Abstract:
A method for enhancing uniformity in the polishing profile of a substrate during chemical mechanical polishing. According to a first embodiment, the method is adapted for a rotary-type chemical mechanical polisher and includes dispensing the polishing slurry onto the rotating polishing pad of the CMP apparatus in a polishing area on the polishing pad that contacts the entire surface area of the substrate. This facilitates substantially equal polishing rates and a substantially uniform polishing profile from the center to the edge regions on the surface of the substrate. According to a second embodiment, the method of the present invention is adapted for a linear-type chemical mechanical polisher and includes increasing the number of nozzles that dispense the slurry onto the polishing pad across the diameter or width of the substrate.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to chemical mechanical polishing apparatus used in the polishing of semiconductor wafers. More particularly, the present invention relates to methods for enhancing uniformity in the polishing profile of substrates during chemical mechanical polishing (CMP).  
       BACKGROUND OF THE INVENTION  
       [0002]     In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in memory devices. The planarization process is important since it enables the subsequent use of a high-resolution lithographic process to fabricate the next-level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.  
         [0003]     A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically-actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.  
         [0004]     A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen, with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing of a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing of a tapered profile onto the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not collinear; however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.  
         [0005]     A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 A and about 10,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.  
         [0006]     Important components used in CMP processes include an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionixed water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.  
         [0007]     As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.  
         [0008]     While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.  
         [0009]     Referring to  FIG. 1A , a conventional rotary-type CMP apparatus  50  includes a wafer carrier  52 , a polishing pad  56 , and a slurry delivery arm  54  positioned over the polishing pad  56 . The wafer carrier  52  is mounted on the bottom end of a vertical shaft  53  which rotates and presses a semiconductor wafer  66 , mounted on the bottom surface of the wafer carrier  52 , against the upper surface  60  of the polishing pad  56  as the polishing pad  56  is rotated. The slurry delivery arm  54  is equipped with slurry dispensing nozzles  62  which are used for dispensing a slurry solution  64  onto the upper surface  60  of the rotating polishing pad  56 . As the wafer carrier  52  rotates the wafer  66  against the upper surface  60  of the polishing pad  56 , the polishing slurry  64  dispensed thereon by the slurry delivery arm  54  travels with the rotating polishing pad  56  until the polishing slurry  64  moves between the wafer  66  and the polishing pad  56 . Accordingly, the polishing slurry  64  substantially polishes or planarizes the surface of the wafer  66 .  
         [0010]     Recently, a chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefore named as a linear chemical mechanical polishing process, in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a more uniform polishing rate across a wafer surface throughout a planarization process for the removal of a film layer from the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus, and this not only reduces the cost of the apparatus but also reduces the floor space required in a clean room environment.  
         [0011]     A typical linear CMP apparatus  10  is shown in  FIG. 1B . The linear CMP apparatus  10  is utilized for polishing a semiconductor wafer  24 , i.e., a silicon wafer in removing a film layer of either an insulating material or a conductive material from the wafer surface. For instance, the film layer to be removed may include insulating materials such as silicon oxide, silicon nitrite or spin-on-glass material or a metal layer such as aluminum, copper or tungsten. Various other materials such as metal alloys or semi-conducting materials such as polysilicon may also be removed.  
         [0012]     As shown in  FIG. 1B , the wafer  24  is mounted on a rotating wafer holder  18 , which rotates at a predetermined speed. The major difference between the conventional linear CMP apparatus  10  and the predecessor rotary CMP apparatus  50  ( FIG. 1A ) is that a continuous, or endless, polishing belt  12  is utilized instead of a rotating polishing pad. The polishing belt  12  moves in a linear, rather than rotational, manner in respect to the rotational surface of the wafer  24 . The linear polishing belt  12  is mounted in a continuous manner over rollers  14  driven by a motor (not shown) at a predetermined rotational speed. The rotational motion of the rollers  14  is transformed into a linear motion  26  in respect to the surface of the wafer  24 . In the conventional linear CMP apparatus  10 , one or more polishing pads  30  are adhesively joined to the continuous polishing belt  12  on its outer surface that faces the wafer  24 . A polishing assembly  38  is thus formed by the continuous polishing belt  12  and the polishing pad or pads  30  glued or otherwise attached thereto.  
