Abstract:
An electron emission device with nano-protrusions is described. Electrons are emitted from the nano-protrusions and directed by one or more conductors into beams. The beams may be shaped to be collimated, diverged, or converged. The shaped beams from one or more nano-protrusions may be focused onto a target spot through the use of additional electron optics.

Description:
RELATED APPLICATIONS  
       [0001]    The following application of the common assignee, incorporated by reference in its entirety, may contain some common disclosure and may relate to the present invention:  
         [0002]    U.S. patent application Ser. No. 09/975,296, filed on Oct. 12, 2001 entitled “APPARATUS AND METHOD FOR FIELD-ENHANCED MIS/MIM ELECTRON EMITTERS” (Attorney Docket No. 10016850-1). 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    This invention relates generally to electron emission devices. In particular, the invention relates generally to electron emission devices with self-aligned extraction and beam shaping capabilities and methods of fabrication and uses thereof.  
         BACKGROUND OF THE INVENTION  
         [0004]    Electron emission technology exists in many forms today. For example, cathode ray tubes (CRT) are prevalent in many devices such as TVs and computer monitors. Electron emission plays a critical role in devices such as x-ray machines and electron microscopes. In addition, microscopic cold cathodes can be employed in electron-beam lithography used, for example, in making integrated circuits, in information storage devices such as those described in Gibson et al, U.S. Pat. No. 5,557,596, in microwave sources, in electron amplifiers, and in flat panel displays. Actual requirements for electron emission vary according to application. In general, electron beams need to deliver sufficient current, be as efficient as possible, operate at application-specific voltages, be focusable, be reliable at the required power densities, and be stable both spatially and temporally at a reasonable vacuum for any given application. Portable devices, for example, demand low power consumption.  
           [0005]    Metal-Insulator-Semiconductor (MIS) and Metal-Insulator-Metal (MIM) electron emitter structures are described in Iwasaki et al, U.S. Pat. No. 6,066,922. In such structures with the application of a potential between the electron supply layer and the thin metal top electrode, electrons are 1) injected into the insulator layer from the electron supply layer (metal or semiconductor), 2) accelerated in the insulator layer, 3) injected into the thin metal top electrode, and 4) emitted from the surface of the thin metal top electrode. Depending upon the magnitude of the potential between the electron supply and thin metal top electrode layers, such emitted electrons can possess kinetic energy substantially higher than thermal energy at the surface of the thin metal film. Hence, these emitters may also be called ballistic electron emitters.  
           [0006]    Shortcomings of MIS or MIM devices include relatively low emission current densities (typically about 1 to 10 mA/cm 2 ) and poor efficiencies (defined as the ratio of emitted current to shunt current between the electron supply layer and the thin metal electrode) (typically approximately 0.1%).  
           [0007]    Electrons may also be emitted from conducting or semiconducting solids into a vacuum through an application of an electric field at the surface of the solid. This type of electron emitter is commonly referred to as a field emitter. Emitted electrons from field emitters possess no kinetic energy at the surface of the solid. The process for making tip-shaped electron field emitters, hereinafter referred to as Spindt emitters, is described in C. A. Spindt, et al, “Physical Properties of Thin-Film Field Emission Cathodes with Molybdenum Cones”, Journal of Applied Physics, vol. 47, No. 12, Dec. 1976, pp. 5248-5263. For a Spindt emitter, the electron-emitting surface is shaped into a tip in order to induce a stronger electric field at the tip surface for a given potential between the tip surface and an anode; the sharper the tip, the lower the potential necessary to extract electrons from the emitter.  
           [0008]    The shortcomings of Spindt emitters include requiring a relatively hard vacuum (pressure&lt;10 −6  Torr, preferably&lt;10 −8  Torr) to provide both spatial and temporal stability as well as reliability. Furthermore, the angle of electron emission is relatively wide with Spindt emitters making emitted electron beams relatively more difficult to focus to spot sizes required for electron-beam lithography or information storage applications. Operational bias voltages for simple Spindt tips are relatively high, ranging up to 1000 volts for a tip-to-anode spacing of 1 millimeter.  
           [0009]    With previous design of electron emitters, aligning electron emitters has been difficult. Also, fabricating emitters that work at low operating voltage have been difficult as well.  
