Patent Publication Number: US-6220944-B1

Title: Carrier head to apply pressure to and retain a substrate

Description:
CROSS-REFERENCE TO RELATED CASES 
     This application is a continuation of U.S. patent application Ser. No. 09/330,243, filed Jun. 10, 1999 now U.S. Pat. No. 6,050,882. 
    
    
     BACKGROUND 
     The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing. 
     Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. 
     Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a polishing surface, e.g., a rotating polishing pad. The polishing pad may be either a “standard” or a fixed-abrasive pad. A standard polishing pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. 
     The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. 
     SUMMARY 
     In one aspect, the invention is directed to a carrier head. The carrier head has a housing, a plurality of substantially independently movable rods, and a first chamber located between the rods and the housing. The chamber is pressurizable to force the rods into contact with a substrate and to surround the substrate to retain the substrate beneath the housing. 
     Implementations of the invention may include the following features. A lower boundary of the first chamber may be defined by a flexible membrane attached to the housing, and the rods may be attached to the flexible membrane. Alternately, the first chamber may apply pressure directly to the rods. The rods may have a circular or hexagonal cross-section, a longitudinal dimension of about 0.06 to 0.5 inches, and a cross-sectional dimension of about 0.03 to 0.25 inches. The longitudinal dimension of the rods may be about twice their cross-sectional dimension. The rods may be spaced apart by about 0.0005 to 0.005 inches. The rods may be positioned around a perimeter portion of the substrate during polishing, and the carrier head further may include a flexible membrane having a mounting surface to contact a central region of the substrate. A second chamber that is pressurizable to apply a load to the central region of the substrate may be located between the flexible membrane and the housing. The rods may be positioned substantially parallel to each other. 
     In another aspect, the invention is directed to a carrier head to hold a substrate on a polishing surface. The carrier head has a housing defining a chamber, a flexible membrane defining a lower boundary of said chamber, and a bundle of independently movable rods secured to the flexible membrane. When a pressure within the chamber is increased, the rods and move into contact with the substrate and the polishing surface to apply a force to the substrate and retain the substrate substantially beneath the housing. 
     In another aspect, the invention is directed to a method of polishing a substrate. In the method, a substrate is positioned between a polishing surface and a plurality of independently movable rods of a carrier head, and a pressure is applied to the plurality of rods. One group of rods contacts a back surface of the substrate, and a second group of rods contacts the polishing surface to surround the substrate to retain the substrate beneath the carrier head. 
     Advantages of the invention may include the following. The spacing between the retainer and the substrate can be reduced, thereby improving polishing uniformity near the edge of the substrate. The carrier head has a large tolerance for misalignment of the substrate at a loading station. The carrier head is also usable with substrates of different sizes and geometries. 
     Other advantages and features of the invention will be apparent from the following description, including the drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a chemical mechanical polishing apparatus. 
     FIG. 2 is a schematic cross-sectional view of a carrier head according to the present invention. 
     FIG. 3A is a perspective view of a rod having a circular cross-section. 
     FIG. 3B is a perspective view of a rod having a hexagonal cross-section. 
     FIG. 4 is a schematic cross-sectional view of the carrier head of FIG. 2 being used to polish a substrate. 
     FIG. 5 is a schematic bottom view of the carrier head of FIG. 2 loaded with a substrate. 
     FIG. 6 is a schematic cross-sectional view of a carrier head in which the rods are not attached to a backing membrane. 
     FIG. 7 is a schematic cross-sectional view of a carrier head that includes both rods and a substrate-backing membrane. 
    
    
     Like reference numbers are designated in the various drawings to indicate like elements. A reference number with a prime or double-prime indicates that an element has a modified function, operation or structure. 
     DETAILED DESCRIPTION 
     Referring to FIG. 1, one or more substrates  10  will be polished by a chemical mechanical polishing (CMP) apparatus  20 . A description of a similar CMP apparatus may be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. 
     The CMP apparatus  20  includes a series of polishing stations  25  and a transfer station  27  for the loading and unloading of the substrates. Each polishing station  25  includes a rotatable platen  30  on which is placed a polishing pad  32 . If substrate  10  is an eight-inch (200 millimeter) or twelve-inch (300 millimeter) diameter disk, then platen  30  and polishing pad  32  will be about twenty or thirty inches in diameter, respectively. Platen  30  and polishing pad  32  may also be about twenty inches in diameter if substrate  10  is a six-inch (150 millimeter) diameter disk. For most polishing processes, a platen drive motor (not shown) rotates platen  30  at thirty to two-hundred revolutions per minute, although lower or higher rotational speeds may be used. Each polishing station  25  may further include an associated pad conditioner apparatus  40  to maintain the abrasive condition of the polishing pad. 