         [0013]     During the CMP process, the wafer holder  18  is normally operated in a rotational mode such that a uniform polishing on the wafer  24  can be achieved. To further improve the uniformity of linear polishing, a support housing  32  is further utilized to provide support to a support platen (not shown) during a polishing process. The support platen provides a supporting platform for the underside of the continuous polishing belt  12  to ensure that the polishing pad  30  makes sufficient contact with the surface of the wafer  24  in order to achieve more uniform material removal from the surface layer of the wafer  24 . Typically, the wafer holder  18  is pressed downwardly against the continuous polishing belt  12  and the polishing pad  30  at a predetermined force such that a suitable polishing rate on the surface of the wafer  24  can be obtained. Air pressure is typically further used to push the support platen upwardly against the polishing belt  12  which, in turn, pushes the polishing pad or pads  30  against the wafer  24 . A desirable polishing rate on the wafer surface can therefore by obtained by suitably adjusting the downward force on the wafer carrier  28 , the upward air pressure against the support platen, and the linear speed  26  of the polishing pad  30 . A slurry dispenser  20 , having multiple, typically eleven, slurry dispensing nozzles  34 , as shown in  FIG. 1C , is further utilized to dispense a slurry solution  36  through the respective slurry dispensing nozzles  34  onto the polishing pad or pads  30 . As further shown in  FIG. 1C , the slurry dispensing nozzles  34  are typically disposed at a distance “D” of 30 mm.  
         [0014]     For Cu CMP applications involving low-K IMD (intermetal dielectric) for planarization, interconnect and gap-fill at 0.13 μm and smaller device generations, both the rotary CMP apparatus and the linear CMP apparatus typically utilize a polishing slurry that contains little or no abrasive in order to prevent or minimize damage to the low-k IMDs. For that type of slurry, the within-wafer slurry distribution is of utmost importance in achieving optimal polishing uniformity among all regions on the wafer surface, particularly with regard to 300 mm-diameter wafers.  
         [0015]     Referring again to  FIG. 1A , the dispensing nozzles  62  of the slurry dispensing arm  54  of the conventional rotary-type CMP apparatus  50  typically dispense the polishing slurry  64 , having little or no abrasive, onto the rotating polishing pad  56  in such a location that the polishing slurry  64  initially contacts the center region of the wafer  66  as the slurry  64  moves beneath the rotating wafer  66 . This causes higher polishing rates at the center relative to the edge regions of the wafer  66 , resulting in an uneven polishing profile across the surface of the wafer  66 .  
         [0016]     Referring again to  FIG. 1B , the slurry dispenser  20  of the conventional linear CMP apparatus  10  typically includes about eleven of the slurry dispensing nozzles  34  that are spaced along the length of the slurry dispenser  20 . Accordingly, higher polishing rates are achieved on those regions of the wafer  24  that initially contact the polishing slurry  36  on the polishing pads  30 , relative to the other regions on the surface of the wafer  24 . This results in an uneven polishing profile across the surface of the wafer  24 . Accordingly, a new and improved method is needed for dispensing a polishing slurry on a polishing pad in such a position or positions on the polishing pad that polishing rates, and thus, polishing profiles, on the wafer surface are more uniform.  
         [0017]     An object of the present invention is to provide a new and improved method for dispensing a polishing slurry onto a polishing pad during a chemical mechanical polishing process.  
         [0018]     Another object of the present invention is to provide a new and improved method for enhancing the polishing rates and polishing profile on the surface of a wafer.  
         [0019]     Still another object of the present invention is to provide a method for enhancing the polishing rates and profile on the surface of a wafer using a rotary-type chemical mechanical polisher.  
         [0020]     Yet another object of the present invention is to provide a method for enhancing the polishing rates and profile on the surface of a wafer using a linear-type chemical mechanical polisher.  
         [0021]     A still further object of the present invention is to provide a method for enhancing the within-wafer distribution of slurry applied to a wafer during a chemical mechanical polishing process using a rotary-type polisher or a linear-type polisher.  
         [0022]     Yet another object of the present invention is to provide a method for providing a substantially uniform polishing profile on a wafer by chemical mechanical polishing.  
         [0023]     Still another object of the present invention is to provide a chemical mechanical polishing method which is well-suited to achieving a substantially uniform polishing profile on a wafer using a polishing slurry having little or no abrasive.  
