Patent Publication Number: US-2005118932-A1

Title: Adjustable gap chemical mechanical polishing method and apparatus

Description:
RELATED APPLICATIONS  
      This application claims priority from Provisional Application Ser. No. 60/484,909 filed Jul. 3, 2003 (NT-303-P), which is incorporated herein by reference. 
    
    
     FIELD  
      The present invention relates to manufacture of semiconductor integrated circuits and, more particularly, to a method and apparatus for polishing substrates.  
     BACKGROUND  
      Chemical mechanical polishing (CMP) of materials for VLSI and ULSI applications has important and broad application in the semiconductor industry. Chemical mechanical polishing is a widely used technique for planarizing metals and dielectrics as well as other types of layers on semiconductor wafers. CMP is generally used to flatten and remove material from surfaces during the wafer fabrication process, for example, during the wafer fabrication process, CMP is often used to flatten/polish the profiles that build up in multilevel metal interconnection.  
      In a typical CMP process, a substrate such as a semiconductor wafer is mounted on a substrate carrier, often called a head. The wafer surface is pressed against a polishing pad and moved with respect to the polishing pad. This is typically performed by rotating the wafer, moving the pad or both. The polishing pad may be a conventional polishing pad or a fixed abrasive polishing pad. Conventional or polymeric polishing pads are usually used with polishing slurries including abrasive particles and chemically reactive agents. During the CMP process, the polishing slurry is supplied onto the polishing pad as the wafer surface is pressed on the pad. The surface of a fixed abrasive polishing pad typically includes abrasive particles that are embedded in a matrix or binder material.  
       FIG. 1  illustrates an exemplary conventional CMP system  10  that includes a polishing pad  12  to polish a front side of the wafer  14 . A wafer carrier  16  holds the wafer  14  and the polishing pad can be moved with respect to the wafer. The wafer carrier  16  may include an array of built in pressure zones  18  that are located behind the wafer. The pressure zones  18  are often formed concentrically to apply localized pressure to the backside of the wafer. By applying pressure to the selected locations of the backside, polishing rate on the corresponding locations of the front side of the wafer can be changed. During the polishing process, the wafer carrier  16  is rotated (clockwise or counter-clockwise) while the pad  12  is moved. A platen  20  with a flat surface supports the polishing pad  12 . Depending on the pressure distribution profile created on the backside of the wafer, polishing rate of the corresponding regions on the wafer can be varied to achieve desired polishing on the wafer. For example, by increasing the pressure around the center of the backside, higher polishing rates are obtained at the center of the front side. However, in such systems, pressures applied by selected pressure zones onto corresponding selected locations on the wafer are not entirely independent from one another. Pressure from neighboring pressure zones may interfere with each other, which situation affects the local material removal rate and cause undesired poor or excessive material removal from the front side.  
      Therefore, a need exists for a chemical mechanical polishing (CMP) system that can provide accurate, stable and controllable polishing rates on a wafer.  
     SUMMARY  
      The present invention provides a polishing system using fluid from a fluid source to push a polishing pad to a workpiece surface during the polishing process. A constant gap is kept between the fluid source and the workpiece surface as the workpiece surface is polished by the polishing pad.  
      In one aspect of the present invention, a polishing apparatus for polishing a surface of a workpiece is provided. The apparatus includes a carrier surface configured to hold the workpiece, a plurality of fluid nozzles placed across from the surface to provide a gap between the nozzles and the surface of the workpiece, and a polishing pad positioned within the gap. The polishing pad is configured to polish the surface of the workpiece when a fluid is applied from the plurality of nozzles to push the polishing pad to the surface.  
      In another aspect of the present invention, a method of polishing a surface of a workpiece surface using a polishing pad is provided. The method includes the steps of placing the polishing pad within a gap defined between the surface of the workpiece and an array of nozzles, emitting a fluid from the array of nozzles to push the polishing pad onto the surface of the workpiece; and polishing the surface with the polishing pad while keeping the gap constant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic illustration of a conventional chemical mechanical polishing apparatus; and  
       FIG. 2  is a schematic illustration of an embodiment of a chemical mechanical polishing system of the present invention. 
    
