Abstract:
The present invention relates to an apparatus and method for polishing semiconductor substrates with improved throughput. Embodiments of the present invention eliminate load cups from a polishing system, hence improve throughput by reducing system footprint and substrate hand off. One embodiment provides an apparatus for polishing a substrate. The apparatus comprises a platen having a polishing pad supported thereon, a carrier head configured to hold the substrate and press the substrate against the polishing pad, and a retaining ring adapted to be attached to and detached from the carrier head, wherein the retaining ring is configured to receive the substrate while positioned on the polishing pad and detached from the carrier head.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention generally relate to an apparatus and method for polishing or planarization of semiconductor substrates. 
     2. Description of the Related Art 
     Sub-micron multi-level metallization is one of the key technologies for the next generation of ultra large-scale integration (ULSI). The multilevel interconnects that lie at the heart of this technology require planarization of interconnect features formed in high aspect ratio apertures, including contacts, vias, trenches and other features. Reliable formation of these interconnect features is very important to the success of ULSI and to the continued effort to increase circuit density and quality on individual substrates and die. 
     In the fabrication of integrated circuits and other electronic devices, multiple layers of conductive, semiconductive, and dielectric materials are deposited on or removed from a surface of a substrate. Thin layers of conductive, semiconductive, and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and electro-chemical plating (ECP). 
     As layers of materials are sequentially deposited and removed, the uppermost surface of the substrate may become non-planar across its surface and require planarization. An example of non-planar process is the deposition of copper films with the ECP process in which the copper topography simply follows the already existing non-planar topography of the wafer surface, especially for lines wider than 10 microns. Planarizing a surface, or “polishing” a surface, is a process where material is removed from the surface of the substrate to form a generally even, planar surface. Planarization is useful in removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches, and contaminated layers or materials. Planarization is also useful in forming features on a substrate by removing excess deposited material used to fill the features and to provide an even surface for subsequent levels of metallization and processing. 
     Planarization is generally performed using Chemical Mechanical Polishing (CMP) and/or Electro-Chemical Mechanical Deposition (ECMP). A planarization method typically requires that the substrate be mounted in a carrier head, with the surface of the substrate to be polished exposed. The substrate supported by the head is then placed against a rotating polishing pad. The head holding the substrate may also rotate, to provide additional motion between the substrate and the polishing pad surface. Further, a polishing composition is supplied to the pad to provide an chemical solution at the interface between the pad and the substrate. 
     Existing polishing tools generally include loading stations configured for robots and polishing heads to drop off and pick up substrates. There are several disadvantages for the loading stations. First, the loading stations increase overall footprint for the polishing tool, therefore, require extra space in a cleanroom which is expensive to maintain. Second, since the polishing heads are dependent on the loading stations, system flexibility is greatly weakened, especially in polishing systems with multiple polishing heads. Third, using loading stations increases substrate hand off which is a source of particle contamination. 
     Therefore, there is a need for an apparatus and method to eliminate loading stations. 
     SUMMARY OF THE INVENTION 
     The present invention provides methods and apparatus for polishing a semiconductor substrate. 
     One embodiment provides an apparatus for polishing a substrate. The apparatus comprises a platen having a polishing pad supported thereon, a carrier head configured to hold the substrate and press the substrate against the polishing pad, and a retaining ring adapted to be attached to and detached from the carrier head, wherein the retaining ring is configured to receive the substrate while positioned on the polishing pad and detached from the carrier head. 
     Another embodiment of the present invention provides a method for polishing a substrate. The method comprises positioning a retaining ring on a polishing pad, positioning the substrate into a recess defined by the retaining ring and the polishing pad, moving a carrier head to engage the retaining ring, loading the substrate on a substrate mounting surface of the carrier head, and polishing the substrate by rotating the substrate against the polishing pad using the carrier head. 
     Yet another embodiment of the present invention provides a method for loading a substrate on a carrier head. The method comprises positioning a retaining ring on a supporting surface, positioning the substrate into a recess defined by the retaining ring and the supporting surface, moving the carrier head to engage the retaining ring, and securing the substrate on a substrate mounting surface of the carrier head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, 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 typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1A  illustrates an polishing system in accordance with one embodiment of the present invention. 
         FIG. 1B  illustrates a partially enlarged view of the polishing system of  FIG. 1A . 
