Abstract:
A tensioning assembly for a polishing belt on a linear chemical mechanical polishing apparatus. The tensioning assembly comprises first and second rollers which are operably engaged by respective air cylinders and exert a selected degree of downward tension on the lower run of the horizontal polishing belt. A third roller biased typically by a spring pushes upwardly on the lower run of the belt between the first and second rollers. Accordingly, the first and second rollers, in conjunction with the third roller, tension the belt on the apparatus to maintain optimum material removal rates and uniformity. The degree of tension exerted on the belt can be varied according to stretching of the belt resulting from prolonged use.

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 an assembly for tensioning a polishing belt on a CMP apparatus to prevent deformation of the belt through prolonged use.  
         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]    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.  
           [0010]    A typical linear CMP apparatus  10  is shown in FIGS. 1A and 1B. The linear CMP apparatus 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.  
           [0011]    As shown in FIGS. 1A and 1B, the wafer  24  is mounted on a rotating platform, or wafer holder  18 , which rotates at a predetermined speed. The major difference between the conventional linear CMP apparatus  10  and the predecessor CMP apparatus (not illustrated) 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 an idle roller  15  and a drive roller  14 , which is driven by a motor (not shown) at a predetermined rotational speed. The rotational motion of the drive roller  14  and idle roller  15  is transformed into a linear motion  26  in respect to the surface of the wafer  24 . This is shown in FIG. 1B.  
           [0012]    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  30  glued or otherwise attached thereto. As shown in FIG. 1A, a plurality of polishing pads  30  are typically utilized and which are frequently supplied in rectangular-shaped pieces with a pressure-sensitive layer coated on the back side.  
           [0013]    The wafer platform  18  and the wafer  24  forms an assembly of a wafer carrier  28  on the conventional linear CMP apparatus  10 . The wafer  24  is normally held in position by a mechanical retainer, commonly known as a retaining ring  16 , as shown in FIG. 1B. The major function of the retaining ring  16  is to fix the wafer  24  in position in the wafer carrier  28  during the linear polishing process and thus, prevent the wafer  24  from moving horizontally as wafer  24  contacts the polishing pad  30 . The wafer carrier  28  is normally operated in a rotational mode such that a more 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  22  during a polishing process. The support platen  22  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 carrier  28  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  22  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  22 , and the linear speed  26  of the polishing pad  30 . A slurry dispenser  20  is further utilized to dispense a slurry solution  34  onto the polishing pad or pads  30 . An air cylinder  34  may be connected to the idle roller  15  through a tensioning shaft  36  to facilitate positioning of the idle roller  15  along a horizontal axis and tightening of the polishing belt  12  on the drive roller  14  and idle roller  15 .  
           [0014]    A common problem associated with the conventional CMP apparatus  10  is that the polishing belt  12  eventually stretches and becomes deformed through prolonged use, as shown in FIG. 2, due to the constant tension exerted on the polishing belt  12  by the support platen  22 , wafer holder  18  and idle roller  15 . This adversely affects the rate of removal of copper from the semiconductor wafer  24 , as well as the removal uniformity, during the CMP process. Consequently, the polishing belt  12  must be frequently replaced, and this requires excessive manpower and costs for preventative maintenance. Accordingly, a device is needed for exerting tension on a polishing belt for a CMP apparatus in order to compensate for enlargement of the belt as a result of prolonged use and maintain proper tension on the belt for effective and uniform polishing of semiconductor wafers.  
           [0015]    An object of the present invention is to provide a tensioning assembly for tensioning a polishing belt on a chemical mechanical polisher.  
           [0016]    Another object of the present invention is to provide a tensioning assembly which exerts a selected degree of tension on a polishing belt of a chemical mechanical polisher in order to compensate for gradual enlargement and loosening or slackening of the belt resulting from prolonged usage.  
           [0017]    Still another object of the present invention is to provide a tensioning assembly which reduces manpower and costs associated with periodic maintenance of chemical mechanical polishers.  
           [0018]    Yet another object of the present invention is to provide a tensioning assembly which extends the useful lifetime of a polishing belt in a chemical mechanical polisher.  
           [0019]    Still another object of the present invention is to provide a tensioning assembly which contributes to enhanced rates of removal of material from semiconductor wafers by maintaining tension on a polishing belt of a chemical mechanical polishing apparatus.  
           [0020]    A still further object of the present invention is to provide a tensioning assembly which facilitates uniform and stable copper removal rates using a polishing belt on a chemical mechanical polisher over prolonged use of the polishing belt.  