       SUMMARY OF THE INVENTION  
       [0024]     In accordance with these and other objects and advantages, the present invention is generally directed to new and improved methods for enhancing uniformity in the polishing profile of a substrate during chemical mechanical polishing, particularly for CMP applications in which a polishing slurry having little or no abrasive is used in low-K IMD copper interconnect applications. According to a first embodiment, the method is adapted for a rotary-type chemical mechanical polisher and includes dispensing the polishing slurry onto the rotating polishing pad of the CMP apparatus in a polishing area on the polishing pad that contacts the entire surface area of the substrate. This facilitates substantially equal polishing rates and a substantially uniform polishing profile from the center to the edge regions on the surface of the substrate. According to a second embodiment, the method of the present invention is adapted for a linear-type chemical mechanical polisher and includes increasing the number of nozzles that dispense the slurry onto the polishing pad across the diameter or width of the substrate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0026]      FIG. 1A  is a perspective view of a typical conventional rotary-type chemical mechanical polishing apparatus;  
         [0027]      FIG. 1B  is a perspective view of a typical conventional linear-type chemical mechanical polishing apparatus;  
         [0028]      FIG. 1C  is a bottom view, partially in section, of a slurry dispenser element of the conventional CMP apparatus of  FIG. 1B ;  
         [0029]      FIG. 2  is a top view of a rotary-type chemical mechanical polishing apparatus in implementation of one embodiment of the present invention;  
         [0030]      FIG. 3  is a top view, partially in section, of a slurry bar element of a rotary-type chemical mechanical polishing apparatus in implementation of another embodiment of the present invention;  
         [0031]      FIG. 4  is a top view of a linear-type chemical mechanical polishing apparatus in implementation of another embodiment of the present invention;  
         [0032]      FIG. 5  is a bottom view, in section, of a pair of slurry bars used in implementation of the present invention as shown in  FIG. 4 ; and  
         [0033]      FIG. 6  is a bottom view of a single slurry bar suitable for implementation of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     The present invention has particularly beneficial utility in the polishing or planarization of semiconductor wafer substrates used in the fabrication of semiconductor integrated circuits. However, the invention is not so limited in application, and while references may be made to such semiconductor wafer substrates, the present invention may be more generally applicable to polishing or planarization of substrates in a variety of mechanical and industrial applications.  
         [0035]     Referring initially to  FIGS. 2 and 3 , a rotary CMP apparatus  70  in implementation of the present invention includes a circular polishing pad  81 . A wafer carrier  72 , typically mounted on the bottom end of a vertical shaft  73 , is disposed above the upper surface  83  of the polishing pad  81 , in conventional fashion. In use, a wafer  78  is mounted on the bottom surface of the wafer carrier  72 , typically in conventional fashion, and the wafer carrier  72  rotates the wafer  78  against the upper surface of the polishing pad  81 , as indicated by the arrow  82 , as the polishing pad  81  rotates as indicated by the arrow  80 , to polish the surface of the wafer  78 , as hereinafter further described. The apparatus  70  further includes an elongated slurry dispensing bar  74  having a proximal segment  75  and a distal segment  77  that extends from the proximal segment  75  at a center point  74   a . The center point  74   a  is disposed directly above a position on the upper surface  83  of the rotating polishing pad  81  which passes beneath the center of the wafer  78 . The proximal segment  75  of the slurry dispensing bar  74  is provided in fluid communication with a supply (not shown) of polishing slurry  79 . The proximal segment  75  and the distal segment  77  each is provided with multiple slurry dispensing nozzles  76  in the bottom thereof for dispensing a polishing slurry solution  79  onto the upper surface  83  of the polishing pad  81  as the polishing pad  81  is rotated. Typically, the proximal segment  75  has a larger number of the slurry dispensing nozzles  76  than does the distal segment  77 . However, in another embodiment of the slurry dispensing bar  84 , shown in  FIG. 3 , the distal segment  87  includes a larger number of slurry dispensing nozzles  86  than does the proximal segment  85 .  
         [0036]     In application, the rotary CMP apparatus  70  is typically used to polish a wafer  78  in low-k IMD, local copper interconnection applications for fabrication of device features on the order of 0.13 μM and smaller. This type of application utilizes a polishing slurry  79  containing little (typically less than about 1% by weight) or no abrasive particles. While the wafer  78  typically has a diameter of 300 mm, it is understood that the present invention may be adapted for wafers having other diameters or widths. The wafer  78  is rotated against the upper surface  83  of the polishing pad  81 , as indicated by the arrow  82 , as the wafer carrier  72  presses the wafer  78  against the polishing pad  81  and the polishing pad  81  is rotated as indicated by the arrow  80 . Simultaneously, the polishing slurry  79  is dispensed from the slurry bar  74 , through the slurry dispensing nozzles  76  of both the proximal segment  75  and the distal segment  77 , and onto the upper surface  83  of the rotating polishing pad  81 . The slurry dispensing bar  74  may be swept in a side-to-side motion as indicated by the double-headed arrow. Because it is dispensed onto the polishing pad  81  in multiple, adjacent slurry lines across a polishing area on the upper surface  83  of the polishing pad  81  that encompasses the diameter of the wafer  78 , the polishing slurry  79  travels with the rotating polishing pad  81  and then contacts the surface of the wafer  78  across the entire diameter thereof as the polishing slurry  79  is moved by the polishing pad  81  beneath the rotating wafer  78 . Consequently, the within-wafer distribution of the polishing slurry  79  is substantially uniform and the polishing rate across the entire surface area on the wafer  78  is substantially uniform, resulting in a substantially uniform polishing profile through the entire polished surface of the wafer  78 .  
