Abstract:
A field emission device ( 10 ) is provided that prevents electrical breakdown. The field emission device ( 10 ) comprises an anode ( 40 ) distally disposed from a cathode plate that includes an insulating substrate ( 12 ) having a portion exposed to the anode ( 40 ), and a cathode metal ( 14 ) overlying another portion of the insulating substrate ( 12 ). A gate electrode ( 26 ) overlies an oxide ( 24 ) above at least a portion of the cathode metal ( 14 ) and optionally above a portion of the substrate. A dielectric layer ( 18 ) is positioned between a resistive layer ( 22 ) and the cathode metal ( 14 ), and substantially all of the exposed substrate, and underlies substantially all of the gate electrode ( 26 ) including its edges ( 34, 46 ), providing a resistance between the cathode metal ( 14 ) and the edges ( 34, 46 ).

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to field emission devices and more particularly to a field emission device structure that prevents electrical breakdown. 
     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 electrons emitted from the pixels 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, for example, 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 has been deposited on or very near a cathode electrode. 
     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 encountered in the process environment 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 insulator 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. 
     Previously known field emission structures, e.g., Spindt tip and some carbon nanotube emitters, often have an unpatterned oxide layer. These structures, comprising metal (cathode)-ballast-oxide-metal (gate), are fabricated by patterning only the top metal layer, leaving an exposed surface of dielectric (oxide). In fact, the manufacturing method of the Spindt tip specifically leaves this dielectric on the surface. This dielectric charges and produces arcs unless a bleed layer is applied to the top surface. To preserve a desired small number of masks, the catalyst or emitter is fabricated with a lift-off process. First, the lift-off photo layer is used as a mask (or to define a lift-off mask) which is used to etch through the oxide layer down to the underlying layers. Next, the emitter or catalyst material is deposited through the same lift-off masking layer into the well and onto the lift-off layer. Removal of the lift-off layer leaves patterned emitter or catalyst in three mask steps. However, the lift-off process does not scale to large size panels. A subtractive etch of the catalyst will be needed. Because these prior art methods use the lift off layer for both local oxide etch (via to bottom metal layer), they need a fourth mask to implement a subtractive etch. 
     Other methods may comprise a process with only three masks; however, there is a metal (cathode)-ballast-oxide-metal (gate) interface outside the emitter well that is susceptible to small defects resulting in arcs which may short out the device due in part to the ballast providing insufficient vertical resistance for suppressing any arcing. The bottom interface is effectively a metal-oxide-vacuum triple junction which is known to be susceptible to arcing. 
     Accordingly, it is desirable to provide a field emission device structure that prevents electrical breakdown with as few mask layers as possible. 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 
     A field emission device is provided that prevents electrical breakdown. The field emission device comprises an anode distally disposed from a cathode plate that includes an insulating substrate having a portion exposed to the anode, and a cathode metal overlying another portion of the insulating substrate. A gate electrode overlies an oxide above at least a portion of the cathode metal and optionally above a portion of the substrate. A resistive layer overlies the cathode metal and substantially all of the exposed substrate, and underlies substantially all of the gate electrode including its edges, providing a resistance between the cathode metal and the edges. 
    
    
     
       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 cross-section of a field emission device cathode of an exemplary embodiment; and 
         FIG. 2  is a partial perspective view of the exemplary embodiment of  FIG. 1 . 
     
    
    
