Abstract:
A field effect electron emitting apparatus is prepared by depositing a plurality of nano-wires  216  onto a substrate  200  having a cathode layer  214 . The deposition occurs by suspending the nano-wires  216  in a plating solution, and plating the substrate with a metal layer  202 , thereby entrapping the nano-wires. The nano-wires  216  are composed of an electrically-conductive magnetic material, and the deposition process is carried out in the presence of a magnetic field perpendicular to the substrate  200  so that the nano-wires  216  are aligned by the field.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electron emitter which can be utilized for a Field Emission Display device (FED), backlight for a liquid crystal display (LCD) or any other devices which require electron emission. 
     BACKGROUND 
     Recently Flat Panel Displays (FPDs) have become popular due to their smaller footprint and larger flatter screen compared to conventional technology. Liquid Crystal Displays (LCD) are replacing Cathode Ray Tubes (CRT) in many domestic applications. However, most LCDs have the disadvantage of a low contrast ratio (5000:1) compared to conventional CRT technology (1,000,000:1). To improve the contrast ratio of an LCD, multi-segment operation of a backlight is being studied actively by many researchers. Multi-segment operation is a method in which a segment of the display which is located in a bright portion of the picture is illuminated more brightly and other segments which are located in darker portions of the picture are less illuminated. The intensity of each segment is controlled according to the displayed picture. The smaller the size of the segments the better the fidelity of the display. Light Emitting Diodes (LED) are usually used as the light sources of a multi-segment operated backlight. For example, it is known for 400 LEDs to be aligned in a 20×20 matrix on the backlight panel. This LED array is divided into 25 segments so that each segment has 16 (=4×4) LEDs. The image which is supposed to be displayed by the LCD panel is analysed in advance and the intensity of each segment is determined so as to maximize the contrast ratio of displayed picture. The LED backlight usually creates a white light from 3 kinds of LEDs, namely Red, Blue and Green, so it is difficult to make the size of the segment too small. If the area of a segment is too small, appropriate white light cannot be obtained because 3 colours are not mixed well. 
     An alternative technology to LCD is a Field Emission Display (FED). A typical FED incorporates a large array of fine metal tips or carbon nano-tubes (CNT), which emit electrons through a process known as field emission. Since a FED works based on a similar principle to a CRT, namely using an electron emitter and a phosphor, it gives a sufficiently high contrast ratio. However, the fabrication of so-called Spindt-type emitters, which are utilized for most FED systems, requires complex processes and increases the cost of the panel. Synthesis of CNT is also costly because it requires expensive equipment. These are the major reasons why FED cannot play a main role in the FPD industry in spite of its potential to achieve a high contrast ratio. 
     It would therefore be desirable to provide a backlight for LCD which has a lot of small segments which can each be operated independently, or an electron emitter array for a FED which enables low production cost. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the invention to provide a new and useful electron emitter display. 
     In general terms, the present invention proposes in a first aspect that in a field effect electron emitting apparatus using nano-wire electron emitters, each nano-wire is made of a magnetic material. 
     This concept provides the advantage that if a magnetic field is applied to the wires perpendicular to the substrate during the fabrication process of the electron emitting apparatus, the magnetic field may align the nano-wires perpendicular to a substrate. Note that conventionally Molybdenum (Mo) is used to form an emitter in Spindt-type FEDs and carbon is used in CNT-type FEDs. Neither is a magnetic material. 
     A second aspect of the invention proposes in general terms a fabrication process for a field effect electron emitting apparatus which includes a step of attaching nano-wires to a substrate in the presence of a magnetic field perpendicular to the substrate, and that the nano-wires are made of a magnetic material, such that they are aligned by the field. 
     Either aspect of the invention makes it easier to control the density and the orientation of the nano-wires on the substrate, so that the fabricated electron emitter has a lower threshold voltage of electron emission. 
     Typically, the nano-wires are attached at the same time that a metallic material is deposited on the substrate using a plating solution, by an electrochemical or non-electric plating process. The magnetic nano-wires are dispersed in the plating solution, aligned with the magnetic field, so they are incorporated in the electrochemically or chemically deposited metallic film maintaining their orientation almost perpendicular to the substrate. Thus, the fixing of the nano-wires is effected by a simple process such as electrochemical or non-electric plating which reduces the production cost relative to known techniques. 
