15, 1989. It is directed at a matrix-addressed flatpanel display using field emission cathodes. The cathodes are integrated into the display backing structure and activate the corresponding cathodoluminescent zones on a faceplate. In the preferred embodiment, the face plate is placed 40 microns away from the cathode arrangement. A vacuum is also provided between the plate (and cathodes) in this space. The spacing is maintained by spacers that look like legs interspersed between the pixels. Electrical connections to the bases of the cathodes can be diffused sections through a backing structure. Spindt et al. Spindt et al. employ a number of micro-tip field emission capacitors in a matrix arrangement with the tips aligned with apertures in the extraction grid above the cathodes. The display described by Spindt and al. can be made with an anode added to the extraction grid. This is a three-decade display.

Micro-tips are difficult to make because they have intricate geometries. If the micro-tips do not have the same geometry throughout the display, there will be variations in the emission from tip to tip, which can lead to uneven illumination. These micro-tip displays can be expensive because of the tight tolerances involved in manufacturing.

U.S. Pat. No. No. 5,038,070 was issued to Bardai and al. on Aug. 6, 1991. It describes a triode display. It discloses a plurality field emitters in hollow pointed cones or pyramids made by molding. A plurality of field emitters extends from an electrically conductive surface. In order to make an electrical connection with the conductive layers, an electrically conductive mesh must be adhered to the opposite surface of the conductive. This is done by high-temperature brazing. The mesh is strong and has good thermal conductivity. To form a field emitting triode array, or similar, additional elements like a gate or anode structure can be added to the conductive layer.

“The field emitter structure described in Bardai and al. is that emitter cones must be photolithographically grown, which is a very complex and expensive procedure.”

“Yet another tride micro-tip structure is shown in Recent Developments at Microtips display at LETI”, published in Technical Digest of IVMC Nagahama 1991. R. Meyer describes a microtip display with two key features. (1) Cold electron emission by field effect of large matrix arrays of micro-guns (or microtips) and (2) low voltage cathodoluminescence, which is a few hundred volts. Meyer again uses micro-tip cathodes, which have the same disadvantages as those mentioned above.

“Another patent to Spindt and al., U.S. Pat. No. No. 5,015,912, published on May 14, 1991, describes a matrix-addressed flat-panel display that uses micro-tip cathodes. Spindt et al. Spindt et al. discloses a grid structure that can be used in conjunction with microtip cathodes.

Spindt and al. It provides its matrix-addressing scheme entirely within its cathode assembly. Each cathode has a number of electron emitting tips that project upwardly from it toward a face structure. To control and generate electron emission from the tips, an electrically conductive gate is placed adjacent to them. This arrangement is perpendicular the base stripes and contains apertures through the which electrons may be emitted from the tips. To produce emission from selected cathodes, the extraction electrode must be used in conjunction with the individual cathodes. Because the extraction field required to produce electron emission is greater than 50 Mv/m, micro-tip cathodes made of silicon, molybdenum, or tungsten require a grid-cathode arrangement. The grid must be within 1 micrometer of the micro-tip cathodes. This tight tolerance requires that gate electrodes are produced using optical lithographic techniques on an electric insulating layer. This electrically separates each pixel’s gates from the common base. This type of photolithography can be expensive and time-consuming due to the high accuracy required to create such displays. Spindt et al. also teaches how to extract grids. It was designed specifically to work with micro-tip cathodes and not other geometries.

Spindt and colleagues have identified two main problems with the device. 1) Formation of micro-tip cathodes; 2) Formation and alignment of extraction electrodes relative to the cathodes. Spindt and colleagues have described the structure. It is difficult to construct large-area displays because of its complexity.

“The prior art is directed to microtip cathodes, despite their enormous manufacturing difficulties. They are advantageously used in conjunction with an extraction grid in the triode (three terminals) structure.