    
     DETAILED DESCRIPTION  
      The present invention provides a CMP system applying fluid flow from a fluid source to the backside of a polishing pad to cause the polishing surface of the polishing pad to be forced against a workpiece surface. During the polishing the workpiece surface with the polishing pad, the workpiece surface is kept at a predetermined distance from the fluid source, to achieve chemical mechanical polishing of the workpiece surface.  
       FIG. 2  depicts an exemplary CMP system  100  according to an embodiment of the invention. The system  100  comprises a fluid source assembly  102  and a carrier surface  104  to hold a workpiece  106 . The workpiece  106  may be a semiconductor wafer. Carrier surface  104  may be a front surface of a wafer carrier or it may be any surface against which the backside of a wafer rests. A polishing pad  108  is positioned between a surface  110  of the wafer  106  and the fluid source assembly  102 . The surface  110  of the wafer may include a conductive layer such as copper or a dielectric layer such as silicon dioxide layer to be planarized using CMP. The polishing pad  108  includes a first surface or a process surface  112  and a second surface or a back surface  114 . The polishing pad  108  may preferably be tensioned by a tensioning mechanism (not shown). Process surface  112  of the polishing pad  108  polishes the surface  110  of the wafer  106  during the CMP process, typically with the help of a process solution or a polishing slurry. A variety of different polishing pads can be used with the present invention. For example, the polishing pad can be a fixed abrasive pad or a more commonly used polymeric pad. The polishing pad  108  may be used with or without a slurry. The carrier surface  104  of the system  100  may rotate or move the wafer laterally or vertically. In this embodiment, fluid source assembly  102  is placed above the wafer  106 . However, other configurations which place the fluid source assembly  102  under the wafer is also possible and within the scope of this invention.  
      In this embodiment, the fluid may be gas such as air, or liquid such as water. During the process, a fluid flow  115  is applied to the back surface  1   14  of the polishing pad  108 . The application of the fluid flow  115  to the back surface  114  of the polishing pad  108  is carried out using the fluid source assembly  102 . The fluid source assembly may include a plurality of fluid nozzles  116 . The fluid nozzles  116  may be arranged into any configuration or array with space  118  among them. For example, the nozzles  116  may form a nozzle array that positions nozzles a predetermined distance from one another thereby creating the space  118  among them. Alternatively, instead of leaving a space among the nozzles, holes or openings may be placed among the nozzles to remove the used fluid from the system. In this approach, the fluid flow assembly may have surface including nozzles and the openings. The nozzles  116  may form discrete zones to create a fluid flow rate distribution profile of the fluid source assembly  102 . The zones may be formed concentrically and each zone may be connected to a fluid flow controller (not shown) to regulate fluid flow for each zone. In an alternative embodiment, a space or holes may exist between the zones. By varying amount of fluid flow rate from the selected fluid flow zones, a fluid flow rate distribution profile including different flow rates from different zones may be generated on the back surface  114  of the polishing pad  108 . Fluid flow rate distribution profile may have high flow rate zones or low flow rate zones. Depending on the fluid flow rate distribution profile, polishing rate of the corresponding regions on the wafer  106  may be varied to achieve desired polishing on the surface  110  since more fluid flow from a given zone pushes the process surface  112  to the surface  110  with a higher force at that zone. For example, by increasing the fluid flow rate from the nozzles around the center of the fluid source assembly  102 , higher polishing rates is obtained at the center of the surface  110  of the wafer.  
      Referring back to  FIG. 2 , in this embodiment, a first end  117  of the nozzles  116  are aligned with an imaginary plane P which is nearly parallel to the surface  110  of the wafer. A gap “t” is left between the first end  117  of the nozzles  116  and the surface  110  of the wafer  106 . During the polishing process, the gap “t” is kept constant. The predetermined height of the gap “t” is important for the polishing process of the present invention and this height is adjustable. The gap “t” may be less than 6 millimeters. If the gap “t” is configured to be large, fluid flow rate must be high to accomplish the desired polishing rate on the surface  110  of the wafer  106 . However, if the gap “t” is configured to be small, reduced flow rates can be used to accomplish desired polishing rates. The gap “t”, however, cannot be too small to allow the back surface  114  of the polishing pad  108  to touch the nozzles  116 .  
      As shown in  FIG. 2 , when fluid flow  115  is applied during the process, the polishing pad  108  moves into a process position  120  within the gap “t” and is forced onto the surface of the wafer with the applied fluid flow. While the gap is kept constant or unchanged, any desired fluid flow rate distribution profile can be applied to the polishing pad  108  to obtain corresponding desired polishing rates on the surface  110 . The space  118  among the nozzles  116  may be used for isolating the nozzles from the neighboring nozzles and may advantageously provide a passage or a drain for the exhausted, or used, fluid. Alternately, the used fluid may leave the system from the edges of the fluid source assembly  102 . After forcing the polishing pad toward the surface of the wafer for the CMP process, the fluid flow from the nozzles  116  exits the fluid source assembly  102  through the space  118  among the nozzles  116  without interfering with the fluid flow from the neighboring nozzles. When the process is over, for example by reaching a predetermined endpoint, the fluid flow is stopped, which causes the preferably tensioned polishing pad to return to its original position within the gap “t”.  
      Accordingly, the present invention provides a substantially independent fluid flow for each nozzle, and if the nozzles are arranged into zones, the present invention provides distinct fluid flow rate distribution profiles. Such well-defined and independent fluid flow rate distribution profiles, in turn, establish well-defined polishing rates on the substrate as the polishing pad polishes the workpiece surface. In one embodiment, the polishing pad  108  is statically held in position with respect to the nozzles  116  and the wafer  106  is moved. In another embodiment, the polishing pad  108  is moved in an orbital direction or a linear direction with respect to the nozzles  116 . In yet another embodiment, the polishing pad  108  is moved in a bi-linear direction with respect to the nozzles  116 . In all cases the wafer  106  may also be moved during the polishing process.  
      Although various preferred embodiments and the best mode have been described in detail above, those skilled in the art will readily appreciate that many modifications of the exemplary embodiment are possible without materially departing from the novel teachings and advantages of this invention.