Abstract:
A method of forming emitter tips for use in a field emission array is disclosed. The tips are formed by utilizing a polymer residue that forms during the dry etch sharpening step to hold the mask caps in place on the emitter tips. The residue polymer continues to support the mask caps as the tips are over-etched, enabling the tips to be etched past sharp without losing their shape and sharpness. The dry etch utilizes an etchant comprised of fluorine and chlorine gases. The mask caps and residue polymer are easily removed after etching by washing the wafers in a wash of deionized water, or Buffered Oxide Etch.

Description:
Government Rights: This invention was made with United States Government support under contract No. DABT63-97-C-0001 awarded by the Advanced Research Projects Agency (ARPA). The United States Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Cross-Reference to Related Applications: 
     This application is a continuation of application Ser. No. 10/153,195, filed May 22, 2002, pending, which is a continuation of application Ser. No. 09/639,357, filed Aug. 14, 2000, now U.S. Pat. No. 6,461,526 B1, issued Oct. 8, 2002, which is a continuation of application Ser. No. 09/026,243, filed Feb. 19, 1998, now U.S. Pat. No. 6,171,164 B1, issued Jan. 9, 2001. 
     This invention relates generally to field emission displays and, more particularly, to the fabrication of an array of atomically sharp field tips for use in field emission displays. 
     The manufacture and use of field emission displays is well known in the art. The clarity, or resolution, of a field emission display is a function of a number of factors, including emitter tip sharpness. 
     One current approach toward the creation of an array of emitter tips is to use a mask to form the silicon tip structure, but not to form the tip completely. Prior to etching a sharp point, the mask is removed or stripped. Next, the tip is etched to sharpness after the mask is stripped from the apex of the tip. 
     It has been necessary to terminate the etch at or before the mask is fully undercut to prevent the mask from being dislodged from the apex. If an etch proceeds under such circumstances, the tips become lopsided and uneven due to the presence of the mask material along the side of the tip, or the substrate, during a dry etch and, additionally, the apex may be degraded, as shown in FIG.  1 . Such a condition also leads to contamination problems because of the mask material randomly lying about a substrate. This mask  30 , when dislodged, masks off a region of the substrate  11  where no masking is desired and allows continued etching in places where the mask  30  is supposedly protected. This results in randomly placed, undesired structures being etched in the material. 
     If the etch is continued after the mask is removed, the tip becomes more dull. This results because the etch chemicals remove material in all directions, thereby attacking the exposed apex of the tip while etching the sides. In addition, the apex of the tip may be degraded when the mask has been dislodged due to physical ion bombardment during a dry etch. 
     Accordingly, current methods perform under-etching, which is to stop the etching process before a fine point is formed at the apex of the tip. Under-etching creates a structure referred to as a “flat top.” An oxidation step is then performed to sharpen the tip. This method results in a nonuniform etching across the array and the tips then have different heights and shapes. Other solutions have been to manufacture tips by etching, but they do not undercut the mask all the way. Furthermore, they do not continue etching beyond full undercut of the mask as this typically leads to degradation of the tip. Rather, they remove the mask before the tip is completely undercut, then sharpen the tips from there. The wet silicon etch methods of the prior art result in the mask being dislodged from the apex of the tip, at the point of full undercut. This approach can contaminate the bath, generate false masking, and degrade the apex. 
     The nonuniformity among the tips can also present difficulties in subsequent manufacturing steps used in the formation of the emission display. This is especially so in those processes employing chemical planarization, mechanical planarization, or chemical mechanical planarization. Nonuniformity is particularly troublesome if it is abrupt, as opposed to a graduated change across the wafer. 
     Fabrication of the uniform wafer of tips using current processes is difficult to accomplish in a manufacturing environment for a number of reasons. For example, simple etch variability across the wafer affects the wafer at the time at which the etch should be terminated with the prior art approach. 
     Generally, it is difficult to obtain positive etches with definitions better than 5%, with uniformities of 10-20% being more common. This makes the “flat top” of an emitter tip etch using conventional methods vary in size. In addition, the oxidation necessary to “sharpen” or point the tip varies as much as 20%, thereby increasing the possibility of nonuniformity among the various tips in the array. 
     Tip height and other critical dimensions suffer from the same effects on uniformity. Variations in the masking conformity and material to be etched compound the problems of etch uniformity. 