“In a triode structure (three terminals) pixel structure, the electron extraction grid structure is interspersed among the corresponding anode and cathode pairs. The grid provides an additional control parameter that gives rise to several benefits in the case of triode displays. The grid can be independently controlled to produce independent cathode anode and cathode electric fields. This allows for electron emission to be achieved by applying a low voltage control voltage to the cathode grid field. However, the grid-anode voltage may be extremely high (several hundred-to several thousand volts) which will result in a higher power efficiency. Because electrons can pass through the anode-phosphor material at a higher potential, and thus be excited by electrons with a greater kinetic energie, this is possible. The second is that voltages can be applied selectively to excite grid-anode pairs (on the order 40 volts), which allows for more traditional electronics in drive circuitry. The lower electric field between the anode and grid (on the order 1-5 volts per millimeter) reduces the dielectric requirements for spacer material to separate cathode from anode assemblies. In order to improve control over electron extraction and emission, the prior art extraction grid structures were designed in conjunction with micro-tip Cathodes.

“In Ser. No. No. 07/851 701, filed Mar. 16, 1992, which was filed on Mar. The Ser. No. 07/851 701 describes a cathode with a relatively flat emission surface. In its preferred embodiment, the cathode employs an emission material with a low effective work function. The material is placed over a conductive layer to form a plurality emission sites that can emit electrons in the presence a low intensity electric field.

Flat cathodes are less costly and more difficult to make in large quantities because they lack the micro-tip geometry. There were many benefits to the flat cathode design. This application is commony assigned with the invention.

“The discovery of amorphic diamond is a relatively recent advancement in materials science. In Thin-Film Diamond (published in the Texas Journal of Science vol. ), the structure and characteristics of an amorphic, are detailed. 41, no. 41, no. 4, 1989, by C. Collins et., whose entire text is incorporated by reference. Collins et al. Collins et al. describe a method for producing an amorphic, diamond-like film using a laser deposition process. Amorphic diamond is composed of a variety of micro-crystallites. Each one has a unique structure that depends on how the film was prepared. It is not clear how these micro-crystallites form and what their properties are.

“Diamond has negative electron affinity in (111) direction. The n-type of diamond therefore has a negative work function. This means that only a low electric field is needed to distort any potential barriers on the diamond’s surface. Diamond is an excellent material to use with field emission cathodes. The prior art actually used diamond films as an emission surface for micro-tip cathodes. The prior art did not recognize that amorphic, which has physical characteristics which are significantly different from other forms, is a good emission material. Ser. No. No. 07/851 701, which was later abandoned, was the first publication to describe the use of amorphic diamond film as an emission material. The preferred embodiment of the invention was actually amorphic diamond film used with a flat cathode to create a completely different field emission cathode design. Amorphic diamond film contains micro-crystallites that can be used as electron emission sites. This depends on their structure. Amorphic diamond microcrystallites are distributed on a flat surface for cathode emission. Some of these will act as localized electron emission spots.

The prior art is primarily focused on triode flat panel displays that are based on microtip cathodes made of silicon, molybdenum and tungsten or similar materials. It has been difficult to find a matrix-addressable flatpanel display that is 1) simple to design and 2) inexpensive to make. It also uses a triode (three terminals) pixel structure, which employs a cathode with a flat emission surface that contains a plurality distributed localized electron emission spots.

“The prior art also fails to address the problem that a grid structure can be used in conjunction with flat cathodes.”

“The present invention builds on the idea that amorphic diamond films can be deposited on relatively flat field emission cathodes. It provides a triode display structure using a novel extraction grid to locate the cathodes and cause emission from them.”

The present invention is a flat panel display that uses a luminescent-phosphor similar to the one used in CRTs. It also has a thin profile. The flat panel display of the invention comprises (1) a plurality light-emitting cathodes and anodes that emit light in response to emission from the respective cathodes. Each cathode includes a layer of low-work function material with a relatively flat emission area that contains a plurality distributed localized electron emission spots. (2) an interspersed grid assembly between the corresponding anodes. These apertures have diameters equal to those of the corresponding corresponding cathodes.

