Abstract:
A system and method for fabricating a FED device is disclosed. The system and method provide for use of PECVD hydrogenation followed by nitrogen plasma treatment of the tip of the current emitter of the FED device. The use of this process greatly reduces the native oxides in the tip of the current emitter. Such native oxides function as undesirable insulators degrading current emission. By reducing the amount of oxides in the tip, this invention provides for an increase in the current emission of the FED device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    I. Field of the Invention  
           [0002]    The present invention relates generally to display devices implementing Field Emission Display (FED) technology. More specifically, the invention relates to a method for increasing the emission current of the current emitters of a Field Emission Display (FED).  
           [0003]    II. Description of the Related Art  
           [0004]    Until recently, the cathode ray tube (“CRT”) had been the primary device for displaying information. While having sufficient display characteristics with respect to color, brightness, contrast, and resolution, CRT&#39;s are relatively bulky and consume large amounts of power. In view of the advent of portable laptop computers, the demand has intensified for a display technology which is light-weight, compact, and power efficient.  
           [0005]    One available technology is flat panel displays, and more particularly, Liquid Crystal Display (“LCD”) devices. LCDs are currently used for laptop computers. However, these LCD devices provide poor contrast in comparison to CRT technology. Further, LCDs offer only a limited angular display range. Moreover, color LCD devices consume power at rates incompatible with extended battery operation. Lastly, a color LCD type screen tends to be far more costly than an equivalent CRT.  
           [0006]    FED technology has recently come into favor as one technology for developing low power, flat panel displays. This technology uses an array of cold cathode emitters and cathodoluminescent phosphors for conversion of energy from an electron beam into visible light. Part of the desire to use FED technology for flatpanel displays is that such technology is conducive to producing flat screen displays having high performance, low power and light weight.  
           [0007]    In FED structures and devices a plurality (array) of microelectronic emission elements are employed to emit a flux of electrons from the surface of the emission clement(s). The emitter surface, referred to as a “tip”, is specifically shaped to facilitate effective emission of electrons, and may for example be conical, pyramidal, or ridge-shaped in surface profile, or alternatively the tip may comprise a flat emitter surface of low work function material.  
           [0008]    In the construction of FED current emitters, various materials are deposited onto a substrate to form the device. Thereafter, a panel containing spaced phosphors is sealed to a panel containing the emitters under conditions where the temperature is approximately 400 degrees Celsius. When the material used to construct the FED current emitter tip, an amorphous silicon doped with boron or phosphorus, is deposited, native oxides form on the tip due to exposure to the atmosphere. This change in the chemical nature of the tip results in an increased work function yielding a decrease in the current emission of the tip nearly ten fold. As a general principal the work function is an instrumental factor in the resulting current emission. The practical effect is the manifestation of a display which is dimmer than that desired or expected, often resulting in an increase in power usage in order to try to achieve a brighter display.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention relates to a system and method for increasing the emission current of the current emitters of a FED device by removing native oxygen from silicon deposited on the tip of the FED device through PECVD hydrogenation and subsequently incorporating nitrogen onto the surface without exposing the tip to the atmosphere.  
           [0010]    In the invention an amorphous silicon tip doped with boron or phosphorus is subjected to PECVD hydrogenation followed by an infusing nitrogen plasma, preferably a NH 3  plasma, which deposits onto the tip surface, while the FED structure is still in the PECVD chamber. PECVD hydrogenation removes oxides from the silicon surface by infusing hydrogen. The result is the tip being free of approximately one third of the native oxides, which formed when the tip was exposed to atmospheric conditions and which would have otherwise remained on the tip increasing the work function and yielding a less than desirable emission current. The nitrogen plasma treatment is used to complete the process. After the PECVD and nitrogen plasma treatment, the FED structure is sealed in a vacuum under high temperature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The foregoing and other advantages and features of the invention will become more apparent from the detailed description of preferred embodiments of the invention given below with reference to the accompanying drawings in which:  
         [0012]    [0012]FIG. 1 is a cross sectional view of the material composition of a FED device in accordance with a preferred embodiment of this invention at an early stage of processing;  
         [0013]    [0013]FIG. 2 is a cross sectional view of the device in FIG. 1 at a subsequent stage of processing.  
         [0014]    [0014]FIG. 3 is a flow chart illustrating the process steps in accordance with a preferred embodiment of this invention;  
         [0015]    [0015]FIG. 4 is a chart comparing the present invention to the prior art in terms of oxygen, nitrogen and silicon present in the FED tip after fabrication;  
         [0016]    [0016]FIG. 5 is a graph plotting the oxygen, nitrogen and silicon concentrations of a FED tip after fabrication according to the prior art;  
         [0017]    [0017]FIG. 6 is a graph plotting the oxygen, nitrogen and silicon concentrations of a FED tip after fabrication according to the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]    Referring now to the drawings, where like reference numerals designate like elements. FIG. 1 is a representative cross-section of a FED device  100 . FED device  100  contains a substrate  102  made of glass upon which the materials making up the functional part of the FED device are deposited. The glass substrate  102  often contains impurities such as sodium, therefore, a “barrier film”  104 , in this instance silicon dioxide (SiO 2 ), is deposited on top of the substrate  102  as an insulator. This barrier film  104  is deposited using PECVD processing. Next, a conductive metal layer  106  is deposited in a desired pattern on top of the barrier film  104 . This conductive metal layer  106  is formed preferably of an aluminum alloy which may contain chromium. This conductive metal layer  106  is patterned to form vacant areas  108  where the conductive metal layer  106  does not cover the barrier layer  104 . These vacant areas  108  will hold the base of a later formed FED tip. After conductive metal layer  106  is formed, a layer of amorphous silicon doped with boron  110  is deposited followed by the deposition of a layer of amorphous silicon doped with phosphorus  114 . Layers  110  and  114  are deposited using PECVD processing.  
