Patent Publication Number: US-6213855-B1

Title: Self-powered carrier for polishing or planarizing wafers

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an apparatus for polishing or planarizing semiconductor workpieces such as silicon wafers. More particularly, the present invention relates to a wafer carrier for planarizing or polishing wafers on a polishing pad. 
     2. Description of the Related Art 
     Silicon workpieces or wafers, which are typically flat and circular in shape, are used in manufacturing semiconductor devices. Wafers are initially sliced from a silicon ingot and, thereafter, undergo multiple masking, etching, and dielectric and conductor deposition processes to create microelectronic structures and circuitry. The surface of a wafer undergoing these processes typically are polished or planarized between processing steps to ensure proper flatness to facilitate the use of photo lithographic processes for building additional dielectric and metallization layers on the wafer surface. 
     Chemical Mechanical Planarization (“CMP”) machines have been developed to polish or planarize silicon wafer surfaces to the flat condition desired for manufacture of integrated circuit components and the like. For examples of conventional CMP processes and machines, see U.S. Pat. No. 4,805,348, issued in February 1989 to Arai, et al.; U.S. Pat. No. 4,811,522, issued in March 1989 to Gill; U.S. Pat. No. 5,099,614, issued in March 1992 to Arai et al.; U.S. Pat. No. 5,329,732, issued in July 1994 to Karlsrud et al.; U.S. Pat. No. 5,476,414, issued in December 1995 to Masayoshi et al.; U.S. Pat. Nos. 5,498,196 and 5,498,199, both issued in March 1996 to Karlsrud et al.; and U.S. Pat. No. 5,558,568, issued in September 1996 to Talieh et al. 
     Typically, a CMP machine includes a wafer carrier configured to hold and to rotate a wafer during the polishing or the planarizing of the wafer. For example, with reference to FIG. 1, a conventional wafer carrier  100  includes an upper housing  101  and a pressure plate  104  mounted underneath a lower or secondary housing  106 . A plurality of fasteners  108  fix pressure plate  104  to lower housing  106 . A plurality of vacuum holes  110  hold the wafer to be planarized to the planar lower surface of pressure plate  104 . Wafer carrier  100  then presses the wafer against a polishing pad (not shown) to polish or to planarize the wafer. More particularly, pressure plate  104  applies pressure to the wafer such that the wafer engages the polishing pad with a desired amount of pressure. The pressure plate and the polishing pad are also rotated, typically with differential velocities, to cause relative lateral motion between the polishing pad and the wafer to produce a more uniform thickness. Additionally, an abrasive slurry, such as a colloidal silica slurry, is often provided to enhance the polishing or planarizing process. 
     Conventional wafer carriers are typically rotated by a drive motor through a central drive shaft and a mechanical bearing assembly. For example, conventional wafer carrier  100  includes a bearing assembly  112  disposed between lower housing  106  and upper housing  101  and a drive shaft  114  connected to a drive motor (not shown). Bearing assembly  112  permits the movement of lower housing  106  and pressure plate  104  relative to upper housing  101  in order to maintain the surface of the wafer in parallel contact with the polishing pad even when the pad deviates from planarity. This motion is often referred to as “gimballing”, and the “gimbal point” is defined as the intersection of the plane in which the pressure plate  104  gimbals and the vertical central axis of the carrier. The gimbal point of wafer carrier  100 , for example, is at point  116 . The location of the gimbal point above the lower or backing surface of the pressure plate, however, can result in excessive tipping of the wafer with respect to the polishing pad, thus causing uneven edge polishing and detracting from uniform pressure distributed across the wafer. 
     Another shortcoming of conventional wafer carriers which arc rotated by a central drive shaft is the lag in response time due to the inertia of the wafer carrier. For example, when a torque is initially applied to drive shaft  114  to begin to rotate wafer carrier  100 , the mass of wafer carrier  100  results in a lag in response time of the wafer carrier  100 . Accordingly, the outer diameter portions of the wafer carrier  100  may initially rotate slower than the inner diameter portions of the wafer carrier  100 , thus contributing to uneven polishing or planarizing of the wafer. Additionally, the mass of wafer carrier  100  may result in undesired vibrations when the rotational speed of drive shaft  114  is increased or decreased, thus further contributing to uneven polishing or planarizing of the wafer. 
     An additional shortcoming of conventional wafer carriers is that the downward pressure applied to the drive shaft is not ideally distributed across the wafer. For example, in carrier  100 , upper housing  101  is connected to outer ring  118  of bearing assembly  112  by fasteners  120 , while inner ring  122  of bearing assembly  112  is connected to lower housing  106  by fasteners  124 . Hence, the pressure distribution path is as follows: downward pressure applied from the drive shaft is transmitted into upper housing  101 , transmitted through fasteners  120  and into outer bearing ring  118 , transmitted through bearing assembly  112  to inner bearing ring  122 , and transmitted through fasteners  124  to the narrow central body portion  126  of lower housing  106  and pressure plate  104 . Consequently, the downward pressure is concentrated at the central portion of the wafer and effects excessive material removal in the inner diameter portions of the wafer, while bowing and inadequate removal occurs at the outside diameter portions of the wafer. 