     Manufacturing environments require processes that produce substantially uniform and stable results. In the manufacture of an array of emitter tips, the tips should be of uniform height, aspect ratio, sharpness, and general shape with minimal deviations, particularly in the uppermost portion. 
     In one approach used to overcome the problems illustrated in the prior art, a mask is formed over the substrate before etching begins. The mask has a composition and dimensions that enable it to remain balanced on the apex of the tips until all the tips are substantially the same shape when the etch is performed. This is disclosed in U.S. Pat. No. 5,391,259, issued Feb. 21, 1995, entitled “Method for Forming a Substantially Uniform Array of Sharp Tips.” Although this process does achieve a more uniform array of sharp tips, there are still problems with the balancing of the mask on the apex of the tips until all the tips have finished etching and reached sharpness. That is, the uniformity of the mask cannot always be guaranteed and slipping of the mask onto the substrate as illustrated in FIG. 1 still occurs, albeit less frequently. Accordingly, what is needed is a method for maintaining the mask above the apex of the tips in a more secure fashion until the desired uniform sharpness is achieved during the etch process. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a method of forming emitter tips for use in a field emission array is disclosed. The tips are formed by utilizing a polymer residue that forms during the dry etch sharpening step to hold the mask caps in place on the apex of the emitter tips. The residue polymer continues to support the mask caps as the tips are over-etched, enabling the tips to be etched past sharp without losing their shape and sharpness. The dry etch utilizes an etchant comprised of fluorine and chlorine gases. The mask caps and residue polymer are stripped after etching by washing the wafers in deionized water. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a cross-sectional schematic drawing of a malformed structure that results when the mask layer is dislodged from the tips of the etch; 
     FIG. 2 is a cross-sectional schematic drawing of a pixel of a flat panel emission display having cathode emitter tips fabricated by the process of the present invention; 
     FIG. 3 is a cross-sectional schematic drawing of a substrate in which is deposited or grown a mask layer and a pattern photoresist layer, according to the process of the present invention; 
     FIG. 4 is a cross-sectional schematic drawing of the structure of FIG. 3, after the mask layer has been selectively removed by plasma dry etch, according to the process of the present invention; 
     FIG. 5 is a cross-sectional schematic drawing of the structure of FIG. 4, during the etch process of the present invention; 
     FIG. 6 is a cross-sectional schematic drawing of the structure of FIG. 5, as the etch proceeds according to the process of the present invention, illustrating that some of the tips become sharp before other tips; 
     FIG. 7 is a cross-sectional schematic drawing of the structure of FIG. 6, as the etch proceeds toward the process of the present invention; and 
     FIG. 8 is a cross-sectional schematic drawing of the structure of FIG. 7, depicting the sharp cathode tip after the etch has been completed and the mask layer has been removed. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A representative portion of a field emission display 10 is illustrated in FIG.  2 . The emission display 10 includes a display segment  22 . Each display segment  22  is capable of displaying a pixel, or a portion of a pixel  19 , as, for example, one green dot of a red/green/blue full-color triad pixel. Preferably, a substrate comprised of glass is used and a material that is capable of conducting electric current is present on the surface of the substrate so that it can be patterned and etched to form micro cathodes or electrode emitter tips  13 . Amorphous silicon is deposited on the glass substrate to form micro cathodes  13 . 
     At a field emission site, a micro cathode  13  has been constructed on top of the substrate 11. The micro cathode  13  is a protuberance that may have a variety of shapes, such as pyramidal, conical, or other geometry that has a fine micro point for the emission of electrons. Surrounding micro cathodes  13  is a grid structure  15 . When a voltage differential, through source  20 , is applied between cathodes  13  and grid structure  15 , a stream of electrons 17 is emitted toward a phosphor coated face plate  16 . Face plate  16  serves as the anode where pixels  19  are charged by electrons  17 . 
     The micro cathode  13  is integral with a substrate  11  and serves as the cathode. Grid structure  15  serves as a grid structure for applying an electrical field potential to its respective cathode  13 . 
     A dielectric insulating layer  14  is deposited on conductive cathode  13 , which dielectric insulating layer  14  can be formed from the substrate or from one or more deposited films, such as a chromium amorphous silicon bilayer. Dielectric insulating layer 14 also has an opening at the field emission site. 
