Abstract:
A method is provided for preventing electron emission from a sidewall ( 34 ) of a gate electrode ( 20 ) and the edge ( 28 ) of the gate electrode stack of a field emission device ( 10 ), the gate electrode ( 20 ) having a surface ( 24 ) distally disposed from an anode ( 40 ) and a side ( 26 ) proximate to emission electrodes ( 38 ). The method comprises growing dielectric material ( 22 ) over the surface ( 24 ) and side ( 26 ) of the gate electrode ( 20 ), and performing an anisotropic etch ( 32 ) normal to the surface ( 24 ) to remove the dielectric material ( 22 ) from the surface ( 24 ) and leaving at least a portion of the dielectric material ( 22 ) on the side ( 26 ) of the gate electrode ( 20 ) and edge ( 28 ) of the gate electrode stack.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to field emission devices and more particularly to a method of preventing electron emission and leakage from a sidewall of a gate electrode and the edge of the gate electrode stack of a field emission device. 
     BACKGROUND OF THE INVENTION 
     Field emission displays include an anode and a cathode structure. The cathode is configured into a matrix of rows and columns, such that a given pixel can be individually addressed. Addressing is accomplished by placing a positive voltage on one row at a time. During the row activation time, data is sent in parallel to each pixel in the selected row by way of a negative voltage applied to the column connections, while the anode is held at a high positive voltage. The voltage differential between the addressed cathode pixels and the anode accelerates the emitted electrons toward the anode. 
     Field effect devices typically comprise a metal cathode on a substrate, with carbon nanotubes grown on the cathode. A metal catalyst may be positioned between the cathode and the carbon nanotubes for facilitating carbon nanotube growth. A gate electrode is positioned between an anode and the tops of the carbon nanotubes for controlling electron emission from the carbon nanotubes. Electrons flow from the metal cathode through the metal catalyst if present, and out the carbon nanotubes to the anode spaced therefrom. 
     Color field emission display devices typically include a cathodoluminescent material underlying an electrically conductive anode. The anode resides on an optically transparent frontplate and is positioned in parallel relationship to an electrically conductive cathode. The cathode is typically attached to a glass backplate and a two dimensional array of field emission sites is disposed on the cathode. The anode is divided into a plurality of pixels and each pixel is divided into three subpixels. Each subpixel is formed by a phosphor corresponding to a different one of the three primary colors, for example, red, green, and blue. Correspondingly, the electron emission sites on the cathode are grouped into pixels and subpixels, where each emitter subpixel is aligned with a red, green, or blue subpixel on the anode. By individually activating each subpixel, the resulting color can be varied anywhere within the color gamut triangle. The color gamut triangle is a standardized triangular-shaped chart used in the color display industry. The color gamut triangle is defined by each individual phosphor&#39;s color coordinates, and shows the color obtained by activating each primary color to a given output intensity. 
     However, vacuum field emission devices are commonly plagued with electrons being emitted (a leakage current) from various types of unintended emission sites. These spurious emission sites are often formed as an unintended consequence of the fabrication process. Unintended emitters can result from anomalously sharp edges of metal electrodes; conductive particles in high field regions, patterning defects, lifting metal, emitters (such as nanotubes) deposited in the wrong place, etc. In addition, many types of field emission cathode structures have a gate electrode stack. This feature typically incorporates a metal gate electrode deposited on top of an insulator, which is then deposited on or very near a cathode electrode. The edges of these features are typically exposed at a sidewall feature. In some cases the wall in vertical, in some cases the ‘wall’ is a gentle slope, and in some cases, the wall is a concave feature. Regardless of the exact structure of the wall, the ‘wall’ feature is a typical location for unintended emission because it is a high electric field region. 
     An example of a type of defect that causes unintended emission is a sharp point on the sidewall of the gate electrode metal. This defect can occur at the edge of a gate electrode stack, but it can also occur at any edge of the gate metal. This defect is typically caused by a wet etch in the manufacturing process, but could also be caused by lithography, stamping, screen printing, or any other process providing gate anomalies. In the case where the anode field alone is sufficient to initiate electron emission, this undesired emission site is commonly referred to as an anode leader. The intensity of electron emission increases with the applied anode voltage. Furthermore, when field emission devices are in their ‘off’ state, the gate electrode potential is driven lower than the cathode electrode potential, creating a reverse bias condition. In this case, the cathode electrode itself provides the field which pulls electrons off the gate metal asperity. This emission site is often called a reverse bias leader. Both cases lead to image defects wherein the sub-pixels are always illuminated, resulting in loss of contrast and brightness, and the inability to operate the device at optimal conditions. 