         FIG. 2  illustrates the polishing system of  FIG. 1A  having a substrate retained in a detachable retaining ring. 
         FIG. 3A  illustrates the polishing system of  FIG. 1A  in a polishing position. 
         FIG. 3B  is a partially enlarged view of  FIG. 3A . 
         FIG. 4  illustrates the polishing system of  FIG. 1A  in a substrate unloading position. 
         FIG. 5  illustrates a detachable retaining ring in accordance with one embodiment of the present invention. 
         FIG. 6A  illustrates a detachable retaining ring in accordance with one embodiment of the present invention. 
         FIG. 6B  illustrates a detachable retaining ring in accordance with one embodiment of the present invention. 
         FIG. 7  illustrates one embodiment of lifting a detachable retaining ring in accordance with the present invention. 
         FIG. 8A  illustrates a sectional view of a detachable retaining ring in accordance with one embodiment of the present invention. 
         FIG. 8B  illustrates a sectional view of a detachable retaining ring in accordance with one embodiment of the present invention. 
         FIG. 9  illustrates a bottom view of a detachable retaining ring and an outer retaining ring in accordance with one embodiment of the present invention. 
         FIG. 10A  illustrates a sectional view of a detachable retaining ring and an outer retaining ring in accordance with one embodiment of the present invention. 
         FIG. 10B  illustrates a bottom view of the detachable retaining ring and the outer retaining ring of  FIG. 10A . 
         FIG. 11  illustrates a polishing system having a detachable retaining ring dedicated to a carrier head. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides methods and apparatus for polishing semiconductor substrates. Polishing systems of the present invention generally comprise a detachable retaining ring which may be detached from a carrier head during loading and unloading of a substrate and attached to the carrier head during polishing. During polishing, the detachable retaining ring may rotate at the same speed as the substrate, retaining the substrate within its inner surface and keep the polishing pad near the edge of the substrate flat to achieve an uniform polishing. A robot may load a substrate into the detachable retaining ring when it is detached from the carrier head and secured on a supporting surface, for example, a polishing pad, therefore, eliminating the needs for a load cup. For a polishing system with multiple polishing stations and/or multiple polishing heads, multiple detachable retaining rings may be used and each of the multiple detachable retaining rings may be dedicated to a polishing station or a carrier head. 
       FIG. 1A  illustrates a perspective view of a polishing system  100  in accordance with one embodiment of the present invention. One polishing tool that may be adapted to benefit from the present invention is polishers available from Applied Materials, Inc. of Santa Clara, Calif. Other polishing tools that may be adapted to benefit from the invention include MIRRA®, MIRRA MESA®, REFLEXION®, REFLEXION® LK, and REFLEXION LK Ecmp™ Planarizing Systems, all available from Applied Materials, Inc. of Santa Clara, Calif. 
     The polishing system  100  generally comprises a carrier head  101 , a polishing station  105 , a detachable retaining ring  104 , and a ring holder  107 . In one embodiment, the polishing system  100  may be a stand alone polishing system. In another embodiment, the polishing system  100  belongs to a polishing system comprising multiple polishing stations and multiple carrier heads. For example, the polishing station  105  may be disposed on a system base having multiple platens and the carrier head  101  may be supported by a rotatable carousel having multiple carrier heads identical or similar to the carrier heads  101 . A detailed description of a polishing system may be found in U.S. Pat. No. 5,804,507, entitled “Radially Oscillating Carousel Processing System for Chemical Mechanical Polishing”, U.S. patent application Ser. No. 10/211,626, entitled “Contacts for Electrochemical Processing”, and U.S. patent application, entitled “Six Headed Carousel”, which are herein incorporated by reference. 
     The polishing station  105  generally comprises a rotatable platen  151  on which a polishing pad  152  is placed. The rotatable platen  151  and the polishing pad  152  are generally larger than a substrate  103  being processed. For example, if the substrate  103  is an eight inch (200 mm) diameter disk, the platen  151  and the polishing pad  152  are about 20 inches in diameter. If the substrate  103  is a twelve inch (300 mm) diameter disk, the platen  151  and the polishing pad  152  are about 30 inches in diameter. In one embodiment, the platen  151  is a rotatable aluminum or stainless steel plate connected by a stainless steel drive shaft  153  to a platen drive motor (not shown). For most polishing processes, the platen drive motor rotates the platen  151  at about thirty to two hundred revolutions per minute, although lower or higher rotational speeds may be used. 