The flat panel display uses a tride (three terminals) pixel structure. Each grid strip and every cathode strip can be individually addressed by the grid and cathode voltage driver. The matrix-addressable display uses grid and cathode assembly arranged in perpendicular relationships. A pixel is the intersection of a grid strip or a cathode strips. This means that each pixel in the display can be individually illuminated.

The grid strips have a unique construction that allows them to work with flat cathodes. The grid strips are made up of a substrate of SiO2, on which is deposited a layer of conductive material, preferable of a metal. The conductive layer is etched to create apertures, which correspond to specific cathode/anode pairs. The edges of the apertures are located substantially higher than the edges of the corresponding cathodes.

“The cathode assembly comprises a plurality of flat cathodes are, in the preferred embodiment of the present invention, photolithographically patterned either (1) through the apertures in the grid or (2) in alignment with the apertures in the grid. Each cathode is composed of a conductive layer that has been deposited on a substrate, and a resistive layer that has been deposited on top. The resistive layer is then covered with a thin film with a low effective work function. The resistive layer acts as an electrical shield between different subdivisions of the cathode strip.

Anode assemblies consist of a conductive material, such as indium-tinoxid in the preferred embodiment, deposited on a substrate with low-energy phosphor (such a zinc oxide in this preferred embodiment), and then deposited on top of the conductive layer. To provide a color display, an alternative embodiment of this invention allows for the addition of a number of red, green, and blue phosphors to the conductive layer.

The anode and cathode assemblies will be joined with a peripheral glass frit seal to a printed circuit board. Spacers made of glass fibers, glass balls, or a fixed spacer using typical deposition technology ensure proper spacing between them. The assemblies are hermetically sealed. A vacuum is drawn through an exhaust tube into the space between the cathode and anode assemblies. These structures have well-known systems for maintaining vacuums. A device known as a “getter” is used to collect the gases in the vacuum.

Flexible connectors are used to access the individual rows and columns in grid strips and cathode stripes. They can be accessed externally using typical semiconductor mounting technology. These connectors attach to grid and cathode driver so that each pixel can be addressed.

An individual pixel is lit when the electrical potential difference between the portions of a cathode or grid strip that correspond to it is sufficient to extract electrons out of the emission material coating the cathode. This causes emission of electrons through the control grid to the anode. The electrons traveling to the anode strike the low-energy phosphor material and produce light.

“The gap between the grid and cathode in a triode display is approximately 1 micrometer. To cause emission, the spacing between the cathode and grid is only 40 volts. There are commercially available devices that can switch 40 volts. These voltage drivers are also known as grid drivers or cathode drivers. When the driver voltage is applied to the corresponding grid strip or cathode strips, a pixel will be addressed and illuminated. This causes the emission of electrons from the area adjacent to the grid strip. Because the threshold potential between grid and cathode is not reached, electrons cannot be emitted from a specific pixel area if the driver voltage is only applied to the appropriate cathode strip.

“The present invention allows for the implementation of grey scale mode display by controlling voltage supply to control grid. This modulates emission of electrons from cathode to anode and, in turn, modifies photon emission of phosphor material deposited at the anode.

“The grid is supported with a layer dielectric material. Anisotropically etched, the dielectric material is removed from between the cathode (and its corresponding aperture). The result is a multitude of mushroom-shaped structures made of dielectric material that support the grid layer. Alternativly, the dielectric layer may be isotropically etched so that the mushroom-shaped structures can be etched away. This will leave the grid suspended locally. This creates an air-bridge structure.

The present invention has many advantages, including low power consumption, high brightness, and low cost. The cathode assembly according to the invention is simpler and cheaper to make, as sophisticated photolithography isn’t required to create the flat cathode arrangement or grid assembly.

