Patent Publication Number: US-11660721-B2

Title: Dual loading retaining ring

Description:
BACKGROUND 
     Field 
     Embodiments of the present disclosure generally relate to an apparatus and method for polishing and/or planarization of substrates. More particularly, embodiments of the disclosure relate to polishing heads utilized for chemical mechanical polishing (CMP). 
     Description of the Related Art 
     Chemical mechanical polishing (CMP) is commonly used in the manufacturing of semiconductor devices to planarize or polish a layer of material deposited on a crystalline silicon (Si) substrate surface. In a typical CMP process, the substrate is retained in a substrate carrier, e.g., polishing head, which presses the back side of the substrate towards a rotating polishing pad in the presence of a polishing fluid. Generally, the polishing fluid comprises an aqueous solution of one or more chemical constituents and nanoscale abrasive particles suspended in the aqueous solution. Material is removed across the material layer surface of the substrate in contact with the polishing pad through a combination of chemical and mechanical activity which is provided by the polishing fluid and the relative motion of the substrate and the polishing pad. 
     The substrate carrier includes a membrane having a plurality of different radial zones that contact the substrate. Using the different radial zones, pressure applied to a chamber bounded by the backside of the membrane may be selected to control the center to edge profile of force applied by the membrane to the substrate, and consequently, to control the center to edge profile of force applied by the substrate against the polishing pad. The polishing head also includes a retaining ring surrounding the membrane. The retaining ring has a bottom surface for contacting the polishing pad during polishing and a top surface which is secured to the polishing head. Pre-compression of the polishing pad under the bottom surface of the retaining ring reduces a pressure spike at the perimeter portion of the substrate by moving an increased pressure region from underneath the substrate to underneath the retaining ring. Thus, the retaining ring can improve the resulting finish and flatness of the substrate surface. 
     Even with the different radial zones and use of the retaining ring, a persistent problem with CMP is the occurrence of an edge effect, i.e., the over- or under-polishing of the outermost 5-10 mm of a substrate, which can result from a knife edge effect, where a leading edge of the substrate is scraped along a top surface of the polishing pad. In certain other instances, conventional CMP processes can suffer from undesirably high polishing rates at the edge of the substrate caused by rebound of the polishing pad. 
     Accordingly, what is needed in the art are apparatus and methods for solving the problems described above. 
     SUMMARY 
     Embodiments of the present disclosure generally relate to an apparatus and method for polishing and/or planarization of substrates. More particularly, embodiments of the disclosure relate to polishing heads utilized for chemical mechanical polishing (CMP). 
     In one embodiment, a substrate carrier is configured to be attached to a polishing system for polishing a substrate. The substrate carrier includes a housing including a plurality of load couplings and a retaining ring coupled to the housing. The retaining ring includes an annular body having a central axis and an inner edge facing the central axis of the annular body. The inner edge has a diameter configured to surround a substrate. The retaining ring includes an outer edge opposite the inner edge. The plurality of load couplings contact the retaining ring at different radial distances measured from the central axis, and the plurality of load couplings are configured to apply a radially differential force to the retaining ring. 
     In another embodiment, a method for polishing a substrate disposed in a substrate carrier includes moving the substrate carrier relative to a polishing pad. A retaining ring of the substrate carrier contacts the polishing pad during the process of moving the substrate carrier. The method includes, during the process of moving the substrate carrier, applying a radially differential force to the retaining ring using a plurality of radially spaced load couplings. 
     In yet another embodiment, a polishing system includes a polishing pad and a substrate carrier configured to press a substrate against the polishing pad. The substrate carrier includes a housing including a plurality of load couplings and a retaining ring coupled to the housing. The retaining ring includes an annular body having a central axis and an inner edge facing the central axis of the annular body. The inner edge has a diameter configured to surround a substrate. The retaining ring includes an outer edge opposite the inner edge. The plurality of load couplings contact the retaining ring at different radial distances measured from the central axis, and the plurality of load couplings are configured to apply a radially differential force to the retaining ring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, may admit to other equally effective embodiments. 
         FIG.  1 A  is a schematic side view of an exemplary polishing station which may be used to practice the methods set forth herein, according to one or more embodiments. 
         FIG.  1 B  is a schematic plan view of a portion of a multi-station polishing system which may be used to practice the methods set forth herein, according to one or more embodiments. 
         FIG.  2 A  is a schematic side cross-sectional view of an exemplary substrate carrier that may be used in the polishing system of  FIG.  1 B . 
         FIG.  2 B  is an enlarged schematic side cross-sectional view of a portion of the substrate carrier of  FIG.  2 A . 
         FIGS.  2 C- 2 E  are schematic top views illustrating different embodiments of the substrate carrier of  FIG.  2 A . 
         FIG.  3 A  is a schematic side cross-sectional view of another exemplary substrate carrier that may be used in the polishing system of  FIG.  1 B . 
         FIG.  3 B  is an enlarged schematic side cross-sectional view of a portion of  FIG.  3 A . 
         FIG.  3 C  is a schematic top view of the substrate carrier of  FIG.  3 A . 