     Another type of unintended emission results from defects at the edge of the gate electrode stack. Conductive particles can be defects at the base of the gate electrode stack. They might result from particles present in the process line, patterning defects, re-deposited material during wet processing, or emitter features (such as nanotubes) erroneously deposited in the wrong place. The base of the gate electrode stack forms a junction between a conductor, an insulator, and vacuum which is commonly termed a triple point. This junction creates an enhanced electric field at the conductive defect, and under the influence of the gate potential and/or the anode potential, the conductive defect can emit electrons. These electrons typically cascade up the sidewall, producing an unwanted leakage current between the anode and the cathode, and often produce emitted electrons at the anode. These defects typically are not ballasted by series resistance in the field emission structure so they contribute to excessive (and non-uniform) light at the sub-pixel. They also become hot and produce a run-away current condition that ends in the explosion of the defect, and sometimes a device shorting defect. 
     Another defect, residual conductive material on the gate electrode stack sidewall, can also produce leakage current between the gate and the cathode electrodes. The residual conductive material allows some electrons to pass between the gate and cathode electrodes along the insulator surface (surface hopping of emitted electrons). With higher bias between the electrodes, more current flows. Sufficient current flow causes the region to heat up and the current to increase in a positive feedback condition that often ends in an explosion of the sidewall region. This may produce a device shorting defect. 
     Accordingly, it is desirable to provide a method of preventing electron emission from various defects at the sidewall of a gate electrode and the edges of the gate electrode stack of a field emission device. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
     BRIEF SUMMARY OF THE INVENTION 
     An apparatus and method is provided for preventing electron emission from a sidewall of a gate electrode and the edge of the gate electrode stack of a field emission device, the gate electrode having a surface distally disposed from an anode and a side proximate to emission electrodes. The method comprises growing dielectric material over the surface and side of the gate electrode and gate electrode stack, and performing an anisotropic etch normal to the surface to remove the dielectric material from the surface and leaving at least a portion of the dielectric material on the side of the gate electrode and edge of the gate electrode stack. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a partial cutaway side view of a known field emission display; 
         FIG. 2  is a partial cutaway side view illustrating a first step of the exemplary embodiment of the present invention; 
         FIG. 3  is a partial cutaway side view illustrating a second step of the exemplary embodiment of the present invention; and 
         FIG. 4  is a partial cutaway side view of an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
     In order to eliminate electron emission from sharp points on the side of a gate electrode, particles at the bottom of the edge of the gate electrode stack, residual conductive material on the insulator surface at the edge of the gate metal stack, and other defects occurring at the edge of the gate metal stack, of a field emission display, a dielectric material is deposited over the gate electrode including its side. An anisotropic etch is then performed to remove the dielectric material from over the gate electrode, leaving a side-wall layer of the dielectric material that presents a smoother surface on all vertical surfaces. This smoother surface is a good insulator. 
     Referring to  FIG. 1 , a previously known process for forming a cathode  10 , which may be used with the present invention, include depositing a cathode metal  14  on a substrate  12 . The substrate  12  comprises silicon; however, alternate materials, for example, silicon, glass, ceramic, metal, a semiconductor material, or an organic material are anticipated by this disclosure. Substrate  12  can include control electronics or other circuitry, which are not shown in this embodiment for simplicity. The cathode metal  14  is molybdenum, but may comprise any metal. A ballast resistor layer  16  of a semiconductor material is deposited over the cathode metal  14  and the substrate  12 . A conformal layer (e.g., dielectric layer  18 ) is deposited over the ballast resistor above the cathode metal  14  to provide spacing for the gate electrode  20 . The gate electrode  20  comprises a conductor, for example, chrome-copper-chrome layers. The above layers and materials are formed by standard thin or thick film techniques known in the industry. The combination of the gate metal layer  20 , dielectric layer  18 , ballast resistor layer  16  and cathode metal  14  may be referred to as a gate electrode stack. The side  26 ,  28  of the gate electrode stack preferably has an angle greater than 80° (may be concave), and more preferably between 80° and 100°, to the top of the substrate  12 . 