“The above has provided a broad overview of the technical features and advantages of the present invention so that you can better understand the detailed description of this invention. The claims of the invention contain additional features and benefits. These will be described in detail here. The inventive concept and its specific embodiment can be used to modify or design other structures. This is a benefit that should be recognized by all who are skilled in the art. Those skilled in the art should realize that similar constructions to the invention do not violate the spirit or scope of the invention, as defined in the appended claims.

“Turning to FIG. 1. This is a top view showing the joined cathode-extraction grid assemblies of this invention. In FIG. 2. The grid structure 101 is made up of electrically isolated and individually addressable sections. These strips are connected in a perpendicular fashion to cathode strip, which together form a cathode architecture 101. Anode strips not shown are parallel to the cathode stripes. This orthogonal arrangement provides a vertically and horizontally-addressable structure that forms the basis of a flat panel display. External connectors 220 allow for electrical access to the grid structure 101 and cathode structures 102. The preferred embodiment of this invention allows for the separation of the grid and cathode strips by using a dielectric layer.

“Turning to FIG. 2. This is a side-view of a pixel’ 100 on a triode flat panel display according to the present invention. Each cathode strip 101 of the cathode architecture 101 of FIG. 1. consists of a substrate 110 and a conductive 150 layer. The resistive 160 layer 160 is used to protect the flat cathodes 170. Each flat cathode 170 is spaced apart, which results in isolation maintained by resistive layer 160. Anode assembly 104 is composed of a substrate 120 made from glass, and a conductive 130 made from indium-tinoxide (ITO). A low energy phosphor 140 such as zinc oxide (ZnO) is also included. If you require a color display, red, green, and blue phosphors may be used in place of the ZnO. A plurality of dielectric spacingrs 190 separate the anode 104 from the grid structure. These spacers maintain the desired separation distance between the anode 104 and grid structure.

The grid structure 102 is found between the anode assembly (104) and cathode strips (103). Through the grid structure 102, electrons are accelerated towards the conductive layer 130. This causes the low-energy phosphor 140 to emit light. The grid structure 102 is separated under the cathode strip 103 by a spacer180. This is, in the preferred embodiment, a layer dielectric material, preferably SiO2. Apertures will be etched through both the grid structure 102 and the SiO2 in order to create a channel that runs from the cathodes through the appropriate apertures in each grid structure to the anodes.

“The pixel 100 can be illuminated when sufficient driver voltage is applied between conductive layer 150 and grid structure 102 that corresponds to that particular Pixel 100. Combining the two driver voltages with the constant DC supply, the constant DC voltage provides sufficient threshold potential between sections of the grid (both of FIG. 1) is associated with the pixel 100. The threshold potential causes electron emission from flat cathodes 170.

Referring to FIG. “Referring to FIG. 10, a biasing circuit has been shown to drive a display pixel with an operating voltage 300 volts using voltage drivers 1000, 1001. One strip 1002 from grid structure 102 can be seen. You can address the pixel (intersection between grid strip 1002 & cathode strips 1003) by either cathode or grid strip 1003, which are orthogonally to one another. Cathode 1003 can be addressed with 25 volt driver 1001 while grid strip 1002 can be addressed using 25 volt drivers 1000. These both float on a 250-volt DC power supply. The DC power supply output voltage is set to slightly lower than the threshold voltage of each pixel. Example: A display pixel that has a threshold voltage 300 volts. It is the 250 volt DC power supply that is used.

“Turning Now to FIG. 3. This is a partial side-view of FIG. 3’s joined extraction grid and cathode assemblies. 2, taken along Section III–III of FIG. 1. FIG. Spacers 180 are used to maintain the correct distance between the grid structure (102) and the substrate (103). The spacers 300 should be a layer dielectric material. The grid structure 102 has a plurality 310 apertures, which can be aligned or not with the corresponding cathodes (not illustrated).