         FIG.  4 A  is a schematic side cross-sectional view of an exemplary retaining ring that may be used with any one of the substrate carriers disclosed herein, according to one or more embodiments. 
         FIG.  4 B  is an enlarged schematic side cross-sectional view of a portion of  FIG.  4 A . 
         FIG.  5 A  is an enlarged schematic side cross-sectional view of another exemplary retaining ring that may be used with any one of the substrate carriers disclosed herein, according to one or more embodiments. 
         FIGS.  5 B- 5 C  are diagrams illustrating downforce/deflection as a function of radial distance from an inner edge to an outer edge of the retaining ring of  FIG.  5 A . 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     Before describing several exemplary embodiments of the apparatus and methods, it is to be understood that the disclosure is not limited to the details of construction or process steps set forth in the following description. It is envisioned that some embodiments of the present disclosure may be combined with other embodiments. 
     Conventional chemical mechanical polishing (CMP) processes can suffer from undesirably high polishing rates at the edge of the substrate caused by rebound of the polishing pad at the edge of the substrate. However, in one or more embodiments of the present disclosure, a downforce of the retaining ring on the polishing pad can be radially controlled. Radial control of the downforce can mitigate the pad rebound effect thereby improving substrate edge uniformity and profile. 
       FIG.  1 A  is a schematic side view of a polishing station  100   a , according to one or more embodiments, which may be used to practice the methods set forth herein.  FIG.  1 B  is a schematic plan view of a portion of a multi-station polishing system  101  comprising a plurality of polishing stations  100   a - c , where each of the polishing stations  100   b - c  are substantially similar to the polishing station  100   a  described in  FIG.  1 A . In  FIG.  1 B  at least some of the components with respect to the polishing station  100   a  described in  FIG.  1 A  are not shown on the plurality of polishing stations  100   a - c  in order to reduce visual clutter. Polishing systems that may be adapted to benefit from the present disclosure include REFLEXION® LK and REFLEXION® LK PRIME Planarizing Systems, available from Applied Materials, Inc. of Santa Clara, Calif., among others. 
     As shown in  FIG.  1 A , the polishing station  100   a  includes a platen  102 , a first actuator  104  coupled to the platen  102 , a polishing pad  106  disposed on the platen  102  and secured thereto, a fluid delivery arm  108  disposed over the polishing pad  106 , a substrate carrier  110  (shown in cross-section), and a pad conditioner assembly  112 . Here, the substrate carrier  110  is suspended from a carriage arm  113  of a carriage assembly  114  ( FIG.  1 B ) so that the substrate carrier  110  is disposed over the polishing pad  106  and faces there towards. The carriage assembly  114  is rotatable about a carriage axis C to move the substrate carrier  110 , and thus a substrate  122  chucked therein, between a substrate carrier loading station  103  ( FIG.  1 B ) and/or between polishing stations  100   a - c  of the multi-station polishing system  101 . The substrate carrier loading station  103  includes a load cup  150  (shown in phantom) for loading a substrate  122  to the substrate carrier  110 . 
     During substrate polishing, the first actuator  104  is used to rotate the platen  102  about a platen axis A and the substrate carrier  110  is disposed above the platen  102  and faces there towards. The substrate carrier  110  is used to urge a to-be-polished surface of a substrate  122  (shown in phantom), disposed therein, against the polishing surface of the polishing pad  106  while simultaneously rotating about a carrier axis B. Here, the substrate carrier  110  includes a housing  111 , an annular retaining ring  115  coupled to the housing  111 , and a membrane  117  spanning the inner diameter of the retaining ring  115 . The retaining ring  115  surrounds the substrate  122  and prevents the substrate  122  from slipping from the substrate carrier  110  during polishing. The membrane  117  is used to apply a downward force to the substrate  122  and for loading (chucking) the substrate into the substrate carrier  110  during substrate loading operations and/or between substrate polishing stations. For example, during polishing, a pressurized gas is provided to a carrier chamber  119  to exert a downward force on the membrane  117  and thus a downward force on the substrate  122  in contact therewith. Before and after polishing, a vacuum may be applied to the chamber  119  so that the membrane  117  is deflected upwards to create a low pressure pocket between the membrane  117  and the substrate  122 , thus vacuum-chucking the substrate  122  into the substrate carrier  110 . 
     The substrate  122  is urged against the pad  106  in the presence of a polishing fluid provided by the fluid delivery arm  108 . The rotating substrate carrier  110  oscillates between an inner radius and an outer radius of the platen  102  to, in part, reduce uneven wear of the surface of the polishing pad  106 . Here, the substrate carrier  110  is rotated using a first actuator  124  and is oscillated using a second actuator  126 . 
     Here, the pad conditioner assembly  112  comprises a fixed abrasive conditioning disk  120 , e.g., a diamond impregnated disk, which may be urged against the polishing pad  106  to rejuvenate the surface thereof and/or to remove polishing byproducts or other debris therefrom. In other embodiments, the pad conditioner assembly  112  may comprise a brush (not shown). 