     SUMMARY OF THE INVENTION 
     In accordance with an exemplary embodiment of the present invention, a wafer carrier for polishing or planarizing semiconductor workpieces or wafers includes a pressure plate, an upper housing, and a lower housing. In accordance with one aspect of the present invention, the pressure plate is configured to hold a wafer to be polished or to be planarized against a polishing pad, and further configured to rotate about the lower housing to rotate the wafer during the polishing or the planarizing process. In accordance with another aspect of the present invention, the wafer carrier includes an electric direct drive motor, with the stators of the motor disposed in the lower housing and the rotors of the motor disposed in the pressure plate, to rotate the pressure plate about the lower housing. Accordingly, when electric power is supplied to the stators of the electric direct drive motor, the rotors of the motor rotate the pressure plate in response to the electromagnetic flux generated by the stators. The torque generated by the motor is developed in close proximity to the wafer, thus lowering the gimballing point of the carrier and thereby reducing the amount of gimballing or tilting force imparted to the wafer. The wafer thus tends to remain essentially parallel with the polishing pad surface. 
     In accordance with still another aspect of the present invention, a compliant material is disposed between the upper housing and the lower housing of the wafer carrier to form a flexible joint, or bellows, which maintains the wafer in substantially parallel and in substantially full contact with the polishing pad. In accordance with yet another aspect of the present invention, the lower housing of the wafer carrier is pressurized to apply pressure across substantially all of the surface area of the pressure plate and substantially uniformly across the surface area of the wafer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, may best be understood by reference to the following description taken in conjunction with the claims and the accompanying drawing, in which like parts may be referred to by like numerals: 
     FIG. 1 is a cross sectional view of a prior art wafer carrier; 
     FIG. 2 is a cross sectional view of a wafer carrier in accordance with various aspects of the present invention; and 
     FIG. 3 is a top plan view of the wafer carrier shown in FIG. 2 taken through lines  5 — 5 . 
    
    
     DETAILED DESCRIPTION 
     The subject matter of the present invention is particularly suited for use in connection with Chemical Mechanical Planarization (“CMP”) of semiconductor workpieces or wafers. As a result, an exemplary embodiment of the present invention is described in that context. It should be recognized, however, that such description is not intended as a limitation on the use or applicability of the present invention, but is instead provided to enable a complete description of an exemplary embodiment. 
     In the relevant art, the terms “polishing” and “planarizing” are used to describe a wide range of both wet and dry processing of semiconductor workpieces or wafers to produce a substantially flat or planar surface thereon. Although the present invention is described in connection with CMP processing of wafers, it should be appreciated that the present invention can be employed with any convenient wafer polishing or planarizing technique, such as chemical-mechanical polishing, lapping, grinding, honing, slurry polishing, and the like. For a more detailed discussion of the CMP process, see U.S. patent application Ser. No. 08/926,700, filed Sep. 10, 1997, the entire content of which is incorporated herein by reference. 
     FIG. 2 is a cross sectional view of a wafer carrier in accordance with various aspects of the present invention. As depicted in FIG. 2, a carrier head  200  according to various aspects of the present invention is suitably employed to polish or to planarize a wafer  102  by applying pressure on wafer  102  to engage the underside of wafer  102  against a polishing pad  206 . Polishing pad  206  (FIG. 2) is preferably attached to polishing table  202 , and is preferably formed from polyurethane, such as the IC and GS series of polishing pads available from Rodel Products Corporation of Scottsdale, Ariz. However, it should be appreciated that polishing pad can be formed from any suitable polishing material depending on the particular application. For example, polishing pad  206  can include a grinding stone, a diamond pellet, a lapping plate, and the like. 
     With reference to FIG. 2, in an exemplary embodiment of the present invention, carrier head  200  includes a pressure plate  210 , an upper housing  220 , and a lower housing  230 . During the polishing process, wafer  102  is held by pressure plate  210 . More particularly, a plurality of vacuum ports  270  formed in pressure plate  210  secure wafer  102  to pressure plate  210 . Methods of providing a vaccum to ports  270  are well known in the art. It should be appreciated, however, that wafer  102  can be secured to pressure plate  210  using various methods, such as, for example, wet surface tension. 