           [0021]    Yet another object of the present invention is to provide a tensioning assembly for a polishing belt on a chemical mechanical polisher, which tensioning apparatus is capable of applying variable tension on the polishing belt depending upon the slack in the polishing belt in order to achieve uniform and optimum removal of copper or other materials from a semiconductor wafer during a CMP process.  
         SUMMARY OF THE INVENTION  
         [0022]    In accordance with these and other objects and advantages, the present invention comprises a tensioning assembly for a polishing belt on a linear chemical mechanical polishing apparatus. The tensioning assembly comprises first and second rollers which are operably engaged by respective air cylinders and exert a selected degree of downward tension on the lower run of the horizontal polishing belt. A third roller biased typically by a spring pushes upwardly on the lower run of the belt between the first and second rollers. Accordingly, the first and second rollers, in conjunction with the third roller, tension the belt on the apparatus to maintain optimum material removal rates and uniformity. The degree of tension exerted on the belt can be varied according to stretching of the belt resulting from prolonged use. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0024]    [0024]FIG. 1A is a perspective view of a typical conventional chemical mechanical polishing apparatus;  
         [0025]    [0025]FIG. 1B is a side view of a typical conventional chemical mechanical polishing apparatus;  
         [0026]    [0026]FIG. 2 illustrates typical stretching and deformation of a polishing belt on a chemical mechanical polishing apparatus as a result of prolonged use of the belt;  
         [0027]    [0027]FIG. 3 illustrates a tensioning assembly of the present invention;  
         [0028]    [0028]FIG. 4 is a graph which illustrates removal rate of copper from a wafer as a function of belt tension; and  
         [0029]    [0029]FIG. 5 is a graph which illustrates removal uniformity of copper from a wafer as a function of belt tension; and  
         [0030]    [0030]FIG. 6 illustrates another embodiment of the tensioning assembly of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    The belt tensioning assembly of the present invention has particularly beneficial utility in the tensioning of a polishing belt in a chemical mechanical polishing apparatus. However, the invention is not so limited in application, and while references may be made to such chemical mechanical polishing apparatus, the invention is more generally applicable to tensioning belts in a variety of industrial and mechanical applications.  
         [0032]    Referring next to FIG. 3, an illustrative embodiment of the belt tensioning assembly of the present invention is generally indicated by reference numeral  42 . The belt tensioning assembly  42  is mounted on a linear CMP apparatus  40  such as a Lam CMP apparatus manufactured by the Lam Research Corp. of Fremont Calif., for example, for tensioning a continuous polishing belt  48  which traverses a drive roller  44  and an idle roller  46  on the linear CMP apparatus  40 . The drive roller  44  and the idle roller  46  define an upper horizontal run  48   a  and a lower horizontal run  48   b  in the continuous polishing belt  48 . The belt tensioning assembly  42  includes an idle end tensioning roller  50  which is located in spaced proximity to the idle roller and engages the inner surface of the lower horizontal run  48   b  of the polishing belt  48 . The idle end tensioning roller  50  is mounted on the lower end of a shaft  52  which is selectively extendible from a hydraulic or pneumatic cylinder  54 . A drive end tensioning roller  56  is located in spaced proximity to the drive roller  44  and, like the idle end tensioning roller  50 , engages the inner surface of the lower horizontal run  48   b  of the polishing belt  48 . The drive end tensioning roller  56  is mounted on the lower end of a shaft  58  which is selectively extendible from a hydraulic or pneumatic cylinder  60 . A static tensioning roller  62 , upwardly biased typically by a roller tensioning spring  64 , engages the outer surface of the lower belt run  48   b,  between the idle end tensioning roller  50  and the drive end tensioning roller  56 . While the static tensioning roller  62 , biased by the roller tensioning spring  64 , exerts an upward static force of typically about 1 lb. against the outer surface of the polishing belt  48 , the idle end tensioning roller  50  and drive end tensioning roller  56  is each capable of exerting a selected pressure of typically from about 20 psi to about 100 psi, and preferably, about 40-60 psi, against the inner surface of the polishing belt  48 .  