     Here, operation of the multi-station polishing system  101  and/or the individual polishing stations  100   a - c  thereof is facilitated by a system controller  136  ( FIG.  1 A ). The system controller  136  includes a programmable central processing unit (CPU  140 ) which is operable with a memory  142  (e.g., non-volatile memory) and support circuits  144 . The support circuits  144  are conventionally coupled to the CPU  140  and comprise cache, clock circuits, input/output subsystems, power supplies, and the like, and combinations thereof coupled to the various components of the polishing system  101 , to facilitate control of a substrate polishing process. For example, in some embodiments the CPU  140  is one of any form of general purpose computer processor used in an industrial setting, such as a programmable logic controller (PLC), for controlling various polishing system component and sub-processors. The memory  142 , coupled to the CPU  140 , is non-transitory including one or more of readily available memory such as random access memory (RAM), read only memory (ROM), floppy disk drive, hard disk, or any other form of digital storage, local or remote. 
     Herein, the memory  142  is in the form of a computer-readable storage media containing instructions (e.g., non-volatile memory), that when executed by the CPU  140 , facilitates the operation of the polishing system  101 . The instructions in the memory  142  are in the form of a program product such as a program that implements the methods of the present disclosure (e.g., middleware application, equipment software application etc.). The program code may conform to any one of a number of different programming languages. In one example, the disclosure may be implemented as a program product stored on computer-readable storage media for use with a computer system. The program(s) of the program product define functions of the embodiments (including the methods described herein). 
     Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, flash memory, ROM chips or any type of solid-state non-volatile semiconductor memory) on which information is permanently stored; and (ii) writable storage media (e.g., floppy disks within a diskette drive or hard-disk drive or any type of solid-state random-access semiconductor memory) on which alterable information is stored. Such computer-readable storage media, when carrying computer-readable instructions that direct the functions of the methods described herein, are embodiments of the present disclosure. 
       FIG.  2 A  is a schematic side cross-sectional view of an exemplary substrate carrier  200  that may be used in the polishing system  101  of  FIG.  1 B .  FIG.  2 B  is an enlarged schematic side cross-sectional view of a portion of  FIG.  2 A  illustrating a plurality of load couplings in more detail. The substrate carrier  200  is similar to the substrate carrier  110  of  FIG.  1 A , except where noted, and corresponding description may be incorporated herein without limitation. The retaining ring  115  is coupled to the housing  111 . In operation, the retaining ring  115  is contacting the polishing pad  106  to retain the substrate  122  in the substrate carrier  110  and to apply pre-compression to the polishing pad  106 . In one or more illustrated embodiments, the retaining ring  115  has an integral molded construction formed from a plastic, e.g., polyurethane (PU), polyethylene terephthalate (PET), polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), other similar materials, or combinations thereof. In some other embodiments (not shown), a lower portion of the retaining ring  115  proximate the polishing pad  106  is formed from a plastic, whereas an upper portion of the retaining ring  115  proximate the housing  111  is formed from a relatively rigid material such as a metal, e.g., stainless steel or anodized aluminum, a ceramic, a plastic, e.g., polyphenylene sulfide (PPS) or polyethylene terephthalate (PET), other similar materials, or combinations thereof. In such embodiments, the retaining ring  115  has a bonded construction. 
     An annular body of the retaining ring  115  includes an inner annular portion  128  and an outer annular portion  130  surrounding the inner annular portion  128 . The inner annular portion  128  has an inner edge  132  facing a central axis  118  of the annular body. The outer annular portion  130  has an outer edge  134  facing opposite the inner edge  132 . The inner and outer annular portions  128 ,  130  are concentric to one another. The inner and outer annular portions  128 ,  130  are defined by a line  116  positioned radially between the inner and outer edges  132 ,  134  of the retaining ring  115 . Here, the line  116  is a centerline equally spaced between the inner and outer edges  132 ,  134  so that the inner and outer annular portions  128 ,  130  have equal width in the radial direction. In some other embodiments, the line  116  is unequally spaced between the inner and outer edges  132 ,  134  so that the inner and outer annular portions  128 ,  130  have different widths in the radial direction. The line  116  is aligned along the z-axis, e.g., vertically aligned in the direction of gravity. A bottom edge  135  of the retaining ring  115  faces the polishing pad  106  and extends between the inner and outer edges  132 ,  134 . The bottom edge  135  is orthogonal relative to the z-axis, e.g., horizontally aligned orthogonal to the direction of gravity, being substantially parallel to a top surface  107  of the polishing pad  106 . In one or more embodiments, the bottom edge  135  includes a plurality of radial grooves (not shown) for facilitating transport of polishing slurry. 
     Here, the inner and outer annular portions  128 ,  130  are integrally formed. In one or more embodiments where the inner and outer annular portions  128 ,  130  are integrally formed, a force applied to one of the inner or outer annular portions  128 ,  130  is at least partially distributed across both portions  128 ,  130 . In some embodiments (not shown), the inner and outer annular portions  128 ,  130  are formed separately. In such embodiments, forces applied to respective ones of the inner and outer annular portions  128 ,  130  are isolated thereto. In some embodiments (not shown), the inner and outer annular portions  128 ,  130  are independently movable with respect to each other. In one or more embodiments, a force applied to one of the inner or outer annular portions  128 ,  130  is operable to generate a torsional moment in the retaining ring  115 . 
