Abstract:
A fixed abrasive chemical polishing method uses an aqueous solution that has a variable pH. During polishing the pH of the aqueous solution is changed so that the polishing process can be more precisely controlled. The removal rate and removal selectivity between oxide and nitride can be controlled by varying the pH of the aqueous solution.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to chemical mechanical polishing (CMP), and more particularly, to pH control during a slurry free CMP process.  
         BACKGROUND OF THE INVENTION  
         [0002]    Chemical mechanical polishing (CMP) has emerged as a crucial semiconductor technology, particularly for devices with critical dimensions smaller than 0.3 microns. CMP machines typically include a means of holding a wafer or substrate to be polished (also referred to as a “wafer chuck”), a polishing pad, and a means to support the pad (also referred to as a “platen”). In many conventional CMP machines, an abrasive slurry is required for polishing and is delivered either directly to the surface of the pad or through holes and grooves in the pad directly to the surface of the wafer. The polishing pad does also is not typically abrasive. The control system on the CMP machine causes motors to press the surface of the wafer against the pad surface with a prescribed amount of force.  
           [0003]    One of the recent developments in CMP technology is slurry free fixed abrasive pad polishing. In this method, an abrasive slurry is not used. Instead, the abrasive material is fixed and integrated into the polishing pad. An example of such a system is described in U.S. Pat. No. 6,069,080 to James et al. The advantage of this type of system is that large amounts of abrasive slurry are not needed, thereby also eliminating the need for time consuming cleaning steps. Although an abrasive slurry is not used, an aqueous fluid is used to facilitate the polishing action. In the &#39;080 patent, the aqueous fluid is maintained at a constant pH.  
           [0004]    Another concern in the CMP process is the accurate detection of the endpoint of the CMP process. In many important applications, the CMP process is used to remove an oxide layer until an underlying stop layer is reached, such as a nitride. For example, the use of a nitride as a stop layer underneath an oxide layer is commonly used in the formation of shallow trench isolation structures. However, it is still relatively difficult to accurately detect when the nitride stop layer has been reached. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0006]    [0006]FIG. 1 is a schematic illustration of a CMP apparatus formed in accordance with the present invention.  
         [0007]    [0007]FIG. 2 is a schematic flow chart of the one embodiment of the present invention.  
         [0008]    [0008]FIG. 3 is a flow diagram illustrating the method of the present invention.  
         [0009]    [0009]FIGS. 4 and 5 are flow diagrams illustrating alternative embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    A schematic representation of the overall system of the present invention is shown in FIG. 1. As seen, a wafer chuck  101  holds a wafer  103  that is to be polished. The wafer chuck  101  preferably rotates about its vertical axis  105 . A pad assembly  107  includes a polishing pad  109  mounted onto a polishing table  111 . The polishing table is secured to a driver or motor means (not shown) that is operative to move the pad assembly  107  is the desired manner.  
         [0011]    In accordance with the present invention, it has been found that by varying the pH of the non-abrasive aqueous solution during polishing, the removal selectivity between oxide and nitride can be adjusted. Note that there are other prior art slurry using methods to increase the removal selectivity between oxide and nitride. However, in these prior art methods, once the removal selectivity has been determined, the removal selectivity cannot be changed during the CMP process. By simply choosing a high selectivity, this results in several detrimental effects, such as oxide residue if there are high topographical features. Also, increased oxide dishing may occur in certain applications, such as during the formation of shallow trench isolation. Thus, as will be detailed below, the present invention teaches that the pH of the aqueous solution is changed dynamically as the endpoint of the CMP process nears in a slurry free fixed abrasive CMP process.  
         [0012]    [0012]FIG. 2 shows a schematic flow diagram on the method of the present invention. First, at box  201 , the wafer that is to be polished is presented. The wafer may have an oxide layer that requires planarization or removal. Next, at box  203 , a film measurement process is carried out. This film measurement process measures the thickness of the oxide layer so that a rough guess of how much polishing is needed. Next, at step  205 , the wafer is loaded into the polishing machine for polishing. The wafer is held in place by the wafer chuck  101 . The polishing apparatus  109  includes a polishing pad  109 . Preferably, the polishing pad  109  is a fixed abrasive polishing pad, such as that described in U.S. Pat. No. 6,069,080, although other commercially available fixed abrasive pads may be used. The above steps are conventional in the art.  
