FIELD EMISSION DEVICE HAVING A COMPOSITE SPACER

Abstract: A field emission display (100) includes a cathode (102) having a plurality of electron emitters (124), an anode (104) opposing the cathode (102), and a composite spacer (108) extending between the anode (104) and the cathode (102). The composite spacer (108) includes a first layer (107), which is made from a dielectric or bulk resistive material, and a conductive layer (109), which is attached to the first layer (107) and is made from a metal, metal alloy, or a ceramic-metal composite material. The height of the composite spacer (108) is greater than 500 micrometers.

Description: FIELD EMISSION DEVICE HAVING A COMPOSITE SPACER Reference to Related Application Related subject matter is disclosed in a U.S. patent application entitled "Method for Fabricating a Field Emission Device Having a Composite Spacer", having an attorney's docket number of FD97091, filed on even date herewith, and assigned to the same assignee. Field of the Invention The present invention pertains to the area of field emission devices and, more particularly, to field emission displays . Background of the Invention It is known in the art to use spacer structures between the cathode and anode plates of a field emission display. The spacer structures maintain the separation between the cathode and the anode. They must also withstand the potential difference between the cathode and the anode. However, spacers can adversely affect the flow of electrons toward the anode in the vicinity of the spacer. Some of the electrons emitted from the cathode can electrostatically charge the surface of the spacer, changing the voltage distribution near the spacer from the desired voltage distribution. The change in voltage distribution near the spacer can result in distortion of the electron flow. It can also result in electrical arcing, as between the spacer and the cathode. In a field emission display, this distortion of the electron flow proximate to the spacers can result in distortions in the image produced by the display. In particular, the distortions render the spacers "visible" by producing a dark region in the image at the location of each spacer. Several prior art spacers attempt to solve the problems associated with spacer charging. For example, it is known in the art to provide a spacer having a surface, which has a sheet resistance that is low enough to remove the impinging electrons by conduction, yet high enough to ameliorate power loss due to electrical current from the anode to the cathode. The resistive surface can be realized by coating the spacer with a film having the desired resistance. However, these films are susceptible to mechanical damage and/or alteration, such as may occur during the handling of the spacers. These films may also introduce chemical incompatibilities with, for example, the cathode. Chemical incompatibilities can adversely affect the emission characteristics of the electron emitters on the cathode. Also, coated spacers can be difficult to manufacture. It is also known in the art to provide additional, independently controlled electrodes along the height of the spacer for controlling the voltage distribution near the spacer. However, this prior art scheme includes additional processing steps for forming the spacer electrodes, which are also mechanically susceptible to damage. This prior art scheme also uses additional voltage sources for applying potentials to the spacer electrodes, which can increase the power requirements of the device. Accordingly, there exists a need for an improved field emission device, which has spacers that reduce distortion of electron flow and that do not result in excessive power losses. Brief Description of the Drawing The sole FIGURE is a cross-sectional view of an embodiment of a field emission device in accordance with the invention. It will be appreciated that for simplicity and clarity of illustration, elements shown in the FIGURE have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other . page intentionally left blank Description The invention is for a field emission device having composite spacers. Each composite spacer has a first layer, which can be made from a dielectric material or a bulk resistive material, and a conductive layer, which is attached to the first layer. The conductive layer is proximate to the cathode, and the first layer is proximate to the anode. The conductive layer causes electrons to be directed away from the composite spacer, so that charging of the spacer surfaces is controlled. The controlled charging provides the benefit of reduced distortion of the trajectory of electrons due to the presence of spacers. The composite spacer also has a height that is greater than 500 micrometers, which renders the composite spacer of the invention useful in high-voltage field emission devices, which are operated at cathode-to-anode potential differences exceeding about 2500 volts. In one embodiment of the invention, the field emission device is a field emission display having composite spacers, which are invisible to a viewer of the field emission display. The sole FIGURE is a cross-sectional view of a field emission display (FED) 100 in accordance with the invention. FED 100 has a cathode 102, which opposes an anode 104. An evacuated region 106 exists between cathode 102 and anode 104. The pressure within evacuated region 106 is less