     In some embodiments, a differential force is applied via the housing  111  of the substrate carrier  200  to the inner and outer annular portions  128 ,  130  such that the retaining ring  115  applies a corresponding differential force to the top surface  107  of polishing pad  106  in contact therewith. In some embodiments, the corresponding differential force is proportional to the differential force applied to the inner and outer annular portions  128 ,  130 . In some embodiments, the differential force applied to the inner and outer annular portions  128 ,  130  generating a torsional moment in the annular body of the retaining ring  115  so that the bottom edge  135  is not perpendicular to the Z-axis, or tilted, relative to the x-y plane. In one or more embodiments, the tilt may be linear or curved. In one or more embodiments, the torsional moment and applied differential force depend on the torsional stiffness of the retaining ring  115 . In one or more embodiments, the torsional stiffness, or torsional constant, of the retaining ring  115  may be from about 1,000 N-m/rad to about 150,000 N-m/rad. In some embodiments, a maximum deflection along the z-axis is about 1 mil or less, such as about 0.1 mil or less, alternatively from about 0.1 mil to about 1 mil, such as from about 0.1 mil to about 0.5 mil. In some embodiments, the tilt angle of the bottom edge  135  relative to the x-y plane is about 1° or less, such as about 0.1° or less. In such embodiments, an interface between the bottom edge  135  of the retaining ring  115  and the top surface  107  of the polishing pad  106  has a tilt corresponding to the torsional moment of the annular body. The torsional moment and resulting tilt of the bottom edge  135  results in a differential force being applied to the polishing pad  106 . 
     In the embodiments of  FIGS.  2 A- 2 B , a plurality of load couplings, e.g., inner and outer load couplings  210 ,  212 , are disposed in the housing  111 . The inner and outer load couplings  210 ,  212  are positioned at different radial distances from the inner edge  132  of the retaining ring  115 . Here, the outer load coupling  212  surrounds the inner load coupling  210 . The inner and outer of load couplings  210 ,  212  are spaced radially from each other. In some other embodiments (not shown), the plurality of load couplings are spaced circumferentially from each other or spaced from each other in the Z direction, e.g., stacked. In some embodiments, the plurality of load couplings includes a number of load couplings equal to the number of forces being independently applied to the retaining ring  115 . In some embodiments, the plurality of load couplings includes from two to five load couplings, such as three, four, or five load couplings. 
     Here, the inner load coupling  210  includes a bladder  214  coupled to the inner annular portion  128  of the retaining ring  115 . The bladder  214  is disposed above the inner annular portion  128 . Likewise, the outer load coupling  212  includes a bladder  216  coupled to the outer annular portion  130  of the retaining ring  115 . The bladder  216  is disposed above the outer annular portion  130 . In some embodiments, each bladder  214 ,  216  extends continuously around the housing  111 . In one or more embodiments, a pressure area of each bladder  214 ,  216  may be from about 20 in 2  to 30 in 2 , such as about 26 in 2 . In one or more embodiments, a pressure range of each bladder  214 ,  216  may be from about 1 psi to about 6 psi. In one or more optional embodiments, each bladder  214 ,  216  is coupled to a respective one of the inner and outer annular portions  128 ,  130  by a respective fastener  218 ,  220 . Here, each bladder  214 ,  216  is independently coupled to a respective pneumatic line  222 ,  224 , where each pneumatic line  222 ,  224  is fluidly coupled to an upper pneumatic assembly (UPA) (not shown). The UPA is fluidly coupled to a pneumatic pressure source (not shown), e.g. a tank or pump for supplying a suitable gas such as air or N 2  to each of the bladders  214 ,  216 . In one or more embodiments, the UPA is operable to supply up to 12 psi. In one or more embodiments, a pneumatic rotary feedthrough (not shown) fluidly couples the pneumatic lines  222 ,  224  between the polishing system  101  and the rotatable housing  111 . 
     In some other embodiments (not shown), each bladder  214 ,  216  includes a plurality of arc-shaped segments each extending partially around the housing  111  (e.g., by about 30°). In such embodiments, loading of the retaining ring  115  may be biased towards a particular annular region of the retaining ring  115 . For example, it may be desirable to apply a first radially differential force on the leading edge and a second radially differential force on the trailing edge of the retaining ring  115  as the polishing pad  106  and platen  102  rotate underneath the substrate carrier  200 . In such embodiments, it may be desirable to utilize a plurality of linear actuators (e.g., solenoids, PZT devices, etc.) that are positioned to apply a force to the retaining ring  115  in a z-direction because pneumatic control may not be actuatable at a rate matching the rotation rate of the substrate carrier  200 . 
     In practice, supplying pneumatic pressure to a respective one of the bladders  214 ,  216  increases a pressure therein. As a result of increasing the pressure in a respective one of the bladders  214 ,  216 , a corresponding increasing force is applied to a respective one of the inner and outer annular portions  128 ,  130  of the retaining ring  115  either directly or indirectly, e.g., through an optional respective fastener  218 ,  220 . In some embodiments, a force applied to each of the inner and outer annular portions  128 ,  130 , which corresponds to the pressure in the bladder multiplied by the pressure area of the bladder, may be from about 20 lbf to about 180 lbf. 
