The present invention generally relates to a field emission display (FED) device and a method for fabricating such device and more particularly, relates to a field emission display device that utilizes nanotube emitters instead of microtips as the electron emission source and a method for fabricating such FED device by a thick film printing technique. 
In recent years, flat panel display devices have been developed and widely used in electronic applications such as personal computers.  One of the popularly used flat panel display device is an active matrix liquid crystal display which provides improved resolution.  However, the liquid crystal display device has many inherent limitations that render it unsuitable for a number of applications.  For instance, liquid crystal displays have numerous fabrication limitations including a slow deposition process for coating a glass panel with amorphous silicon, high manufacturing complexity and low yield for the fabrication process.  Moreover, the liquid crystal display devices require a fluorescent backlight which draws high power while most of the light generated is wasted.  A liquid crystal display image is also difficult to see under bright light conditions or at wide viewing angles which further limit its use in many applications. 
Other flat panel display devices have been developed in recent years to replace the liquid crystal display panels.  One of such devices is a field emission display device that overcomes some of the limitations of LCD and provides significant advantages over the traditional LCD devices.  For instance, the field emission display devices have higher contrast ratio, larger viewing angle, higher maximum brightness, lower power consumption and a wider operating temperature range when compared to a conventional thin film transistor (TFT) liquid crystal display panel. 
One of the most drastic difference between a FED and a LCD is that, unlike the LCD, FED produces its own light source utilizing colored phosphors.  The FEDs do not require complicated, power-consuming backlights and filters and as a result, almost all the light generated by a FED is visible to the user.  Furthermore, the FEDs do not require large arrays of thin film transistors, and thus, a major source of high cost and yield problems for active matrix LCDs is eliminated. 
In a FED, electrons are emitted from a cathode and impinge on phosphors on the back of a transparent cover plate to produce an image.  Such a cathodoluminescent process is known as one of the most efficient methods for generating light.  Contrary to a conventional CRT device, each pixel or emission unit in a FED has its own electron source, i.e., typically an array of emitting microtips.  A voltage difference existed between a cathode and a gate extracts electrons from the cathode and accelerates them toward the phosphor coating.  The emission current, and thus the display brightness, is strongly dependent on the work function of the emitting material.  To achieve the necessary efficiency of a FED, the cleanliness and uniformity of the emitter source material are therefore very important. 
In order for the electron to travel in a FED, most FEDs are evacuated to a low pressure such as 10xe2x88x927 torr, in order to provide a log mean free path for the emitted electrons and to prevent contamination and deterioration of the microtips.  The resolution of the display can be improved by using a focus grid to collimate electrons drawn from the microtips. 
In the early development for field emission cathodes, a metal microtip emitter of molybdenum was utilized.  In such a device, a silicon wafer is first oxidized to produce a thick silicon oxide layer and then a metallic gate layer is deposited on top of the oxide.  The metallic gate layer is then patterned to form gate openings, while subsequent etching of the silicon oxide underneath the openings undercuts the gate and creates a well.  A sacrificial material layer such as nickel is deposited to prevent deposition of nickel into the emitter well.  Molybdenum is then deposited at normal incidence such that a cone with a sharp point grows inside the cavity until the opening closes thereabove.  An emitter cone is left when the sacrificial layer of nickel is removed. 
In an alternate design, silicon microtip emitters are produced by first conducting a thermal oxidation on silicon and then followed by patterning the oxide and selectively etching to form silicon tips.  Further oxidation or etching protects the silicon and sharpens the point to provide a sacrificial layer.  In another alternate design, the microtips are built onto a substrate of a desirable material such as glass, as an ideal substrate for large area flat panel display.  The microtips can be formed of conducting materials such as metals or doped semi-conducting materials.  In this alternate design for a FED device, an interlayer that has controlled conductivity deposited between the cathode and the microtips is highly desirable.  A proper resistivity of the interlayer enables the device to operate in a stable condition.  In fabricating such FED devices, it is therefore desirable to deposit an amorphous silicon film which has electrical conductivity in an intermediate range between that of intrinsic amorphous silicon and n+doped amorphous silicon.  The conductivity of the n+doped amorphous silicon can be controlled by adjusting the amount of phosphorous atoms contained in the film. 
