Abstract:
An apparatus is provided for focusing electrons being emitted from a field emission device. The apparatus comprises a substrate ( 12,41,51 ) having first and second portions, and a cathode metal layer ( 20,44,52 ) formed over the substrate ( 12,41,51 ) in the first portion to partially define a sidewall ( 23 ) for a trench ( 25 ) in the second portion. A ballast layer ( 22,46,53 ) is formed over the substrate ( 12,41,51 ) in the second portion, the cathode metal layer ( 20,44,52 ), and the sidewall ( 23 ). A first dielectric layer ( 24,47,54 ) is formed over the ballast layer ( 22,46,53 ) in the first portion. A gate extraction metal layer ( 26,48,55 ) is formed over the first dielectric layer. At least one emitter ( 30 ) comprising a high aspect ratio conductive material is formed above the substrate and in the trench ( 25 ) having a sidewall ( 23 ) defined by the first dielectric layer ( 24,47,54 ) and the cathode metal layer ( 20,44,52 ). The ballast layer ( 22,46,53 ) extends along the sidewall and conductively contacts the cathode metal layer and the at least one emitter. An anode ( 32 ) is positioned to receive electrons from the at least one emitter ( 30 ). The ballast layer ( 22,46,53 ) provides a force that counteracts the sidewise pull of the gate extraction metal layer ( 26,48,55 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to field emission displays and more particularly to a field emission display structure that focuses the electrons from the emitter.  
       BACKGROUND OF THE INVENTION  
       [0002]     Field emission devices that generate electron beams from electron emitters such as carbon nanotubes at an anode plate are well known in the art. Each of the electron beams are received at a spot on the anode plate and define a corresponding spot size. The separation distance between the cathode plate and the anode plate determine, in part, the spot size. It is known in the art to control the spot size by using focusing structures to collimate the electron beams.  
         [0003]     To achieve adequate display brightness, the anode should be maintained at a high voltage relative to the cathode. Consequently, the anode plate and cathode plate should be spaced far enough apart that nanotubes due not emit electrons from the anode field alone, and the spacers that separate the anode and cathode plates do not break down. This separation distance that is sufficient to prevent unwanted electrical events can result in an undesirably large spot size. As the voltage on the cathode plate generates the electrons which are attracted by a gate electrode from the emitter toward the anode plate, the gate electrode voltage tends to cause the beam to diverge. Thus, focusing structures are frequently employed in field emission devices.  
         [0004]     However, prior art focusing structures often employ dielectric layers to support a focusing electrode and to separate the focusing electrode from the other electrodes, such as gate extraction electrodes, of the field emission device. Such prior art focusing structures suffer from disadvantages. For example, the capacitance between the focusing structures and the gate extraction electrodes increases the power requirements of the device. Many focusing structures add additional layers and processing steps to reduce the beam spot size, and this significantly increases fabrication cost and reduces yield. Processing geometries are reduced for many methods, thus reducing the available emitter area, increasing the sensitivity to defects, and increasing processing and equipment costs associated with decrease in feature sizes. For example, a typical focusing scheme incorporates a separate perforated sheet placed between the anode and cathode. The extra sheet adds costs, the precision alignment step reduces yield, and the device requires a double set of spacers, which is extremely complex and costly due to difficulty and low yield.  
         [0005]     Prior art focusing structures also reduce the size of the region that can incorporate electron emitters, thereby reducing the overall device current. Prior art focusing structures have also been placed too close to the emitting material, thereby screening the emitter material at the edge of the emitter material region entirely from the gate field. While this reduces the beam size, it also reduces the overall current by orders of magnitude. Prior art focusing structures which place focusing electrodes in proximity to the electron emitters with a distance less than the average height of the emitters risk having the emitters stick to the focusing structure via van der Waals forces, thereby rendering the emitters less useful. Many prior art focusing concepts do not describe a method for implementation and typically require significantly more patterning layers than are economically viable. Prior art focusing structures often do not address the importance of the relationship of the focusing structure to the height of the emitting features. They are typically proposed for electron focusing and fail to account for the re-orientation of the actual emitting nanostructures in the electric field.  
         [0006]     Accordingly, it is desirable to provide a field emission device structure that focuses the electron beam from the emitter area away from the gate extraction field without significantly increasing the device cost, thereby providing for a small spot size required for high resolution displays. Such a device would require no new patterning steps, no increased complexity, and would not require more complicated spacer technology. It is desirable for the structure to orient the emitter structures more perpendicular to the anode, thereby reducing the initial lateral velocity. It would also be advantageous to create a structure which increases the focusing function for regions with the highest emission current, since these are regions that would see the largest absolute intensity away from the beam center. 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  
       [0007]     An apparatus is provided for focusing electrons being emitted from a field emission device. The apparatus comprises a substrate having first and second portions, and a cathode metal layer formed over the substrate in the first portion to partially define a sidewall for a trench in the second portion. A ballast layer is formed over the substrate in the second portion, the cathode metal layer, and the sidewall. A first dielectric layer is formed over the ballast layer in the first portion. A gate extraction metal layer is formed over the first dielectric layer. At least one emitter comprising a high aspect ratio conductive material is formed above the substrate and in the trench having a sidewall defined by the first dielectric layer and the cathode metal layer. The ballast layer extends along the sidewall and conductively contacts the cathode metal layer and the at least one emitter. An anode is positioned to receive electrons from the at least one emitter. The ballast layer provides a force that counteracts the sidewise pull of the gate extraction metal layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0009]      FIGS. 1-3  are cross sections of the structure of a first embodiment during successive periods of fabrication.  
         [0010]      FIG. 4  is a cross section of the structure of a second embodiment.  
         [0011]      FIG. 5  is a cross section of the structure of a third embodiment.  
         [0012]      FIG. 6  is a graph representing the results of an FEA simulation of the embodiment as shown in  FIG. 3 , illustrating electric potential lines and electric field lines.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     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.  
         [0014]     Referring to  FIGS. 1-3 , the field emission device  10  according to a first embodiment of the present invention includes a dielectric layer  14  deposited on the substrate  12 . The substrate  12 , for example, comprises glass, silicon, or ceramic. The dielectric layer  14 , for example, comprises silicon oxide and would preferably be approximately 1 micron thick. A hard mask  16  of diamond-like carbon or nitride of approximately 0.05-0.5 microns is deposited on the dielectric layer  14  and is used for a self-limiting etch stop to aid in fabrication of a trench structure. Another dielectric layer  18  of silicon oxide and approximately 0.1-2.0 microns thick is deposited on the hard mask  16 . A metal layer  20  of molybdenum or copper, for example, and of sufficient thickness to limit the voltage drop over the length of the cathode metal, approximately 0.2-2 microns thick, is deposited on the dielectric layer  18 .  
         [0015]     Referring to  FIG. 2 , a photo etch is performed on the metal  20  and then on the dielectric  18  in a manner well known to those skilled in the art. The etch of these two layers is limited by the etch stop layer  16  of diamond like carbon, nitride or similar material, offering a selectivity to the metal  20  and dielectric  18 . A ballast layer  22  comprising, for example, Carbon, Silicon Carbon, Silicon Carbide, Tantalum Silicon Nitride, Tantalum Nitride, Chrome Oxi-nitride, Aluminum Nitride, or Silicon doped with one or more of Boron, Phosphorous, Arsenic, Antimony, Tantalum, or Nitrogen is deposited on the hard mask  16 , dielectric layer  18 , and metal layer  20  creating a side wall  23 . The ballast layer  22  is self aligned within the emitter area and coats all exposed surfaces. The ballast resistance selected is such that it limits the current to each emitter (via voltage drop) and as the emitter current increases, the delta cathode voltage increases between emitter pad and trench sidewall  23 , reducing the gate voltage electric field influence for electrons leaving the emitters. A dielectric layer  24  of silicon oxide and approximately 1.0 micron thick is deposited on the ballast layer  22 . A metal layer  26  of molybdenum or copper, for example, and approximately 0.5 microns thick, is deposited on the dielectric layer  24 . The total trench depth created preferably would have a depth to height ratio of 1:1 to 1:20. The emitter length to prevent the anode field from generating emission from the emitters is a ratio of trench depth less gate metal thickness plus the catalyst thickness to the emitter of 2:1 to 1:2.  