     In one or more embodiments, the inner load coupling  210  is operable to apply a first downforce  202  to the inner annular portion  128 . Likewise, the outer load coupling  212  is operable to apply a second downforce  204  to the outer annular portion  130 . In one or more embodiments, the loading axes of the first and second downforces  202 ,  204  may be spaced in the radial direction by from about 0.5 inches to about 1 inch. In one or more embodiments, it may be desirable to maximize or increase the spacing between the loading axes in order to impart a maximum or increasing torsion moment, respectively, on the retaining ring  115  under the same load. In some embodiments, the first downforce  202  applied to the inner annular portion  128  is greater than the second downforce  204  applied to the outer annular portion  130 . In some embodiments, the second downforce  204  is zero. In embodiments where the first downforce  202  is greater, the retaining ring  115  shifts its orientation so that the inner annular portion  128  is tilted toward the top surface  107  of the polishing pad  106  by a greater degree than the outer annular portion  130  (i.e., a positive taper). In embodiments where the first downforce  202  is greater, the corresponding force applied to the polishing pad  106  by the inner annular portion  128  is greater than the corresponding force applied to the polishing pad  106  by the outer annular portion  130 . As a result, greater deflection of the polishing pad  106  occurs under the inner annular portion  128 . In other words, greater deflection of the polishing pad  106  occurs at the inner edge  132  of the retaining ring  115 , i.e., adjacent to an outer edge of the substrate  122 , relative to the outer edge  134  of the retaining ring  115  due to the forces applied by the bladders or actuators, which creates a torsional moment. 
     In some other embodiments, the second downforce  204  applied to the outer annular portion  130  is greater than the first downforce  202  applied to the inner annular portion  128 . In some embodiments, the first downforce  202  is zero. In embodiments where the second downforce  204  is greater, the retaining ring  115  shifts its orientation so that the outer annular portion  130  is tilted toward the top surface  107  of the polishing pad  106  by a greater degree than the inner annular portion  128  (i.e., a negative taper). In embodiments where the second downforce  204  is greater, the corresponding force applied to the polishing pad  106  by the outer annular portion  130  is greater than the corresponding force applied to the polishing pad  106  by the inner annular portion  128 . As a result, greater deflection of the polishing pad  106  occurs under the outer annular portion  130 . In other words, greater deflection of the polishing pad  106  occurs at the outer edge  134  of the retaining ring  115  relative to the inner edge  132  of the retaining ring  115  due to the forces applied by the bladders or actuators, which creates a torsional moment. 
     Beneficially, the substrate carrier  110  can control deflection of the polishing pad  106  along a radial direction through modulation of the first and second downforces  202 ,  204 . In some embodiments, one or more additional downforces are independently applied to the retaining ring  115 , such as from two to five total independently applied downforces at different radial distances, such as three, four, or five independently applied downforces. Beneficially, the substrate carrier  110  can improve substrate non-uniformity without replacement or redesign of the retaining ring  115 . In some embodiments, a pre-load force is applied to the retaining ring  115  in addition to the first and second downforces  202 ,  204  described herein. 
       FIGS.  2 C- 2 E  are schematic top views illustrating different embodiments of the substrate carrier  200  of  FIG.  2 A . In  FIGS.  2 C- 2 E , certain parts of the housing  111  and certain other internal and external components of the substrate carrier  200  are omitted to more clearly show the positioning of the load couplings  210 ,  212  relative to the retaining ring  115 . Referring to  FIG.  2 C , each of the inner and outer load couplings  210 ,  212  extends continuously around the housing  111 . In such embodiments, each load coupling  210 ,  212  is independently coupled to a respective pneumatic line  222 ,  224 . In such embodiments, the radially differential force and torsional moment of the retaining ring  115  are substantially uniform about the circumference thereof. 
     Referring to  FIG.  2 D , each of the inner and outer load couplings  210 ,  212  includes a plurality of arc-shaped segments (e.g., two arc-shaped segments) each extending partially around the housing  111  (e.g., by about 180°). In the illustrated embodiments, the inner load coupling  210  includes arc-shaped segments  210   a ,  210   b . Likewise, the outer load coupling  212  includes arc-shaped segments  212   a ,  212   b . In some embodiments, each of the plurality of load couplings  210 ,  212  includes from one to twelve arc-shaped segments, such as one, two, three, four, five, six, seven, eight, nine, ten, eleven, or twelve arc-shaped segments. Here, each of the segments in the plurality of load couplings  210 ,  212  are equally sized, e.g., having the same arc length. In some other embodiments, the segments have sizes different from each other. Here, the arc-shaped segments  210   a ,  210   b  of the inner load coupling  210  are fluidly coupled to the same pneumatic line  222  so that the pressure applied to each of the arc-shaped segments  210   a ,  210   b  is equal. In such embodiments, the radially differential force and torsional moment of the retaining ring  115  are substantially uniform about the circumference thereof. Likewise, the arc-shaped segments  212   a ,  212   b  of the outer load coupling  212  are fluidly coupled to the same pneumatic line  224  so that the pressure applied to each of the arc-shaped segments  212   a ,  212   b  is equal. Beneficially, the substrate carrier  200  can control deflection of the polishing pad  106  along a radial direction or within sectors of the retaining ring through modulation of the downforces applied by one or more of the arc-shaped segments  210   a ,  210   b ,  212   a , or  212   b.    
