Abstract:
A method of forming an electron emitter structure for use in a field emission display, or as a field emission backlight for an LCD display is provided. The electron emitter structure is formed by depositing mask elements onto an laminar Al substrate, and etching the Al substrate chemically through gaps between the mask elements, such that a spikes are formed on the substrate. These spikes are then covered with an electron emitter material. The spikes can be formed with a desired pitch/height ratio.

Description:
[0001]    Methods for producing an electron emitter structure for a field emission display (FED), or a field emission backlight for a liquid crystal display (LCD), and also to the electron emitter structures, FEDs and field emission backlights so produced. 
       BACKGROUND  
       [0002]    Field emission displays (FED) are among the most promising candidates for the next generation of displays. Field Emission Backlights for liquid crystal displays (LCD) also attract the interest of many researchers today. 
         [0003]    One well-known type of electron emitter structure of a FED, a Spindt type emitter, is shown schematically in  FIG. 1 . A lower portion of the emitter comprises a glass substrate  1 , covered by a lower electrode  2 . A number of insulator elements  3  support gate electrode elements  4 . This type of emitter has a small Mo cone  5 , in electric contact with the lower electrode  2 , as a cathode. When a large enough negative voltage is applied to the cone  5  (via the lower electrode  2 ) the electric field concentration at the top of the Mo cone  5  becomes high enough to expel electrons. The electrons are attracted across a gap to an upper portion of the FED, composed of an upper electrode (anode)  7 , which is sandwiched by a phosphor layer  6  and a second glass substrate  8 . The generation of light by the phosphor layer  6  can be controlled by controlling the voltages applied to the gate electrode elements  4 , in particular so as to make certain areas brighter than others. If the gate elements  4  are omitted, the structure generates uniform light, and may also for example function as a uniform field emission backlight of an LCD. 
         [0004]    This emitter array is fabricated using vacuum-processing machinery in a photo-lithographic process which needs many masks. The Mo cone  5  is deposited with the substrate tilted, and then by rotating it. The series of process steps is very complicated and expensive, and it is difficult to make exactly the same cone shape for each of the many emitters in the array, which leads to uneven electron emission properties. 
         [0005]    Another type of emitter, a Carbon Nano Tube (CNT), also attracts much interest.  FIG. 2  shows a schematic diagram of the CNT type FED. Elements having the same meaning as those in  FIG. 1  are denoted by the same reference numerals. The only difference is that a plurality of CNTs  9  are used as each cathode. 
         [0006]    The CNTs are grown perpendicular to the surface of the substrate  1  and the electric field concentration is high at the top of the CNTs. Carbon is well known for its high electron emission property, so CNT is a suitable material for an electron emitter. However, the CNT are normally grown in a vacuum and usually high temperature is required to make good quality CNTs, so the process is expensive, and the substrate also expensive because it has to withstand high temperature during the process. 
         [0007]    To solve the CNT-related problems which are mentioned above, some researchers have proposed a binder having the consistency of a paste and containing independent CNTs. Such a binder is pasted onto the substrate in place of the CNT of  FIG. 2 , and some of the CNTs in the paste happen to project above the binder surface, and work as cathodes. Unfortunately, this method makes it difficult to achieve even electron emission in the array, and also there is also high contact resistance between the electrode  2  below the CNT paste and each CNT. Additionally, the independent CNTs themselves are an expensive material. 
       SUMMARY  
       [0008]    A mask with a plurality of spaced-apart openings is formed on a surface of a substrate. The substrate is chemically etched through the openings, whereby portions of the substrate proximate each of the openings are removed, leaving the surface with a plurality of spikes. Each of the spikes may function as an electron emitter. If the substrate itself is not formed of an electron emission substance having suitable electron emission properties, a layer of such a material is deposited onto the structure. An exemplary electron emission substance is DLC. 
         [0009]    By selecting the properties of the mask and the etching conditions, the geometrical properties of the spikes can be selected. Thus, certain embodiments of the invention provides spike structures which have an optimal pitch/height ratio. Isotropic chemical etching naturally tends to form a structure having a pitch/height ratio of substantially 2, which has been shown elsewhere to be ideal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]    A more complete appreciation of the inventions and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. However, the accompanying drawings and their exemplary depictions do not in any way limit the scope of the inventions embraced by this specification. The scope of the inventions embraced by the specification and drawings are defined by the words of the accompanying claims. 