     In the present exemplary embodiment, pressure plate  210  is substantially circular and appropriately sized to apply pressure across substantially the entire upper surface of wafer  102 . Accordingly, the specific shape and size of pressure plate  210  can vary depending on the shape and size of wafer  102 . According to various aspects of the present invention, pressure plate  210  may be formed using any convenient method, such as casting, milling, and the like. Additionally, pressure plate  210  can be formed from any suitably rigid material, such as metal, ceramic, and the like. Furthermore, pressure plate  210  can be coated with protective material such as urethane to protect the upper surface of wafer  102 . 
     As indicated above, during the polishing or planarizing process, pressure plate  210  rotates wafer  102  to more uniformly remove material therefrom and to accelerate the polishing or planarizing process. In the present exemplary embodiment of the invention, pressure plate  210  is preferably configured to rotate around lower housing  230 . More particularly, in the present exemplary embodiment, pressure plate  210  includes an arm  212  and a groove  208 , which is preferably formed in lower housing  230  to receive arm  212 . Although in FIG. 2 arm  212  and pressure plate  210  are depicted as separate pieces, it should be appreciated, however, that arm  212  and pressure plate  210  can be formed as a single piece using any convenient method. For example, arm  212  and pressure plate  210  can be cast together as a single piece. Additionally, although in FIG. 2 groove  208  is depicted as having a substantially square shaped profile, it should be appreciated that groove  208  and arm  212  can be configured with profiles having various shapes depending on the particular application. For example, groove  208  can be configured with a substantially concave shaped profile and arm  212  can be configured with a substantially matching convex shaped profile. Alternatively, groove  208  can be configured with a substantially convex shaped profile and arm  212  can be configured with a substantially matching concave shaped profile. 
     A bearing ring  290  according to various aspects of the present invention is preferably disposed within groove  208  (shown as having an upper portion  208 U and a lower portion  208 L) to facilitate the movement of arm  212  within groove  208 . Although groove  208  is shown in FIG. 2 as a continuous void for the purpose of clarity, in practice, surface  209  of arm  212  rests slidably on bearing ring  290 . In the present exemplary embodiment, bearing ring  290  is preferably configured as an o-ring formed from any suitable low friction material with a low coefficient of friction, such as polytetrafluoroethylene, (commercially known as TEFLON®). Alternatively, a mechanical system, such as ball-bearings, bushings, and the like, can be employed to facilitate the movement of arm  212  within groove  208 . Although bearing ring  290  is depicted in FIG. 2 as being a ring disposed between the lower surface of arm  212  and groove portion  208 L, it should be appreciated that bearing ring  290  can be configured with various shapes and dimensions depending on the particular application. For example, bearing ring  290  can be configured with a profile substantially similar to the profile of groove  208 . Alternatively, an additional bearing ring  290  can be disposed between the upper portion of arm  212  and groove portion  208 U. 
     FIG. 3 is a top plan view of the wafer carrier shown in FIG. 2 taken through lines  5 — 5 . In accordance with various aspects of the present invention, an electric direct drive motor comprising a plurality of rotors  250  and stators  260  is employed to rotate pressure plate  210  about lower housing  230  of wafer carrier  200 . With reference to FIG. 3, in the present exemplary embodiment of the present invention, a plurality of rotors  250  are disposed about the circumference of arm  212 , and a plurality of stators  260  are disposed about the circumference of lower housing  230 . The configuration of rotors  250  and stators  260  about the circumference of arm  212  and the circumference of lower housing  230 , respectively, is particularly advantageous in that a torque can be applied directly to the outer circumference of pressure plate  210  (FIG.  2 ), thus reducing the lag time which can result if a torque is applied to the center of wafer carrier  200  as in conventional systems. Additionally, less torque is required to rotate pressure plate  210  in comparison to conventional system in which the entire wafer carrier  200  is rotated. Accordingly, the present invention facilitates faster acceleration and response time in rotating pressure plate  210  which in turn facilitates a more uniform polishing or planarizing of wafer  102 . 
     With reference to FIG. 2, in the present exemplary embodiment, plurality of rotors  250  include permanent magnets ranging in diameter from about 8 to 12 inches in diameter and about ¾ inch wide. It should be recognized, however, that the plurality of rotors  250  can include magnets with various dimensions and shape depending on the particular application. For example, increasing the size of the magnets used as plurality of rotors  250  can increase the overall torque applied to pressure plate  210 . This exemplary configuration of using permanent magnets as plurality of rotors  250  has the advantage in that no electrical wires need to be provided to the rotating portion of the pressure plate  210  to magnetize the plurality of rotors  250 . It should be appreciated, however, that plurality of rotors  250  can be configured as electromagnets. In such a configuration, a rotary slip-joint or the like may be used for applying current to the electromagnets. 