         [0037]     Referring next to  FIGS. 4-6 , a linear CMP apparatus  90  in implementation of the present invention includes an endless polishing belt  91 , typically fitted with one or multiple olishing pads (not shown) and driven by a roller or rollers (not shown), in conventional fashion. A wafer holder  92  is mounted above the polishing belt  91 , on the bottom end of a shaft  93 , in conventional fashion. In use, a wafer  94  to be polished is mounted on the bottom surface of the wafer holder  92 , typically in conventional fashion, and the wafer holder  92  rotates the wafer  94  as indicated by the arrow  88  as the polishing belt  91  is driven linearly by the rollers (not shown) as indicated by the arrow  89 . A slurry delivery conduit includes a pair of adjacent slurry dispensing bars  95  disposed above the polishing belt  91 , perpendicular to the longitudinal axis thereof, at the “upstream” end of the polishing belt  91 . Each of the slurry dispensing bars  95  is provided in fluid communication with a supply (not shown) of polishing slurry  98 . Each of the slurry dispensing bars  95  is provided with multiple, typically eleven, slurry dispensing nozzles  96 , each having a nozzle opening  97  in the bottom of the corresponding slurry dispensing bar  95 , for dispensing the polishing slurry  98  onto the linearly-traveling polishing belt  91 .  
         [0038]     As shown in  FIG. 5 , the nozzle openings  97  in each slurry bar  95  are offset or staggered with respect to the nozzle openings  97  in the adjacent slurry bar  95 . The distance “A” between each nozzle opening  97  in one of the slurry dispensing bars  95  and the next nozzle opening  97  in the adjacent slurry dispensing bar  95  is less than about 30 mm. In a preferred embodiment, the slurry dispensing bars  95  have a total of twenty-two nozzle openings  97  and the spacing “A” between adjacent nozzle openings  97  is about 14.28 mm apart. However, it is understood that the slurry dispensing bars  95  may have a greater or lesser number of the nozzle openings  97 , with the spacing “A” between adjacent nozzle openings  97  less than about 30 mm. Each of the nozzle openings  97  has a diameter or width of typically about 2-3 mm. The nozzle openings  97  in the adjacent slurry dispensing bars  95  span an area above the polishing belt  91  that approximates the diameter of the wafer  94 .  
         [0039]     In an alternative embodiment, shown in  FIG. 6 , a single slurry dispensing bar  99  replaces the two adjacent slurry dispensing bars  95  shown in  FIGS. 4 and 5 . Adjacent nozzle openings  100  in the slurry dispensing bar  99  are disposed at a spacing “B” of less than about 30 mm with respect to each other.  
         [0040]     In application, the linear CMP apparatus  90  is typically used to polish a wafer  94  in low-k IMD, local copper interconnection applications for fabrication of device features on the order of 0.13 μM and smaller and utilizes a polishing slurry  98  containing little (typically less than about 1% by weight) or no abrasive particles. While the wafer  94  typically has a diameter of 300 mm, it is understood that the present invention may be adapted for wafers having other diameters or widths. The wafer holder  92  rotates the wafer  94  against the polishing belt  91 , as indicated by the arrow  88 , as the wafer holder  92  presses the wafer  94  against the polishing belt  91  and the polishing belt  91  is driven in a linear direction as indicated by the arrow  89 . Simultaneously, the polishing slurry  98  is dispensed from the adjacent slurry dispensing bars  95 , through the nozzle openings  97  of the respective nozzles  96 , and onto the moving polishing belt  91 . Because it is dispensed onto the polishing belt  91  in adjacent slurry lines across a polishing area on the polishing belt  91  that substantially encompasses the diameter of the wafer  94 , the polishing slurry  98  travels with the polishing belt  91  and then contacts the surface of the wafer  94  across the entire diameter thereof as the polishing slurry  98  is moved by the polishing belt  91  beneath the rotating wafer  94 . Consequently, the within-wafer distribution of the polishing slurry  98  is substantially uniform and the polishing rate across the entire surface area on the wafer  94  is substantially uniform, resulting in a substantially uniform polishing profile through the entire polished surface of the wafer  94 .  
         [0041]     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.