         [0016]     Referring to  FIG. 3 , a photo etch is performed on the metal  26 , and then on the dielectric layer  24 . A pattern (not shown) is performed with the catalyst  28  being subsequently grown or deposited in the trench  25  created by the etching of the dielectric layer  18 , metal layer  20 , dielectric layer  24 , metal layer  26 , and ballast layer  22 , and including the side wall  23 . Alternatively, the catalyst  28  could be grown and then a pattern etch performed. The catalyst  28  preferably comprises nickel, but could comprise any one of a number of other materials including cobalt, iron oxide, and a transition metal or alloys thereof. A carbon nanotube  30  is then grown on the catalyst  28  in a manner known to those skilled in the art. Although only one carbon nanotube  30  is shown, those skilled in the art understand that any number of carbon nanotubes could be grown on the catalyst layer  28 . While the preferred emitter is a carbon nanotube, it should be understood that any emitter having a height to radius ratio of greater than 100, for example, would function equally well with the present invention. The distance between the ballast layer  22  on the sidewall  23  and the carbon nanotube  30  is approximately 1-5 microns. Although a trench is preferred, it should also be understood that a well could alternatively be formed. As used herein, trench is considered to include a well. The sidewall  23  height should be greater than 0.5 um to provide sufficient field to straighten, align, and focus emitters and electrons.  
         [0017]     When voltages are applied to the metal layer  20  (cathode), the metal layer  26  (gate electrode), and the anode  32 , voltage is supplied through the ballast layer  22  from the metal layer  20  to the catalyst  28  and nanotube  30 . Typical voltages on the metal layer  20 , metal layer  26 , and anode  32  are 0.0-60.0 volts, 0-100.0 volts, and 1,000-10,000 volts, respectively. Electrons are extracted from the nanotube  30  by the local electric field which is dominated by the field generated by the metal layer  26  (gate electrode) and directed to the anode  32  as indicated by arrow  35 . The field generated by metal layer  26 , in addition to extracting electrons in a beam directed toward the anode  32 , also exerts a lateral force (arrow  34 ) toward the metal layer  26 , resulting in divergence of the electron beam from the preferred direct path to the anode and creating a larger beam spot on the anode  32 . However, the structure of the present invention, reduces the lateral force exerted on the electrons by the gate electrode field by modifying the electric potential profile and thereby modifying the electric field profile. The relatively zero potential of the cathode field minimizes the lateral influence off the gate extraction filed on the emitted electrons.  
         [0018]     This focusing, or directing, of the electrons from the carbon nanotube  30  by the ballast layer  22  reduces the spot size on the anode  32 . Ballast resistance, from cathode metal to catalyst metal, between 1×10 6  and 1×10 10  ohms is preferred. Electrons emitted from the carbon nanotube  30  closest to the gate electric field have a much lower lateral momentum due to the low potential of the trench sidewall  23  pinned to the cathode potential. Further, the ballast layer  22  in concert with the emission current from the nanotubes  30  increases the potential difference between the emitter pad and the cathode metal  20  (rises above ground relative to cathode metal) with the effect of reducing the spot size at higher emission currents. As a result, those nanotubes  30 , which statistically emit with higher current, experience a greater beam divergence reduction than nanotubes with lower emission current. This is important because nanotubes  30  which emit with more current produce a larger beam size on the anode, for a given intensity detection threshold (even though the full width at half maximum of the beam full spatial distribution on the anode may be the same), and consequently, they benefit from additional divergence reduction. A collateral benefit of the improved divergence, especially in the near-emitter region is reduced leakage current which would result from excessively diverging electrons intercepting the sidewall and/or gate. Device  10  efficiency is increased by reducing the number of stray electrons impacting the area outside the phosphor subpixel region (i.e., adjacent phosphor regions, the black area surrounding the spot, and spacers (not shown) separating the anode  32  and device  10 ).  