     The polishing pad  152  has a roughened polishing surface  157  configured to polish the substrate  103  using a chemical mechanical polishing (CMP) method or an electrical chemical mechanical polishing (ECMP) method. In one embodiment, the polishing pad  152  may be attached to the platen  151  by a pressure-sensitive adhesive layer. The polishing pad  152  is generally consumable and may be replaced. A detailed description of a polishing pad may be found in U.S. Pat. No. 6,991,528, entitled “Conductive Polishing Article for Electrochemical Mechanical Polishing”, which is herein incorporated by reference. 
     The polishing station  105  may further comprise a polishing composition supplying tube  155  configured to provide sufficient polishing composition  154  to cover and wet the entire polishing pad  152 . The polishing composition  154  generally contains a reactive agent, e.g. deionized water for oxide polishing, abrasive particles, e.g., silicon dioxide for oxide polishing, and a chemical-reactive catalyzer, e.g., potassium hydroxide for oxide polishing. 
     The polishing station  105  may further comprise a pad conditioner  156  configured to maintain the condition of the polishing pad  152  so that it will effectively polish any substrate pressed against it. In one embodiment, the pad conditioner  156  may comprise a rotatable arm holding an independently rotating conditioner head and an associated washing basin. 
     In another embodiment, the platen  151  may be replaced by a polishing structure having a belt pad made of CMP or ECMP materials. 
     The carrier head  101  is generally configured to hold the substrate  103  against the polishing pad  152  during polishing and evenly distribute a downward pressure across the back surface of the substrate  103 . One embodiment of the carrier head  101  is illustrated in  FIG. 1B  which is  FIG. 1A  partially enlarged. 
     The carrier head  101  generally comprises a housing  112 , a base assembly  114 , a loading chamber  129 , an outer retaining ring  111 , and a retaining ring engaging assembly  140 . A description of a similar carrier head may be found in U.S. Pat. No. 6,183,354, entitled “Carrier Head with Flexible Membrane for Chemical Mechanical Polishing”, and U.S. patent application Ser. No. 11/054,128, filed on Feb. 8, 2005, entitled “Multiple-Chamber Carrier Head with a Flexible Membrane”, which are incorporated herein by reference. 
     The housing  112  is generally circular in shape and can be connected to a drive shaft (not shown) to rotate and or sweep therewith during polishing. A vertical bore  121  may be formed through the housing  112 , and passages  124  and  125  may extend through the housing  112  for pneumatic control of the carrier head. 
     The base assembly  114  is a vertically movable assembly located beneath the housing  112 . The base assembly  114  comprises a generally rigid annular body  127 , an outer clamp ring  113 , and a gimbal mechanism  136 . The gimbal mechanism  136  comprises a gimbal rod  122  which slides vertically the along bore  121  to provide a vertical motion of the base assembly  114 , and a flexure ring  137  which bends to permit the base assembly  114  to pivot with respect to the housing  112  so that the outer retaining ring  111  and the detachable retaining ring  104  (will be described later) may remain substantially parallel with the polishing surface  157  of the polishing pad  152 . 
     The loading chamber  129  is located between the housing  112  and the base assembly  114  to apply a load, i.e., a downward pressure or weight, to the base assembly  114 . The vertical position of the base assembly  114  relative to the polishing pad  152  is also controlled by the loading chamber  129 . An inner edge of a generally ring-shaped rolling diaphragm  120  may be clamped to the housing  112  by an inner clamp ring  128 . An outer edge of the rolling diaphragm  120  may be clamped to the base assembly  114  by the outer clamp ring  113 . 
     The outer retaining ring  111  may be a generally annular ring secured at the outer edge of the base assembly  114 . The bottom surface  139  may be substantially flat with multiple recesses  126  configured for vacuum chuck a substrate. When fluid is pumped into the loading chamber  129  and the base assembly  114  is pushed downwardly, the outer retaining ring  111  is also pushed downwardly to apply a load to the polishing pad  152 . A bottom surface  146  of the outer retaining ring  111  may be substantially flat, or it may have a plurality of channels to facilitate transport of polishing composition from outside the outer retaining ring  111  to the substrate. 