     Disposed between face plate  16  and base plate  21  are spatial support structures  18  that function as support for atmospheric pressure that exists on the electrode face plate  16 . The atmospheric pressure is the result of the vacuum created between the base plate  21  and face plate  16  for the proper functioning of the micro cathodes  13 . 
     Base plate  21  comprises a matrix addressable array of cold micro cathodes  13 , a substrate  11  where cathodes  13  are formed, insulating layer  14 , and anode grid structure  15 . 
     In the process of the present invention, the mask dimensions, the balancing of the gases and parameters in the plasma etch enable the manufacturer to determine and significantly control the dimensions of micro cathode  13 . Compositions of the mask affects the ability of mask 30 (see FIG. 3) to remain balanced at the apex of the micro cathode  13  and to remain centered on the apex of micron cathode  13  during the over-etching of micro cathode  13 . This is achieved by using a combination of gases that forms a polymer support between the apex of cathode 13 and the subsurface of dielectric insulating layer  14 , rather than merely relying upon mask  30  to balance precariously on the micro cathode  13  during the etching process. Over-etching refers to the time period when the etch process is continued after a substantially full undercut is achieved. Full undercut refers to the point at which the lateral removal of material is equal to the original lateral dimension of the mask 30. 
     FIG. 3 depicts the substrate  11 , which is amorphous silicon overlying glass, polysilicon, or any other material from which micro cathode  13  can be fabricated. Substrate 11 has a mask  30  deposited or grown thereon. Mask  30  is typically a 0.2 micrometer (μm) layer of silicon dioxide formed on the substrate 11. Tip geometries and dimensions and conditions for the etch process will vary with the type of materials used to form cathodes 13. 
     Mask 30 can be made of any suitable materials such that its thickness is great enough to avoid being completely consumed during the etching process, but not so thick as to overcome the adherent forces that maintain it in the correct position with respect to cathode 13 throughout the etch process. 
     A photoresist layer  32 , or other protective element, is patterned on mask 30 if the desired masking material cannot be directly patterned or applied. When photoresist layer 32 is patterned, the preferred shapes are dots or circles. 
     The next step in the process is selective removal of mask 30 that is not covered by photoresist layer  32  as shown in FIG.  4 . The selective removal of mask 30 is accomplished preferably through a wet chemical etch. An aqueous HF solution can be used in a case of a silicon dioxide mask; however, any suitable technique known in the industry may also be employed, including physical removal techniques or plasma removal. 
     In a plasma etch, the typical etches used to etch the silicon dioxide include, but are not limited to: Chlorine and Fluorine. And typical gases and compounds include: CF 4 , CHF 3,  C 2 F 6  and C 3 F 8 . Fluorine with oxygen can also be used to accomplish the oxide mask 30 etch step. The etchant gases are selective with respect to silicon and the etch rate of oxide is known in the art, so that the point of the etch step can be calculated. 
     Alternatively, a wet oxide etch can also be preformed using common oxide etch chemicals. At this stage, the photoresist layer 32 is stripped. FIG. 5 depicts the mask 30 structure prior to the silicon etch step. 
     A plasma etch, with selectivity to the etch mask 30, is then employed to form cathodes 13. The plasma contains a fluorinated gas, such as NF 3,  in combination with a chlorinated gas, such as Cl 2 , and forms a polymer residue that supports the mask during the etch process. Preferably, the plasma comprises a combination of NF 3  and Cl 2 , and an additive, such as helium. The combination of NF 3  and Cl 2  is in such a ratio that during the etching process, a polymer 34 is formed underneath mask 30 and on the cathode  13 . Polymer 34 is used to build a mask support of mask 30 as cathode  13  goes from before sharp, shown in FIG. 5, to etch sharp, shown in FIG. 6, and past sharp, shown in FIG.  7 . Sharpness is defined as “atomically sharp” and refers to a degree of sharpness that cannot be defined clearly by the human eye when looking at a scanning electron microscope (SEM) micrograph of the structure. The human eye cannot distinguish where the peak of cathode 13 actually ends. The measured apex of a sharp tip is typically between 7Å and 10Å. 
     The following are the ranges of parameters for the process as described in the present application. Included is a range of values investigated during the characterization of the process, as well as the range of values that provides the best results for cathodes 13 that were from 1 μm to 2 μm in height and 1.3 μm to 2.0 μm at the base, with 1.5 μm preferred. One having ordinary skill in the art will realize that the values can be varied to obtain a cathode 13 having other height and width dimensions as previously stated. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Parameters 
                 Investigative Range 
                 Preferred Range 
               