     In accordance with an exemplary embodiment of the present invention, a dielectric material  22  is deposited over the surface  24  and the side  26  of the gate electrode  20 , as well as the side  28  of the dielectric layer  18  and over the ballast resistor  16  in the well  30  (a blanket deposition not requiring a mask). The dielectric material  22  is deposited using a low pressure technique such as PECVD resulting in a uniform thickness in the range of 100 Angstroms to 10,000 Angstroms for example. Other techniques such as sputtering may be used, but the thickness may not be as uniform. The dielectric material preferably comprises silicon oxide or silicon nitride, but may comprise any dielectric material including at least silicon dioxide, silicon oxynitride, and a spin-on glass. 
     In a preferred embodiment, the anisotropic, or directional, etch is a dry etch represented by the arrows  32  in  FIG. 3  is then performed normal, or perpendicular, to the surface  24  of the gate electrode  20 , resulting in the removal of the dielectric material  22  from the surface  24  and from the ballast resistor  16 . The dry etch preferably comprises chlorine, but may comprise any material used in the industry as a dry etch. The dry etch, for example, may be applied at, for example, 350 W RF, 70 mTorr, 20 sccm Ar, 7 sccm CHF 3 , with etch pressure and Ar to CHF 3  ratio being critical to selectivity of planar etching to sidewall etching. The RIE parameters produce a polymer which blocks sidewall etch by forming a polymer on the sidewall at a rate faster than it etches the sidewall polymer. The process parameters maintain a planar surface etch rate higher than the polymer build up rate making the etch anisotropic. A sidewall  34  of the dielectric material remains after the dry etch on the side  26  of the gate electrode  20  (as well on the side  28  of the dielectric layer  18 ) due to the physical property of a dry etch removing a much larger (ten times for example) amount of the dielectric material  22  when impacted normal to the surface as opposed to vertically. Preferably, 50% to 80% of the thickness of the dielectric material  22  remains as the sidewall  34  after the dry etch. The sidewall  34  must be thick enough to lower the electric field potential of the gate electrode  20 . Alternatively, a wet etch may be used, such as when the conformal layer  22  comprises a vertical grain structure or a multi-layer stack. 
     In accordance with known methods, the catalyst  36  is deposited on the ballast resistor  16 . The catalyst  36  preferably comprises nickel, but could comprise any one of a number of other materials including cobalt, iron, and a transition metal or oxides and alloys thereof. Additionally, the catalyst  36  may be formed by any process known in the industry, e.g., evaporation, sputtering, precipitation, wet chemical impregnation, incipient wetness impregnation, adsorption, ion exchange in aqueous medium or solid state, before having the present invention applied thereto. One preferred method would be to form a relatively smooth film and subsequently etching the film to provide a rougher surface. 
     Carbon nanotubes  38  are then grown from the catalyst  36  in a manner known to those skilled in the art. Although only a few carbon nanotubes  38  are shown, those skilled in the art understand that any number of carbon nanotubes  38  could be formed. It should be understood that any nanotube or electron emitter having a height to radius ratio of greater than 100, for example, would function equally well with some embodiments of the present invention. 
     Anode plate  40  includes a solid, transparent material, for example, glass. Typically, a black matrix material (not shown) is disposed on the anode plate to define openings (not shown) representing pixels and sub-pixels containing a phosphor material (not shown) in a manner known to those in the industry. The phosphor material is cathodoluminescent and emits light upon activation by electrons, which are emitted by carbon nanotubes  38 . 
     As used herein, carbon nanotubes include any elongated carbon structure. Preferably, the carbon nanotubes  38  are grown on a line from the cathode  10  (more particularly the catalyst  36  in this exemplary embodiment) towards the anode  40 . 
     The sidewall spacer  34  of dielectric material  22  isolates the gate electrode  20  from the cathode  14  (e.g., through the ballast resistor  16  and catalyst material  36 ). Since the sidewall spacer  34  is positioned in a non-active area of the field emission device  10 , it does not negatively impact the display from the anode  40 . 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.