     A flexible membrane  119  is generally clamped on a bottom surface  139  of the base assembly  114 . The flexible membrane  119  and the base assembly  114  may form multiple chambers, for example, chambers  134  and  135 , which apply pressure or generate vacuum between the flexible membrane  119  and a backside of the substrate  103  to engage the substrate  103 . In one embodiment, the chambers  134  and  135  may be inflated and deflated through the passages  125  and  124  respectively. 
     The detachable retaining ring  104  may be engaged by a plurality of methods, for example by pressure, friction, dowel pins, and electromagnetic approach. In one embodiment, the retaining ring engaging assembly  140  is used. The retaining ring engaging assembly  140  is generally a circular structure attached to the base assembly  114  inside the outer retaining ring  111  and outside the flexible membrane  119 . The retaining ring engaging assembly  140  is configured to engage the detachable retaining ring  104  and transfer torque from the carrier head  101  to the detachable retaining ring  104 . In one embodiment, the retaining ring engaging assembly  140  comprises a bladder  117  clamped on the rigid annular body  127 . The bladder  117  may be inflated and deflated via a passage  118 . An engaging member  116  is generally attached to the bladder  117 . The engaging member  116  configured to engage the detachable retaining ring  104  by an upper surface  144  of the detachable retaining ring  104 . 
     The detachable retaining ring  104  is a generally annular ring adapted to be attached to the retaining ring engaging assembly  140  of the carrier head  101 . The detachable retaining ring  104  may be pushed downwardly by the carrier head  101  or the ring holder  107  to apply a load to the polishing pad  152 . A bottom surface  143  of the detachable retaining ring  104  may be substantially flat, or it may have a plurality of channels to facilitate transport of polishing composition from the outer retaining ring  111  to the substrate  103 . An inner surface  141  of the detachable retaining ring  104  engages the substrate to prevent it from escaping. In one embodiment, the inner surface  141  may be tapered near the upper surface  144  to increase a tolerance for loading the substrate  103 . In another embodiment, an outer surface  142  of the detachable retaining ring  104  and/or an inner surface  115  of the outer retaining ring  111  may also be tapered for easy attaching. 
     Referring back to  FIG. 1A , the ring holder  107  may be positioned near the polishing station  105 . The ring holder  107  is configured to grasp the detachable retaining ring  104  when it is detached and to position the detachable retaining ring  104  in a predetermined location so that a robot assembly  106  may drop a substrate within the inner surface  141  of the detachable retaining ring  104 . In one embodiment, the ring holder  107  may comprise a gripper  171  configured to hold the detachable retaining ring  104  by the outer surface  142 . In one embodiment, the ring holder  107  may also apply a downwardly force to the detachable retaining ring  104  to prevent a substrate retained therein from escaping, especially when the platen  1511  is rotating. In another embodiment, the ring holder  107  is also configured to remove the detachable retaining ring  104  from the polishing pad  152  when the polishing pad  152  is maintained and/or replaced. The ring holder  107  may also be capable of transferring the detachable retaining ring  104  to a wash station for cleaning. 
       FIG. 1A  illustrates the polishing system  100  in a substrate loading position. The carrier head  101  is in an up position. The detachable retaining ring  104  is detached from the carrier head  101  and is disposed in a designated location by the ring holder  107 . In one embodiment, the platen  151  and the polishing pad  152  may maintain a rotating movement for a higher throughput. In this case, the ring holder  107  also prevent the detachable retaining ring  104  from drifting away from the designated location. The robot assembly  106  transfers the substrate  103  by holding the substrate  103  with an end effecter  161 , for example, a gripper. The substrate  103  is transferred by the robot assembly  106  to a position above the detachable retaining ring  104  where the end effecter  161  drops the substrate  103  onto a recess defined by the inner surface  141  of the detachable retaining ring  104  and the polishing pad  152 . 
     As shown in  FIG. 2 , the substrate  103  is dropped onto the polishing pad  152  and within the inner surface  141  of the detachable retaining ring  104  with a polishing surface  131  facing the polishing pad  152 . The robot assembly  106  retracts allowing the carrier head  101  to lower down and perform polishing to the substrate  103 . 