     In  FIG.  2 E , the arc-shaped segments  210   a ,  210   b  of the inner load coupling  210  are independently coupled to different pneumatic lines  222   a ,  222   b  so that the pressures applied to each of the arc-shaped segments  210   a ,  210   b  are independently controllable. In such embodiments, the pressures applied to each of the arc-shaped segments  210   a ,  210   b  can be the same or different. Likewise, the arc-shaped segments  212   a ,  212   b  of the outer load coupling  212  are independently coupled to different pneumatic lines  224   a ,  224   b  so that the pressures applied to each of the arc-shaped segments  212   a ,  212   b  are independently controllable. In such embodiments, the pressures applied to each of the arc-shaped segments  212   a ,  212   b  can be the same or different. In such embodiments, the radially differential force and torsional moment of the retaining ring  115  can be the same or different about the circumference thereof. Beneficially, having independent control of each of a plurality of arc-shaped segments provides more precise control of the differential force applied to the retaining ring  115 , and consequently, more precise control of the differential force applied by the retaining ring  115  to the polishing pad  106 . 
       FIG.  3 A  is a schematic side view of another exemplary substrate carrier  300  that may be used in the polishing system  101  of  FIG.  1 B .  FIG.  3 B  is an enlarged schematic side view of a portion of  FIG.  3 A  illustrating a plurality of load couplings in more detail. The substrate carrier  300  is similar to the substrate carrier  200  of  FIGS.  2 A- 2 B , except where noted, and corresponding description may be incorporated herein without limitation. Referring to  FIG.  3 B , the inner and outer load couplings  310  include a lower clamp  314  fixedly coupled to the housing  111  and an upper clamp  316  movably coupled to the housing  111 . The lower and upper clamps  314 ,  316  have a mating, relatively movable engagement therebetween. Here, the lower and upper clamps  314 ,  316  are vertically movable relative to each other. In some other embodiments, the lower and upper clamps  314 ,  316  have one or more additional degrees of relative motion. The lower clamp  314  includes a plurality of channels  318 , here a pair, to accommodate the vertical relative movement between the lower and upper clamps  314 ,  316 . The upper clamp  316  is further fixedly coupled to the retaining ring  115  and movable therewith. In some embodiments, the upper clamp  316  is fixedly coupled to the retaining ring  115  by one or more fasteners  320 . The upper clamp  316  further includes a push rod  322  extending above the lower clamp  314 . 
     In contrast to the substrate carrier  200  of  FIGS.  2 A- 2 B , the substrate carrier  300  includes a plurality of independent actuators, e.g., first and second actuators  306 ,  308 . The first actuator  306  is operably coupled to the push rod  322  of the inner load coupling  310  so that linear movement of the first actuator  306  applies a force to the push rod  322  which is transferred to the retaining ring  115 . In this way, the first downforce  302  is generated by the first actuator  306 . Likewise, the second actuator  308  is operably coupled to the push rod  322  of the outer load coupling  312  so that linear movement of the second actuator  308  applies a force to the push rod  322  which is transferred to the retaining ring  115 . In this way, the second downforce  304  is generated by the second actuator  308 . 
       FIG.  3 C  is a schematic top view of the substrate carrier  300  of  FIG.  3 A . In  FIG.  3 C , certain parts of the housing  111  and certain other internal and external components of the substrate carrier  300  are omitted to more clearly show the positioning of the actuators  306 ,  308  and the load couplings  310 ,  312  relative to the retaining ring  115 . Here, each of the first and second actuators  306 ,  308  are in circumferential alignment. As illustrated, the plurality of actuators  306 ,  308  are disposed in a ring around the housing  111 . In such embodiments, the plurality of actuators  306 ,  308  are operable to apply a radially differential force to the retaining ring  115  which is substantially uniform about the circumference of the each of the inner and outer annular portions  128 ,  130  thereof. In one or more embodiments, the plurality of actuators  306 ,  308  are independently actuatable. In such embodiments, the plurality of actuators  306 ,  308  may be operable to apply differential forces in both the radial and circumferential directions. Here, each of the inner and outer load couplings  310 ,  312  extend continuously around the housing  111 . In some other embodiments (not shown), each of the inner and outer load couplings  310 ,  312  include a plurality of arc-shaped segments aligned with each one of the plurality of actuators  306 ,  308 . In such embodiments, pairing each of the plurality actuators  306 ,  308  with a respective arc-shaped segment provides precise control of the torsional moment generated in the retaining ring  115  within each of a plurality of distinct annular regions at any point in time. For example, it may be desirable to generate a first torsional moment on the leading edge and a second torsional moment on the trailing edge of the retaining ring  115  as the polishing pad  106  and platen  102  rotate underneath the substrate carrier  300 . In some other embodiments (not shown), each of the inner and outer load couplings  310 ,  312  include a plurality of arc-shaped segments each extending partially around the housing  111  (e.g., by about 30°). 