Generally, in the fabrication of a FED device, the device is contained in a cavity of very low pressure such that the emission of electrons is not impeded.  For instance, a low pressure of 10xe2x88x927 torr is normally required.  In order to prevent the collapse of two relatively large glass panels which form the FED device, spacers must be used to support and provide proper spacing between the two panels.  For instance, in conventional FED devices, glass spheres or glass crosses have been used for maintaining such spacings in FED devices.  Elongated spacers have also been used for such purpose. 
Referring initially to FIG.  1A wherein an enlarged, cross-sectional view of a conventional field emission display device 10 is shown.  The FED device 10 is formed by depositing a resistive layer 12 of typically an amorphous silicon base film on a glass substrate 14.  An insulating layer 16 of a dielectric material and a metallic gate layer 18 are then deposited and formed together to provide metallic microtips 20 a cathode structure 22 is covered by the resistive layer 12 and thus, a resistive but somewhat conductive amorphous silicon layer 12 underlies a highly insulating layer 16 which is formed of a dielectric material such as SiO2.  It is important to be able to control the resistivity of the amorphous silicon layer 12 such that it is not overly resistive but yet, it will act as a limiting resistor to prevent excessive current flow if one of the microtips 20 shorts to the metal layer 18. 
A completed FED structure 30 including anode 28 mounted on top of the structure 30 is shown in FIG.  1B. It is to be noted, for simplicity reasons, the cathode layer 22 and the resistive layer 12 are shown as a single layer 22 for the cathode.  The microtips 20 are formed to emit electrons 26 from the tips of the microtips 20.  The gate electrodes are provided with a positive charge, while the anode 28 is provided with a higher positive charge.  The anode 28 is formed by a glass plate 36 which is coated with phosphorous particles 32.  An intermittent conductive layer of indium-tin-oxide layer 34 may also be utilized to further improve the brightness of the phosphorous layer when bombarded by electrons 26.  This is shown in a partial, enlarged cross-sectional view of FIG.  1C. The total thickness of the FED device is only about 2 mm, with vacuum pulled inbetween the lower glass plate 14 and the upper glass plate 36 sealed by sidewall panels 38 (shown in FIG.  1B). 
The conventional FED devices formed by microtips shown in FIGS.  1Axcx9c1C produces a flat panel display device of improved quality when compared to liquid crystal display devices.  However, a major disadvantage of the microtip FED device is the complicated processing steps that must be used to fabricate the device.  For instance, the formation of the various layers in the device, and specifically the formation of the microtips, requires a thin film deposition technique utilizing a photolithographic method.  As a result, numerous photomasking steps must be performed in order to define and fabricate the various structural features in the FED.  The CVD deposition processes and the photolithographic processes involved greatly increase the manufacturing costs of a FED device. 
It is therefore an object of the present invention to provide a FED device that does not have the drawbacks and shortcomings of the conventional FED devices. 
It is another object of the present invention to provide a FED device that is not fabricated by thin film deposition techniques and photolithographic masking techniques. 
It is a further object of the present invention to provide a FED device that can be fabricated by a low cost thick film printing technique. 
It is another further object of the present invention to provide a FED device that can be fabricated by a screen printing technique and screens for printing various patterns of material layers. 
It is still another object of the present invention to provide a FED device that contains a nanotube emitter layer of nanotubes of carbon, diamond or diamond-like carbon. 
It is yet another object of the present invention to provide a FED device that contains a multiplicity of emitter stacks insulated by insulating rib sections of a dielectric material. 
It is still another further object of the present invention to provide a method for fabricating a FED device by a thick film printing technique in which a nanotube emitter material is screen printed on a high electrical resistivity layer to form a multiplicity of spaced-apart emitter stacks. 
It is yet another further object of the present invention to provide a method for fabricating a FED device by using a thick film printing technique to form a cathode layer, a resistivity layer, a nanotube emitter layer, a dielectric layer filling the gaps between the emitter stacks, a dielectric layer on top of the emitter stacks, and a conductive metal layer as the gate electrode. 
In accordance with the present invention, a field emission display device and a method for fabricating such device are disclosed. 
In a preferred embodiment, a field emission display device is provided which includes a first electrically insulating plate, a cathode formed on the first electrically insulating plate of a material that includes silver, a layer formed on the cathode of a high electrical resistivity material, a layer of nanotube emitter on the resistivity layer formed of a material selected from the group consisting of carbon, diamond and diamond-like carbon; the cathode, the resistivity layer and the nanotube emitter layer form an emitter stack insulated by an insulating rib section from adjacent emitter stacks, a dielectric material layer perpendicularly over flying a multiplicity of emitter stacks, a gate electrode on top of the dielectric material layer, and an anode formed on a second electrically insulating plate overlying the gate electrode. 
In the field emission display device, the first electrically insulating plate may be a glass plate and the second electrically insulating plate may be a phosphor coated glass plate.  The cathode may be formed of a layer of silver paste.  The high electrical resistivity material may be RuO2.  The nanotube emitter layer may be formed of a carbon paste that includes between 20 wt % and 80 wt % of carbon and the remainder of a solvent-containing binder, or about 50 wt % carbon and about 50 wt % solvent-containing binder.  The nanotube emitter layer may further include a carbon paste that has about 50 wt % carbon fibers having diameters between about 30 nanometer and about 50 nanometer and about 50 wt % solvent-containing binder. 
In the field emission display device, the emitter stack may have a thickness between about 10 xcexcm and about 100 xcexcm, and preferably between about 20 xcexcm and about 40 xcexcm.  The insulating rib section may have a thickness substantially similar to a thickness of the emitter stack.  The insulating rib section may be formed of a dielectric material such as a glass frit.  The dielectric material overlying the multiplicity of emitter stacks may include glass powder and a solvent.  The gate electrode may be formed of a silver paste.  The dielectric material layer perpendicularly overlying a multiplicity of emitter stacks may have a width of about 150 xcexcm, the gate electrode on top of the dielectric material layer may have a width of about 110 xcexcm, and the nanotube emitter layer not overlaid by the dielectric material layer may have a width of about 120 xcexcm and a length of about 110 xcexcm. 
The present invention is further directed to a method for fabricating a field emission display device by a thick film printing technique which can be carried out by the operating steps of providing a first electrically insulating plate, screen printing an electrically conductive plate on the first electrically insulating plate forming a cathode, screen printing a layer of high electrical resistivity material on top of the first electrically insulating plate, screen printing a nanotube emitter material on the high electrical resistivity material layer forming a multiplicity of spaced-apart emitter stacks with gaps thereinbetween, screen printing a layer of insulating material in the gaps forming insulating rib sections, screen printing a layer of dielectric material in elongated strips overlying and perpendicularly intersecting the multiplicity of spaced-apart emitter stacks, screen printing a layer of electrically conductive material in elongated strips on top of the layer of dielectric material forming gate electrode, and mounting an anode formed on a second electrically insulating plate overlying the layer of electrical conductive material. 
In the method for fabricating a field emission display device by a thick film printing technique, the first electrically insulating plate may be a glass plate that is substantially transparent.  The method may further include the step of screen printing the electrically conductive paste in a silver paste on a glass plate to form a cathode.  The method may further include the step of screen printing a layer of RuO2 on top of the first electrically insulating plate as the high electrical resistivity material.  The method may further include the step of forming the nanotube emitter material with carbon at between about 20 wt % and about 80 wt % and a solvent-containing binder for the remainder. 
The method for fabricating a field emission display device by a thick film printing technique may further include the step of forming the nanotube emitter material with about 50 wt % of carbon fibers that have diameters of between about 30 and about 50 nanometers and about 50 wt % of a solvent-containing binder.  The method may further include the step of forming the emitter stack to a total thickness between about 20 xcexcm and about 40 xcexcm.  The method may further include the step of forming the insulating rib sections to a thickness substantially similar to a thickness of the emitter stack, or the step of screen printing elongated strips of the layer of dielectric material on top of the multiplicity of emitter stacks by a material including a glass powder and a solvent.  The method may further include the step of forming the gate electrode with a silver paste, or the step of forming the anode on a phosphor coated plate.