       FIG. 3A  illustrates the polishing system  100  of  FIG. 1A  in a polishing position. In the polishing position, the carrier head  101  lowers down and the ring holder  107  retracts so that the detachable retaining ring  104  is attached to the carrier head  101 .  FIG. 3B  is a partially enlarged view of  FIG. 3A . As shown in  FIG. 3B , the carrier head  101  is in a lowered position that the outer retaining ring  111  is in contact with the polishing pad  152 . The bladder  117  is inflated to apply a downward force to the polishing pad  152  through the attached detachable retaining ring  104 . The amplitude of the downward force may be adjusted by the pressure inside the bladder  117 . In one embodiment, the engaging member  116  may engage the detachable retaining ring with friction and the downward force from the bladder  117  provides a pressure to enable the friction which may be large enough to transfer torque from the carrier head  101  to the detachable retaining ring  104  so that the detachable retaining ring  104  rotates at the same speed as the carrier head  101 . During polishing, the substrate  103  is generally rotated by the carrier head  101  and pushed against the polishing pad  152  by the carrier head  101 . To rotate the substrate  103 , the carrier head  101  must secure the substrate  103  by generating vacuum pouches  145  between a backside of the substrate  103  and the flexible membrane  119 . In one embodiment, the vacuum pouches  145  are generated by first pressing the substrate  103  with the chamber  135  of the flexible membrane  119  inflated to eliminate the air between the substrate  103  and the flexible membrane, and then deflating the chamber  135  while inflating the chamber  134 . 
       FIG. 4  illustrates the polishing system  100  of  FIG. 1A  in a substrate unloading position. Generally, after a polishing process is finished, the carrier head  101  releases the detachable retaining ring  104 , for example by deflating the bladder  117  and removing the pressure between the engaging member  116  and the detachable retaining ring  104 . The carrier head  101  then raises up with the substrate  103  secured on the flexible membrane  119  and leaving the detachable retaining ring  104  on the polishing pad  152 . In one embodiment, the gripper  171  of the ring holder  107  comes in to grasp the detachable retaining ring  104  before the carrier head  101  completely releases the detachable retaining ring  104 . 
     In one embodiment, after the detachable retaining ring  104  is released, the end effecter  161  of the robot assembly  106  may come under the carrier head  101  to pick up the substrate  103 . In one embodiment, the chamber  135  may be inflated and the vacuum pouches  145  may be eliminated and the substrate  103  is released from the flexible membrane  119 . After the substrate  103  is picked up by the robot assembly  106 , a new substrate may be loaded into the detachable retaining ring  104  and a new cycle of polishing process may start. 
     In another embodiment, the carrier head  101  may take the substrate  103  to another polishing station configured to perform a different polishing step to the substrate  103 . 
     As shown in  FIGS. 1–4 , the detachable retaining ring  104  is dedicated to the polishing station  105 . In another embodiment, the detachable retaining ring  104  may also be dedicated to the carrier head  101 . In the later configuration, the detachable retaining ring  104  will stay attached to the carrier head  101  and travel with the carrier head  101  in the case of multiple step polishing. The detachable retaining ring  104  is only detached from the carrier head  101  when loading a substrate or cleaning the detachable retaining ring  104 . An embodiment of a detachable retaining ring dedicated to a carrier head is shown in  FIG. 11  and will be described later. 
     It should be noted that the method of loading a substrate to a carrier head using a detachable retaining ring may be used in situations other than on a polishing pad, for example, on a cleaning station. 
       FIGS. 5–7  illustrate embodiments for attaching a detachable retaining ring to a carrier head and/or transferring torque from the carrier head to the detachable retaining ring. 
       FIG. 5  illustrates a schematic view of the detachable retaining ring  104  in accordance with the polishing system  100  of  FIGS. 1–4 . The detachable retaining ring  104  has a generally circular shape with an inner surface  141  configured to confine a substrate therein. An upper surface  144  of the detachable retaining ring  104  comprises a high friction material that provides easy engagement with and form torque transference from part of the carrier head  101 . In one embodiment, the engaging member  116  is made of a ring of friction material, for example, a form of roughed plastic. The upper surface  144  may comprise similar friction material as the engaging member  116 , or may be a frictional metal or plastic surface. When the bladder  117 , shown in  FIG. 1B , is inflated, the engaging member  116  moves down and contacts the upper surface  144 . Torque transfer between the carrier head  101  and the detachable retaining ring  104  occurs as the bladder  117  is further inflated causing more downforce on the detachable retaining ring  104 , and more friction to help lock the engaging member  116  and the upper surface  144  together. 