         [0019]     In this first embodiment, a dielectric layer  18  under the ballast layer  22  controls the length of the lateral ballast to the defined cathode  20  to catalyst pad  28 . The focusing effect will be enhanced by a minimal amount by the voltage drop caused by the resistance in the trench and the length which is the sidewall  23  height. When the cathode metal  20  is over the dielectric  18 , the device operates as described above. An added benefit of cathode metal  20  over ballast layer  22  is the trench depth can be used to control the ballast resistance value. The ballast resistance, R, is determined by the relation R=ρ*((L 1 +L 2 )/A), where ρ is the resistivity of the ballast material, L 1  is the trench depth, L 2  is the lateral distance from a pad edge to the nearest sidewall, and A is cross-sectional area for current flow from the cathode to the catalyst pad through the ballast layer. In other embodiments, ballast resistance is determined by R=ρ*((L 2 )/A). Changing the depth of the trench or the slope of the trench sidewall  23  will change the ballast resistance without a change in material resistivity, or cathode  20  to catalyst pad  28  spacing. In both cases the available emitter area (catalyst pad  28 ) is not reduced by adding other features or layers. The proposed device utilizes existing layers in a manner to simplify fabrication, minimize cost, and implement novel methods of electron trajectory control. There are several possible ways to configure the order and thicknesses of the layers in the first set of layers to achieve the desired sidewall potential. The apparatus reduces the beam size both by creating a more advantageous electrical potential profile, thereby reducing the divergence of the beam of emitted electrons; and by creating in the near-emitter pad region an electric field profile which is aligned more closely to perpendicular to the anode plane. It has been demonstrated that nanotubes will align to a local electric field by a mechanism whereby the electric field induces dipole moments in the nanotube, the electric field in turn exerts torques on these dipole moments, tending to align the nanotube in the field. By orienting the nanotubes more perpendicular to the anode, the emitted electrons have a reduced initial lateral velocity.  
         [0020]     In a second embodiment, shown in  FIG. 4 , the first set of layers is ordered with cathode metal  44  on the bottom, a dielectric layer  45  in the middle, and the ballast layer  46  on top. A typical fabrication sequence would involve providing a substrate  41  with a barrier layers  42  deposited thereover (for example SiO2 or SiN), depositing a cathode metal layer  44 , depositing a dielectric layer  45 , depositing a photoresist, patterning and developing the photoresist, etching the dielectric layer  45  and then the cathode metal layer  44 . The ballast layer  46  is deposited as the top layer, thereby covering the sidewall, and a dielectric layer  47  and a gate layer  48  are formed thereon. It is not patterned in the vicinity of the trench. The ballast sidewall height is desired to be at most 2× the emitter height, less the catalyst thickness. Emitters greater than 1.5× the total trench depth risk contacting the gate prior to biasing and becoming attached by Van der Waal&#39;s forces. The catalyst pad distance from the trench sidewall can abate the problem to some extent but lateral gate field strength is compromised at large distances. One advantage of this embodiment is that it provides more isolation between the gate and cathode electrodes and reduces capacitance.  
         [0021]     In a third embodiment, shown in  FIG. 5 , the first set of layers is ordered with a thick cathode metal  52  on the bottom, and the ballast layer  53  on top. A dielectric layer is not used. A typical fabrication sequence would involve providing a substrate  51  with barrier layers on it of, for example, SiO2 or SiN, depositing a thick cathode metal layer  52 , depositing a photoresist, patterning and developing the photoresist, etching the cathode metal layer  52 . The ballast is deposited as the top layer, thereby covering the sidewall, and a dielectric layer  54  and a gate extraction layer  55  are formed thereon. The ballast sidewall height is desired to be at most 2× the emitter height, less the catalyst thickness.  