         SUMMARY OF THE INVENTION  
         [0010]    According to an embodiment of the present invention, an electron emitting device comprises an electron supply structure; at least one nano-protrusion integrally formed on a top of the electron supply structure; an emitter insulator formed above the electron supply structure; and a top conductor formed above the emitter insulator such that the at least one nano-protrusion is exposed.  
           [0011]    According to another embodiment of the present invention, an electron beam focusing device comprises a plurality of electron beam emitters and an electron beam focusing lens configured to focus electron beams emitted from the plurality of electron beam emitters.  
           [0012]    According to yet another embodiment of the present invention, a method for forming electron emitting device comprises forming an electron supply structure; integrally forming at least one nano-protrusion on a top of the electron supply structure; forming an emitter insulator above the electron supply structure; forming a top conductor above the emitter insulator; and exposing the at least one nano-protrusion.  
           [0013]    According to a further embodiment of the present invention, a method for forming an electron beam focusing device comprises forming a plurality of electron beam emitters and forming an electron beam focusing lens configured to focus electron beams emitted from the plurality of electron beam emitters. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Features of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings, in which:  
         [0015]    [0015]FIGS. 1A-1B illustrate electron emitters according to first and second embodiments of the present invention;  
         [0016]    [0016]FIG. 2 illustrates a top view of an emitter with multiple nano-protrusions according to an embodiment of the present invention;  
         [0017]    [0017]FIGS. 3A-3B illustrate electron emitters according to third and fourth embodiments of the present invention;  
         [0018]    [0018]FIGS. 4A-4C illustrate example shaping effects of nano-lens on the emitted electron beam;  
         [0019]    [0019]FIG. 5 illustrates an electron beam focusing device according to an embodiment of the present invention;  
         [0020]    [0020]FIGS. 6A-6C illustrate an exemplary method to form the electron emitter according to the first embodiment of the present invention shown in FIG. 1A;  
         [0021]    [0021]FIGS. 7A-7C illustrate an exemplary method to form the electron emitter according to the second embodiment of the present invention shown in FIG. 1B;  
         [0022]    [0022]FIGS. 8A-8D illustrate an exemplary method to form the electron emitter according to the third embodiment of the present invention shown in FIG. 3A; and  
         [0023]    [0023]FIG. 8A-2 and  8 D- 2  illustrate exemplary modifications to the steps shown in FIGS. 8A-8D to form the electron emitter according to the fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]    For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. However, it is to be understood that the same principles are equally applicable to many types of electron emitters.  
         [0025]    [0025]FIG. 1A illustrates an electron emitter  100  according to a first embodiment of the present invention. As shown, the emitter  100  may include a conductive substrate  110  with a nano-protrusion  120  formed integrally with the conductive substrate  110 , i.e. the conductive substrate  110  and the nano-protrusion  120  are made from the same material. The emitter  100  may also include an emitter insulator  170  above the conductive substrate  110  and a top conductor  180  above the emitter insulator  170 . The emitter insulator  170  and the top conductor  180  are formed such that the nano-protrusion  120  is exposed.  
         [0026]    The conductive substrate  110  and the nano-protrusion  120  may be formed from any combination of metal, doped polysilicon, doped silicon, graphite, a metal coating on glass, a metal coating on ceramic, a metal coating on plastic, an ITO coating on glass, an ITO coating on ceramic, an ITO coating on plastic, and the like. Note that glass, ceramic, and plastic may be considered as an insulating substrate upon which the metal is coated. In an embodiment, the height of the nano-protrusion  120  substantially ranges from 5-50 nm.  
         [0027]    The metal or metal coating may include any combination of aluminum, tungsten, titanium, copper, gold, tantalum, platinum, iridium, palladium, rhodium, chromium, magnesium, scandium, yttrium, vanadium, zirconium, niobium, molybdenum, silicon, beryllium, hafnium, silver, and osmium and alloys and multilayered films thereof.  
         [0028]    The emitter insulator  170  may be formed from any combination of diamond-like carbon and oxides, nitrides, carbides, and oxynitrides of silicon, aluminum, titanium, tantalum, tungsten, hafnium, zirconium, vanadium, niobium, molybdenum, chromium, yttrium, scandium, nickel, cobalt, beryllium, polyimide, and magnesium. In an embodiment, the emitter insulator  170  substantially ranges in thickness from 5-1000 nm.  