     In another embodiment, the bladder  117  shown in  FIG. 1B  may be made of rubber and may contact the upper surface  144  directly when it inflates. The upper surface  144  may be made of roughened plastic. When the bladder  117  contacts the upper surface  144 , the bladder  117  pushes down on the detachable retaining ring  104 . This pressure/down force provides a means of torque transfer from the rotating carrier head  101  to the detachable retaining ring  104  which may rotate at the same speed with the carrier head  101 . 
       FIG. 6A  illustrates a schematic view of a detachable retaining ring  204  in accordance with one embodiment of the present invention. The detachable retaining ring  204  generally has at least two “L” shaped slots  248  which open to a top surface  244  of the detachable retaining ring  204 . Each of the “L” shaped slots  248  are configured to house a dowel pin  247  from a carrier head  201 . In one embodiment, the dowel pins  247  may be extending from an outer retaining ring  211  of the carrier head  201 . During engaging, the carrier head  201  lowers to insert the dowel pins  247  into entrances of the “L” shaped slots  248 . After the dowel pins  247  are inserted into the “L” shaped slots  248 , the carrier head  201  may rotate along direction  1  or  2  to transfer torque to the detachable retaining ring  204 . To disengage the detachable retaining ring  204  from an attached position, the carrier head  201  may be rotated along direction  2  so that the dowel pins  247  are in line with the entrances of the “L” shaped slots  248 , and raise up to remove the dowel pins  247  from the “L” shaped slots  248 . To lift up the detachable retaining ring  204 , the carrier head  201  may rotate along direction  1  to position the dowel pins  247  deep in the “L” shape slots  248 , and then raise up with the detachable retaining ring  204 . 
       FIG. 6B  illustrates a detachable retaining ring  304  in accordance with one embodiment of the present invention. The detachable retaining ring  304  has at least two vertical apertures  347  which open to an upper surface  344  of the detachable retaining ring  304 . Each of the vertical apertures  347  is configured to hold a vertical dowel pin  348  extending from a carrier head (not shown). The vertical dowel pins  348  may be lowered into the vertical apertures  347  to attach and transfer torque to the detachable retaining ring  304 . The vertical dowel pins  348  may be raised up from the vertical apertures  347  to detach the detachable retaining ring  304 . 
       FIG. 7  illustrates a partial sectional view of one embodiment for lifting a detachable retaining ring in accordance with the present invention. A carrier head  401  is partially shown in  FIG. 7 . The carrier head  401 , similar to the carrier head  101  of  FIG. 1A , comprise a rigid annular body  427 , a flexible membrane  419  configured to secure a substrate, and an outer retaining ring  411 . In one embodiment, the outer retaining ring  411  has coils  480  embedded therein. The coils  408  are configured to generate an upward electromagnetic force F when biased. The upward electromagnetic force F may be adjusted to pick up an detachable retaining ring  404  which is at least partially made of metal. 
       FIG. 8A  illustrates a sectional view of a detachable retaining ring  504  in accordance with one embodiment of the present invention. The detachable retaining ring  504  generally comprises an inner ring  181  and an outer shell  182 . The inner ring  181  is configured to provide a rigid structure to the detachable retaining ring  504 , and may be formed of a rigid material, such as a metal, for example, stainless steel, molybdenum, or aluminum, or a ceramic, for example, alumina, or other materials. The outer shell  182  may be formed of a material which is chemically inert in a CMP or ECMP process, durable with a low wear rate, and sufficient compressible so that contact of a substrate edge against the detachable retaining ring  504  does not cause the substrate to chip or crack. Suitable materials for the outer shell  182  may be polyphenylene sulfide (PPS), polyetheretherketone (PEEK), carbon filled PEEK, Teflon® filled PEEK, polyethylene therephthalate (PET), polybutylene terephthalate (PBT) polytetrafluoroethylene (PTFE), polybenzimidazole (PBI), polyetherimide (PEI), or a composite material. 