     A slurry  50  containing a reactive agent (e.g., deionized water for oxide polishing) and a chemically-reactive catalyzer (e.g., potassium hydroxide for oxide polishing) may be supplied to the surface of polishing pad  32  by a combined slurry/rinse arm  52 . If polishing pad  32  is a standard pad, slurry  50  may also include abrasive particles (e.g., silicon dioxide for oxide polishing). Typically, sufficient slurry is provided to cover and wet the entire polishing pad  32 . Slurry/rinse arm  52  includes several spray nozzles (not shown) which provide a high pressure rinse of polishing pad  32  at the end of each polishing and conditioning cycle. 
     A rotatable multi-head carousel  60  is supported by a center post  62  and rotated thereon about a carousel axis  64  by a carousel motor assembly (not shown). Multi-head carousel  60  includes four carrier head systems  70  mounted on a carousel support plate  66  at equal angular intervals about carousel axis  64 . Three of the carrier head systems position substrates over the polishing stations. One of the carrier head systems receives a substrate from and delivers the substrate to the transfer station. The carousel motor may orbit carrier head systems  70 , and the substrates attached thereto, about carousel axis  64  between the polishing stations and the transfer station. 
     Each carrier head system  70  includes a polishing or carrier head  100 . Each carrier head  100  independently rotates about its own axis, and independently laterally oscillates in a radial slot  72  formed in carousel support plate  66 . A carrier drive shaft  74  extends through slot  72  to connect a carrier head rotation motor  76  (shown by the removal of one-quarter of a carousel cover  68 ) to carrier head  100 . There is one carrier drive shaft and motor for each head. Each motor and drive shaft may be supported on a slider (not shown) which can be linearly driven along the slot by a radial drive motor to laterally oscillate the carrier head. 
     During actual polishing, three of the carrier heads, are positioned at and above the three polishing stations. Each carrier head  100  lowers a substrate into contact with a polishing pad  32 . Generally, carrier head  100  holds the substrate in position against the polishing pad and distributes a force across the back surface of the substrate. The carrier head also transfers torque from the drive shaft to the substrate. 
     Referring to FIGS. 2, carrier head  100  includes a housing  102 , a rod-backing membrane  104  secured to the housing, and an array or bundle  106  of independently vertically-movable rods  108  attached to the underside of the membrane. 
     Housing  102  can be connected to drive shaft  74  to rotate therewith during polishing about an axis of rotation which is substantially perpendicular to the surface of the polishing pad during polishing. Housing  102  may be generally circular in shape to correspond to the circular configuration of the substrate to be polished. A vertical passage  112  may be formed through the housing to provide pneumatic control of the carrier head. Unillustrated O-rings may be used to form a fluid-tight seal between the passage through the housing and a corresponding passage through the drive shaft. Fluid coupling between the drive shaft and carrier head is discussed in pending U.S. application Ser. No. 08/861,260, filed May 21, assigned to the assignee of the present application, the entire disclosure of which is incorporated herein by references. 
     Membrane  104  is a generally circular sheet formed of a flexible and elastic material, such as silicone. An edge  114  of membrane  104  can be secured to housing  102  to form a fluid-tight seal, e.g., by an unillustrated clamp, adhesive, or the like. The sealed volume between membrane  104  and housing  102  defines a loading chamber  110 . Loading chamber  110  can be pressurized to apply a load, i.e., a downward pressure, to membrane  104  and thus to rods  108 . A pump (not shown) may be fluidly connected to loading chamber  110  by passage  112  to control the pressure in the loading chamber and, thus, the load applied to the rods. 
     The rods  108  are attached to membrane  104 , e.g., by an adhesive or mechanical fasteners, to form bundle  106 . Specifically, the rods are arranged with their longitudinal axes generally parallel to each other and perpendicular to the plane of the polishing pad. The rods in bundle  106  are sufficiently densely packed that small gaps between individual rods do not affect the polishing uniformity, yet sufficiently loosely packed that the rods can slide vertically relative to each other. Furthermore, membrane  104  is sufficiently flexible that each rod can move vertically independently by at least the substrate thickness (about 27 mils for an “eight-inch” substrate). In short, the underside of bundle  106  formed by the bottom surfaces of the individual rods provides a collection of individually vertically adjustable surfaces. 
     Referring to FIG. 3A, rods  108  may be elongated circular shafts formed of a low-friction material, such as Delrin™, available from DuPont of Newark, Del., or polyphenylene sulfide (PPS). Each rod has a top surface  116  that is adjacent the membrane, a bottom surface  118 , and a side surface  119  that slides against the corresponding side surface of adjacent rods. As illustrated, rods  108  can have a circular cross-section, a longitudinal dimension L of about 0.06 to 0.5 inches, and a cross-sectional dimension D of about 0.03 to 0.25 inches. The longitudinal dimension of the rod should be about twice its cross-sectional dimension. Of course, the rods can have other cross-sectional shapes. For instance, they may be hexagonal (see FIG. 3B) or square. 