     In some embodiments, as shown in  FIGS.  3 B- 3 C , the first and second actuators  306 ,  308  are disposed in the housing  111 . In some other embodiments (not shown), the first and second actuators  306 ,  308  are disposed outside the housing  111  such as being coupled to the carriage arm  113  or the carriage assembly  114  ( FIG.  1 B ). In some embodiments, the first and second actuators  306 ,  308  can be solenoids, pneumatic actuators, hydraulic actuators, piezo-electric actuators, voice coils, stepper motors, other linear actuators, other similar actuators, or combinations thereof. 
     In the embodiments illustrated in  FIGS.  2 A- 2 B and  3 A- 3 B , the plurality of load couplings are disposed in the housing  111 . In some other embodiments (not shown), the plurality of load couplings are disposed outside the housing  111  such as being coupled to the carriage arm  113  or the carriage assembly  114  ( FIG.  1 B ). In the embodiments illustrated in  FIGS.  2 A- 2 B and  3 A- 3 B , the plurality of load couplings are radially aligned, e.g., along the Z axis, with respective inner and outer annular portions  128 ,  130  of the retaining ring  115 . In some other embodiments (not shown), one or more of the plurality of load couplings are not in alignment with, e.g., being radially offset from, the inner and outer annular portions  128 ,  130 . In one or more embodiments described herein, the bottom edge  135  of the retaining ring  115  wears down during use due to contact with the polishing pad  106 . In some embodiments, the wear is measured using one or more in situ sensors. In such embodiments, the radially differential force applied to the retaining ring  115  and the resulting generated torsional moment are controlled based on the measured wear of the bottom edge  135 . In some other embodiments, wear of the bottom edge  135  is controlled based on a material of the retaining ring  115 . For example, in one or more embodiments, respective bottom edges  135  of each of the inner and outer annular portions  128 ,  130  may be formed from different materials having different hardnesses and/or wear resistance. In one or more embodiments, the materials may be selected to mitigate grooving the inner edge  132  of the retaining ring  115 , e.g., where the inner edge  132  and bottom edge  135  intersect. 
       FIG.  4 A  is a schematic side view of an exemplary retaining ring  415  that may be used with any one of the substrate carriers  110 ,  200 ,  300 ,  400  disclosed herein.  FIG.  4 B  is an enlarged schematic side view of a portion of  FIG.  4 A . The retaining ring  415  is shown in combination with an exemplary substrate carrier  400  for illustrative purposes only. The substrate carrier  400  is not particularly limited to the illustrated embodiments, and the retaining ring  415  may be combined with any one of the substrate carriers  200 ,  300  disclosed herein without limitation. Therefore, corresponding description of the substrate carriers  200 ,  300  may be incorporated herein without limitation. Referring to  FIGS.  4 A- 4 B , the retaining ring  415  has a circumferential groove  420 . The circumferential groove  420  is formed in the bottom edge  135  of the retaining ring  415 . In one or more embodiments, the circumferential groove  420  is a continuous annular groove around the retaining ring  415 . In some other embodiments, the circumferential groove  420  consists of a plurality of arc-shaped segments. In one example, the arc-shaped segments are separated by a radially oriented groove and have an arc length relative to a central axis of the retaining ring that is between about 5 degrees and about 175 degrees in swept length. Here, the circumferential groove  420  has a square profile in cross-section. For example, in one or more illustrated embodiments, the circumferential groove  420  has inner and outer edges  422 ,  424  which are substantially orthogonal to the bottom edge  135 . The circumferential groove  420  has a top edge  426  extending between the inner and outer edges  422 ,  424  where the top edge  426  is substantially parallel to the bottom edge  135 . A width of the circumferential groove  420  from the inner edge  422  to the outer edge  424 , that is in the radial direction, is from about 0.1 inches to about 0.5 inches. In some embodiments, a height of the circumferential groove  420  from the bottom edge  135  to the top edge  426  is from about 0.1 inches to about 0.5 inches. 
     In some other embodiments (not shown), the circumferential groove  420  may have a rectangular, rounded, or oval profile in cross-section. As shown in  FIG.  4 B , the circumferential groove  420  is symmetrically aligned with the line  116 . In this configuration, the circumferential groove  420  is equally spaced between the inner and outer edges  132 ,  134  of the retaining ring  115  so that bottom edges  135   a ,  135   b  of the inner and outer annular portions  128 ,  130 , respectively, have equal width in the radial direction. In some other embodiments, the circumferential groove  420  is unequally spaced between the inner and outer edges  132 ,  134  so that bottom edges  135   a ,  135   b  of the inner and outer annular portions  128 ,  130 , respectively, have different widths in the radial direction. The circumferential groove  420  can create the effect of two independent retaining rings within a single integral retaining ring, e.g., the retaining ring  415 , namely by forming separate bottom edges  135   a ,  135   b  for each of the inner and outer annular portions  128 ,  130 , respectively. The circumferential groove  420  increases the torsional moment of the retaining ring  415  under the same load and improves the capability of the retaining ring  415  to apply and control differential force to the polishing pad  106  along the radial direction. 