     The material of the nano-wires may be an electrically conductive material. However, it would also be possible to form them from a non-electrically conductive magnetic material (such as a metal oxide), a non-electrically conductive magnetic material and subsequently coat them with conductive material, or an electrically conductive magnetic material and subsequently coat them with another electrically conductive or non-conductive material. 
     Some embodiments of the invention are electron emitters which can be divided into small segments and operated independently by patterning cathodes and gate electrodes. Such an emitter is particularly useful as a multi-segmented backlight for a LCD or FED panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will now be described, with reference to the following figures, in which: 
         FIG. 1  is a cross sectional diagram of a known Field Emission Display (FED) or multi-segmented backlight for a LCD. 
         FIG. 2  is a cross section of an electron emitter array which is an embodiment of the invention and which can be employed in the structure of  FIG. 1 . 
         FIG. 3  is a top view of the emitter array in  FIG. 2 . 
         FIG. 4  is the calculated field emission behavior of an electron emitter. 
         FIG. 5  is a process for fabricating magnetic nano-wires, which can be used as a first step of a process to fabricate a device according to the embodiment of  FIGS. 2-3 . 
         FIG. 6  is electrochemical plating process of this invention. 
         FIG. 7  is an alternative setup for the electroplating. 
         FIG. 8  is a top view of a second embodiment of the emitter array of the invention. 
         FIG. 9  is a cross sectional diagram of a third embodiment of the emitter array of the invention. 
         FIG. 10 , which is composed of  FIGS. 10(   a ) to  10 ( c ), shows steps in the formation of a further embodiment of the invention. 
         FIG. 11 , which is composed of  FIGS. 11(   a ) to  11 ( c ), shows steps in the formation of a further embodiment of the invention. 
         FIG. 12  shows a preferred property of all embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , the principle of a Field Emission Display (FED) or multi-segmented backlight for LCD  100  is shown, including an emitter array  102  and a phosphor coated screen  104  in a housing  108 . The phosphor coated screen  104  is parallel to the emitter array  102  and spaced apart from it by a series of spacers  106 . The cavity surrounded by the housing  108  and the screen  104  is maintained under vacuum. The phosphor coated screen  104  consists of a glass face plate, a phosphor layer and an anode layer. The accelerated electrons from the emitter array  102  collide against the phosphor coated screen  104  and fluorescent light is generated. 
     A first embodiment of the invention, illustrated in  FIG. 2 , is a novel electron emitter assembly  220  which may be used to replace the emitter array  102  in  FIG. 1 . The electron emitter assembly  220  includes a substrate  200 , cathode  214 , insulation layer  203 , a metal layer  202 , nano-wire electron emitters  216  and gate electrode  204 . In the fabrication process, the cathodes  214  are deposited on the substrate  200  as a series of parallel strips on the substrate  200 . The insulation layer  203  is then applied, defining apertures  201  extending over the cathodes  214 . Then a series of gate electrodes  204  are deposited on the insulation layer  203  so that they extend perpendicular to the cathodes  214  as shown in  FIG. 3 . Thus an electrode assembly  220  is fabricated. Thereafter, the nano-wires  216  made of a magnetic material are attached on the cathode layer  214 , together with an electroplated metal layer  202 . Note that the metal layer  202  need not be contiguous: instead it is partitioned by the bodies of insulating material  203 , so as to include a plurality of regions overlying different ones of the cathodes  214 . As shown in  FIG. 2 , the upper tips of the nano-wires  216  are substantially co-planar with the gate electrodes  204 . 
     It is well-known that the ratio S/L, where S is the separation between nano-wires and L is the length of exposed nano-wires, affects electron emission properties. For example, Jean-Marc Bonard et al. [1] clarified that the relationship between S/L and β m /β s  can be expressed by the following equation,
 
β m /β s =1−exp(−2.3172 S/L ),  (1)
 
where β m  is a field enhancement factor for a series of nano-wires which are uniformly bristled on a plane and δ s  is a field enhancement factor for a single nano-wire. This expression is plotted in  FIG. 4 . The field enhancement factor β is defined by eqn. (2),
 
F=βE,  (2)
 
where F is the electric field at the tip of a nano-wire and E is the applied external electric field. As  FIG. 4  shows, β m  is very much smaller than β s  when S/L is smaller than 1. From this point of view, the thickness of metal layer  202 , the length of nano-wire  216  and density of nano-wires should be determined so that S/L becomes larger than 1.