         [0029]    The top conductor  180  may be formed from any combination of a metal, conductive oxides, nitrides and carbides of metals, doped polysilicon, graphite, and alloys, and multilayered films thereof. Like the conductive substrate  110 , the metal of the top conductor  180  may be any combination of aluminum, tungsten, titanium, molybdenum titanium, copper, gold, silver, tantalum, platinum, iridium, palladium, rhodium, chromium, magnesium, scandium, yttrium, vanadium, zirconium, niobium, molybdenum, hafnium, silver, and osmium and any alloys and multilayered films thereof. In an embodiment, the top conductor  180  substantially ranges in thickness from 5-1000 nm.  
         [0030]    [0030]FIG. 1B illustrates an electron emitter  100 - 2  according to a second embodiment of the present invention. The electron emitter  100 - 2  is similar to the first embodiment  100  in that it includes a conductive substrate  110 , a nano-protrusion  120 , an emitter insulator  170 , and a top conductor  180 . The types of materials that may be used to form the conductive substrate  110 , the emitter insulator  170 , and top conductor  180  and exemplary dimensions thereof are similar to the emitter  100  and thus are not repeated here.  
         [0031]    The emitter  100 - 2  of the second embodiment may include an electron supply layer  115  above the conductive substrate  110  and the nano-protrusion  120  may be integrally formed with the electron supply layer  115 . The electron supply layer  115  and the nano-protrusion  120  may be formed from a doped or from an undoped semiconductor. The thickness of the electron supply layer may range substantially from 5- 1000 nm and the nano-protrusion whose diameter may range substantially from 5 to 60 nm.  
         [0032]    Note that a junction may be formed between the electron supply layer  115  and the conductive substrate  110 . The characteristics of the junction may be tailored to be optimal for controlling beam current for applications such as E-beam lithography, displays, storage devices, and microwave sources. Also, as will be made clear below, the conductive substrate  110  of the emitter  100  and a combination of the conductive substrate  110  and the electron supply layer  115  of the emitter  100 - 2  may be referred to as the electron supply structure.  
         [0033]    While FIGS. 1A and 1B illustrate examples of a single nano-protrusion structure, emitters may include multiple nano-protrusions. FIG. 2 illustrates a top view of an emitter  200 , which includes multiple nano-protrusions  220  above an electron supply structure  215 . The emitter insulator and the top conductor have been omitted for clarity. The density of the nano-protrusions  220  may substantially range from 20-200 per μm 2 . However, the density range may differ from the range listed depending on the type of application envisioned.  
         [0034]    The nano-protrusions  220  may be randomly spaced (not shown). Also, the nano-protrusions  220  may be substantially regularly spaced as shown in FIG. 2. In other words, if the nano-protrusions  220  are regularly spaced, the placements of the nano-protrusions  220  are such that the horizontal and vertical spacings between the nano-protrusions are substantially the same within some predefined tolerance. Also, the periodicity in the x and y directions may be different. In addition, the periodicity may be in any angle and not just in the x and y directions.  
         [0035]    [0035]FIG. 3A illustrates an electron emitter  300  according to a third embodiment of the present invention. As shown, the emitter  300  may include a conductive substrate  310  with a nano-protrusion  320  above the conductive substrate  310 . The nano-protrusion  320  may be formed integrally with the conductive substrate  310 . The emitter  300  may also include an emitter insulator  370  and a top conductor  380  above the emitter insulator  370 . In between the emitter insulator  370  and the top conductor  380 , there may be one or more pairs of intervening conductors  360  and insulators  350 , wherein the conductors  360  and the insulators  350  alternate. Again, the nano-protrusion  320  is exposed. The top conductor  380  may also be called a nano-lens  380 .  
         [0036]    The types of materials that may be used to form the conductive substrate  310 , nano-protrusion  320 , insulators  350  and  370 , and conductors  360  and  380  and exemplary dimensions thereof are similar to the emitters  100  and  100 - 2  discussed above and thus are not repeated here.  
         [0037]    Any combination of the nano-lens  380  and the intervening conductors  360  may be used to shape the beam of electrons emitted from the nano-protrusion  320 . FIGS. 4A-4C illustrate various shaping effects of nano-lens on the emitted electron beam. (In these figures, the emitter insulator and the intervening insulators and conductors have been omitted for clarity.) For example, in FIG. 4A, the emitted beam of electrons from the nano-protrusion  420  is collimated by the nano-lens  480  and intervening conductors (not shown). In FIG. 4B, the electron beam is shaped to be divergent, and in FIG. 4C, the beam is shaped to be convergent.  