         [0022]     This invention, in addition to focusing the electrons, also aligns the nanotubes perpendicular to the electric field lines. The presence of the sidewall layer  23  at near the cathode  20  potential directs the electric field lines to be more parallel to the substrate  12 . Consequently, the sidewall  23  electrode layer has the effect of orienting the nanotubes  30  more perpendicular to the anode  32  and cathode plates  20 , and the nanotubes  30  thereby direct the initially emitted electrons in a direction towards the anode  32 , with reduced initial lateral velocity towards the gate  26 . The feature is also very important in creating a small beam size on the anode  32 .  
         [0023]     One added benefit of the invention is that the gate electrode  26  is positioned higher relative to the nanotube  30  height than without the added sidewall height. This reduces the influence of the anode  32  electric field on the nanotube  30 . This is important because high anode fields can initiate field emission without extraction from the gate, leading to display pixels that are always in the ‘on’ state. Some displays are operated with an offset voltage on the gate. The combination of the offset voltage and the anode field can also be sufficient to initiate field emission without extraction from the gate, leading to display pixels that are always in the ‘on’ state. This invention can reduce the influence of the anode  32  electrode. In some prior art cases, attempts to focus the beam result in significantly higher gate extraction voltage requirements. This is particularly noticeable when the electric field is substantially higher at the edge of the nanotube-containing area than in the middle of the nanotube containing area. In the case of this invention, the sidewall electrode layer  23  has the effect of reducing the field at the edge of the nanotube-containing region by less than 20% for a given gate voltage, but the field in the middle of the nanotube containing region is nominally equal to or higher than the field in the case without the sidewall electrode layer. As a result, electrons are emitted from a larger region of the pad, so the extracted current from the pad is only minimally changed from the case where there is no sidewall electrode layer. In contrast, in some prior art configurations, the current is reduced by  12  orders of magnitude. Often prior art cites a large tradeoff between emission current and reduction in the beam size. In this invention, there is very little tradeoff if any.  
         [0024]     An important point of this invention which is lacking in prior art is that the reduction of the beam size on the anode is accomplished without increasing the number of patterning steps, changing the method for inserting spacers in the display, adding new materials, adding complex focusing layers, adding additional electrodes, adding additional inputs and outputs, or modifying any other important parameter. The invention maintains the integrity of the entire display system and does not violate design rules for the other parts. It does not substantially alter the gate extraction voltage. The invention does not require a significant change to the pattern alignments.  
         [0025]     Referencing  FIG. 6 , the graph represents the results of an FEA simulation of the exemplary embodiment described in the preceding text and as shown in  FIG. 3 , illustrating electric potential lines and electric field lines. The plane of  FIG. 6  represents a cross-section through the length of a single emitter trench such that the x-axis is perpendicular to both the length of the trench and a line normal to the planes of the cathode and anode (this line is the z-axis) and such that the z-axis is perpendicular to both the length of the trench and the previously defined x-axis. The current invention provides a substantial improvement over previously known art, including, but not limited to, the following specific advantages:  
         [0026]     a. More uniform electric field magnitude in the near-emitter region as evidenced by more uniform spacing of the electric potential lines.  
         [0027]     b. Electric field vectors aligned more closely to the normal coincident with symmetry axis of the emitter area.  
         [0028]     c. Increased influence of the gate-moderated electric field relative to the anode field. This has direct implications for the requirement that the device not emit electrons when the gate is not biased, i.e., the influence of the anode field is reduced to a point at which diode emission is negligible.  
         [0029]     d. Extraction field (for the electric field in the proximity of the emitter, the component of that field normal to the anode) is reduced by no more than 20% which compares favorably with modeled prior art which exhibited extraction field losses of 50%-80%.  
         [0030]     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.