 
     Referring to  FIGS. 5-6 , a first fabrication process which is an embodiment of the present invention is explained in detail. Firstly, nano-wires made of a magnetic material such as Ni, Co, Fe or any other metal, alloy or its oxide which shows soft or hard magnetic properties are formed ( FIG. 5 ). It is convenient to use a sheet of Anodized Aluminum Oxide (AAO)  502  as a template for the nano-wires. For example, Ni nano-wires can be electroplated in the pores of an AAO template. In one specific example, a 200 nm thick Cu layer  501  is deposited as a seed layer on one side of a 50 μm thick AAO sheet  502  which has through-holes  505  which are 20 nm in diameter  505 . By applying current between the Cu seed layer  501  and a counter electrode  503  made of a material such as Pt in a plating solution  504 , Ni nano-wires  500  having a diameter of 20 nm and a length of 10 μm, for example, are obtained. The diameter and length are adjustable by changing the pore size of AAO template and a plating time. For the plating solution, the following mixture can be used, namely, 240 g/L of NiSO 4 -6H 2 O, 45 g/L of NiCl 2 -6H 2 O and 35 g/L of H 3 BO 3 . After plating, the AAO template is removed by etching in NaOH solution. The Cu seed layer can be removed by treating the result with (NH 4 ) 2 S 2 O 8  or FeCl 2  solution. Thus only individual Ni nano-wires remain. Typically, the length of the nano-wires is within the range from several micrometers to several dozens micrometers. In this document the term nano-wire is used to mean an elongate conductor less than 1 micron in diameter, and preferably less than 500 nm in diameter. Experiments carried out by the inventors indicate that metal nano-wire less than 200 nm in diameter gives a reasonable threshold voltage. 
     Secondly, the electroplating process of the embodiment will be explained. An assembly  220  having a substrate  200  with a patterned insulator layer  203 , electrodes  204  and cathodes  214 , is dipped in a Cu plating solution  601  which contains, for example, 200 g/L of CuSO 4 -5H 2 O, 50 g/L of H 2 SO 4  and 100 mg/L of HCl. The magnetic nano-wires  500  formed by the process shown in  FIG. 5  are dispersed in the plating solution. A uniform magnetic field which is generated by a magnet  604  is applied in the plating solution. The direction of the applied magnetic field is perpendicular to the substrate. Then the magnetic nano-wires which are dispersed in the solution start to align to the direction of the magnetic field. At the same time that the magnetic field is applied, an electric current is also applied between the cathode  214  and a counter electrode  602 . Thus a Cu layer is deposited on the cathode layer  214  through the apertures  201  together with the Ni nano-wires. Since the Ni nano-wires align perpendicularly to the substrate in the solution, most of incorporated Ni nano-wires in the Cu film are oriented in the direction perpendicular to the major surface of the substrate (as shown in  FIG. 2 ) It will be seen that the alignment need not be exact. All that is required is a strong correlation between the length direction of the nano-wires and the vertical direction in the figure. The difference between the above-mentioned two directions is preferably smaller than 10 degrees on average. The average difference in the direction of adjacent nano-wires is preferably no more than 5 degrees. Note that these two figures are merely averages: a small proportion of the nano-wires may fall over or incline at exceptional angles. The density of the nano-wires on the fabricated electron emitter is adjusted by optimizing the amount of dispersed nano-wires in the plating solution, the magnetic field, agitation of the plating solution and the plating current, so that the ratio S/L becomes larger than 1. This fabrication process results in a reasonable threshold voltage of field emission. The materials for the nano-wires and a metal layer do not have to be limited to Ni and Cu. Indeed both the nano-wires and the metal layer may be formed of the same material, e.g. Ni. Referring now to  FIG. 7 , an alternative way of performing the step of  FIG. 6  is shown. A permanent magnet  704  is attached behind the substrate  200 . A magnetic yoke  705  may be effective to make the magnetic field uniform. The Ni nano-wires  500  are attracted by the magnet  704  and attached on the cathode layer  214  through the apertures  201  of the insulation layer  203 . The nano-wires  500  are aligned with their longitudinal direction perpendicular to the substrate. An electric current is applied between the cathode layer  214  and a counter electrode  702  at the same time. Then Cu film is deposited on the cathode  214  and the nano-wires  500  are fixed on the cathode layer  214  by the electroplated Cu film  202 . The density of the nano-wires on the fabricated electron emitter is adjusted by optimizing the amount of dispersed nano-wires in the plating solution, the magnetic field, agitation of the plating solution and the plating current, so that the ratio S/L becomes larger than 1. 
