Patent Publication Number: US-6659846-B2

Title: Pad for chemical mechanical polishing

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of semiconductor device fabrication, and more particularly to the field of chemical mechanical polishing of semiconductor wafers, and specifically to an improved polishing pad for chemical mechanical polishing of a semiconductor wafer. 
     BACKGROUND OF THE INVENTION 
     The fabrication of microelectronics devices involves the deposition and removal of multiple layers of material on a semiconductor substrate to form active semiconductor devices and circuits. Device densities currently exceed 8 million transistors per square centimeter, and they are expected to increase by an order of magnitude within the next decade. Such devices utilize multiple layers of metal and dielectric materials which can selectively connect or isolate device elements within a layer and between layers. Integrated circuits using up to six levels of interconnects have been reported and even more complex circuits are expected in the future. Device geometries have gone from 0.5 micron to 0.12 micron and will soon be 0.08 micron. Multi-levels of metallization are required in such devices to achieve the desired speeds, and each inter-metal level must be planarized during the manufacturing process. The only known process with the ability to create a sufficiently planar surface is chemical mechanical polishing (CMP). CMP may be used to remove high topography and/or to remove defects, scratches or embedded particles from the surface of a semiconductor wafer as part of the manufacturing process. 
     The CMP process generally involves rubbing a surface of a semiconductor wafer against a polishing pad under controlled pressure, temperature and rotational speed in the presence of a chemical slurry. An abrasive material is introduced between the wafer and the polishing pad, either as particles affixed to the polishing pad itself or in fluid suspension in the chemical slurry. The abrasive particles may be, for example, alumina or silica. The chemical slurry may contain selected chemicals which function together with the abrasive to remove a portion of the surface of the wafer in a polishing action. The slurry also provides a temperature control function and serves to flush the polishing debris away from the wafer. 
     As may be seen in FIG. 1, a chemical mechanical polishing system  10  may include a carrier  12  for holding and moving a semiconductor wafer  14  against a polishing pad  16  supported on a rotatable platen  18 . A slurry  20  is used to provide the desired chemical interaction and abrasion when the wafer  14  is pressed and rotated against the polishing pad. As is known in the art, the rate of material removal from the wafer  14  will depend upon many variables, including the amount of force F exerted between the wafer  14  and the polishing pad  16 , the speeds of rotation R 1  of the carrier and R 2  of the platen, the transverse location of the carrier  12  relative to the axis of rotation of the platen  18 , the chemical composition of the slurry  20 , the temperature, and the composition and history of use of the polishing pad  16 . Numerous configurations of CMP machines are known and are available in the industry. One manufacturer of such CMP machines is Applied Materials, Inc. of Santa Clara, Calif. (www.appliedmaterials.com) 
     It is known in the art that polishing pads  16  may be made of various materials and compositions. One or more layers of material may be used to form a polishing pad. For example, one style of polishing pad includes both a rigid pad layer in contact with the wafer and a compliant pad layer underlying the rigid pad layer. In one example, a cast polyurethane pad is backed by a polyester felt pad stiffened with polyurethane resin. Other pads having various material compositions are known and are available in the industry. One manufacturer of prior art polishing pads is Rodel, Inc. of Phoenix, Ariz. (www.rodel.com) Polishing pads are known to have a porous surface that interacts with the wafer surface in the presence of the slurry to provide the necessary material removal for the polishing process. The porous surface will capture the micro particles of wafer materials that are removed during the CMP process. It is well known that as a polishing pad is used, the porous surface of the pad will gradually become clogged with particles and the rate of removal of wafer material will decrease with use. Yet another style of polishing utilizes a fixed abrasive pad wherein, as the name suggests, abrasive material is fixed on the surface of a polishing pad. A fixed abrasive pad will accumulate debris between the abrasive particles as it is used, and the hard mineral particles used as the abrasive will wear and may become dislodged from the pad surface. Such changes reduce the rate of material removal and cause the polishing performance to be non-reproducible from wafer to wafer. Once the material removal rate has dropped to a predetermined value, a fixed abrasive pad must be replaced and a porous surface pad must be conditioned to restore its full functionality. Pad conditioning is a integral part of prior art CMP processes. Pad conditioning may be performed by exposing the polishing pad to a sonically agitated stream of fluid with or without chemical additive, or it may be performed by rubbing a hard abrasive surface against the polishing pad to remove embedded debris and to restore a desired degree of roughness and porosity to the polishing pad surface. Pad conditioners may be metal plates having industrial diamonds affixed to their surface. Rodel, Inc. is one supplier of pad conditioners to the semiconductor manufacturing industry. In a typical CMP operation, a polishing pad may have to be conditioned after polishing only one or a few wafers. Conditioning requires that the carrier  12  be moved to a conditioning position or station, and it may consume from 5-60 seconds of critical path time during the fabrication process. During the conditioning operation, the polishing pad and its associated carrier are not available for CMP operations, thus impacting the overall productivity of a semiconductor manufacturing line. Under even the best circumstances, it is unusual to be able to perform more than ten polishing operations between conditioning operations. Pads must be replaced after polishing from 350-1,000 wafers, depending upon the polishing parameters. Accordingly, a more efficient CMP process is needed wherein the critical path time spent conditioning a polishing pad is reduced. 
