FIELD OF THE INVENTION  
       [0001]     This invention relates generally to field emission electron sources and more particularly to field emission elements formed from a silicon-based semiconductor material and a method for forming the field emission elements.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the technology of field emission devices and structures, an electric potential applied to or near a pointed surface of an emission element or emitter (or a plurality of such emission elements or emitters configured in an array) stimulates the emission of electrons from the pointed surface. A shape of the emitting surface, e.g. a pointed emitter tip, is selected to concentrate the electric field formed by the potential and thus maximize electron emissions into a vacuum surrounding the emitter. Increasing the electrical field intensity increases a current density of the emitted electrons, and the intensity is inversely further related to a radius of curvature of the emitting surface shape. Extremely pointed field emission tips are therefore desired.  
         [0003]     In a field emission display, electrons emitted from the emission element are accelerated in a vacuum to impinge a phosphor screen that glows when struck by the electron. By contrast, in a cathode ray tube display, the electrons are generated by thermal emission from a heated cathode surface. In the field emission display the electrons are emitted from a “cold” cathode surface.  
         [0004]     As illustrated in  FIG. 1 , in a field emission display  6 , electrons are generated by the field emission process from a cathode electrode  8  comprising an array of millions of sub-micrometer emission elements  10  formed within openings  11  in an insulator layer  12 . Application of a voltage V g  between the cathode electrode  8  (overlying a cathode substrate  14 ) and a gate electrode  16  forms an electric field between the cathode electrode  8  and the gate electrode  16 . The electric field causes the emission of electrons from the emission elements  10 . In  FIG. 1 , the emitted electrons are represented by arrowheads  20 .  
         [0005]     A shape of the emission elements  10  is selected to maximize electron emission, as sharper emission elements produce more electrons and thus a brighter image. As the number of emission elements supplying electrons to each display pixel increases, the display reliability also increases, as it is known that the electron emissions from an emission element can decrease with time.  
         [0006]     A voltage V a  (greater than the voltage V g ) applied between the cathode electrode  8  and an anode electrode  24  accelerates the electrons toward a phosphor screen  25  (or other electroluminescent display device). The phosphor screen  25  and the anode electrode  24  are supported by a transparent anode substrate  26 . Responsive to the impinging electrons, phosphor pixels comprising the phosphor screen  25  emit light observable from a surface  30  of the anode substrate  26 . Typically, a plurality of emission elements  10  supply impinging electrons for a single pixel, wherein the plurality of emission elements  10  are insulated from other pluralities of emission elements  10 , such that each plurality is independently controllable for emitting electrons that strike a single pixel.  
         [0007]     For producing a color image, each pixel comprises a color pixel triad, further comprising a red sub-pixel, a green sub-pixel and a blue sub-pixel. The emission elements  10  associated with a pixel are segregated into a matrix of insulated addressable arrays, such that a first array is associated with the red sub-pixel, a second array is associated with the green sub-pixel and a third array is associated with the blue sub-pixel. To produce a blue color on the display, for example, the third emitter group is activated to emit electrons that impinge on the blue sub-pixel.  
         [0008]     To permit operation at relatively low operating voltages, the emission elements  10  are typically constructed from a material exhibiting a low work function (such as molybdenum, where the work function is a measure of the amount of energy required for an electron to escape from the metal into the surrounding vacuum) to increase the electron emissions and shaped in the form of points  34 . As can be seen from  FIG. 1 , the emission elements  10  (also referred to as cones) have a generally triangular shape with each emission element  10  pointed in a direction of the phosphor screen  25  such that electrons emitted from the emission elements  10  are directed toward the screen  25 .  
         [0009]     Application of the voltage V g  between the gate electrode  16  and the cathode electrode  8  controls emission of electrons from the emission elements  10 . As can be seen in  FIG. 1 , the gate electrode  16  is disposed above the cathode electrode  8 . To permit proper electron flow from the emitter emission elements  10  to the anode electrode  24 , the openings  11  formed in the gate electrode  16  and the insulating layer  12  must be properly positioned with respect to the emission elements  10 . A size and location of the openings affect not only the magnitude of electron flow from the emission elements  10 , but also determine the shape and direction of the electron flux. The opening size and circumferential proximity to each emission element  10  determines the voltage V g  that is required for effective control of the electron emissions, while alignment of a hole axis with respect to an element axis controls the electron beam direction.  