         [0019]    Next, as shown in FIG. 2, layers  110  and  114  are etched to form a current emitter  116  in an extended shape. Preferably emitter  116  is formed in a conical shape, but other shapes can be formed as well.  
         [0020]    To finish the construction of the FED device  100 , silicon dioxide is deposited using PECVD processing to form a insulating layer  112  around the sides of the current emitter  116 . The insulating layer  112  is provided around the sides of the current emitter  116  so that current does not radiate out of the sides of the current emitter  116  and provide cross-talk to nearby current emitters. Furthermore, this insulating layer  112  helps direct the current to the tip  118  of the current emitter  116  which is desired.  
         [0021]    A grid layer  120  is then deposited using PECVD. The grid layer  120  is composed of amorphous silicon doped with phosphorus. Another metal layer  122  is deposited using DC magnetron sputtering on top of grid layer  120 . Lastly, a passivation layer  124 , containing nitride, is deposited on top of the metal layer  122 . To ensure an opening for emission current to pass from the tip  118 , an open area  126  is etched from the passivation layer  124  down to the insulating layer  112 .  
         [0022]    At this point native oxides are present in the tip  118  as a result of the silicon at tip  118  being exposed to the atmosphere. If left untreated, these natural oxides will reduce the emission current at the tip  118  approximately ten fold. To combat this problem, this invention, treats the tip  118  of FIG. 2 with a PECVD hydrogenation process and subsequently with a nitrogen plasma process while the FED device  100  is still in the PECVD chamber. The nitrogen plasma treatment should occur while the FED device  100  is still in the PECVD chamber to reduce the possibility of atmosphere contamination.  
         [0023]    This PECVD hydrogenation is performed with about 1000 sccm silane gas flow, with the RF power set between about 200-300 watts, and the PECVD chamber pressure at about 1200 mtorr for a period of about 5 to 10 minutes. The nitrogen plasma treatment is performed with about 500 sccm NH3 (ammonia) gas flow, with the RF power set between about 300-400 watts, and the PECVD chamber pressure at about 1200 mtorr for a period of about 10 to 15 minutes. This treatment changes the chemical nature of the current emitter tip  118 .  
         [0024]    After the PECVD and nitrogen plasma treatment, a panel containing a plurality of FIG. 2 current emitters is heat sealed to a facing faceplate panel containing phosphors  130  with a top surface substrate  132 , which oppose the current emitters  116  using conventional techniques under a temperature as high as 400-420 degrees Celsius. The resulting FED device has a lower work function and increased current emission as a consequence of the PECVD hydrogenation and nitrogen plasma treatment.  
         [0025]    [0025]FIG. 4 illustrates, in a tabular format, the surface atomic concentrations of an oxygen, nitrogen and silicon for a conventional emitter tip without tip surface treatment in accordance with the invention (1) and with the surface treatment in accordance with the invention (2). It shows that the surface treatment of this invention greatly reduces the atomic concentration of silicon and oxygen on the tip (which can form silicon dioxide in the presence of heat). This data was derived by using x-ray photoelectron spectroscopy. FIGS. 5 and 6 are graphs of the results of a x-ray photoelectron spectroscopy (XPS) analysis of the emitter tips. These graphs show the atomic percentages of nitrogen, oxygen, and silicon verses sputter time for a conventional emitter tip without surface treatment in accordance with this invention and for an emitter tip with tip surface treatment in accordance with this invention, respectively. These graphs were generated by the XPS inspection apparatus after the FED devices  100  were already fabricated. Thus, “sputter time” as illustrated in FIGS. 5 and 6 pertains solely to the sputter time of the XPS inspection apparatus not to sputter time in relation to the fabrication of the FED device  100 .  
         [0026]    A comparison of FIGS. 5 and 6, focusing on the data to the left of lines  500  and  600 , shows that treatment in accordance with this invention does change the surface chemistry of the current emitter. The most obvious chemical changes being the reduction of oxygen and presence of nitrogen in FIG. 6. This confirms the data illustrated in FIG. 4.  
         [0027]    The overall process of the invention is illustrated in FIG. 3. The barrier film layer  104  is first deposited on the substrate  102  using PECVD processing  302 . The conductive layer  106  is then deposited using DC magnetron sputtering, where patterning is included for the base of the current emitter  116  to be formed  304  and for electrode contact with the current emitters  116 . The current emitter  116  is constructed by the deposition of silicon doped with boron  110 ,  306  and silicon doped with phosphorus  114 ,  308 . The current emitter  116  is then etched forming a tip  118  at the top of the structure  310 . The insulating layer  112  is then deposited using PECVD processing  312 . Next, the grid  120  is deposited also using PECVD  314 . A second metal layer  122 ,  316  and the deposit of a passivation layer  124 ,  318  complete fabrication of the structure. An area is then etch through the layers formed in steps  314  through  318 , so that the current emission from the tip  118  can reach the upper surface of the FED device  100 . The tip  118  is then treated with PECVD hydrogenation  322  followed by an infusion of nitrogen on the tip  324  while the tip  118  is still in the PECVD chamber. Lastly, a panel containing the formed FED device  100  is sealed under a high temperature  326  to a faceplate panel area containing phosphors  130 , where the areas containing phosphors  130  are positioned to align with a respective current emitter.  
         [0028]    It is to be understood that the above description is intended to be illustrative and not restrictive. Many variations to the above-described method and structure will be readily apparent to those having ordinary skill in the art. For example, the micropoint structures may be manufacture with more than one insulating layer.  
         [0029]    Accordingly, the present invention is not to be considered as limited by the specifics of the particular structures which have been described and illustrated, but is only limited by the scope of the appended claims.