       FIG. 8B  illustrates a sectional view of a detachable retaining ring  604  in accordance with one embodiment of the present invention. The detachable retaining ring  604  generally comprises an upper ring  183  and a lower ring  184 . The upper ring  183  is attached to the lower ring  184  by various ways, for example by an adhesive layer, fasteners, pressing to fit, or the combination thereof. The upper ring  183  may be formed of a material similar to that for the inner ring  181  of the detachable retaining ring  504 . The lower ring  184  may be formed of a material similar to that for the outer shell  182  of the detachable retaining ring  504 . 
     As shown in  FIG. 3A , during polishing, the polishing composition  154  supplied to the polishing pad  152  needs to pass the outer retaining ring  111  and the detachable retaining ring  104  to reach the substrate  103 . Since both the outer retaining ring  111  and the detachable retaining ring  104  are pressed against the polishing pad  152  to reduce the “edge effect” (a tendency of the substrate edge to be polished at a different rate than the substrate center), a plurality of channels may be formed on the bottom surface  146  of the outer retaining ring  111  and the bottom surface  143  of the detachable retaining ring  104 . 
       FIG. 9  illustrates a bottom view of the detachable retaining ring  104  and the outer retaining ring  111  in accordance with one embodiment of the present invention. A plurality of channels  186  are formed on the bottom surface  143  of the detachable retaining ring  104 . In one embodiment, the plurality of channels  186  are distributed at equal angular intervals around the detachable retaining ring  104 . Each of the plurality of channels  186  may be oriented at an angle α, for example 45°, relative to a radial segment R extending through the center of the detachable retaining ring  104 . It should be noted that the channels  186  may have other orientation, for example, with the angle α may be between about 30° to about 60°. Similarly, a plurality of channels  185  are formed on the bottom surface  146  of the outer retaining ring  111 . The plurality of channels  185  are distributed at equal angular intervals around the outer retaining ring  111 . In one embodiment, the channels  185  are oriented at the same angle α as the channels  186  and the number of the channels  186  equals the number of the channel  185 . During polishing, the outer retaining ring  111  and the detachable retaining ring  104  may be positioned in such an angle that each of the plurality of channels  186  is aligned with a corresponding channel  185  for easy flow of the polishing composition. 
       FIG. 10A  and  FIG. 10B  illustrate a sectional view and a bottom view of the detachable retaining ring  104  and the outer retaining ring  111  in accordance with one embodiment of the present invention. In this configuration, radial channels are combined with a circular channel to facilitate polishing composition flow through the outer retaining ring  111  and the detachable retaining ring  104  to a substrate position within the detachable retaining ring  104 . A plurality of radial channels  189 , similar to the plurality of channels  185  of  FIG. 9 , are formed on the bottom surface  143  of the detachable retaining ring  104 . A plurality of radial channels  188  are formed on the bottom surface  146  of the outer retaining ring  111 . A circular channel  196  is formed on the bottom surface  146  and open to the inner surface  115  of the outer retaining ring  111 . Each of the plurality of radial channels  188  opens to the circular channel  196 . During polishing, polishing solution generally flows through the plurality of radial channels  188  to the circular channel  196 , where the polishing solution may be temporarily stored and redistributed to the plurality of radial channels  189  on the detachable retaining ring  104 . The circular channel  196  facilitates fluid flow between the outer retaining ring  111  and the detachable retaining ring  104  when the radial channels  189  and  188  are not aligned as shown in  FIG. 9 . The radial channels  189  and  188  may be different in number and/or in direction. 
       FIG. 11  illustrates a polishing system  700  having a detachable retaining ring  704  dedicated to a carrier head  701 . The carrier head  701  is similar to the carrier head  101  of  FIG. 1A , except that the carrier head  701  comprise at least two dowel pins  187  extending from an outer retaining ring  711 . The dowel pins  187  are configured to lift up the detachable retaining ring  704  as well as transferring torque to the detachable retaining ring  704 . In one embodiment, the detachable retaining ring  704  may be similar to the detachable retaining ring  204  of  FIG. 6A . As shown in  FIG. 11 , after a polishing step is completed in the polishing system  700 , the carrier head  701  may raise up with the detachable retaining ring  704  and the substrate  103  and move to another polishing station for another polishing step. The detachable retaining ring  704  may travel with the carrier head  701  in a multi-station polishing system. A multiple step polishing process may particularly benefit from this configuration since it only loads the substrate once and also eliminates load cups in the system. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.