         [0010]     Opening/element alignment and opening size have been difficult to control in the prior art due to the extremely small geometries and tolerances associated with the openings  11  and the emission elements  10 . Typically, to obtain opening/element alignment it has been necessary to employ a difficult and time-consuming masking step to form the openings  11 , but slight errors in either the mask or the mask alignment relative to the substrate  14  can detrimentally affect the opening/element alignment and thus the emission of electrons. The difficulties encountered in fabricating such arrays increase significantly as the dimensions of the emitter emission elements  10  are reduced to a sub-micrometer or nanometer scale.  
         [0011]     In addition to opening/element alignment concerns, according to the prior art the emission elements  10  are fabricated using known photolithographic masking, patterning and etching steps. This process limits element density and element quality. In particular, the density is limited by resolution of the photolithographic process. Also, since the emission elements are tapered, each occupies a larger area at a bottom surface than at a tip apex. Thus the required tapered base limits the emission element density, which lowers the image brightness. A higher element density is therefore desired to achieve a higher image brightness.  
         [0012]     In an effort to overcome the disadvantages associated with the use of the photolithographic process for forming emitter emission elements, current research efforts form the emission elements  10  by directing a laser beam toward a substrate surface. When the laser beam strikes the surface material is removed therefrom, with the material remaining forming the emission elements  10 . This process requires a laser scan over the entire substrate and thus can be time consuming. Disadvantageously, the emission elements  10  produced by the laser technique may not be uniform throughout the substrate.  
         [0013]     Etching techniques to remove material layers from a silicon substrate are commonly used in semiconductor fabrication processes. Various dry and wet etchants are available, with each etchant offering specific etching characteristics, including material selectivity, etch uniformity and edge profile control. Plasma etching is one form of dry etching that employs a gas and plasma energy to create a chemical reaction that etches the desired material layer.  
         [0014]     A conventional plasma etching system comprises a chamber, a vacuum system, a gas supply and a power source. After loading a silicon wafer onto a pedestal in the chamber, the vacuum system reduces the pressure and a reactive gas is supplied to the chamber. An electrode in the chamber is energized by a radio frequency power source to energize the gas to a plasma state, producing ions, electrons and radicals. A radio frequency bias applied to the substrate develops an electric field proximate the substrate to attract ions of the reactive gas to the substrate. These ions and the radicals synergistically etch the substrate according to a pattern in a mask overlying the substrate.  
         [0015]     Selection of a specific reactive gas is based on the material to be removed during the etch process. For example, for etching a silicon dioxide material layer, CF 4  and oxygen are typically used. In the energized state, the CF 4  is disassociated into highly reactive carbon and fluorine radicals, in addition to a number of ions. The radicals and ions interact with the substrate, where the fluorine attacks the silicon dioxide, converting the silicon dioxide to a volatile material that is removed from the chamber by the vacuum system. Typically, the plasma etch process is performed at a temperature between about 15 and 45° C., and at a pressure between about 5 and 100 mTorr, depending on the reactor type employed for the process.  
       BRIEF SUMMARY OF THE INVENTION  
       [0016]     One embodiment of the present invention comprises a method for fabricating field emission elements within a silicon substrate. The method comprises providing a plasma etching chamber, supplying oxygen to the chamber, supplying a silicon etchant to the chamber, controlling a ratio of the oxygen to the silicon etchant and etching silicon from the silicon substrate to form the emission elements in the substrate, wherein an upper surface of the emission elements exhibits a generally convergent shape.  
         [0017]     According to another embodiment the invention comprises a field emission display further comprising an anode, a doped silicon substrate, emission elements randomly disposed on a surface of the silicon substrate and having a convergent tip region in a direction of the anode, an insulating layer overlying the substrate, wherein the tip region of each emission element is below an upper surface of the insulating layer and a gate overlying the insulating layer, wherein openings disposed through the insulating layer and the gate expose the tip region of certain ones of the emission elements, and wherein in regions of the substrate absent openings the tip region of other ones of the emission elements remain covered by the insulating layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The foregoing and other features of the invention will be apparent from the following more particular description of the invention, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0019]      FIG. 1  is a cross-sectional illustration of prior art field emission elements.  
         [0020]      FIGS. 2-9  are cross-sectional illustrations of a substrate during sequential processing steps for forming field emission elements according to one method of the present invention.  
         [0021]      FIG. 10  is a top view of a plurality of emission elements formed according to the methods depicted in  FIGS. 2-9 .  
         [0022]      FIGS. 11-13  are additional cross-sectional illustrations of the substrate during subsequent sequential processing steps for forming field emission elements according to one method of the present invention.  
         [0023]      FIG. 14  is a top view of a plurality of emission elements formed according to the method depicted in  FIGS. 2-9  and  11 - 13 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     Before describing in detail the particular method and apparatus for forming field emission elements according to the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of elements and process steps. So as not to obscure the disclosure with details that will be readily apparent to those skilled in the art, certain conventional elements and steps have been presented with lesser detail, while the drawings and the specification describe other elements and steps pertinent to understanding the invention in greater detail.  
         [0025]     A method for forming emission elements  10  according to the present invention begins as illustrated in  FIG. 2 , wherein a substrate  50  comprises a heavily-doped single crystalline silicon layer  52 , having an upper surface  53 , and an overlying silicon nitride layer  54 . Typically, the doping density of the silicon layer  52  produces a sheet resistance of at least 10-30 ohms square or a doping density as required to impart sufficient conductivity to the silicon layer  53 , according to a field emission display into which the silicon layer  53  is incorporated.  
         [0026]     A photoresist layer is deposited overlying the silicon nitride layer  54  and patterned according to known techniques to form a patterned photoresist layer  56 . The pattern in the photoresist layer  56  is determined by a desired pattern for the field emission elements  10 .  
         [0027]     Using the pattern of the photoresist layer  56 , the underlying silicon nitride layer  54  is etched according to known techniques (for example, using a CF 4  chemistry) to form silicon nitride regions  54 A (see  FIG. 3A ) that during a subsequent etch process (using a different etch chemistry from the silicon nitride etch chemistry) prevent formation of the field emission elements  10  in regions of the silicon layer  52  immediately below the silicon nitride regions  54 A. Thus  FIG. 3A  is a cross-sectional view after formation of the silicon nitride regions  54 A, removal of the photoresist layer  56  by plasma etching or by other techniques known in the art and etching of the substrate  52  to form the emission elements  10 . In another embodiment, the silicon nitride regions  54 A may be of a different size than illustrated or may be absent.  
         [0028]     According to the present invention, the emission elements  10  are formed in the silicon layer  52  using a plasma etch process without the use of a photolithographic mask, thus reducing emission element fabrication costs. In addition, the present invention provides higher density and higher aspect ratio emission elements than the prior art techniques, resulting in better element uniformity and a brighter display image. During the plasma etch process, oxygen (O 2 ) and sulfur hexafluoride (SF 6 ) are supplied to the etching chamber in a ratio of oxygen to sulfur hexafluoride of about 1.5:1. Preferred flow rates are about 30 sccm for the oxygen and about 20 sccm for the sulfur hexafluoride. Hydrogen bromine (HBr) is also supplied to the etch chamber at a flow rate of about 50 sccm. In another embodiment, a chlorine-based compound (or other compounds including an element from Column VIIA of the periodic table) can be used in lieu of the hydrogen bromine and/or the sulfur hexafluoride.  
         [0029]     During the etch process, a chamber pressure is maintained at about 30 mTorr. A radio frequency current generating about 60 W of power biases the substrate  50 . A radio frequency source supplies about 1500 W to the plasma-forming electrode in the chamber.  
         [0030]     The stated etch parameters are merely exemplary. Those skilled in the art recognize that variations of up to at least 20% from the stated parameters may produce desired results, i.e., formation of the emission elements  10 . Further, the etch parameters may vary due to the design of the etching tool and the conditions of the chamber.  
         [0031]     During the etch process, oxygen radicals combine with silicon on the upper surface  53  to form silicon dioxide regions  55 , also referred to as micro-masks. These silicon dioxide regions  55  are not easily etched due to the material selective nature of the etchants employed, i.e., a higher etch selectivity to silicon than to silicon dioxide. Thus the emission elements  10  are formed as regions of the silicon layer  52  adjacent the silicon dioxide regions  55  are etched, while silicon regions masked by the silicon dioxide regions  55  remain substantially intact (i.e., are etched at a much slower rate).  
         [0032]     This phenomenon of forming the silicon dioxide regions  55  and etching regions of the silicon layer  52  that are not masked by the silicon dioxide regions  55  is referred to as micro-masking. The process occurs when the etch chemistry is such that both etching (of the silicon) and deposition (of silicon dioxide to form the silicon dioxide micro-masks) occur simultaneously at a ratio of the rate of deposition to the rate of etching determined by the reactants employed during the process.  
         [0033]     Both the SF6 gas and the HBr gas, in an embodiment in which it is present, participate in the silicon etching process. The SF6 etches faster but is less selective to the silicon dioxide and more isotropic (i.e., the resulting etch profile lacks the perpendicularity of a substantially anisotropic etch). The combination of the fluorine and the silicon form volatile SF4 that is removed from the etch chamber. The HBr gas is more selective to the silicon dioxide and etches very anisotropically, because the bromine is less reactive than fluorine and requires a greater ion bombardment energy to form volatile SiBr4.  
         [0034]     The ratio of SF6 to HBr determines the degree of selectivity to the silicon dioxide and the anisotropic features of the resulting etch. Some of the oxygen ions and radicals combine with the silicon to form the silicon dioxide regions  55 , since silicon dioxide is not a volatile material.  
         [0035]     The ions and radicals that etch the substrate  50  are derived from both the SF6 and the HBr (in an embodiment where it is present). The ions strike the surface of the silicon layer  52  substantially normally or anisotropically because they are attracted by the negative potential applied to the substrate  50 . Further, since the ions strike the surface at about 90 degrees to the surface, they tend to drive the etch process vertically, rather than laterally, resulting in a predominantly vertical etch process, creating the emission elements  10  with a higher density than the prior art processes. The free radicals, which carry no charge, strike the silicon layer  52  from substantially all directions because they are not attracted to the substrate  50 . Instead the motion of the radicals is influenced by collisions with other atoms in the chamber and therefore is essentially random in all directions. As the ions impinge the exposed silicon surface, they tend to accelerate the etch process that was begun by the radicals in the first several monolayers of the silicon layer  52 .  
         [0036]     As the etching process begins, the upper surface  53  of the silicon layer  52  comprises a relatively flat surface. As the silicon dioxide regions  55  are formed, the etch process removes material adjacent the silicon dioxide regions  55 , forming substantially rectangular vertical structures  10 A as illustrated in the close-up view of  FIG. 3B . As the etch process continues, enhanced ion bombardment at corners  10 B of the rectangular vertical structures  10 A, due to a larger electric field at corners than on flat surfaces, forms generally convergent emission elements  10 , e.g., conical or pointed emission elements. Formation of a polymer material on sidewalls  10 C of the region  10 A can also contribute to formation of the convergent tips of the emission elements  10 , as the polymer masks the side surfaces  10 C from the bombarding ions and radicals. Following formation of the emission elements  10 , the silicon dioxide regions  55  are removed by selective isotropic etching.  