     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 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 resistive layer acts as a lateral resistor for arc suppression. The resistive layer may be the same as a ballast layer that is used to couple the electron emitters to the cathode metal. A metal-oxide-ballast-oxide-metal stack is fabricated having the first metal with a sloped sidewall contacting the resistive layer. The first metal-oxide stack is patterned in one step with a slope on both layers, requiring no additional masks. The resistive layer also acts as a bleed layer for charges that accumulate on the cathode surface from operation of the device. Electrons and ions generated from operation of the device tend to collect on the cathode surface. Thus, it is important that either a metal layer or a resistive charge bleed layer cover all non-vertical surfaces of the cathode. Additionally, the resistive layer effectively acts as a spacer landing zone with the ability to bleed charge from the spacer. 
     Referring to  FIGS. 1 and 2 , a process for forming a cathode  10  in accordance with an exemplary embodiment includes depositing a cathode metal  14  over a substrate  12 . The substrate  12  comprises glass; however, alternate materials, for example, silicon, 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  preferably is molybdenum, but may comprise any metal, or group of conductors such as chrome-copper-chrome layers. An optional buried oxide or dielectric layer  16  may be formed in the top portion of the substrate  12  prior to depositing the cathode metal  14  as a diffusion barrier for contaminants and for providing a pure surface layer. The combination of this buried layer and the substrate is typically considered the ‘substrate’. In  FIG. 2 , the optional buried oxide layer  16  is not shown. A dielectric layer  18  is formed over the cathode metal  14 . 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. The thickness of this layer  18  is important. If it is too thin, it may break down at a defect site. If it is too thick, it can change the field applied to the emitters. Typical thickness range from 500 angstroms to 10 micrometers. In the preferred thin film case, the thickness lies between 2000 angstroms and one micrometer. In a thick film device, the preferred thickness for electrical integrity is approximately 4 to 10 micrometers. The cathode metal  14  and dielectric layer  18  are etched using known lithograph processes to form a structure  20 . 
     A ballast resistor layer  22  of a semiconductor material is deposited over the dielectric layer  18  and the substrate  12 . A conformal layer (e.g., dielectric layer  24 ) is deposited over the ballast resistor above the cathode metal  14  to provide spacing for the gate electrode  26 . The gate electrode  26  comprises a conductor, for example, molybdenum or 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  26 , dielectric layer  24 , ballast resistor layer  22 , dielectric layer  18 , and cathode metal  14  may be referred to as a gate electrode stack. The gate electrode  26  and dielectric layer  24  define a well  28 . 
     In accordance with known methods, the optional catalyst layer  30  is deposited on the ballast resistor  22 . The catalyst  30  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  30  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. 
     In  FIG. 1 , the exemplary embodiment shows the gate-oxide stack edge  34  closer to the emitter pad  30  than the cathode metal edge  36 . Resistive material  22  lies under the gate-oxide stack edge  34  and electrically connects the electron emitter pad  30  to the cathode metal  14 . The position of the gate-oxide stack edge  34  relative to the cathode metal edge  36  is important because there is now a resistive path  38  between the gate-oxide stack edge  34  and the cathode metal  36 . Any defect that occurs on that edge  34  will not form a point short. In some cases, electron focusing can be improved by moving the gate-oxide stack edge  34  approximately over the top of the cathode metal step edge  36 . This is most preferably done with a thick dielectric layer  18  on top of the cathode metal  14 , but underlying the resistive layer  22 . The ballast  22  at the edge  36  of this dielectric layer  18  then forms a lateral resistance path  38  between the gate-oxide stack edge  34  and the cathode metal  14 , offering some protection against leakage currents and spurious electron emission from defects. 
     Carbon nanotubes  32  are then grown from the catalyst  30  in a manner known to those skilled in the art. Although only a few carbon nanotubes  32  are shown, those skilled in the art understand that any number of carbon nanotubes  32  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  32 . 
     As used herein, carbon nanotubes include any elongated carbon structure. Preferably, the carbon nanotubes  32  are grown on a line from the catalyst  30  (in this exemplary embodiment) towards the anode  40 . 
     Referring to  FIGS. 1 and 2 , the side  42  of the cathode metal  14  and dielectric layer  18  opposed to the carbon nanotubes  32  is covered by the ballast resistor  22 . The ballast resistor  22  further extends onto and covers part of the buried oxide layer  16  at surface  44 . As shown in  FIG. 2 , the ballast resistor  22  may be continuous. It is this portion of the ballast layer  22  overlying the dielectric layer  18  and the side  42  that acts as a lateral resistor for arc suppression. The resistive layer  22  connects to the underlying cathode metal  14  only at the sides. Consequently, the gate-oxide layer stack edge  46 , which sits on the resistive layer  22 , is electrically connected to the cathode metal  14  through a large span  48  of the resistive material  22 . For effective prevention of spurious emission from defects, the value of this resistance needs to be larger than about 100,000 ohms. For example, this means that a defect that begins to emit one microampere of current under a gate bias of 80 volts, will experience an effective 10 volt drop. This is enough negative feedback to prevent runaway emission. More preferably, the typical resistive path would be larger than one mega-ohm in most regions, so that a small current drops nearly all of the gate voltage in the resistive layer  22 , leaving no effective bias on the defect. 
     It is clearly preferable to have every gate-oxide stack edge  34 ,  46  separated by a high resistance. However, in practice, the gate metal  26  bus bar must cross the cathode metal edge  42  to proceed to the next pixel. There are several points along the gate oxide stack edge  46  where the resistive path between the gate-oxide stack edge  46  is separated from the cathode electrode  14  by only the thickness of the dielectric layer  18  on top of the cathode metal. At this location, it is often not feasible to provide sufficient resistivity. Obviously, the thicker the dielectric layer is over the cathode metal, the more protection is accomplished at these points in the device. However, the probability of having a defect in that exact location is extremely low, so the overall solution still provides good protection. Clearly, it is desirable to protect a majority of the gate-oxide edges  46  with a highly resistive path. Good protection is obtained when more than 90% of the gate-oxide stack edge  46  length is coupled to the cathode metal  14  through a resistance greater than 100K. More preferably, more than 95% of the gate-oxide stack edge  46 S length is coupled to the cathode metal  14  through a resistance greater than 100K. 
     The exemplary embodiment, incorporating lateral resistances to the cathode metal  14  at predominantly all the gate-oxide stack edges, can be fabricated with only three mask steps. This is an important feature of a low cost field emission device technology. An example of the fabrication sequence is as follows. The cathode metal layer  14  and the dielectric layer  18  are deposited and then patterned using the same mask step (mask  1 ). They are etched with a sloped sidewall etch technique. The resistive layer  22  is deposited over the top. The oxide layer  24  and the gate layer  26  are deposited on the top and patterned using the same mask step (mask  2 ) and then etched. Finally, the nanotube catalyst layer  30  (or alternatively the nanotube-containing layer  32 ) is deposited and patterned using mask step  3 . Consequently, the structure can be fabricated with only three mask steps. Alternatively, a fourth low resolution mask might be used to pattern the resistive layer  22  between the cathode metal bus lines  44  (mask  4 ). In some cases, where the expense of more mask layers can be tolerated, variations to this embodiment may exist. For example, although the dielectric layer  18  extends to the right edge of the cathode metal  14  and the ballast layer  22  extends over the dielectric layer  18  and the cathode metal  14 , the dielectric layer  18  and ballast layer  22  may optionally only extend over part of the cathode metal  14 . 
     Another benefit of this structure is that it allows laser repair of structures. In previously known devices, the act of laser repair generates unwanted metal fragments as a laser is used to evaporate metal to excise defective areas. The structure  10  described in this invention prevents such defects from causing point shorts, thereby enabling laser repair. 
     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.