               
                   
               
             
             
               
                 Cl 2 :NF 3  ratio 
                 10 to 60% 
                 30 to 40% 
               
               
                 Cl 2 :NF 3   
                 150-620 SCCM 
                 290-340 SCCM 
               
               
                 Helium 
                 60-250 SCCM 
                 110-140 SCCM 
               
               
                 Power 
                 2500 w 
                 2500 w 
               
               
                 Pressure 
                 5-100 mTorr 
                 50-70 mTorr 
               
               
                 Bottom Electrode Power 
                 0-400 w 
                 200-300 w 
               
               
                 Spacing Time 
                 1.5-3.5 min 
                 140-150 seconds 
               
               
                 Temperature 
                 15-70° C. 
                 35-45° C. 
               
               
                   
               
               
                 *SCCM—Standard Cubic Centimeters per Minute.  
               
             
          
         
       
     
     Experiments were conducted on a LAM continuum etcher with enhanced cooling. The lower electrode was maintained substantially in the range of 40° C.. The etched time that received the best results was between 140-150 seconds with 145 seconds being optimal. 
     The use of the polymer 34 created during the etching allows the cathodes to achieve an aspect ratio of 2.5-3.2 using the preferred parameter ranges. Aspect ratio = downward etch rate/undercut etch rate. 
     The ability to etch to its conclusion past full undercut with minimal changes to the functional shape between the first cathode 13 to become sharp and the last cathode to become sharp provides a process in which all of the cathodes in the array are essentially identical in characteristics. Cathodes of uniform height and sharpness are carefully selected based on the ratio of NF 3  to Cl 2  used during the mask etch step. This is important in that the combination of NF 3  to Cl 2  forms the polymer 34 that provides support for mask 30 during the etching of micro cathodes  13 . 
     After the array of micro cathodes  13  has been fabricated, the oxide mask  30  can be removed along with the polymer 34. This is illustrated in FIG.  8 . Mask 30 and polymer 34 are stripped off by a simple wet etch utilizing deionized water, or a Buffered Oxide Etch. As the mask has been etched away from each cathode 13, no harsh chemicals need to be used during a subsequent etch removal of mask 30. 
     Ideally, the NF 3 -Cl 2  gas is provided at 310 SCCMs while the helium gas is provided at 125 SCCMs during etching. 
     As shown in FIG. 8, the yield of cathodes results in a uniformity of 20%, or within plus or minus 10%, of the average height and shape for each cathode 13. Further, the yield is improved such that a fewer number of cathodes per pixel are necessary as more and more useful cathodes are provided. Additionally, with the more uniform height and sharpness, the turn-on voltage during operation of a field emission display can be lowered. Further, the number of shorter cathodes that are much shorter than the dimension desired are greatly reduced or eliminated, which means shorting to the grid is also reduced or eliminated. 
     While the particular process for forming sharp micro cathodes to use in flat panel displays as herein shown and disclosed in detail is fully capable of obtaining the desired effects stated above, it is to be understood that it is to be illustrated as the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the depending claims. For example, the process of the present invention was discussed with regards to the fabrication of uniform arrays of sharp micro cathodes and flat panel displays; however, one of ordinary skill in the art will realize that such a process can be applied to other field ionizing and electron emitting structures, and to micro-machining of structures in which it is desired to have a sharp point, such as a probe tip or other device.