Abstract:
An apparatus for slurry distribution during semiconductor wafer polishing operations. The slurry is gravity fed or fed under pressure and broadcast under an angle across the entire face of the polishing pad by either a rotating slurry nozzle arrangement or by a rotating slurry shaft arrangement. This as opposed to the conventional slurry supply lines, which are stationary in design.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of Chemical Mechanical Polishing (CMP). More particularly, the present invention relates to methods and apparatus for chemical mechanical polishing of substrates, such as semiconductor substrates, on a rotating polishing pad in the presence of a chemically and/or physically abrasive slurry, and providing fresh supply of slurry onto the surface of the substrate which is mounted on the polishing pad while the substrate is being polished. 
     DESCRIPTION OF THE PRIOR ART 
     Chemical Mechanical Polishing is a method of polishing materials, such as semiconductor substrates, to a high degree of planarity and uniformity. The process is used to planarize semiconductor slices prior to the fabrication of semiconductor circuitry thereon, and is also used to remove high elevation features created during the fabrication of the microelectronic circuitry on the substrate. One typical chemical mechanical polishing process uses a large polishing pad that is located on a rotating platen against which a substrate is positioned for polishing, and a positioning member which positions and biases the substrate on the rotating polishing pad. Chemical slurry, which may also include abrasive materials therein, is maintained on the polishing pad to modify the polishing characteristics of the polishing pad in order to enhance the polishing of the substrate. 
     The use of chemical mechanical polishing to planarize semiconductor substrates has not met with universal acceptance, particularly where the process is used to remove high elevation features created during the fabrication of microelectronic circuitry on the substrate. One primary problem which has limited the used of chemical mechanical polishing in the semiconductor industry is the limited ability to predict, much less control, the rate and uniformity at which the process will remove material from the substrate. As a result, CMP is labor intensive process because the thickness and uniformity of the substrate must be constantly monitored to prevent overpolishing or inconsistent polishing of the substrate surface. 
     One factor, which contributes to the unpredictability and non-uniformity of the polishing rate of the CMP process, is the non-homogeneous replenishment of slurry at the surface of the substrate and the polishing pad. The slurry is primarily used to enhance the rate at which selected materials are removed from the substrate surface. As a fixed volume of slurry in contact with the substrate reacts with the selected materials on the surface of the substrate, this fixed volume of slurry becomes less reactive and the polishing enhancing characteristics of that fixed volume of slurry is significantly reduced. One approach to overcoming this problem is to continuously provide fresh slurry onto the polishing pad. 
     This approach presents at least two problems. Because of the physical configuration of the polishing apparatus, introducing fresh slurry into the area of contact between the substrate and the polishing pad is difficult. Providing a fresh supply of slurry to all positions of the substrate is even more difficult. As a result, the uniformity and the overall rate of polishing are significantly affected as the slurry reacts with the substrate. 
     Current practice uses a straight line tubing arrangement to dispense the slurry directly into the center of the polishing pad. This leads to uneven distribution of the slurry onto the polishing pad while covering the entire polishing pad with slurry requires a significant amount of time. Current practice therefore leads to higher usage of slurry and longer process time since the polishing process can only start after the entire polishing pad has been covered with slurry. 
     The polishing process is carried out until the surface of the wafer is ground to a highly planar state. During the polishing process, both the wafer surface and the polishing pad become abraded. After numerous wafers have been polished, the polishing pad becomes worn to the point where the efficiency of the polishing process is diminished and the rate of removal of material from the wafer surface is significantly decreased. It is usually at this point that the polishing pad is treated and restored to its initial state so that a high rate of uniform polishing can once again be obtained. 
     FIG. 1 shows a Prior Art CMP apparatus. A polishing pad  20  is affixed to a circular polishing table  22  which rotates in a direction indicated by arrow  24  at a rate in the order of 1 to 100 m RPM. A wafer carrier  26  is used to hold wafer  18  face down against the polishing pad  20 . The wafer  18  is held in place by applying a vacuum to the backside of the wafer (not shown). The wafer carrier  26  also rotates as indicated by arrow  32 , usually in the same direction as the polishing table  22 , at a rate on the order of 1 to 100 RPM. Due to the rotation of the polishing table  22 , the wafer traverses a circular polishing path over the polishing pad  20 . A force  28  is also applied in the downward vertical direction against wafer  18  and presses the wafer  18  against the polishing pad  20  as it is being polished. The force  28  is typically in the order of 0 to 15 pounds per square inch and is applied by means of a shaft  30  that is attached to the back of wafer carrier  26 . Slurry  21  is provided to the top of the polishing pad to further enhance the polishing action of polishing pad  20 . 
     U.S. Pat. No. 5,775,983 (Shendon et al.) shows a conical roller pad. 
     U.S. Pat. No. 5,738,573 (Yueh) shows a slurry distribution method with slurry distribution in the core. However, this reference differs from the present invention. 
     U.S. Pat. No. 5,245,794 (Salusugan) shows a conventional slurry tube. 
     SUMMARY OF THE INVENTION 
     The present invention teaches a gravity fed slurry distribution system. Slurry is fed to the polishing pad via a slurry tube. The slurry tube of the present invention rotates; this rotation of the slurry tube is caused by a special design of the outlet of the slurry tube that dispenses the slurry onto the polishing pad. The rotation of the slurry tube outlet is in a direction that is opposite to the direction of rotation of the polishing pad. This allows for quick and efficient broadcasting of the slurry across the entire surface of the polishing pad. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a Prior Art CMP apparatus. 
     FIG. 2 shows an overview of the implementation of the present invention with the relative positioning of the polishing table, the polishing pad, the wafer carrier and the slurry distribution tube. 
     FIG. 3 a  and FIG. 3 b  show the design of the outlet of the rotating slurry distribution tubing. 
