Abstract:
A method for removing a surface protrusion projecting from a layer of a first material deposited on a surface of a substrate. In accordance with one embodiment of the invention, a layer of a second material is applied on the layer of first material. A sufficient quantity of the second material is removed to expose the surface protrusion. The first material exposed through the surface protrusion is then removed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 08/726,955, filed Oct. 7, 1996, now U.S. Pat. No. 5,902,491. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under Contract No. DABT63-93-C-0025 ordered by Advanced Research Projects Agency (ARPA). The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present invention is directed generally to the formation of a layer on a substrate and specifically to removing surface protrusions from such a layer which inherently occur during its formation. 
     BACKGROUND OF THE INVENTION 
     Processes for depositing a thin film layer on a substrate are known in the art. An example of one such process is physical vapor deposition (PVD). Inherent in the PVD process is the formation of surface protrusions on the thin film during deposition of the material forming the thin film. These surface protrusions can be many times the size of components to be later deposited on the thin film. As a result, the surface protrusions may project into layers of material formed on the thin film layer. In such cases, the surface protrusions may result in an unwanted short circuit between the thin film layer and a layer formed on top of the thin film layer. For example, in the baseplate of a field emission display (FED), such surface protrusions could result in a short circuit between the thin film layer on which emitters are formed and an extraction grid positioned above the emitters. This is true because a surface protrusion can have a height which is much greater than the height of an insulating layer positioned between the thin film layer and the extraction grid. 
     Various techniques have been attempted in the prior art in an effort to alleviate the adverse effects of surface protrusions. First, the parameters of the PVD process have been adjusted to try and prevent formation of the surface protrusions. This technique has not been entirely successful in that some surface protrusions are inherently formed during the PVD process. Given that some surface protrusions are formed, chemical-mechanical planarization (CMP) has been utilized to try and remove these protrusions from the thin film layer. When CMP is used directly on the thin film layer, however, the larger surface protrusions sometimes break lose and scratch the surface of the thin film layer. These scratches can result in unacceptably large areas of the thin film being unsuitable for the formation of the desired components. 
     In view of the problems associated with these processes for removing surface protrusions from a thin film, it is desirable to develop a process which removes the surface protrusions from the thin film without detrimentally affecting the surface of the thin film layer. 
     SUMMARY OF THE INVENTION 
     The present invention is a method for removing a surface protrusion projecting from a layer of a first material deposited on a surface of a substrate. In one embodiment, the method comprises the steps of applying a layer of a second material on the layer of first material. A sufficient quantity of the second material is removed to expose the protrusion. The first material exposed through the protrusion is then removed. 
     The step of removing a sufficient quantity of the second material to expose the protrusion can comprise mechanical planarization of the second material, chemical mechanical planarization of the second material, or can comprise the steps of removing the second material above a predetermined distance from the surface of the substrate so that the thickness of the second material above the first material is greater adjacent the protrusion than above the protrusion and isotropically removing the second material to expose the protrusion. 
     In accordance with another embodiment of the present invention, a field emission display (FED) is constructed from a process comprising the following steps. First, a thin film layer is deposited on a substrate. The thin film layer may have one or more surface protrusions. The thin film layer is covered with a sacrificial layer having a top surface. Through a leveling material removal process, such as chemical-mechanical planarization, a portion of the top surface of the sacrificial layer is removed until the sacrificial layer has a predetermined thickness D to thereby expose on the top surface all surface protrusions having a height of at least D. The exposed surface protrusions are then etched to remove the surface protrusions from the thin film layer. The sacrificial layer is etched to remove the sacrificial layer from the thin film layer. Emitters are then constructed on the thin film layer, and an extraction grid is formed above the emitters. Finally, a screen is constructed above the extraction grid, the screen having a phosphor-coated surface facing the extraction grid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a substrate with a thin film layer having surface protrusions deposited on the substrate; 
     FIG. 2 is a cross-sectional view of the substrate and thin film layer of FIG. 1 showing a sacrificial layer deposited on the thin film layer; 
     FIG. 3 is a cross-sectional view showing the sacrificial layer after it had been planarized to remove the tips of the surface protrusions; 
     FIG. 4 is a cross-sectional view of the substrate of FIGS. 1-3 showing the removal of the surface protrusions from the thin film layer; and 
     FIG. 5 is a cross-sectional view of the substrate and thin film layer of FIGS. 1-4 depicting the thin film layer after its surface protrusions have been removed in accordance with the preferred embodiment of the present invention. 
     FIG. 6 illustrates a FED formed according to the methods of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a substrate  10  having a top surface  12 . The substrate  10  may be glass or other materials known in the art which are suitable for use as a substrate. A thin film layer  14  is deposited on the top surface  12  of the substrate  10 . The thin film layer  14  may be a conductive layer on which are formed elements of a device, such as a field emission display baseplate. In one embodiment, the thin film layer  14  is chromium. Other suitably conductive materials can also be used. The depositing of the thin film layer  14  may be done using any of a number of processes known in the art, such as physical vapor deposition (PVD). Inherent in many of these processes is the unwanted formation of surface protrusions  18  on the top surface  16  of the thin film layer  14 . Only one surface protrusion  18  is shown in FIG. 1, but in reality such protrusions may be scattered randomly over the entire surface  16  of the thin film layer  14 . 
