Abstract:
A field emission lighting device and a method making the field emission lighting device are disclosed. The field emission lighting device includes a cathode, an anode, at least one emitter base, at least one conductive emitter tip, and a fluorescence layer. The emitter base is formed on the cathode and is made of diamond-like carbon. Each emitter tip is formed on a respective emitter base. The fluorescence layer is formed on the anode. In addition, each emitter tip has a bottom portion and a top portion. The bottom portion of the emitter tip has a cross-section size (i.e., diameter/width) essentially equal to the cross-section size of the emitter base. The emitter tip and the emitter base have a nanometer-magnitude size. The emitter base and a corresponding single emitter tip together form an emitter unit of the field emission lighting device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is related to a copending U.S. patent application filed on Jul. 29, 2005 and entitled “FIELD EMISSION LIGHTING DEVICE” with the same assignee. The disclosure of the above-identified application is incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to electronic lighting technology and, particularly, to a field emission lighting device and a method for making the field emission lighting device.  
         [0004]     2. Discussion of the Related Art  
         [0005]     Conventional light sources generally include combustion-based light sources, incandescent light sources, and fluorescent light sources. The fluorescent light sources include, for example, high-intensity discharge (HID) lamps and light emitting diode (LED) lamps.  
         [0006]     As regards a typical HID lamp, a low voltage has to be converted into a high voltage of about 2300V so as to excite an inert gas to thereby produce electric arc light. In operation, a constant working voltage of about 8000 V has to be maintained so as to keep the HID lamp lighting. Therefore, such HID lamp has to be equipped with a voltage converter. The need for such a converter increases power consumption and the complexity of the configuration of the HID lamp. Nowadays, the LED lamps have been widely used in small sized lamps. The LED lamps generally have low luminosity, however.  
         [0007]     Field emission devices operate based on emission of electrons in a vacuum. In operation, electrons are extracted from micron-sized (or less) emitter tips under a strong electric field. The electrons are accelerated under the electric field and then bombard a fluorescent material. The fluorescent material then emits visible light. Field emission devices are generally compact in size, light in weight, and capable of providing a high brightness.  
         [0008]     Emitter bases of a typical field emission device are generally comprised of metal or semiconductor materials, such as silicon. Nevertheless, such metal or semiconductor materials typically have a lower mechanical strength, relative to known metallic structural materials, such as aluminum alloys, titanium alloys, steel and iron.  
         [0009]     What is needed, therefore, is a field emission lighting device that has an excellent mechanical strength and a high luminous efficiency.  
         [0010]     What is also needed, therefore, is a method for making the above-described field emission lighting device.  
       SUMMARY OF THE INVENTION  
       [0011]     In a preferred embodiment, a field emission lighting device includes a cathode, an anode, at least one diamond-like carbon emitter base, at least one conductive emitter tip, and a fluorescence layer. The emitter base is formed on the cathode. The emitter tip is formed on the emitter base. The fluorescence layer is formed on the anode.  
         [0012]     Preferably, the cathode includes a cathode main body and a functional layer disposed on the cathode main body The cathode main body is made, e.g., of a material selected from the group consisting of copper, silver, and gold (i.e., a noble metal).  
         [0013]     In addition, the field emission lighting device includes a nucleation layer, a transparent upper substrate, a lower substrate, and a number of sidewalls. The nucleation layer is disposed on the cathode. The cathode is disposed on the lower substrate. The anode is disposed under the upper substrate. The sidewalls are interposed between the upper substrate and the lower substrate. Therefore, a chamber is defined by the sidewalls, the lower substrate and the upper substrate. The chamber generally is vacuumized for optimal operation of the field emission process.  