       FIG. 8  shows a top view of a second embodiment of the emitter array  220 . Reference numerals have the same meaning as in  FIG. 3 . This embodiment may be used also in the manner shown in  FIG. 1 . In contrast, to the first embodiment shown in  FIG. 3 , the apertures  201  are replaced by an array of smaller apertures  1201 . 
       FIG. 9  is a cross-sectional view of a third embodiment of the emitter array  220 . Reference numerals have the same meaning as in  FIG. 2 . This embodiment may be used also in the manner shown in  FIG. 1 . In contrast to the first embodiment shown in  FIG. 2 , the nano-wires  216  of the third embodiment are shorter and narrower, The upper tips of the uppermost nano-wires  216  are substantially co-planar with the gate electrodes  204 . 
     By applying an electric field between one cathode and one gate electrode, field electron emission can be induced at the intersection of the cathode and the gate electrode selectively. Since the induced electron emission results in a local luminescence, it can be utilized for a pixel of a FED or a segment of a multi-segmented backlight for a LCD. 
       FIG. 10  shows the production of a further embodiment of the invention. Elements which have the same meaning as the first embodiment are shown by the same reference numerals as in  FIG. 2 . In contrast to the first embodiment, a second metal layer  1001  deposited over the first layer  202 . For example, Ni can be used for the first metal layer  202  and Cu can be used for the second metal layer  1001 . The Cu of layer  1001  can be lapped or polished on a flat whetstone, to give the structure of  FIG. 10(   b ). Then the Cu of layer  1001  can be etched selectively (e.g. by one of the suitable materials mentioned above), and then the gate electrodes  204  formed, to give the structure of  FIG. 10(   c ). 
       FIG. 11  shows the production of a further embodiment of the invention. Elements which have the same meaning as the first embodiment are shown by the same reference numerals as in  FIG. 2 . In contrast to the first embodiment, the layer  202  is much thicker. Optionally, Ni can be used for the nano-wires  216  and Cu can be used for the metal layer  202 . There is then a lapping step, to produce the structure of  FIG. 11(   b ). Then, the Cu of layer  202  can be etched selectively (e.g. by one of the suitable solutions mentioned above) and then the gate electrodes formed, to give the structure of  FIG. 11(   c ). 
     Both of the embodiments of  FIGS. 10 and 11  tend to give more correlated nanowires than the first embodiment and/or the exposed length of the nanowires is more uniform. 
       FIG. 12  shows a preferred property of all the embodiments explained above. Specifically, the lengths of the exposed portions of the nanowires from metal layer in which they are embedded is approximately the same. Preferably, about 90% of the nanowires are such that, their if the average exposed length of those nanowires is L A  on average, the deviations ΔL 1  and ΔL 2  respectively above and below this average, are within 10% of L A . A relatively small proportion of nano-wires have an average length which deviates from L A  by an amount ΔL 3  which is more than 10%. Note that when S/L=1, (βm/βs) changes by less than 3% if the length of nano-wire fluctuates by no more than 10%. In most applications of the embodiment, a 3% change in (β m /β s ) does not affect the luminescence of the phosphor plate significantly, so a variation in average length of +10% is preferable. 
     REFERENCE 
     
         
         [1] Jean-Marc Bonard et al. “Tuning the Field Emission Properties of Patterned Carbon Nanotube Films”,  Advanced Material  2001, 13, No. 3, February 5, pp. 184-188