     FIG. 2 shows the first step of the process of the preferred embodiment of the present invention in which a sacrificial layer  20  is deposited on the top surface  16  of the thin film layer  14 . The sacrificial layer  20  typically consists of a material which is selectively removable from the thin film layer  14 . In one embodiment of the present invention, the sacrificial layer  20  consists of silicon dioxide. The sacrificial layer  20  shown in FIG. 2 is deposited as a conformal layer on the thin film layer  14 . The sacrificial layer  20  need not, however, be a conformal layer. If the sacrificial layer  20  is not a conformal layer, the surface protrusion  18  may extend above the top surface of the sacrificial layer (see FIG.  3 ). The sacrificial layer  20  may be deposited so that it has a predetermined thickness d 1  above the top surface  16  of the thin film layer  14 . 
     A bump  22  is formed on the top surface  24  of the sacrificial layer  20  wherever there is a surface protrusion  18  on the thin film layer  14 . These bumps  22  are removed from the surface  24  by mechanical planarization or chemical-mechanical planarization (CMP). Although CMP is discussed as the preferred method used to remove the bump  22 , one skilled in the art will realize that other known processes can also be utilized. The CMP process is performed until a sufficient amount of the sacrificial layer  20  has been removed to expose the tip of the surface protrusion  18 . FIG. 3 shows the sacrificial layer  20  having a planarized surface  26  after the CMP process is complete. The tip of the surface protrusion  18 —illustrated by the dotted line—is removed by the CMP process. The removal of the tip of the surface protrusion  18  results in an island  28  of the material comprising the thin film layer  14  being exposed on the planarized surface  26 . Such islands  28  occur wherever there was a surface protrusion  18  on the thin film layer  14  having a height greater than d 2 . 
     The next step of the present process is the removal of the surface protrusions  18  as shown in FIG.  4 . The surface protrusions  18  are removed from the thin film layer  14  through a process, such as etching, which is known in the art. An etchant is disposed on the planarized surface  26 . The etchant is preferably chosen so that it will selectively remove the material of the protrusions  18  exposed through the island  28  to a greater degree than the material of the sacrificial layer  20 . In this way, the sacrificial layer  20  protects the portions of the thin film layer  14  not having surface protrusions  18  while allowing removal of the surface protrusions. The etching process results in a void  30 , which is the space formerly occupied by the surface protrusion  18 . 
     In FIG. 5, the sacrificial layer  20  is removed from the top surface  16  of the thin film  14 . This removal of the sacrificial layer  20  may be accomplished using known etching processes. In the etching process, an etchant is preferably used which selectively removes the material of the sacrificial layer  20  to a greater degree than the material of the thin film layer  14 . A CMP process could alternatively be used to remove the sacrificial layer  20 . The CMP process will not harm the top surface  16  of the thin film layer  14  since the large surface protrusions  18  (i.e., having a height of at least d 2 ) have been removed from the thin film layer. Although the sacrificial layer  20  is depicted and described as being removed from the thin film layer  14 , the sacrificial layer need not always be removed. For example, if the sacrificial layer  20  is an insulating layer which is part of the device being fabricated, the sacrificial layer could remain and further layers deposited on the top surface  26  of the sacrificial layer. 
     It should be noted that after the process of the present invention has been performed on the thin film layer  14 , the thin film layer has a void  30  in the same location where a surface protrusion  18  was previously located. Such voids  30 , however, generally do not adversely affect the utility of the thin film layer  14 . These voids  30  occupy a small percentage of the total surface area of the thin film layer  14 , which means the remaining area can be utilized for the formation of the desired elements on the thin film layer. Furthermore, the voids  30  do not pose the threat of short circuiting, as did the surface protrusions  18 , to layers disposed above the surface  16  of the thin film layer  14 . 
     An FED device formed by the methods provided herein is illustrated in FIG.  6 . After removal of the protrusion  18  and formation of the void  30  in the thin film layer  14 , an emitter  32  is constructed on the upper surface  16  of the thin film layer  14 . An insulating material  34  is formed atop the film layer  14  and fills the voids  30  therein. AD extraction grid  36  is formed on the insulating material  34  above the emitter  32 . Finally, a transparent screen  38  is constructed above the extraction grid  36 , the transparent screen  38  having an anode  40  and a cathodoluminescent coating  42  facing the extraction grid  36 . 
     The present invention has particular utility in the area of processing field emission displays and flat panel displays. In addition, the process is well suited for application to large area substrates in the range of twelve inches. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.