     FIGS. 4 a  and  4   b  show cross sections of an alternate implementation of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the first embodiment of the present invention the means of distributing slurry evenly across the surface of a polishing pad consists of a slurry feed tubing arrangement whereby the last orb lowest section of this arrangement rotates and in so doing distributes the slurry over the face of the polishing pad. FIG. 3 a  and FIG. 3 b  address the first embodiment of the present invention. 
     In the second embodiment of the present invention the means of distributing slurry evenly across the surface of a polishing pad consists of a rotating shaft contained within the slurry supply reservoir. FIG. 4 a  and FIG. 4 b  address the second embodiment of the present invention. 
     First Embodiment of the Present Invention 
     Referring now specifically to FIG. 2, there is shown an overview of the implementation of the present invention with the relative positioning of the polishing table  22 , the polishing pad  24 , the wafer carrier  26  and the slurry supply tube  35 . One polishing pad  24  is shown, however a set of more than one abrasive polishing pads having a suspension medium can be used. The slurry supply tube  35  consists of two sections, the upper section  28  which is fixed or stationary in position and the lower section  36  which rotates around its axis  31  which is also the axis of the stationary slurry tube  28 . The wafer-polishing table rotates in direction  34 ; the lower section  36  of the slurry supply tube  35  rotates in direction  32 , the wafer carrier rotates in direction  33 . The wafer carrier holds wafer  37  in place. Note that directions  34  and  32  are in opposite directions. The slurry is fed into the slurry supply tube  35  at point  30 ; the slurry gravity feeds down through the stationary upper section  28  and the rotating lower section  36  and exits the lower section  36  at point  36 . The slurry spay  39  is, due to the counter directive rotations of the lower section  36  and the wafer polishing table  22 , that is  32  and  34 , efficiently broadcast and distributed across the surface of the wafer polishing pad  24 . 
     FIG. 3 provides further detail of the construction of the slurry supply tube  35 . FIG. 3 a  shows the cross section of the stationary upper section  28  of the slurry supply tube  35 . FIG. 3 b  shows a cross section of the rotating lower section  36  of the slurry supply tube  35 . The slurry  30  enters upper section  28  at the top of section  28  and gravity flows down through the tube  28 . The concave section  42  provided at outside surface of the lower portion of tube  28  matches the convex section  52  provided at the inside surface of the upper portion of the rotating section  36  of the slurry supply tube  35 . These matching sections  42  and  52  are operationally combined by affixing the concavity of  42  within the convexity of  52 , this affixing is done in such a manner that the two sections, that is the stationary and the rotating sections, exhibit little or no motion with respect to each other in the vertical or Z direction while the two sections maintain freedom of rotation with respect to each other around the axis of rotation  31 . The gravity fed slurry  30  is, on its downward path, routed through a channel  54  of spiral shape within the inside of the rotating section  36  thus causing and maintaining the rotation of this section  36 . By providing a seal  56  within section  36  across the lower extremity of  36  and in a plane perpendicular to the axis  31  of lower section  36 , the slurry is prevented from exiting section  36  in the direction of its axis  31 . The exit spout  53  forces the slurry  30  to exit ( 58 ) section  36  under an angle with its axis  31  thus causing section  36  to rotate around its axis  31 . 
     Second Embodiment of the Present Invention 
     FIG. 4 a  shows and alternate method of implementing the rotational aspect of the slurry distribution arrangement. The slurry  60  enters the slurry reservoir  62  and is forced to flow through a channel  66 , which is shaped in the form of a spiral. The downward or Z-axis directional motion of the slurry  60  is in this manner translated into a rotational motion  70 , which is transferred to a rotational motion of a shaft  76 . The shaft  76  is held in a fixed or stationary position with respect to the slurry reservoir  62  by means of two membranes  64  within the reservoir  62 . The shaft  76  does however have freedom of rotation  70  within these membranes  64 . An opening or exit point provided within the bottom part  72  of shaft  76  allows the slurry to escape as indicated by  68 . This opening is provided under an angle with respect to the X-Y-Z direction forcing the slurry to be broadcast or distributed across the surface of the polishing pad and to impact this surface under and angle. This angle which can be selected and designed to suit any particular application of the present invention. 
     FIG. 4 b  shows a top plan view of the implementation of the second embodiment of the present invention. Indicator  80  further highlights that the slurry reservoir  62  and the membranes  64  within this reservoir are stationary while the head  74  of shaft  76  has freedom of rotation  70 . 
     The invention, which provides a method for chemical mechanical planarization of a semiconductor wafer, can be summarized as follows: 
     a rotating platform is provided for mounting semiconductor wafers over the surface thereof 
     a rotating platform is provided for mounting semiconductor polishing pads over the surface thereof 
     a means is provided for evenly distributing slurry across a polishing pad, this means comprising a slurry supply shaft which is mounted within a slurry supply reservoir and wherein a lower extremity of a rotating slurry supply shaft has a means for distributing the slurry, the means for distributing the slurry comprising at least one opening in a lower extremity of the rotating slurry supply shaft whereby downward motion of the slurry is transformed into a rotating motion of the rotating slurry supply shaft, the means for distributing the slurry can further use pressure applied to the slurry while the slurry exits the rotating slurry supply shaft, and 
     a means is provided for controlling rate of slurry flow. 
     The means for distributing the slurry comprises a multiplicity of openings in a lower extremity of a rotating slurry supply shaft wherein the direction of the axis of the openings does not coincide with the X-Y-Z direction of the rotating slurry supply shaft. 
     From the foregoing it will be clear that, although a specific embodiment of the present invention has been described herein for purposes of illustration, various modifications to the present invention may be made without deviating from the spirit and scope of the present invention. Accordingly, the present invention is not limited except as by the appended claims.