Abstract:
An array of carbon-based emitters is provided having more uniform electron emission over the area of the array. This is made possible by a resistive layer that is present below each of the emission tips. Both organic and inorganic resistive layers may be grown under the emitting carbon-based material. A conductive backing layer is in contact with the resistive layer. Methods for making the improved array are provided. The methods include growth of carbon-based tips in a mold, removal of various films or portions of films by etching, and other techniques.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to field emission of electrons. More particularly, apparatus and method for its manufacture are provided for improving emission uniformity across an array of carbon-based emitters.  
           [0003]    2. Description of Related Art  
           [0004]    Present field emitter arrays tend to have non-uniform current emission from tips in the array. This deficiency is caused by slight variations in the geometry and work functions of the emitter tips and variations in extraction gate sizes. Accordingly, only five to ten percent of the emitter tips in an array normally carry the current supplied from the array while the remainder of the tips are inactive. The relatively large amount of current passing through some of the emitting tips shortens the lifetime of the tips and the array overall.  
           [0005]    Another drawback to existing array designs is their susceptibility to arcing or catastrophic failure. Failure can occur when an event such as a gas burst caused by a backscattered ion initiates a plasma discharge. The plasma discharge can destroy one or more individual tips and gates.  
           [0006]    These problems have been alleviated in applications using Spindt tips through the use of a resistive layer to balance the current between tips and pinch off current flow during micro-discharges. For manufacturing techniques where the tips are deposited onto a substrate, current technology allows for a resistive layer to be deposited on the substrate prior to the deposition of the tips. For example, U.S. Pat. Nos. 6,091,188, 6,084,341, 6,060,823, 6,031,322, 5,910,701, 5,905,330, 5,894,187 and 4,940,916 disclose emitter arrays with a resistive layer beneath the Spindt tips.  
           [0007]    Where carbon-based emitter tips are fabricated through the deposition or growth of material in a mold, such as that disclosed in U.S. Pat. No. 6,132,278, there is a need for apparatus and method for producing a reliable resistive layer to make possible field emitter arrays with more evenly distributed emission current.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a field emission apparatus with improved emission uniformity across an array of carbon-based emitters and a method for the manufacture of the apparatus. In one embodiment, apparatus having a layer with an electrical resistance greater than the resistance of the array of emitter tips is in contact with the bottom of emitting tips. The resistive layer is also in contact with an electrically conductive backing layer. In accordance with a second embodiment, the resistive layer is a resistive carbon-based layer that is grown integrally with the carbon-based -emitting material. In yet another embodiment, a thin layer of emitting material is present between the resistive layer and the emitting tips.  
           [0009]    The invention further provides methods for making arrays of carbon-based emitters having a resistive layer. One method begins with the formation of a mold with an array of indentations or pits on a selected surface of the mold. An array of tips of carbon-based material is formed when a layer of carbon-based material is grown on the mold to fill the pits with carbon-based material and produce a layer of excess carbon-based material. The layer of excess carbon-based material may then be completely removed from the mold and a resistive layer of material with greater resistance than the emitting tips may be deposited on the mold and remaining carbon-based material. An electrically conducting support or backing layer is placed in contact with the resistive layer. Finally the mold is removed to expose the tips of the carbon-based material. In another embodiment, a thin conducting layer is formed, which may be the emitting material, and remains between the resistive layer and the tips.  
           [0010]    In another method for making the apparatus, the resistive layer is resistive carbon that is integrally formed with the layer of emitting material. In yet another method for making, carbon-based emitting tips are formed by growing on diamond seed material in pits in a mold. No excess carbon-based material layer is grown. A resistive layer is then grown over the isolated pits, a conductive layer is placed over the resistive layer and the mold is removed to leave exposed emitting tips. 
       
    
    
     DESCRIPTION OF THE FIGURES  
       [0011]    For a more complete understanding of the invention and the advantages thereof, reference is now made to the following description taken in conjunction with the following drawings in which the like reference numbers indicate like features and wherein:  
         [0012]    [0012]FIG. 1 shows a silicon mold with inverse pyramidal depressions.  
         [0013]    [0013]FIG. 2 shows a silicon mold with a thin, continuous carbon-based film.  
         [0014]    [0014]FIG. 3 shows a silicon mold and carbon-based tips after the excess carbon-based material is removed.  
         [0015]    [0015]FIG. 4 shows a silicon mold and carbon-based tips after a resistive layer is added.  
         [0016]    [0016]FIG. 5 shows carbon-based tips, a resistive layer, and a backing layer after the mold is removed.  
         [0017]    [0017]FIG. 6 shows a silicon mold with carbon-based films of varying resistance.  
         [0018]    [0018]FIG. 7 shows carbon-based films of varying resistance after the mold is removed with an aluminum layer deposited on the tip side of the carbon-based film.  
         [0019]    [0019]FIG. 8 shows a layer of photoresist deposited on top of the aluminum layer once the photoresist is etched to reveal the aluminum layer on the tips.  
         [0020]    [0020]FIG. 9 shows a titanium layer deposited on top of the aluminum and remaining photoresist.  
         [0021]    [0021]FIG. 10 shows a titanium layer coating the tips after the photoresist is removed.  
         [0022]    [0022]FIG. 11 shows tips and the resistive layer after the aluminum and conductive carbon-based material are etched.  
         [0023]    [0023]FIG. 12 shows the resulting emitter after the titanium is removed from the tips.  
         [0024]    [0024]FIG. 13 shows a device with isolated tips and a self-aligned gate layer.  
         [0025]    [0025]FIG. 14 shows the silicon mold with isolated carbon-based tips grown into the pyramidal depressions.  
         [0026]    [0026]FIG. 15 shows the carbon-based tips in the mold after a resistive layer has been added to the back side of the tips.  
         [0027]    [0027]FIG. 16 shows the carbon-based tips in the mold after a backing layer has been added to the back side of the resistive layer.  
         [0028]    [0028]FIG. 17 shows a device structure. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]    In a first embodiment of the apparatus, a resistive layer is in contact with the bottom of a layer of emitting material having molded emitter tips. The process of making this structure begins with forming a mold in which the layer of emitting material will be grown. FIG. 1 shows mold  20 , which may be formed from silicon and may be produced using the following standard photolithographic techniques. Initially, a thin silicon oxide or silicon nitride film is grown onto the surface of a silicon wafer. A template is then created by etching a plurality of openings through the oxide film using standard photolithographic processes. The openings may be in the range of about 2 microns per side and the openings are preferably arranged in groups such that each group forms an array having a selected number of openings. After the aperture areas are defined in the template, the silicon oxide film is isotropically etched in a buffered hydrofluoric acid mixture to form apertures in the oxide layer. The exposed silicon within the apertures is then anisotropically etched using a mixture of potassium hydroxide and normal propanol to produce pits or inverted pyramids  22  in the silicon. This process forms the basic mold on which the carbon-based emitter tips will be grown. After the pits are formed, the remaining silicon oxide film is removed and the wafer is nucleated for carbon-based film growth using standard techniques.  
         [0030]    Next, mold  20  is placed in a Chemical Vapor Deposition (CVD) growth reactor. A commercially-available reactor such as an ASTeX 5 kW Microwave CVD Reactor may be used to grow a carbon-based film on mold  20 . Growth conditions for such a carbon-based film are described in U.S. Pat. No. 6,181,055B1 and in co-pending patent application Ser. No. 09/169,908, which are hereby incorporated by reference herein. Such films contain a mixture of sp 2  and sp 3  carbon bonds, and are sometimes referred to herein as “diamond” and sometimes as “carbon-based.” It should be understood that the carbon-based material may be any film grown by the methods described in the documents incorporated by reference or by any other methods forming a carbon-based film having electron emission properties. As shown in FIG. 2, carbon-based material is grown on mold  20  in the diamond reactor. This material forms both emitter tips  24  in the mold and layer  23  of excess carbon-based material connecting the tips on what will become the back side.  
         [0031]    After the carbon-based film growth is complete, the excess layer  23  of carbon-based material on the back side is removed by polishing the back side down to the plane of silicon mold  20 , leaving tips  24 , as shown in FIG. 3.  
         [0032]    [0032]FIG. 4 shows a thin layer of resistive material  25 , preferably sputter-deposited polysilicon, formed on mold  20  and the back side of diamond tips  24 . Other materials such as doped silicon carbide, amorphous silicon, or high-resistance carbon (diamond) may also be used. Resistive layer  25  should have a resistance through the layer between about 1,000 ohms and about 5,000,000 ohms for a 10 square micron cross section and a thickness between about 0.5 and about 50 microns. For high-resistance carbon, the resistance may be in the range of 1,000 to 10,000,000 ohms for a 10 square micron cross section. The higher the resistance of this resistive layer, the more uniform emission current will be across a particular array; however, increasing the resistance of the resistive layer can broaden the energy distribution of electrons emitted from the gated device.  
         [0033]    Then, referring to FIG. 5, conducting backing layer  26 , which may be formed from silicon or carbon, for example, is attached to the resistive layer. Backing layer  26  can be deposited directly onto resistive layer  25  or independently fabricated and bonded, sintered, adhered, welded or alloyed to resistive layer  25 . Finally, silicon mold  20  is removed using well-known techniques, leaving carbon-based tips  24  attached to resistive layer  25  and supported by backing layer  26 , as shown in FIG. 5.  
         [0034]    A second embodiment of this invention involves controlling conditions during the growth process of the carbon-based material growth process to produce an intermediate layer of high-resistance carbon-based material. In FIG. 6, mold  20  is created as in the first embodiment. It may then be placed into an ASTeX 5 kW Microwave CVD reactor, as previously described. Initially, the carbon-based material of layer  23  should be grown in conditions such that it is conductive and emissive. Again, growth conditions for conductive carbon-based material are taught in U.S. Pat. No. 6,181,055B1 and in Ser. No. 09/169,908.  
         [0035]    After layer  23  is formed, film growth conditions are changed to enable the growth of layer  27  of carbon-based material, which is much less conductive than the material in layer  23 . Optimally, resistive layer  27  will be less than about two microns thick and have a resistance through the film on the order of 100,000 to 1,000,000 ohms for a 10 square micron cross-section, but may have a resistance in the range of about 1,000 ohms to about 10,000,000 ohms for a 10 square micron cross-section. Growth conditions for producing such high-resistance films are also taught in U.S. Pat. No. 6,181,055B1 and in Ser. No. 09/169,908.  
         [0036]    After layer  27  is complete, growth conditions may be changed again to produce higher conductivity carbon-based material, indicated in FIG. 6 as layer  23 A. Once the diamond growth is complete, silicon mold  20  is removed to expose tips  24 . An advantage of the isolated tips  24  and layer  23  grown in conjunction with a diamond resistive layer  27  and the conductive diamond layer  23 A is that the entire structure can be grown in a single deposition cycle, with no further processing required, to produce isolated conductive diamond tips.  
         [0037]    At this point, conductive carbon-based material in layer  23  will join tips  24 . In one embodiment, this layer may be grown with limited thickness, such that electrical resistance between tips will be sufficiently large to achieve an effective amount of emission uniformity, even though the tips are not connected directly to a resistive layer. An effective amount of emission uniformity may be determined by observing the variation of emission current over an array of emitters, using well known techniques. Layer  23  of FIG. 6 may have a thickness in the range from about 1 micron to about 10 microns; the preferred thickness will vary with resistivity of layer  23 . Preferably, the resistance will be in the range from about 10 to about 5,000,000 ohms for a 10 square micron cross-section.  