“Turning to FIG. 4. This is a partial view of the emitter array, without supporting pillars prior to cathode deposit. The emitter array consists of the substrate, cathode-conductive layer and resistive layers. All are illustrated and detailed with regard to FIG. 1. A SiO2 dielectric coating 400 is placed over the substrate. This provides a base for the extraction gate conductive layer (102). FIG. 4, layer 102 has already been deposited on layer 400 and apertures photolithographically etched therein. FIG. FIG. 4. is a cross-section. The apertures are indicated as spaces in layer 102. After the apertures are etched, the SiO2 layers is isotropically removed from the part of layer 102 that is between the dielectriclayer 400. The plurality of gate apertures that correspond to a specific pixel are close-knit in the region of the pixels, so isotropic etching the SiO2 layer creates an air-bridge structure in which the layer 102 is suspended locally over the pixel without any support from the pillars. In the preferred embodiment, a particular pixel may have a number of gate apertures and cathodes. However, the layer 102 is supported by the layer 400 on all sides, as illustrated in FIG. 4. Notably, however, the SiO2 isotropic etch results in layer 102 being slightly etched back from the edges and apertures. This important feature is a key feature of the present invention. We will discuss it in detail in FIG. 5.”

“Turning to FIG. 5 shows a partial side-view of an emitter array that has no supporting pillars following cathode deposit. The cathodes 500 are shown having been deposited through the apertures as well as on the resistive layers. It is important that the cathodes do not exceed the apertures of the grid structure. The cathodes are located entirely below the apertures. This is an important feature of the invention. The grid creates an electric field around a cathode that is uniformly distributed over its surface. The surface is able to emit electrons. Because electrons are not able to strike the anode but only the cathodes, they do not tend to be emitted directly below the grid. Because electrons that fall to the anode will not waste power, this results in a greater display efficiency.

“Turning to FIG. 6 shows a partial side-view of an emitter array and supporting pillars prior to cathode deposit. After apertures have been etched into the grid layer 101, the SiO2 dielectric layers 400 beneath is anisotropically heated until all of the SiO2 has been etched from below the apertures. Between each aperture, there are a multitude of mushroom-shaped columns 600.

“Turning to FIG. 7 shows a partial side-view of an emitter array and supporting pillars following cathode deposit. Important to notice that the cathodes 700 measure the same width as the apertures in grid layer. Important to also note is that 600 pillars are etched slightly back from the grid layer’s edges. As in FIG. 5, it is important to recall that the cathodes deposited are of the same diameter as the apertures in grid layer. Recall, as in FIG. 5, that the cathodes are the same size as the apertures. It is extremely undesirable for the dielectric layer to touch cathodes 700 directly. This creates a “triple junction” of SiO2 space, cathode and cathode. Otherwise electrons emitted by the cathodes700 have a tendency climb the walls the dielectric layers, creating a low resistance path that inhibits emission of electrons to the anode. Display inefficiency is the result, just as in the above case. This phenomenon can be minimized by providing a dielectric coating etched back from apertures and so removed at a small distance to the cathode.

The preferred method for producing the invention is to deposit the cathodes 700 through apertures in the grid-conductive layer using the grid-conductive layer as a mask. Alternative methods to those shown in FIGS. Alternative to the one shown in FIGS. 4-7, the cathodes may be formed on top of the cathode-conductive layer before deposition of the grid conductor layer and the dielectric layer. This alternative method has one drawback: the cathodes must be aligned with the grid conductive layers apertures. Inefficiency or inoperability could result from misalignment.

“Turning Now to FIG. 8 is an ineffective grid structure. The structure is generally known as 801. It consists of a cathode substrate 802. On this layer, is deposited a cathode conductor layer 803 along with strips of cathode emission material layers 804. To form individual cathode-anode pair, a dielectric layer 805 of conductive material is placed on the material layer 804. Next, a grid layer 806 of conductive material will be deposited on top of the material layer 805. The grid layer 806 is formed in the same strips as the dielectric layers 805 and has corresponding apertures. An anode assembly 807 containing a phosphor 809 is placed above grid layer 806. It is held at a controlled distance to grid layer 806 by a number of fibrous dielectric spacesrs 808.