         [0013]    After the wafer is loaded, the polishing process begins. The polishing machine includes a delivery system for delivering an aqueous solution to the polishing pad and wafer surface. The aqueous solution lubricates and otherwise facilitates the polishing process. In one embodiment, the aqueous solution is a solution of deionized water and various other chemicals that control the pH of the deionized water. Furthermore, other chemicals may be included in the aqueous solution that facilitates the polishing of the wafer. A pipe  209  delivers the aqueous solution to the wafer and polishing pad. Additionally, the polishing machine includes an endpoint detection (EDP) system  207  that can detect when the polishing process should terminate.  
         [0014]    Still referring to FIG. 2, a pH modification system  211  closely controls the pH of the aqueous solution. The pH modification system  211  includes a deionized water reservoir  213 , a pH controller  215 , an acid/base reservoir  217 , and a pH mixing chamber  223 . The acid/base reservoir  217  includes an acid source  219  and a base source  221 .  
         [0015]    In operation, the pH modification system  211  can change the pH of the aqueous solution by adding either an acid from the acid source  219  or a base from the base source  221 . The pH controller  215  monitors the pH of the aqueous solution from the pH mixing chamber  223 . Depending upon the polishing process required, the pH controller  215  selectively modifies the pH level of the aqueous solution. Various acids or bases (stored in the acid/base reservoir  217 ) can be added to modify the pH of the aqueous solution. KOH, NH 4 OH, CsOH, TMAH, or amines may be added singularly or in combination to change the pH of the aqueous solution. To lower the pH of the aqueous solution, deionized water may be used to dilute the solution or an acid added to the solution.  
         [0016]    Turning to FIG. 3, in one embodiment where oxide is to be removed over a nitride stop layer, during the initial polishing at box  301 , the pH of the aqueous solution is maintained to be relatively high, e.g., greater than 11. In one embodiment, the pH is on the order of 11.5 or higher. It has been found that a relatively high pH provides an excellent rate of removal of the oxide, increasing throughput. It has also been found that there is a linear relationship between pH and removal rate. However, the high pH also results in a relatively low removal selectivity between oxide and nitride, on the order of 1:1.  
         [0017]    Therefore, in accordance with the present invention, at box  303 , after the bulk of the oxide layer has been removed, the pH of the aqueous solution is changed to a lower level. This lowers the removal rate of the oxide and also increases the removal selectivity between oxide and nitride. This in turn results in greater process control.  
         [0018]    In one embodiment, the transition from a high pH to a low pH is governed by a signal from the EPD system  207 . The EPD system  207  is conventional to most polishing machines and is operative to determine when an endpoint is reached during the polishing process. For example, the EPD system  207  may operate in a time mode. In the above example, when the EPD system  207  first detects that the underlying nitride layer may be reached, the EPD system  207  provides a signal to the pH controller  215  to lower the pH of the aqueous solution. At that point, further polishing for a limited time (e.g. 40 seconds) is performed using the lower pH aqueous solution.  
         [0019]    Additionally, in one embodiment, during the bulk polishing at box  301 , the rotational speed of the polishing pad relative to the wafer may be increased. During the fine polish at box  303 , the relative rotational speed may be decreased to lower the number of polishing defects. Finally, after the fine polish is complete, at box  225 , the wafer is removed from the polishing machine.  
         [0020]    In an alternative embodiment shown in FIG. 4, where oxide is to be removed over a nitride stop layer, during the initial polishing at box  401 , the pH of the aqueous solution is maintained to be relatively low, e.g., lower than  10 . The bulk polishing is continued until the EPD system  207  outputs a signal indicating that the nitride layer has been reached. Next, at box  403 , after the bulk of the oxide layer has been removed, the pH of the aqueous solution is changed to a higher level, e.g. above 11.5, and a fine polish is performed for a limited time, such as 10 seconds. In this embodiment, the fine polish quickly removes any oxide residue and a portion of the nitride layer.  
         [0021]    In another alternative embodiment shown in FIG. 5, where oxide is to be removed over a nitride stop layer, during the initial polishing at box  501 , the pH of the aqueous solution is maintained to be relatively low, e.g., lower than 10. The bulk polishing for a predetermined amount of time (e.g. 100 seconds), without regard to the EPD system  207 . Next, at box  503 , after the bulk of the oxide layer has been removed, the pH of the aqueous solution is changed to a higher level, e.g. above 11.5, and a fine polish is performed for a limited time, such as 10 seconds. In this embodiment, the fine polish quickly removes any oxide residue and a portion of the nitride layer.  
         [0022]    While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.