         [0038]    [0038]FIG. 3B illustrates an electron emitter  300 - 2  according to a fourth embodiment of the present invention. The electron emitter  300 - 2  is similar to emitter  300  in that it may include a conductive substrate  310 , a nano-protrusion  320 , an emitter insulator  370 , one or more pairs of intervening conductors  360  and insulators  350 , and a nano-lens  380 .  
         [0039]    Like the emitter  100 - 2 , the emitter  300 - 2  includes an electron supply layer  315  above the conductive substrate  310  and the nano-protrusion  320  may be integrally formed with the electron supply layer  315 . The electron supply layer  315  and the nano-protrusion  320  may be formed from a doped or from an undoped semiconductor, which as discussed above, may be tailored to provide an optimal junction between the electron supply layer  315  and the conductive substrate  310  or a series resistor between the conductive substrate  310  and the electron emission surface. Also as discussed above, any combination of the nano-lens  380  and the conductors  360  of the emitter  300 - 2  may be used to shape the emitted beam of electrons.  
         [0040]    Again, the types of materials used to form the elements of the electrons emitters and exemplary dimensions thereof have been discussed and thus are not repeated.  
         [0041]    Also, like the situation depicted in FIG. 2, an emitter structure may be formed that includes multiple nano-protrusions of type illustrated in FIGS. 3A-3B may be used. Also, the nano-protrusions may be randomly spaced or regularly spaced.  
         [0042]    The beams emitted from one or more electron emitters may be focused to a particular target spot. For example, in order to prevent crosstalk between pixels, field emission displays employ appropriate electron optics to focus the beams from a plurality of electron emitters to a single pixel. Each display pixel is thereby illuminated solely with electrons from a corresponding multitude of emitters.  
         [0043]    [0043]FIG. 5 illustrates an electron beam focusing device  500  according to an embodiment of the present invention. As shown, the focusing device  500  may include a plurality of electron beam emitters  510 . The beam emitters  510  may be any combination of the emitters  100 ,  100 - 2 ,  300 , and  300 - 2  as discussed above or other types of emitters. The focusing device  500  may also include an electron focusing lens  520  configured to focus the electron beams emitted from the plurality of electron beam emitters  510  on to a target spot  530  of a medium  540 .  
         [0044]    The focusing lens  520  may be formed from any combination of metal, conductive oxides, nitrides, carbides and oxynitrides of a metal and metal alloys, doped silicon, doped amorphous silicon, doped polysilicon, graphite, and alloys, and multilayered films thereof. The types of metal may include any combination of aluminum, tungsten, titanium, molybdenum titanium, copper, gold, silver, tantalum, platinum, iridium, palladium, rhodium, chromium, magnesium, scandium, yttrium, vanadium, zirconium, niobium, molybdenum, hafnium, silver, and osmium and any alloys and multilayered films thereof.  
         [0045]    In an embodiment, the focusing lens  520  substantially ranges in thickness from 100-2000 nm. Also the diameter of an aperture  525  of the focusing lens  520  may range substantially from 0.1 to 300 μm depending on application. Additionally, a vertical distance d 1  from the emitters  510  and the focusing lens  520  and a vertical distance d 2  from the focusing lens to the target medium  540  may range substantially between 0.1 to 300 μm and 0.1 to 5000 μm respectively depending on application. In addition, the beam emitters  510  may be randomly or substantially regularly spaced.  
         [0046]    [0046]FIGS. 6A-6C illustrate an exemplary method to form the electron emitter  100  according to the first embodiment of the present invention shown in FIG. 1A. As shown in FIG. 6A, the conductive substrate  110  and the nano-protrusion  120  are formed, for example, by low pressure chemical vapor deposition (LPCVD) of doped polysilicon. The deposition process creates the nano-protrusions  120  integrally with the conductive substrate  110 . Note that many other materials and processes may be used to form the conductive substrate  110  and the nano-protrusion  120 .  