         [0038]    In another embodiment, the layer of conductive material between tips is removed, such that each emitter tip is electrically connected only to resistive layer  27 . FIG. 7 shows the first step of this removal process. Aluminum layer  28  is deposited on tips  24  and layer  23  of carbon-based material. Other materials such as nickel may be used in place of aluminum. Next, referring to FIG. 8, photoresist  29  is spun onto the surface of aluminum layer  28  and baked. The photoresist will be thinner over the tops of the tips than in between the tips due to the photoresist spinning process. Dry etching may then be used to remove the photoresist to reveal aluminum layer  28  over the tips, as shown in FIG. 8.  
         [0039]    Next, referring to FIG. 9, the exposed aluminum-covered tips are wet etched to remove the aluminum and leave a surface of carbon-based material on tips  24  and a surface of photoresist  29  between the tips. Protective layer  30 , which may be formed from titanium, gold or other similarly reactive materials, is then vapor-deposited onto the tips and remaining photoresist  29 . As shown in FIG. 10, removing the remaining photoresist  29  will leave protective layer  30  only on tips  24  and expose aluminum layer  28  between the tips.  
         [0040]    In FIG. 11, the remaining aluminum layer has been wet etched to expose the surface of the underlying layer  23  of carbon-based material between tips  24 . Also, the layer  23  of carbon-based material has been dry etched between tips to expose layer  27  of resistive diamond between tips. During this dry etching step, protective layer  30  protects carbon-based material in tips  24  such that they are not etched. Finally, FIG. 12 shows conductive diamond tips  24  after protective layer  30  has been removed. The tips  24  now have less of a pyramidal shape and sides more nearly perpendicular to resistive diamond layer  27 . A layer  23 A of carbon-based material with higher conductivity than layer  27  may serve as an electrically conductive support for the structure. Alternatively, layer  23 A may be attached to other electrically conductive support materials such as layer  26  of FIG. 5.  
         [0041]    After the structure of FIG. 5 or of FIG. 12 is formed with a resistive layer in contact with carbon-based emission tips  24 , an array of tips may be used as a source of electrons by placing an anode in proximity to the tips and supplying a voltage between the array and the anode.  
         [0042]    [0042]FIG. 13 depicts an alternative structure with self-aligned gates  34  formed in proximity to tips  24 . The gate structure may be formed by any process for forming self-aligned gates, such as those described in U.S. Pat. No. 6,181,055 and patent application Ser. No. 09/169,908.  
         [0043]    In yet a third embodiment of the present invention, isolated diamond tips are deposited in the pits of a silicon mold. Mold  20  of FIG. 1 is produced, as explained previously, by etching through a silicon oxide film on a silicon wafer to produce pits that may be in the shape of inverted pyramids  22 . In this third embodiment, the silicon template is seeded with a proper nucleating agent such as diamond powder before the silicon oxide film is removed from the wafer area surrounding the pits. Seed particles between the pits are then removed as the silicon oxide hard mask is removed. By this procedure, the nucleation can be confined to the pits. A short growth period in a diamond reactor results in isolated diamond tips  31  in the pits, as shown in FIG. 14. An ASTeX 5 kW Microwave CVD Reactor may be used. Growth conditions are preferably as disclosed in U.S. Pat. No. 6,181,055B1 or U.S. patent application Ser. No. 09/169,908.  
         [0044]    After growth of isolated tips  31 , resistive layer  32  is deposited onto the back of the tips, as depicted in FIG. 15. Preferably, the resistive layer is silicon carbide or diamond. The through-resistance of the resistive layer is preferably between about 1,000 ohms and about 5,000,000 ohms for a 10 square micron cross section and its thickness is preferably in the range from about 0.5 to about 2 microns.  
         [0045]    After the thin resistive layer is deposited, thick backing layer  33  is deposited, adhered, welded, bonded or sintered on the resistive layer, as shown in FIG. 16. The thick backing layer is preferably polysilicon or carbon. Finally, the silicon mold is removed, using well-known procedures. The final tip structure having a resistive layer is shown in FIG. 17.  
         [0046]    While particular preferred embodiments of the present invention have been described, it is not intended that these details should be regarded as limitations upon the present invention, except as and to the extent they are included in the following claims.