         [0037]     Beginning in  FIG. 4  and continuing through  FIG. 12 , the substrate  50  undergoes a series of processing steps to form electrically conductive paths to the emission elements  10 , through which current is supplied to cause the emission of electrons. As illustrated in  FIG. 4 , a layer of silicon dioxide  60  is deposited by a high-density plasma or conformal chemical vapor deposition technique. Plasma deposition is preferred due to its excellent gap filling results.  
         [0038]     A chemical/mechanical polishing step (CMP) is performed to planarize an upper surface  64  of the substrate  50 . See  FIG. 5 .  
         [0039]     As illustrated in  FIG. 6 , a silicon dioxide layer  66  is formed overlying the upper surface  64 . A photoresist layer  70  is formed overlying the silicon dioxide layer  66  and patterned to form an opening  72  therein. A corresponding opening is etched in the silicon dioxide layer  66  and the silicon nitride region  54 A, after which the photoresist layer  70  is removed.  
         [0040]     As illustrated in  FIG. 7 , a conductive plug  76  (for example, comprising tungsten) and a barrier layer  78  (for example, comprising titanium or titanium nitride) are formed according to known techniques in the opening in the silicon dioxide layer  66  and the silicon nitride region  54 A. The conductive plug  76  provides an electrical connection to the emission elements  10  through the highly doped silicon layer  52 . In one embodiment, a plurality of electrically insulated emission element arrays are formed in the silicon layer  52 , wherein each element array is associated with a display sub-pixel. Such arrays can be formed by fabricating insulating regions, such as trench isolation regions, in the silicon layer  52 . A tungsten plug, such as the conductive plug  76 , is formed in electrical contact with each array to independently control the emission of electrons from that array. Thus selected arrays can be energized to emit electrons while others remain inactive, thereby producing images on the display.  
         [0041]      FIG. 8  depicts in stacked relation overlying the silicon dioxide layer  66 , a barrier layer  80 , an aluminum layer  82  and a photoresist layer  84 , the latter patterned to form an opening  88  therein. The barrier layer  80  typically comprises a bilayer further comprising a titanium layer and a titanium-nitride layer to avoid migration of the aluminum into the silicon dioxide.  
         [0042]     Using the opening  88  as a pattern, an opening  89  is formed in the aluminum layer  82 , using a chlorine-based etch chemistry, for example. Through the opening  89 , an opening  90  is formed in the material layer  80  and the silicon dioxide layer  66 . As can be seen in  FIG. 9 , the opening  90  exposes the emission element  10 A. The opening  90  is preferably formed using a dry cold fluorine-based isotropic etch to remove material from an upper region of the silicon dioxide layer  66 , stopping prior to reaching material of the silicon layer  52 , such as the emission element  10 . A subsequent silicon-selective dry anisotropic etch removes additional material of the silicon dioxide layer  66 . The described etch is known as a champagne glass etch, which is isotropic in the first step and non-selective to silicon. In the second step the etch is anisotropic and selective to silicon, otherwise the field emission elements would be eroded. During both etch steps the silicon dioxide is etched, while removal of underlying silicon in the silicon layer  52  and the emission elements  10  is minimized.  
         [0043]     After formation of the opening  90  the photoresist layer  84  is removed. Note that a plurality of emission elements  10  are formed within each opening  90 , although only a single emission element  10  is illustrated in  FIG. 9 , which differentiates the present invention from the prior art techniques for forming emission elements.  FIG. 10  illustrates a top view of a region of the substrate  50  depicting a plurality of emission elements  10  within each of a plurality of openings  82 / 90 . A plurality of openings  82 / 90  (and the emission elements disposed therein) form an array element, with each such element providing electrons for a color pixel of the display. Thus a brighter image with a more uniform electron distribution and a more reliable display is provided according to the teachings of the present invention.  
         [0044]     A physical deposition process (according to one embodiment) deposits a material layer  96  over the tip  10 A through the opening  90 , and deposits a conductive layer  98  over the aluminum layer  82 . See  FIG. 11 . A material of the material layer  96  exhibits a low work function for electron emissions such that electrons are emitted from the material layer  96  at relatively low voltages. In one embodiment, the material layer  96  extends to a surface  100  formed in the silicon dioxide layer  60 . In one embodiment the material layer  96  provides a continuous coating over the emission elements  10 ; in another embodiment only the tips  10 A are covered by the material layer  96 . In yet another embodiment, both the material layer  96  and the conductive layer  98  are absent and electors are emitted from the emission elements  10  through the silicon layer  52 .  