“The structure 801 can be used with flat cathodes but it does have some disadvantages. The first is that the electric field between the grid layers 806 and 806 strips is higher than that which exists under the grid layer 806. This results in many emitter electrons being directed to the grid layer 806 instead of the anode 807 as previously stated. These electrons do not strike a phosphor so the energy they produce is lost.

“Second: The ratio of the electric fields at and within the apertures of the grid layers 806 strips is dependent upon the dimensions of the grid layer806 apertures, and the thickness of dielectric layer 805. Display operation should be as good as possible. The aperture diameter and thickness of the dielectric layers 805 should not differ from one another. The preferred embodiment of this invention has the apertures measuring approximately 1 to 20 millimeters in diameter.

Third, because the emission layer 804 extends completely across the aperture, excess emission is generated from parts of the emission material proximate to the dielectric material (at “triple junction \”).). The emission layer 804 emits different amounts of emission. It is stronger at the edges of cathodes. This causes leakage currents to form along the dielectric layers 805, which causes the emission layer 804 to shorten across the dielectric layers 805, thereby preventing or completely disabling the operation of the pixels. The structure 801 therefore is defective.

“The main difference between FIG. 8 and the preferred structures shown in FIGS. The difference between FIGS. 5 and 7 is the fact that the emission layer 804 has a uniform layer with triple junctions, while individual cathodes can be seen in FIGS. 5, 7 and 8, respectively, the cathodes have been either deposited through the gate apertures (or previously placed in alignment with them). The cathodes are located directly beneath the apertures. They do not extend below the gate conductors. This is a disadvantage that has been described previously and is illustrated in FIG. 8.”

“Furthermore in the case FIG. “Furthermore, in the case of FIG. 7, wherein the individual cathodes have mushroom-shaped SiO2 Dielectric Supports, these supports are separated from each other so that triple junctions can be eliminated and surface current leakage is reduced. The emitters are not able to extend beyond the grid layer’s aperture and do not contact the dielectric layer. This minimizes the possibility of leakage currents. Instead, the cathodes are separate units that are deposited on the conductive layer.

“Turning Now to FIG. 9 shows a perspective view showing the cathode-extraction grid assembly with an interfacing dielectric layer. The substrate 901 is shown, upon which is deposited the conductive layer 902, described previously. As shown, the conductive layer 902 can be deposited in strips. The dielectric layer 903 will be deposited as a blanket over the conductive layers 902 and parts of the substrate 901. The control grid layer 904 will be deposited next on the dielectric 903. It is composed of strips that are perpendicular to the conductive layers 902 strips. Each strip has a plurality apertures that correspond to those in the dielectric 903. The dielectric layer 903 contains a plurality of apertures 906, which correspond to the cathodes that were created or are to be created in conductive layer 902. The grid layer 904 ends in a plurality end conductors 905, which can be coupled with drive circuitry. This allows the grid layer 904 and the conductive layer 902. FIG. 9 The anode layer as well as the fibrous spacing material are not shown. However, they would be located over the grid layer 904 if shown.”

The above description reveals that the present invention is unique in providing a flat panel display consisting of (1) a plurality light-emitting and field-emission anodes, each anode emitting light in response emission from the respective cathodes. Each cathode also includes a layer made of low-work function material with a relatively flat emission area. (2) A grid assembly interspersed between each anode and cathode to control the emission levels to the anodes of the thereby controlling the anodes.

“Even though the advantages of the present invention have been explained in detail, it is important to understand that many modifications, substitutions, and alterations may be made without departing from its spirit and scope as described by the attached claims.”

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