     SUMMARY OF THE INVENTION 
     An improved polishing pad for a chemical mechanical polishing process is described herein as including a plurality of particles of abrasive material disposed in a matrix material. This is referred to as an embedded abrasive pad, wherein the matrix material may be a polymeric material such as polyurethane and the abrasive material may be an inorganic material such as silica, calcium carbonate, alumina silicate, feldspar, calcium sulfate, glass or sintered carbon. The matrix can be visualized as a three-dimensional grid in which the distribution of particles of abrasive material per unit volume of matrix material may be constant throughout the pad, or it may vary from a first portion of the pad to a second portion of the pad. In one embodiment, an edge portion of a polishing pad may contain fewer or more abrasive particles, thereby serving to better control the polishing performance across the pad diameter. As the polishing surface of this improved pad wears during wafer polishing operations, a new surface containing a fresh population of abrasive particles will be exposed, thereby maintaining polishing performance consistent from wafer to wafer. In this manner, as many as 100-500 polishing operations may be accomplished without the need for conditioning of the pad. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which: 
     FIG. 1 is a schematic illustration of a prior art chemical mechanical polishing system. 
     FIG. 2 is a partial cross-sectional view of a polishing pad having abrasive particles embedded in a matrix material. 
     FIG. 3 is a partial top view of the polishing pad of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 is a partial cross-sectional view of a polishing pad  22  having a plurality of abrasive particles  24  embedded in a matrix material  26 . Polishing pad  22  provides a desired degree of roughness and hardness for accomplishing a wafer polishing operation regardless of the state of wear of the polishing pad  22 . As can be seen from FIG. 2, abrasive particles  24  are distributed throughout a thickness T of the polishing pad  22  within a matrix material  26 . Although viewed in two dimensions in FIG. 2, one may appreciate that the matrix material  26  defines a three-dimensional micro-grid or mesh for supporting a three-dimensional array of abrasive particles  24 . As polishing surface  28  is used to polish one or more semiconductor wafers, a top portion of the matrix material  26  and some of the uppermost abrasive particles  24  will be worn away, thereby reducing the thickness T of the pad  22 . As T is reduced, a different population of abrasive particles  24  will become exposed at the newly exposed polishing surface  28 ′. 
     The abrasive particles  24  are selected to provide a desired degree of polishing action considering the materials to be removed and the desired surface finish. Stiff inorganic particles may be selected, for example, silica, calcium carbonate, alumina silicate, feldspar, calcium sulfate, glass or sintered carbon. For a typical semiconductor polishing operation, the particle size must be very small to achieve the desired degree of smoothness, for example on the order of 10 −9  meters, such as a range of 50-200 microns. Particles  24  may be distributed evenly or randomly throughout the matrix material  26  in order to provide consistent polishing properties across the thickness T of the pad  22 . Alternatively, a systematic array of abrasive particles  24  may be may be desired, with variations in the distribution of the particles  24  possible through the thickness T or across a diameter of the polishing surface  28 . FIG. 3 illustrates a partial top view of such an uneven distribution wherein pad  22  has more particles per unit volume toward a center area  23  of the polishing pad  22  and less particles per unit volume toward an edge area  25  in order to counteract an edge effect. In another embodiment, there may be more abrasive particles per unit volume of matrix material as a function of the pad depth T. The number of particles per unit volume may be selected in conjunction with the specification of the other pad properties in order to achieve a desired material removal performance for a particular application. It would be expected that the weight percentage of abrasive particles in the pad may be of the same order of magnitude as the weight percentage of the abrasives in a prior art abrasive slurry, for example 5-40% and preferably 10-25%. The abrasive particles  24  may be treated with a surface chemical coupling agent, such as organo-silicates, organo-titanates, organo-zirconates, etc. to enhance adhesion to the matrix material  26 . 
     The matrix material  26  may be a bulk polymer, for example, polyurethane, poly alkyd (alcohol plus acid), poly vinylester, epoxy, or polyester. The matrix material  26  may be selected to have a desired degree of elasticity, porosity, density, hardness, etc. in order to provide predetermined polishing and wear performance in conjunction with the selected abrasive particles  24 . 
     Polishing pad  22  may be used to replace the prior art polishing pad  14  in the prior art CMP system illustrated in FIG.  1 . Polishing pad  22  may be used with a fluid slurry  20  for temperature and chemistry control and debris removal but without abrasives suspended in the slurry  20 . Alternatively, a polishing process utilizing polishing pad  22  may include one step wherein an abrasive is introduced with slurry  20  and a second step wherein no abrasive is included in the slurry  20 . Any other element of the composition of the slurry  20  may be changed from a first period of polishing to a second period of polishing, such as a chemical additive or the temperature of the slurry. Such a multi-step process may be used to provide distinct material removal rates during different portions of a polishing process, such as when a first, faster rate of material removal is used to achieve a desired level of planarity, then a second, slower rate of material removal is used to achieve a desired surface finish. 
     The CMP system  10  of FIG. 1 may be operated without a conditioning step when the prior art polishing pad  14  is replaced by the embedded particle polishing pad  22 . As the polishing pad  22  is used, the wear surface  28  will recede into the thickness of the pad  22 , removing some of the abrasive particles  24  and matrix material  26 . However, the newly exposed surface  28 ′, indicated by the dashed line in FIG. 2, will contain a fresh population of abrasive particles and exhibit the same polishing properties as the original surface  28 . The polishing performance properties are thus uniform throughout the life of the pad  22  without the need for conditioning operations. In one embodiment, the original thickness of the pad  22  may be 0.050-0.150 inches and the pad may be used until its thickness is reduced to about 0.015-0.025 inches. During the useful life of such a pad, it would be expected that approximately 100-250 conditioning operations would be eliminated when compared to prior art polishing pads, thereby saving approximately 60-90 minutes of critical path processing time per pad. Such performance would require pad changes no more often than for prior art porous surface pads. 
     Polishing pad  22  may be manufactured by methods well known in the art, such as with sintering/powder metallurgy, injection molding, or molding/baking/cutting. To achieve a pad having a variable density of abrasive particles per unit volume at different locations on the pad, it may be preferred to utilize a dry sintering/powder metallurgy process, as the distribution of abrasive particles could be controlled as the powders are mixed and applied. 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.