         [0045]     As shown in  FIG. 12 , a photoresist layer  106  is deposited overlying the substrate  50  and patterned to form an opening  108  therein.  
         [0046]     As illustrated in  FIG. 13 , the conductive layer  98  and the aluminum layer  82  are patterned according to the opening  108 , forming an opening  111  therein that isolates the conductive plug  76  and a region  82 A of the aluminum layer  82  from regions  82 B of the aluminum layer  82 . The regions  82 B (which are connected in a third dimension not illustrated in  FIG. 13 ) function as the gate electrode  16 , i.e., one terminal of the voltage source V g  is connected to the regions  82 B. The other terminal of the voltage source V g  is connected to the silicon layer  52  and thus to the emission tip  10 A through the region  82 A and the conductive plug  76 . Electrons are emitted from the emissive material layer  96  in response to the applied voltage V g . The sharp point of each silicon emission tip  10 A creates an electric field that facilitates electron emission from the material layer  96 . In another embodiment, the material layer  96  is absent and the electrons are emitted directly from emitter elements  10  formed in the silicon layer  52 . Exemplary materials suitable for use as an emissive material include diamonds, (either chemical vapor deposited, natural diamond grits or synthetic diamonds, doped or undoped) graphite, metals such as molybdenum, tungsten or cesium, compounds such as LaB6, YB6, AIN or combinations of these materials, and other low work function materials.  
         [0047]     A top view of the completed structure is illustrated in  FIG. 14 , including grid conductors  113  and emission element conductors  115  for supplying the voltage V g  between the gate electrode  16  and the emission elements  10 . A controller, not shown, controls application of the voltage V g  to certain of the emission element conductors  115  for causing emission elements  10  associated with those conductors to emit an electron flux.  
         [0048]     A red sub-pixel array  120  comprises a plurality of emission elements  10  that when energized emit electrons that strike a red sub-pixel for producing a red color on the phosphor screen  25 . Similarly, electrons emitted from a blue sub-pixel array  122 , comprising a plurality of emission elements  10 , impinge a blue sub-pixel to produce a blue color and electrons emitted from a green sub-pixel array  124 , comprising a plurality of emission elements  10 , impinge a green sub-pixel to produce a green color. As illustrated in  FIG. 14 , each pixel array  120 ,  122  and  124  comprises an array of openings  82 / 90 , and each opening comprises a plurality of emission elements  10 , although only one emission element  10  is depicted in each opening  82 / 90  for clarity.  
         [0049]     An architecture and process have been described as useful for forming field emission elements in a semiconductor substrate. Specific applications and exemplary embodiments of the invention have been illustrated and discussed, which provide a basis for practicing the invention in a variety of ways and in a variety of circuit structures. Numerous variations are possible within the scope of the invention. Features and elements associated with one or more of the described embodiments are not to be construed as required elements for all embodiments. The invention is limited only by the claims that follow.
A method for forming high density emission elements for a field emission display and field emission elements and field emission displays formed according to the method. Oxygen and a silicon etchant are introduced into a plasma etching chamber containing a silicon substrate. The oxygen reacts with the silicon surface to form regions of silicon dioxide, while the silicon etchant etches the silicon to form the emission elements. The silicon dioxide regions mask the underlying silicon during the silicon etch process. High density and high aspect ratio emission elements are formed without using photolithographic processes as practiced in the prior art. The emission elements formed according to the present invention provide a more uniform emission of electrons than the prior art techniques. Further, a display incorporating emission elements formed according to the present invention provides increased brightness. Further, the reliability of the display is increased due to the use of a plurality of emission elements to supply electrons for stimulating the phosphor substrate material to produce the image.