     Referring to FIGS. 4 and 5, rod bundle  106  provides the functions of both a retaining ring and a substrate backing member. During polishing, substrate  10  is placed on polishing pad  32  beneath carrier head  100 . Fluid is pumped into chamber  110  via passage  112  to force flexible membrane  118  and rods  108  downwardly. The rods  108   a  positioned above substrate  10  (which are obscured by the substrate in the view of FIG. 5) press against the backside of the substrate. However, the rods  108   b  positioned outside the region directly above the substrate are forced into contact the polishing pad and surround the substrate. During polishing, frictional forces from the polishing pad will force the substrate against the sides of the “innermost” rods  108   b , i.e., the rods adjacent the substrate. Thus, the rod bundle both applies pressure and retains the substrate beneath the carrier head. The closer the “fit” between the rods and the substrate, the less room the polishing pad has to decompress, thereby providing improved polishing uniformity at the substrate edge. 
     As explained below, the cross-sectional shape and dimensions of the rods are selected to provide a small gap with the substrate while ensuring that the rods can slide relative to each other. It should be noted that the greater the frictional forces between the rods, the more likely it is that the rods will “stick” rather than slide. Three main factors contribute to these frictional forces and the fit of the rods to the substrate: the spacing between the rods, the cross-sectional dimension (D) of the rods, and the contact area between the side surfaces of adjacent rods. 
     With respect to the spacing between adjacent rods, which may be about 0.0005 to 0.005 inches, closely packed rods will provide smaller substrate gap and more uniform pressure profile, but exhibit a higher coefficient of friction. Conversely, loosely packed rods will exhibit a lower coefficient of friction, but will provide a wider substrate gap and a more nonuniform pressure profile. 
     With respect to the cross-sectional dimension (D) of the rods, decreasing this cross-sectional dimension will increase the rod density, thereby improving the substrate fit. However, since the surface area of the a rod&#39;s side surface scales linearly to D, whereas the surface area of a rod&#39;s top surface scales to the square of D, decreasing the cross-sectional dimension will increase the frictional forces relative to the pressure on the rod. Conversely, increasing the cross-sectional dimension will result in a worse fit to the substrate, but will decrease the frictional forces. 
     The contact area between the side surfaces of the rods also depends on their cross-sectional shape. For example, circular rods will contact each other only along a relatively narrow strip, whereas hexagonal or square rods will contact each other across the entire face of the rod. Using a cross-sectional shape that provides a larger contact area (e.g., by using a hexagonal rod instead of a circular rod) will improve the substrate fit, but will also increase the frictional forces. Conversely, decreasing the contact area of will result in a worse substrate fit, but will decrease the frictional forces. Circular rods may be used in a densely packed bundle to reduce the frictional forces, whereas hexagonal rods may be used in a loosely packed bundle to improve the substrate fit. 
     The rods that surround the substrate will be pressed into contact the polishing pad to form a retainer. Thus, the carrier head is self-fitting to substrates having different diameters and different geometries (e.g., flatted or notched wafers). Since the rods are self-fitting, it should be possible to significantly reduce the gap between the substrate and retainer edge as compared to a conventional retaining ring (shown by solid line A in FIG.  5 ). Furthermore, the carrier head has a large tolerance for misalignment of the substrate. When the substrate is loaded at the transfer station or at a polishing station, the rods will adjust to surround the substrate, regardless of its horizontal position. In addition, the pressure on the top surface of the rods will cause them to move downwardly as their bottom surfaces are worn away. Thus, the rod bundle provides a retainer that is less subject to uneven wear patterns. 
     Referring to FIG. 6, in another embodiment, carrier head  100 ′ does not include a flexible membrane. Instead, pressure is applied directly to the top surfaces of rods  108 ′. When the carrier head is lifted away from the polishing pad, vacuum is applied to chamber  110 ′ to hold the bundle in the carrier head. In this implementation, the vacuum source needs a sufficiently high flow rate to compensate for pressure leaks between the rods. 
     Referring to FIG. 7, in another embodiment, carrier head  100 ″ includes both a flexible membrane  120  that contacts a back surface of the substrate, and a bundle  106 ″ of rods  108 ″. Specifically, rods  108 ″ may be positioned in an annular region around membrane  120 . A lower surface  122  of membrane  120  provides a mounting surface to apply pressure to a central region of the substrate. Rods  108 ″ function as the retainer and apply pressure to a perimeter region of the substrate. The volume between rods  108 ″ and housing  102 ″ defines an annular first pressurizable chamber  110 ″, and a first pump (not shown) may be fluidly connected to chamber  110 ″ by passage  112 ″ to control the pressure in the chamber and thus the downward force on rods  108 ″. The sealed volume between flexible membrane  120  and housing  102 ″ defines a second pressurizable chamber  124 . A second pump (not shown) may be fluidly connected to chamber  124  by a passage  126  in housing  102 ″ to control the pressure in chamber  124  and thus the downward force of flexible membrane  120  on the substrate. In addition, chamber  124  may be evacuated to pull flexible membrane  120  upwardly and thereby vacuum-chuck the substrate to the carrier head. 
     The present invention has been described in terms of a number of embodiments. The invention, however, is not limited to the embodiments depicted and described. Rather, the scope of the invention is defined by the appended claims.