         [0014]     Furthermore, the emitter base has a cross-section size in the range from about 10 nanometers to about 100 nanometers. Preferably, the emitter base has a base cross-section size (i.e., diameter/width) in the range from about 10 nanometers to about 50 nanometers. Each emitter tip has a bottom portion and a top portion. The bottom portion of the emitter tip has an emitter cross-section size (i.e., diameter/width) approximately equal to the base cross-section size. The top portion of the emitter tip has cross-section size in the range from about 0.5 nanometers to about 10 nanometers. The emitter base and a corresponding single emitter tip together have a total height in the range from about 100 nanometers to about 2000 nanometers.  
         [0015]     A method for making a field emission lighting device, in accordance with another preferred embodiment, includes the steps of: providing a first substrate and forming a cathode thereon; forming a layer comprised of a diamond-like carbon on the cathode; forming a conductive layer on the diamond-like carbon layer; etching the conductive layer and the diamond-like carbon layer, thereby forming at least one diamond-like carbon emitter base and at least one conductive emitter tip on the respective emitter base; providing a second substrate and forming an anode thereon; depositing a fluorescence layer on the anode; assembling the anode to the cathode and forming a chamber therebetween and thereby attaining a field emission lighting device.  
         [0016]     Preferably, the method includes a step of forming a nucleation layer on the cathode after providing the cathode. The diamond-like carbon layer is formed by a deposition method selected from the group consisting of a chemical vapor deposition method, a plasma-enhanced chemical vapor deposition method, and an ion-beam sputtering deposition method. The conductive layer is formed by a method selected from the group consisting of a vacuum sputtering, a magnetron sputtering method, and an ion-beam sputtering deposition method.  
         [0017]     In addition, because the combined emitter tip and the emitter base have good mechanical strength, good electrical conductance, and excellent field-emission capability, the combined emitter tip and the emitter base may operate under relatively high-voltage electrical fields without the risk of being damaged.  
         [0018]     A high-voltage electric field may be applied so as to obtain a high current of field emission. The high current of field emission enables the field emission lighting device to provide a high luminous efficiency and a satisfactory brightness. The brightness provided by the present field emission lighting device may reach about 10 to about 1000 times that of a comparable light emitting diode (LED) or high intensity discharge (HID) lamp.  
         [0019]     Other advantages and novel features of the embodiments will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     Many aspects of the field emission lighting device having the combined emitter tip and the emitter base can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present field emission lighting device. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
         [0021]      FIG. 1  is a schematic, cross-sectional view of a field emission lighting device in accordance with a preferred embodiment; and  
         [0022]      FIG. 2  is an enlarged view of an exemplary emitter unit of the field emission lighting device of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]     Embodiments of the present field emission lighting device will now be described in detail below and with reference to the drawings.  
         [0024]      FIG. 1  illustrates a field emission lighting device  10  in accordance with a preferred embodiment. The field emission lighting device  10  mainly includes a cathode  11 , a nucleation layer  13 , a transparent upper substrate  14 , a fluorescence layer  15 , an anode  16 , a low substrate  17 , at least one electrically conductive emitter base  18 , and at least one emitter tip  19 . The cathode  11  is formed on the low substrate  17 . The anode  16  is formed on an interior surface of the upper substrate  14 . The fluorescence layer  15  is formed on the anode  16 . The emitter base  18  is essentially made of diamond-like carbon (DLC). Each emitter tip  19  is formed on a respective emitter base  18 .  
         [0025]     The cathode  11  includes a cathode main body  11   b  and a functional layer  11   a  disposed on the cathode main body  11   b . The cathode main body  11   b  is generally made, e.g., of a material selected from the group consisting of copper (Cu), silver (Ag), and gold (Au) (i.e., a noble metal, featuring high conductivity and superior oxidation resistance). The low substrate  17  may be made of a metal such as Cu or Ag, or a metal alloy such as a Cu—Ag alloy. Alternatively, the low substrate  17  may be made of nonmetal material, for example, silicon or silicon dioxide. The low substrate  17  advantageously has a surface configured to be smooth and not prone to crack, for facilitating formation of the cathode  11  thereon. The functional layer  11   a  preferably has a thickness of less than about 1 micrometer. The low substrate  17  advantageously has a thickness in the range from about 1 millimeter to about 10 millimeters.  