     In the present exemplary embodiment, plurality of stators  260  include a plurality of electric coils suitably configured to produce a magnetic flux sufficient to rotate pressure plate  210  at a rotational speed of at least 50 rpm. The direct drive motor comprising rotors  250  and stators  260  preferably generates a minimum of 0.2 horsepower with 95 ft-lbs of torque at 10 rpm, and 0.85 horsepower with 89 ft-lbs of torque at 50 rpm. It should be recognized, however, that the plurality of stators  260  can include electric coils configured to produce various amounts of magnetic flux depending on the particular application. For example, increasing the amount of magnetic flux produced by stators  260  can increase the overall torque applied to pressure plate  210 . 
     With reference to FIG. 3, when electric power is provided to plurality of stators  260  sequentially in the desired rotational direction, the magnetic flux generated by plurality of stators  260  exerts a force on the plurality of rotors  250  to rotate pressure plate  210  (FIG. 2) in the same direction. For example, when electric power is provided to stators  260  sequentially in a clockwise direction, pressure plate  210  also rotates in a clockwise direction. Similarly, when electric power is provided to stators  260  sequentially in a counter-clockwise direction, pressure plate  210  also rotates in a counter-clockwise direction. Additionally, the direction in which power is provided to stators  260  may be alternated, thus oscillating pressure plate  210 . 
     Although eight rotors  250  and eight stators  260  are depicted in FIG. 3, it should be appreciated that any number of rotors  250  and stators  260  can be employed depending on the particular application. For example, the torque applied to pressure plate  210  can be increased or decreased by employing more or fewer rotors  250  and stators  260 . This aspect of the present invention is particularly advantageous in that the torque applied to pressure plate  210  can be increased without necessarily increasing the size of the existing rotors  250  and stators  260  which would increase the vertical profile of wafer carrier  200 . 
     Additionally, although rotors  250  and stators  260  are depicted in FIG. 3 as being disposed in equally spaced increments, it should be appreciated that rotors  250  and stators  260  can be disposed in various patterns depending on the particular application. Disposing rotors  250  and stator  260  in equally spaced increments, however, has the advantage of equally distributing the torque applied to the pressure plate  210 , thus facilitating a more uniform polishing and planarizing of wafer  102 . 
     Furthermore, it should be appreciated that pressure plate  210  can be rotated using any convenient electric motor depending on the particular application without deviating from the spirit or scope of the present invention. The direct drive motor assembly described above, however, has the particular advantage of providing fast response time and high rate of acceleration, which is essentially limited by the adhesion/retention between the wafer  102  and carrier  200 . 
     With reference to FIG. 2, in accordance with another aspect of the present invention, carrier head  200  preferably includes a compliant member  240  disposed between upper housing  220  and lower housing  230 . The flexible joint formed between upper housing  220  and lower portion  230  facilitates a floating joint whereby pressure plate  210  can pivot along its x-, y- and z-axes relative to upper housing  220 . Hence, pressure plate  210  is able to mimic movement of the polishing pad  206  in the x-, y- or z-directions to thereby dynamically and continuously adjust the plane of wafer  102  held by wafer carrier  200  relative to polishing pad  206  and maintain wafer  102  in substantially parallel and in substantially full contact with polishing pad  206 , thus facilitating a more uniform polishing and planarizing of wafer  102 . The use of compliant member  240  to form a flexible joint has the advantage that no lubricants, which can contaminate wafer  102 , are needed as in conventional mechanical bearing assemblies. In the present exemplary embodiment of the present invention, compliant member  240  functions as a bellows. Compliant member  240  can be formed from any suitable compliant material, such as rubber, plastic, or metal. 
     In accordance with another aspect of the present invention, chamber  235  of wafer carrier  200  is pressurized to apply a desired polishing pressure on pressure plate  210 . The pressure is applied across substantially all of the surface area of pressure plate  210  and substantially uniformly across the surface area of pressure plate  210 . Accordingly, the pressure applied by pressure plate  210  to wafer  102  is applied across substantially all of the surface area of wafer  102  and substantially uniformly across the surface area of wafer  102  to facilitate a more uniform polishing or planarizing of wafer  102 . In the exemplary embodiment, chamber  235  is pressurized with approximately 5 to 10 psi of pressure. It should be appreciated, however, that various amounts of pressure can be employed depending on the particular application. 
     It is to be noted that the wafer carrier  200  of the present invention can be retrofitted to existing CMP machines, and advantageously employed in conjunction with a wide range of polishing or planarizing operations. 
     Although the present invention is set forth herein in the context of the appended drawing figures, it should be appreciated that the invention is not limited to the specific forms shown. Various other modifications, variations, and enhancements in the design, arrangement, and implementation may be made without departing from the spirit and scope of the present invention set forth herein. Furthermore, one of skill in the art will appreciate that various other applications and uses exist for the wafer carrier  200  besides the specific examples given.