than about 10 " e r p orr _ A composite spacer 108 extends between cathode 102 and anode 104. Composite spacer 108 provides mechanical support to maintain the separation between cathode 102 and anode 104. Composite spacer 108 also has features that ameliorate electrostatic charging of composite spacer 108. By controlling the electrostatic charging of composite spacer 108, distortions of the trajectory of an electron current 132 within FED 100 are also controlled. In the embodiment of the FIGURE, composite spacer 108 has features that render it invisible to a viewer of FED 100 during its operation. Cathode 102 includes a substrate 116, which can be made from glass, silicon, and the like. Upon substrate 116 is disposed a cathode conductor 118, which can include a thin layer of molybdenum. A dielectric layer 120 is formed on cathode conductor 118. Dielectric layer 120 can be made from, for example, silicon dioxide. Dielectric layer 120 defines a plurality of emitter wells 122, in which are disposed one each a plurality of electron emitters 124. In the embodiment of the FIGURE, electron emitters 124 include Spindt tips. However, a field emission device in accordance with the invention is not limited to Spindt tip electron sources. For example, an emissive carbon film can alternatively be employed for the electron source of the cathode. Cathode 102 further includes a plurality of gate extraction electrodes. A first gate extraction electrode 126 and a second gate extraction electrode 128 are illustrated in the FIGURE. In general, the gate extraction electrodes are used to selectively address the electron emitters. Anode 104 includes a transparent substrate 110, upon which is disposed an anode conductor 112, which is transparent and can include a thin layer of indium tin oxide. A plurality of phosphors 114 is disposed upon anode conductor 112. Phosphors 114 oppose electron emitters 124. A first voltage source 136 is connected to anode conductor 112. A second voltage source 138 is connected to second gate extraction electrode 128. A third voltage source 140 is connected to first gate extraction electrode 126, and a fourth voltage source 142 is connected to cathode conductor 118. Composite spacer 108 extends between cathode 102 and anode 104 to provide mechanical support. The height of composite spacer 108 is sufficient to aid in the prevention of electrical arcing between anode 104 and cathode 102. For example, for a potential difference between anode 104 and cathode 102 of greater than about 2500, the height of composite spacer 108 is greater than about 500 micrometers, preferably within a range of 700 - 1200 micrometers. One end of composite spacer 108 contacts anode 104, at a surface that is not covered by phosphors 114; the opposing end of composite spacer 108 contacts cathode 102, at a portion that does not define emitter wells 122. In accordance with the invention, composite spacer 108 includes a first layer 107, which extends from a point intermediate the ends of composite spacer 108 toward anode 104. First layer 107 has a height, H. Composite spacer 108 also includes a conductive layer 109, which is connected to first layer 107 and extends toward cathode 102. Conductive layer 109 has a conductive surface 111, which is disposed within evacuated region 106. Conductive layer 109 has a height, h. First layer 107 includes a dielectric material or a bulk resistive material. The material of first layer 107 is selected to withstand the applied potential. The material of first layer 107 preferably has a high work function in order to ameliorate spurious electron emission from first layer 107. First layer 107 also preferably has very smooth surfaces in order to reduce charge accumulation and spurious emission. Dielectric materials useful for first layer 107 include ceramic, glass, sapphire, quartz, and the like. Bulk resistive materials useful for first layer 107 include iron- containing glass ceramics, tin oxide, nickel oxide, silicon nitride, neodymium titanate, zirconium oxide, and the like. In general, the bulk resistive material has a resistivity within a range of 10 5 - 10 13 Ωcm. Conductive layer 109 includes a conductive material. Preferably, conductive layer 109 is made from a metal, such as aluminum, gold, copper, and the like. It can also be made from a metal alloy. In another embodiment of the invention, conductive layer 109 includes a ceramic-metal composite material. The conductive nature of conductive layer 109 is necessary for causing the electron deflection or focusing, as described above . The material comprising conductive layer 109 can be selected to provide additional advantages. For example, a ductile metal reduces the risk of damage to cathode 102 during assembly of FED 100. The material of conductive layer 109 can have a high work function in order to control spurious electron emission from conductive layer 109. The material chosen for conductive layer 109 must also be very inert with respect to the materials of cathode 102, to prevent the formation of intermetallics and other undesirable chemical reactions . The height, h, of conductive layer 109 is selected to provide sufficient deflection of electron current 132 to ameliorate electrostatic charging of composite spacer 108 and to direct electron current 132 toward the appropriate phosphor. That is, the height, h, is selected to cause the electrons to strike the phosphor that opposes the originating electron emitter. By deflecting the electrons, conductive layer 109 prevents excessive electrostatic charging of composite spacer 108, which would otherwise produce undesirable results, such as excessive distortion of the trajectory of electron current 132, visibly discernable spacers, electrical arcing, and the like. In the preferred embodiment of the invention, conductive layer 109 is connected to a potential, which is useful for removing the electrostatic charge developed on composite spacer 108 during operation of FED 100. In the embodiment of the FIGURE, the discharging potential is provided at a second conductive layer 130. Second conductive layer 130 is disposed on dielectric layer 120 and includes a thin layer of a conductive material, such as molybdenum, aluminum, and the like. The potential at second conductive layer 130 can be independently controlled. Most preferably second conductive layer 130 is connected to electrical ground. The connection to electrical ground provides enhanced electron deflection, while not requiring additional power. However, it can also be connected to a fifth potential source (not shown) for applying a potential selected to provide the desired extent of electron deflection and/or charge dissipation characteristics of composite spacer 108. Alternatively, second conductive layer 130 can be, connected to or be a portion of one of the gate extraction electrodes of FED 100, which does not require an additional potential source. In the embodiment of the FIGURE, electron current 132 is controlled to an extent sufficient to render composite spacer 108 invisible to a viewer of FED 100 during its operation. Specifically, conductive layer 109 is required for shaping the electric field in the vicinity of composite spacer 108. To provide this field shaping function, it is critical that the conductive material of conductive layer 109 be exposed to evacuated region 106. An exemplary configuration of a field emission device in accordance with the invention will now be described with reference to the FIGURE. It is desired to be understood that a field emission device embodying the invention is not limited to the geometric configuration described with reference to the FIGURE. This configuration is useful for operation of FED 100 at potential differences between cathode 102 and anode 104, which are greater than about 300 volts, and preferably within a range of about 2500 - 10,000 volts. It also includes a VGA configuration. It is desired to be understood, however, that a field emission display embodying the invention is not limited to a VGA configuration. In the embodiment of the FIGURE, transparent substrate 110 and substrate 116 each have a thickness of about one millimeter. Composite spacer 108 includes a rectangular platelet, which has a length (into the page) of about 5 millimeters, a height (extending between cathode 102 and anode 104) of about 1 millimeter, and a thickness, t, of about 0.07 millimeters. The center-to-center distance between first and second gate extraction electrodes 126, 128 is about 0.3 millimeters. FED 100 can be operated at a potential difference between anode conductor 112 and first and second gate extraction electrodes 126, 128 within a range of about 2500 - 10,000 volts. For this voltage range, it is essential that the distance between anode 104 and cathode 102 be greater than 500 micrometers in order to reduce the risk of electrical arcing between anode 104 and cathode 102. During the operation of FED 100, potentials are applied to first and second gate extraction electrodes 126, 128, cathode conductor 118, and anode conductor 112 to cause selected electron emission at electron emitters 124 and to direct the electrons through evacuated region 106 toward phosphors 114. Phosphors 114 are caused to emit light by the impinging electrons . Conductive layer 109 shapes the electric field in the vicinity of composite spacer 108, so that electrons emitted proximate to composite spacer 108 are directed toward phosphors 114 and do not impinge upon composite spacer 108. For the exemplary configuration described with reference to the FIGURE and for a potential difference between anode conductor 112 and first and second gate extraction electrodes 126, 128 within a range of about 2500 - 10,000 volts, the height, h, of conductive layer 109 is preferably within a range of about 75 - 150 micrometers. A composite spacer in accordance with the invention can be made using very economical and simple methods . The composite spacer of the invention does not require photolithographic steps, expensive x-ray lithography, or highly directional etching and deposition techniques . It also does not require steps that coat the electron emitters, which can risk the integrity of the electron emitters. Composite spacer 108 can be made by first providing a sheet of dielectric or bulk resistive material. Such sheets are commercially available. A metal layer is formed on the dielectric or bulk resistive sheet by one of a number of convenient methods, such as silk screening, brazing of a metal foil, electroplating, electroless plating, acoustophoresis, and the like. The composite sheet, which has the metal layer attached to the dielectric or bulk resistive layer, is then cut into individual spacers, as by cutting with a wire saw or dicing saw. Preferably, the cutting operation is initiated at the surface of the dielectric or bulk resistive material, in order to avoid coating the surfaces of first layer 107 with the conductive material . To fabricate the embodiment in which conductive layer 109 is made from a ceramic-metal composite material, first, the ceramic-metal composite material is made by dispersing an insulating ceramic phase in a metallic powder. The mixture is formed into thin sheets via a standard ceramic forming method, such as tape casting, dry pressing, slip casting, and the like. From the forming operation, a monolith is cut out. The monolith is laminated to a layer of the selected dielectric or bulk resistive material. The lamination step is performed by applying pressure at the bonding interface at a temperature less than about 200°C. Alternatively, a bond can be achieved by heat treatment at temperatures above 200°C in a reducing atmosphere to prevent oxidation of the metal component . By varying the compositions of the metal and the ceramic, the mechanical and thermal properties can be tailored to achieve the desired characteristics, such as the desired thermal expansion coefficient of conductive layer 109. This embodiment is useful, for example, if greater mechanical strength, rather than compliance, is desired at conductive layer 109. Methods for forming anode 104 and cathode 102 are known to one skilled in the art. After anode 104 and cathode 102 are made, composite spacers 108 can be bonded to second conductive layer 130 by, for example, thermal compression bonding to maintain a perpendicular configuration with respect to cathode 102. Anode 104 is then placed upon composite spacers 108 and the package is hermetically sealed in a vacuum environment . In summary, the invention is for a field emission device having composite spacers. The field emission device of the invention can be operated at anode-to-cathode potential differences greater than 300 volts, preferably within a range of about 2500 - 10,000 volts. A field emission display in accordance with the invention has "invisible spacers", which are not visibly discernable to a viewer of the display. The composite spacer of the invention can be fabricated using simple, economical methods. While we have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. We desire it to be understood, therefore, that this invention is not limited to the particular forms shown, and we intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.

Claims: CLAIMS 1. A field emission device (100) comprising: a cathode (102) having a plurality of electron emitters (124) , wherein the plurality of electron emitters (124) are designed to emit an electron current (132) ; an anode (104) disposed to receive the electron current (132) emitted by the plurality of electron emitters (124); an evacuated region (106) disposed between the cathode (102) and the anode (104); and a composite spacer (108) extending between the anode (104) and the cathode (102) and comprising a first layer (107) and a conductive layer (109), wherein the conductive layer (109) of the composite spacer (108) defines a conductive surface (111) disposed within the evacuated region (106) , and wherein the composite spacer (108) has a height greater than 500 micrometers. 2. The field emission device (100) as claimed in claim 1, wherein the first layer (107) extends between the anode (104) and the conductive layer (109), and wherein the conductive layer (109) extends between the first layer (107) and the cathode (102) . 3. The field emission device (100) as claimed in claim 1, wherein the conductive layer (109) of the composite spacer (108) comprises a metal. 4. The field emission device (100) as claimed in claim 1, wherein the conductive layer (109) of the composite spacer (108) comprises a ceramic-metal composite material. 5. The field emission device (100) as claimed in claim 1, wherein the first layer (107) of the composite spacer (108) comprises a bulk resistive material. 6. The field emission device (100) as claimed in claim 1, wherein the first layer (107) of the composite spacer (108) comprises a dielectric material. 7. The field emission device (100) as claimed in claim 1, wherein the conductive layer (109) of the composite spacer (108) has a height selected to cause deflection of the electron current (132) to an extent sufficient to prevent excessive electrostatic charging of the composite spacer (108) by the electron current (132). 8. The field emission device (100) as claimed in claim 1, wherein the cathode (102) includes a dielectric layer (120), wherein the cathode (102) further includes a second conductive layer (130) disposed on the dielectric layer (120) thereof, and wherein the second conductive layer (130) is connected to the conductive layer (109) of the composite spacer (108) whereby electrostatic charge developed on the composite spacer (108) during operation of the field emission device (100) is dissipated through the second conductive layer (130) . 9. The field emission device (100) as claimed in claim 8, wherein the cathode (102) comprises a plurality of gate extraction electrodes (126, 128), and wherein the second conductive layer (130) is connected to one of the plurality of gate extraction electrodes (126, 128) .

Inventors: AGENO SCOTT K, SMITH PETER A, AGENO, SCOTT, K., SMITH, PETER, A.
Applicants: MOTOROLA INC, MOTOROLA INC.
Application year: 1998
Main classification: H01J1
IPC codes: H01J9, H01J29, H01J31