       FIG.  5 A  is an enlarged schematic side view of another exemplary retaining ring  115  that may be used with any one of the substrate carriers  110 ,  200 ,  300 ,  400  disclosed herein. Here, a first downforce  502   a  is applied to the inner annular portion  128 , and a second downforce  504   a  is applied to the outer annular portion  130 .  FIGS.  5 B- 5 C  are diagrams illustrating downforce/deflection as a function of radial distance from the inner edge  132  to the outer edge  134  of the retaining ring  115  of  FIG.  5 A . Each of the diagrams illustrated in  FIGS.  5 B- 5 C  is aligned radially with the schematic view of the retaining ring  115  of  FIG.  5 A . 
       FIG.  5 B  illustrates downforce and deflection of the retaining ring  115  and the polishing pad  106 , respectively, where a first downforce  502   b  is greater than a second downforce  504   b . The first and second downforces  502   b ,  504   b  are applied to the inner and outer annular portions  128 ,  130 , respectively, in the −Z direction, generating a torsional moment in the retaining ring  115 . The torsional moment of the retaining ring  115  applies a differential pressure to the polishing pad  106  by contact between the bottom edge  135  of the retaining ring  115  and the top surface  107  of the polishing pad  106 . A force  506   b  applied to the polishing pad  106  in the −Z direction increases from the outer edge  134  to the inner edge  132  of the retaining ring  115 . Here the force  506   b  varies linearly. In some other embodiments, the force  506   b  varies non-linearly. As a result of the force  506   b , a deflection  508   b  of the polishing pad  106  in the −Z direction increases from the outer edge  134  to the inner edge  132  of the retaining ring  115 . Here, the deflection  508   b  of the polishing pad  106  is directly, linearly proportional to the applied force  506   b . In some other embodiments, the applied force  506   b  and deflection  508   b  are non-linearly proportional to each other. 
       FIG.  5 C  illustrates downforce and deflection of the retaining ring  115  and the polishing pad  106 , respectively, where a first downforce  502   c  is less than a second downforce  504   c . The first and second downforces  502   c ,  504   c  are applied to the inner and outer annular portions  128 ,  130 , respectively, in the −Z direction, generating a torsional moment in the retaining ring  115 . The torsional moment of the retaining ring  115  applies a differential pressure to the polishing pad  106  by contact between the bottom edge  135  of the retaining ring  115  and the top surface  107  of the polishing pad  106 . A force  506   c  applied to the polishing pad  106  in the −Z direction increases from the outer edge  134  to the inner edge  132  of the retaining ring  115 . Here the force  506   c  varies linearly. In some other embodiments, the force  506   c  varies non-linearly. As a result of the force  506   c , a deflection  508   c  of the polishing pad  106  in the −Z direction increases from the outer edge  134  to the inner edge  132  of the retaining ring  115 . Here, the deflection  508   c  of the polishing pad  106  is directly, linearly proportional to the applied force  506   c . In some other embodiments, the applied force  506   c  and deflection  508   c  are non-linearly proportional to each other. 
     In one or more embodiments, the system controller  136  ( FIG.  1 A ) is operable to control a plurality of radially and/or circumferentially differential forces on a retaining ring. In one or more embodiments, the control can be based on a pre-determined polishing plan. In some embodiments, the system controller  136  is operable to independently monitor a plurality of applied forces and adjust the applied forces in real time. In some embodiments, the system controller  136  is operable to receive inputs from one or more sensors, e.g., optical sensors, to measure wafer thickness and/or wafer non-uniformity in situ. In some embodiments, sensors on or within the platen  102  sense the wafer thickness. In some embodiments, the system controller  136  is operable to output signals to control each of the plurality load couplings or actuators based on the in situ measurements. The system controller  136  is a general use computer that is used to control one or more components found in the processing system(s) disclosed herein. The system controller  136  is generally designed to facilitate the control and automation of one or more of the processing sequences disclosed herein and typically includes a central processing unit (CPU) (not shown), memory (not shown), and support circuits (or I/O) (not shown). Software instructions and data can be coded and stored within the memory (e.g., non-transitory computer readable medium) for instructing the CPU. A program (or computer instructions) readable by the processing unit within the system controller determines which tasks are performable in the processing system. For example, the non-transitory computer readable medium includes a program which when executed by the processing unit are configured to perform one or more of the methods described herein. Preferably, the program includes code to perform tasks relating to monitoring, execution and control of the movement, applied forces/loads, and/or other various process recipe variables and various CMP process recipe steps being performed. In summary, aspects of the present disclosure at least enable precise control of radially and/or circumferentially differential forces applied to a retaining ring, thereby enabling precise control of the compression of the polishing pad in contact therewith. As a result, embodiments of the present disclosure enable improved control over polishing pad deflection and resultant mitigation of substrate profile issues. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.