         [0026]     In general, the cathode main body  11   b , which is most advantageously made of silver or gold, has a relatively low mechanical strength. The functional layer  11   a  is advantageously comprised of a material having a relatively high mechanical strength, such as copper, nickel, and an alloy thereof. The functional layer  11   a  can effectively improve the mechanical strength of the cathode  11 . The functional layer  11   a  is generally disposed on at least one of opposite surfaces of the cathode main body  11   b.    
         [0027]     The nucleation layer  13  preferably has a thickness of less than about 1 micrometer. The nucleation layer  13  is configured for facilitating deposition of an emitter base layer thereon. The emitter base layer and a conductive layer, which is subsequently deposited on the emitter base layer, are provided for forming the emitter base  18  and the emitter tip  19 , respectively. The nucleation layer  13  is advantageously comprised of a material selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), and other transition metals.  
         [0028]     Referring also to  FIG. 2 , each emitter base  18  and the respective emitter tip  19  cooperatively form an emitter assembly or an emitter unit for the field emission lighting device  10 . In the illustrated exemplary embodiment, the emitter base  18  is essentially made of DLC. The emitter base  18  formed of DLC has many valuable characteristics, such as excellent mechanical strength, good electrical conductivity, high hardness, high chemical stability, and high thermal conductivity. For example, the emitter base  18  can effectively dissipate heat generated from the cathode  11 .  
         [0029]     The emitter tip  19  is generally made of an emissive material and is advantageously selected from the group consisting of graphite-like carbon (GLC), niobium (Nb), and molybdenum (Mo). The emitter tip  19  is preferably formed of GLC. (It is noted that DLC and GLC are commonly used terms in the art with respect to such forms of carbon and thus are definitive within such context.)  
         [0030]     The emitter base  18  is generally in a form of a cylinder. The emitter base  18  has a diameter d 2  in the range from about 10 nanometers to about 100 nanometers. Preferably, the emitter base  18  has a diameter d 2  in the range from about 10 nanometers to about 50 nanometers. The emitter tip  19  is in a form of a frustum of a cone. The emitter tip  19  includes a bottom portion and a top portion. The bottom portion has a diameter d 3  approximately equal to a diameter d 2  of the emitter base  18 . The top portion of the emitter tip  19  has a diameter d 1  in the range from about 0.5 nanometers to about 10 nanometers. The emitter tip  19  has an aspect ratio in the range from about 10 to about 4000. The aspect ratio is preferably in the range from about 20 to about 400. The emitter unit, including the emitter base  18  and the corresponding emitter tip  19 , has a total height h in the range from about 100 nanometers to about 2000 nanometers.  
         [0031]     Referring back to  FIG. 1 , the upper substrate  14  is formed of a transparent material, such as glass or silicon dioxide. The upper substrate  14  is generally in a form of a planar substrate.  
         [0032]     Alternatively, the upper substrate  14  can be configured as a tubular substrate. The cathode  11  may be a metal filament disposed between the sidewalls  12 . The cathode  11  could then be interposed between the sidewalls  12 , thereby obviating the need for using a lower substrate  17  for supporting the cathode  11  thereon. The cathode  11  would preferably extend along a central axis of the tubular substrate. The emitter base  18  and the emitter tip  19  would be formed on an outside surface of the cathode  11 . In fact, a field emission lighting device in this form would be a tubular light source.  
         [0033]     The anode  16  is formed on the upper substrate  14  by a DC reactive sputtering technique or an RF reactive sputtering technique. The anode  16  is generally made of an indium tin oxide (ITO) material, for its favorable conductivity and transparency. The fluorescence layer  15  is formed on the anode  16 . The fluorescence layer  15  is generally made of a phosphor material.  