           [0011]      FIG. 1  is a schematic diagram of the known Spindt type FED; 
           [0012]      FIG. 2  is a schematic diagram of the known CNT type FED; 
           [0013]      FIG. 3 , which is composed of  FIGS. 3(   a ) to  3 ( h ), shows, using cross-sectional views, the steps of an exemplary process which is an embodiment of the present invention, to form spike structures on an Al substrate using AAO; 
           [0014]      FIG. 4  shows schematically the anodization setup used in the embodiment of  FIG. 3 ; and 
           [0015]      FIG. 5  shows schematically the filtered-cathodic-vacuum-arc (FCVA) deposition system used in the embodiment of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION  
       [0016]    Referring firstly to  FIG. 3(   a ), the starting point of a method which is an embodiment of the present invention is a laminar substrate  10  formed of Al. The thickness of the Al substrate is 0.4 mm (so that the substrate is an Al foil) and the purity of Al is 99.999%. As shown in  FIG. 3(   b ), a layer  11  of anodized aluminum oxide (AAO) is formed on one major surface of the Al substrate  11  by an anodization step, followed by pore widening. The thickness of AAO layer is preferably less than 1 μm. The AAO layer  11  is formed with many through-holes  12  which are perpendicular to the surface of the substrate  10 . 
         [0017]      FIG. 4  shows schematically the setup used to perform the anodization. The Al foil  10  is supported by a support structure  19  with one of its two faces exposed to an acidic solution  15  within a bath  14 . 0.4M H3PO4 is used as the acidic solution  15 . The bath also contains a Pt wire  16 , and an electric process is carried out in which the Al foil  10  functions as an anode and a Pt wire  16  as the cathode. A magnetic stirrer  17  is driven by a magnetic driver  18 . A DC voltage is applied between the Pt wire  16  and the Al foil  10 , for example 100V, and a layer of AAO is formed on the substrate  10 . The AAO thickness is controlled by the anodization time, which may be 2 mins. 
         [0018]    After the anodization, the pore widening process is conducted. In this process the surface of the Al substrate  10  having the AAO layer is soaked in 10 wt % H3PO4 solution for 70 mins. This process enlarges the pores, to the extent that the pores become through holes  12 . The resulting thickness of the AAO elements  11  (in the vertical direction of  FIG. 3(   b )) is about 1 μm, the pore diameter is about 200 nm and the pore pitch (i.e. periodicity of the pores) is about 250 nm, such that the width of the elements  11  is about 50 nm. As shown in  FIG. 3(   c ), Ti layers  20  are then deposited in the respective pores by sputtering. Their thickness is about 300 nm. 
         [0019]    As shown in  FIG. 3(   d ), the AAO  11  is then removed completely using 10wt % H3PO4 solution. 
         [0020]    As shown in  FIG. 3(   e ), the Al substrate  10  is etched using 10 wt % H3PO4 solution. In this process, the Ti layers  20  work as a mask and the etching proceeds isotropically as shown in this figure. 
         [0021]    As shown in  FIG. 3(   f ), the parts of the Al substrate  10  below the Ti masks  20  are made thinner by etching. Eventually, as shown in  FIG. 3(   g ), the parts of the Al substrate  10  below the Ti masks  20  become too thin to support the Ti masks  20 , and finally the Ti masks  20  are separated. The ratio of the pitch of the spikes and the height of the spikes (pitch/height) becomes about 2 because of the isotropic etching. This is significant because it has been reported in the paper Appl. Phys. A 83, 111-114 (2006), by Kim et al, entitled “Numerical study on the field emission properties of aligned carbon nano tubes using the hybrid field enhancement scheme”, that when the pitch/height ratio of CNT is about 2, the maximum current density is obtained if the top of the spike is very sharp. 
         [0022]    As shown in  FIG. 3(   h ), a layer of DLC (diamond-like carbon)  21  is deposited on the Al substrate  10  by the filtered-cathodic-vacuum-arc (FCVA) method. DLC formed by FCVA is known for its high electron emission properties. In other embodiments, other low work function materials or low electron affinity materials may be deposited instead of DLC. 
         [0023]    The FCVA deposition system is shown in  FIG. 5 , where a carbon plasma  22  is produced through high current arcing  24  of a graphite cathode  23  in high vacuum conditions. A magnetic field produced by electromagnets  25  is created to steer the carbon plasma in a curved path towards the surface of the substrate  10 , eliminating undesired macro-particles in order to produce a purer and denser film structure. Further electromagnets  26  perform a focusing function. The ion energy is controlled by the bias voltage, applied by a voltage generator  27 , which is applied to a metal substrate holder (not shown) which holds the substrate  10 . By optimizing the voltage bias during the deposition process, the film hardness, the film stress and the strength of adhesion can be controlled (as disclosed in U.S. Pat. No. 6,031,239, which is hereby incorporated by reference). Also the bonding structure of carbon ions can be controlled. 
         [0024]    The structure produced in  FIG. 3(   h ) can now be employed in a structure as shown in  FIG. 1 , replacing the cone  5  and the lower electrodes  2 . If the gates  4  are omitted, the structure operates as a light-generator, such as a uniform field emission backlight for an LCD. 
         [0025]    Note, however that nowadays many researches are trying to segment the backlight into many areas and to control the brightness of each area depending on a shown picture in order to increase the contrast of the shown picture and decrease the power consumption. Therefore even in systems in which the embodiment is employed as a backlight, the backlight may also be required to be controlled area by area. 
         [0026]    It is to be understood that both the foregoing general description of the invention and the following detailed description are exemplary, but are not restrictive, of the invention.