         [0047]    Then as shown in FIG. 6B, an emitter insulator layer  170 ′ and a top conductor layer  180 ′ may be formed. For example, to form the emitter insulator layer  170 ′, an oxide layer may be grown by thermal oxidation. Other means of forming the emitter insulator layer  170 ′ may include physical vapor deposition (PVD) and/or chemical vapor deposition (CVD). Note that the emitter insulator layer  170 ′ may be conformal to the nano-protrusion  120 . To form the top conductor layer  180 ′, conductive materials may be deposited, for example, by a PVD process. The top conductor layer  180 ′ may be planarized.  
         [0048]    Then as shown in FIG. 6C, the emitter insulator layer  170 ′ and the top conductor layer  180 ′ may be may be etched to form the emitter insulator  170  and the conductor  180  as well as to expose nano-protrusion  120 . For example, the conductor  140  may be formed by ion etching the top conductor layer  180 ′ above the nano-protrusion  120 . Then the nano-protrusion  120  may be exposed by reactive ion etching or wet etching the emitter insulator layer  170 ′, which also forms the emitter insulator  170 . Other etching processes may be utilized to expose the nano-protrusion  120 .  
         [0049]    [0049]FIGS. 7A-7C illustrate an exemplary method to form the electron emitter  100 - 2  according to the second embodiment of the present invention shown in FIG. 1B. The steps are similar to the method illustrated in FIGS. 6A-6C, except an electron supply layer  115  is formed above the conductive substrate  110  and nano-protrusion  120  may be formed above the electron supply layer  115  and may be formed integrally with the electron supply layer  115 .  
         [0050]    [0050]FIGS. 8A-8E illustrate an exemplary method to form the electron emitter  300  according to the third embodiment of the present invention shown in FIG. 3A. As shown in FIG. 8A, the conductive substrate  310  and the nano-protrusion  320  may be formed, for example, by low pressure chemical vapor deposition of metal or polysilicon. The deposition process creates the nano-protrusions  320  integrally with the conductive substrate  310 . Note that many other materials and processes may be used to form the conductive substrate  310  and the nano-protrusion  320 .  
         [0051]    Then as shown in FIG. 8B, an emitter insulator layer  370 ′ and one or more intervening conductor layers  360 ′ and insulator layers  350 ′ may be formed. For example, to form the emitter insulator layer  370 ′, an oxide layer may be grown by thermal oxidation. Other means of forming the emitter insulator layer  370 ′ may include PVD and/or CVD. Note that the emitter insulator layer  370 ′ may be conformal to the nano-protrusion  120 . The intervening conductor layers  360 ′ may be formed, for example, by a PVD process. The insulator layers  350 ′ may be formed, for example, by PVD or CVD. Both the intervening insulating and conductor layers  350 ′ and  360 ′ may be planarized.  
         [0052]    Then as shown in FIG. 8C, the nano-lens layer  380 ′ may be formed by using the process similar to form the intervening conductor layer  360 ′. Again, the nano-lens layer  380 ′ may be planarized.  
         [0053]    Then as shown in FIG. 8D, etching may take place to form intervening insulator(s)  350 , intervening conductor(s)  360 , emitter insulator  370 , and the nano-lens  380  such that the nano-protrusion  320  is exposed. For example, the nano-lens  380  may be formed by ion beam etching the nano-lens layer  380 ′ above the nano-protrusion  320 . Also the emitter insulator layer  370 ′, the intervening conductor layers  360 ′, and the intervening insulator layers  350 ′ may be wet etched or reactive ion etched.  
         [0054]    [0054]FIG. 8A-2 and  8 D- 2  illustrate an exemplary modification to the steps shown in FIGS. 8A-8D to form the electron emitter  300 - 2  according to the fourth embodiment of the present invention shown in FIGS. 3B. As shown in FIG. 8A-2, the step illustrated in FIG. 8A may be modified in that the electron supply layer  315  is formed above the conductive substrate  310  and the nano-protrusion  320  is formed above the electron supply layer  315 . The remaining steps may be similar to the steps shown in FIGS. 8B-8E to arrive at the result shown in FIG. 8D-2.  
         [0055]    While the invention has been described with reference to the exemplary embodiments thereof, it is to be understood that various modifications may be made to the described embodiments of the invention without departing from the spirit and scope of the invention. The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. In particular, although the methods of the present invention has been described by examples, the steps of the method may be performed in a different order than illustrated or may be performed simultaneously. These and other variations are possible within the spirit and scope of the invention as defined in the following claims and their equivalents.