         [0034]     A number of sidewalls  12  are interposed between the lower substrate  17  and the upper substrate  14 . A chamber  30  is bounded by the sidewalls  12 , the lower substrate  17 , and the upper substrate  14 . The chamber  30  is generally vacuumized, thereby minimizing the resistance met by the electrons emitted from the emitter tip  19  prior to reaching the fluorescence layer  15 .  
         [0035]     In operation, a bias voltage is applied between the cathode  11  and the anode  16 , thereby establishing an electric field. Electrons are extracted and accelerated from the emitter tip  19  and then bombard the fluorescence layer  15 . As a result, the fluorescence layer  15  generates visible light after being bombarded by the electrons.  
         [0036]     A method for making a field emission lighting device, in another preferred embodiment, comprises the steps of (a) providing a first substrate and forming a cathode thereon; (b) forming a layer made of diamond-like carbon (hereinafter, DLC layer) on the cathode; (c) forming a conductive layer on the DLC layer; (d) etching the conductive layer and the DLC layer, thereby forming at least one DLC emitter base and a corresponding conductive emitter tip on the respective emitter base; (e) providing a second substrate and forming an anode thereon; (f) forming a fluorescence layer on the anode; and (g) assembling the anode to the cathode and forming a chamber therebetween, thereby obtaining a field emission lighting device.  
         [0037]     The cathode includes a cathode main body and a functional layer disposed on the cathode main body. The functional layer may be deposited on at least one of opposite surfaces of the cathode main body, for improving a mechanical strength of the total cathode. Preferably, a nucleation layer is formed on the cathode by a sputtering method subsequent to the step (a). The nucleation layer has a thickness of less than about  1  micrometer. The nucleation layer facilitates deposition of a DLC layer. A conductive layer is then deposited on the DLC layer.  
         [0038]     The DLC layer and the conductive layer is formed by a deposition method such as a chemical vapor deposition method, a plasma-enhanced chemical vapor deposition method, or an ion-beam sputtering deposition method. The conductive layer is formed, e.g., by a sputtering method, such as a vacuum sputtering method, a magnetron sputtering method, or an ion-beam sputtering deposition method.  
         [0039]     Furthermore, the conductive layer and the DLC layer are partially/selectively etched at the same time. The conductive layer and the DLC layer is etched by a method selected from a group consisting of a chemical etching method, a plasma etching method, a photo etching method, and a dry etching method. After the etching step, each emitter base has a respective emitter tip remaining thereon, thereby resulting in an emitter unit.  
         [0040]     In the step (e), the second substrate is formed of a transparent material such as glass and silicon oxide. The anode may be made of an indium tin oxide (ITO) material or another transparent conductor by, e.g., a DC reactive sputtering technique or an RF reactive sputtering technique. The fluorescence layer, which is made of a phosphor material, is formed, for example, by a deposition method.  
         [0041]     Moreover, sidewalls are interposed between the first and second substrates for forming a chamber therebetween, subsequent to the step (g). The chamber is vacuumized so as to minimize resistance encountered by emitted electrons, prior to reaching the fluorescence layer.  
         [0042]     In the above-described preferred embodiments, because the emitter unit has excellent mechanical strength, good electrical conductance, and excellent field-emission capability, the emitter unit may operate under relatively high voltage electrical fields without the risk of being damaged.  
         [0043]     When a high voltage electric field is applied, a high current of field emission can be obtained. The high current of field emission enables the field emission lighting device to provide a high luminous efficiency and a satisfactory brightness. The brightness provided by the present field emission lighting device may reach about 10 to about 1000 times that of a comparable light emitting diode (LED) or high intensity discharge (HID) lamp.  
         [0044]     The field emission lighting device of the above-described preferred embodiments may be implemented into various illumination products. For example, the field emission lighting device may be employed as a headlight for an automobile or incorporated into residential or industrial lighting units.  
         [0045]     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.