         [0033]    In application, the CMP apparatus  40  is operated to remove copper or other material from a semiconductor wafer  70  mounted on a wafer holder  68  of the CMP apparatus  40 . Accordingly, as the polishing belt  48  is driven on the drive roller  44  and the idle roller  46  at a desired linear speed in the direction indicated by the arrow  66 , the wafer holder  68  rotates the wafer  70  against polishing pads (not illustrated) attached to the outer surface of the moving polishing belt  48 . The idle end tensioning roller  50  and the drive end tensioning roller  56  each exerts a downward pressure against the polishing belt  48 . Consequently, the lower horizontal run  48   b  of the polishing belt  48  is crimped by the idle end tensioning roller  50 , the static tensioning roller  62  and the drive end tensioning roller  56 , as illustrated. This crimped configuration tends to tighten the polishing belt  48  on the drive roller  44  and the idle roller  46 , and the upper run  48   a  of the polishing belt  48  is thus maintained at a sufficiently tight configuration for optimal polishing rates and uniformity in the polishing of a copper layer, for example, from the semiconductor wafer  70  as the drive roller  44  drives the polishing belt  48  along the linear motion  66 . The downward pressure exerted on the polishing belt  48  by the idle end tensioning roller  50  and the drive end tensioning roller  56 , and thus, the tension of the polishing belt  48 , is increased as needed by extending the shaft  52  and shaft  58  from the cylinder  54  and cylinder  60 , respectively. Conversely, the tension of the polishing belt  48  is decreased as needed by retracting the shaft  52  and shaft  58  into the cylinder  54  and cylinder  60 , respectively, thereby decreasing the downward pressure exerted on the polishing belt  48  by the idle end tensioning roller  50  and the drive end tensioning roller  56 , respectively. The downward pressure of the idle end tensioning roller  50  and the drive end tensioning roller  56  against the polishing belt  48  is typically between about 20 psi and about 100 psi, and preferably, between about 40 psi and about 60 psi. Over a period of prolonged usage, the polishing belt  48  tends to gradually lengthen and become slack due to the tension exerted thereon during the CMP process. Accordingly, the longer the length of the polishing belt  48 , the greater the length of extension of the shaft  52  and the shaft  58  from the cylinder  54  and the cylinder  60 , respectively, and the more compressed the roller tensioning spring  64  will become to maintain the desired tension on the polishing belt  48 .  
         [0034]    Referring next to FIG. 6, in another embodiment of the belt tensioning assembly indicated by reference numeral  72 , the shaft  52  and shaft  58  are extendible from a common hydraulic or pneumatic cylinder  74 , with the idle end tensioning roller  50  rotatably mounted on the bottom end of the shaft  52  and the drive end tensioning roller  56  rotatably mounted on the bottom end of the shaft  58 . The static tensioning roller  62 , upwardly biased by the roller tensioning spring  64 , engages the outer surface of the lower run  48   b  of the polishing belt  48 . Accordingly, the idle end tensioning roller  50  and the drive end tensioning roller  56  are pressed against the inner surface of the lower run  48   b  of the polishing belt  48  in order to tighten the polishing belt  48  by simultaneously extending the shaft  52  and shaft  58  from the cylinder  74 . Conversely, the polishing belt  48  is loosened by retracting the shaft  52  and shaft  58  simultaneously into the cylinder  74 , thereby reducing pressure of the idle end tensioning roller  50  and the drive end tensioning roller  56  against the polishing belt  48 .  
         [0035]    A graph illustrating rates of removal of copper from a semiconductor wafer as a function of tension in pounds per square inch applied the polishing belt of a CMP apparatus is illustrated in FIG. 4. Accordingly, copper removal rates using a polishing belt having a tension of 50 psi is represented by the connected diamonds and copper removal rates using a polishing belt having a tension of 60 psi is represented by the connected squares. It can be seen from the graph that copper removal rates using a polishing belt having a tension of 60 psi significantly enhances the rate of removal of copper from the wafer as compared to using a polishing belt having a tension of only 50 psi.  
         [0036]    A graph illustrating uniformity in removal of copper from a semiconductor wafer as a function of tension in pounds per square inch applied the polishing belt of a CMP apparatus is illustrated in FIG. 5. In the graph, copper removal uniformity using a polishing belt having a tension of 50 psi is represented by the connected diamonds, whereas copper removal uniformity using a polishing belt having a tension of 60 psi is represented by the connected squares. It can be seen from the graph that copper removal uniformity using a polishing belt having a tension of 60 psi is more stable than copper removal uniformity using a polishing belt having a tension of 50 psi.  
         [0037]    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.  
         [0038]    Having described our invention with the particularity set forth above, we claim: