Abstract:
A substrate polishing apparatus and method are described. A base includes at least one movable platen to engage a polishing pad. At least one carrier head assembly presses a substrate against the polishing pad substantially within a polishing area during a polishing operation. A polishing solution dispenser applies a polishing solution to the polishing pad substantially within the polishing area during the polishing operation. A polishing solution retaining mechanism is attached to one of the base or the carrier head assembly. The retaining mechanism engages a top surface of the polishing pad and retains the polishing solution substantially within the polishing area during the polishing operation. Some implementations may reduce polishing solution consumption and allow for increased angular velocity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to Provisional Application No. 60/590,683, filed on Jul. 22, 2004. 
     
    
     TECHNICAL FIELD  
       [0002]     This document relates to retaining polishing solution during chemical mechanical polishing (CMP) or electrochemical mechanical polishing (ECMP).  
       BACKGROUND  
       [0003]     An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface, and planarizing the filler layer until the non-planar surface is exposed. For example, a conductive filler layer, such as copper, can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. The filler layer is then polished until the raised pattern of the insulative layer is exposed. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate. In addition, planarization is needed to planarize the substrate surface for photolithography.  
         [0004]     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 rotating polishing disk pad or belt pad. The polishing pad can be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment medium. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing solution, including at least one chemically-reactive agent, is supplied to the surface of the polishing pad. The polishing solution can optionally include abrasive particles, e.g., if a standard pad is used.  
         [0005]     A variation of CMP, which is particularly useful for copper polishing, is ECMP. In ECMP techniques, conductive material is removed from the substrate surface by electrochemical dissolution while concurrently polishing the substrate, typically with reduced mechanical abrasion as compared to conventional CMP processes. The polishing solution includes an electrolyte. The electrochemical dissolution is performed by applying a bias between a cathode and the substrate surface and thus removing conductive material from the substrate surface into the surrounding electrolyte solution.  
         [0006]     In both CMP and ECMP, a polishing solution is applied to a rotating surface. Due to centrifugal forces, the polishing solution disperses across the surface of the polishing pad, causing much of it to spill over the edge of the pad before its functional capacity is exhausted. These polishing solutions are expensive consumables. The per substrate cost of these processes could be reduced considerably by reducing the amount of polishing solution used. In addition, as the polishing pad&#39;s angular velocity is increased, smoother substrate surfaces and increased substrate planarization efficiency and consistency from center to edge result. Increased angular velocity, however, leads to stronger centrifugal forces, which lead to more polishing solution spillover.  
       SUMMARY  
       [0007]     This document describes a polishing solution retaining mechanism that reduces the amount of polishing solution required in CMP and ECMP and allows the polishing pad&#39;s angular velocity to be increased. In one implementation, a substrate polishing apparatus includes several components. A base includes at least one movable platen to engage a polishing pad. At least one carrier head assembly presses a substrate against the polishing pad substantially within a polishing area during a polishing operation. A polishing solution dispenser applies a polishing solution to the polishing pad substantially within the polishing area during the polishing operation. A polishing solution retaining mechanism is attached to one of the base or the carrier head assembly. The retaining mechanism engages a top surface of the polishing pad and retains the polishing solution substantially within the polishing area during the polishing operation.  
         [0008]     In another implementation, a substrate polishing method includes several steps. The method includes rotating a polishing pad in a first direction. The method also includes applying a polishing solution to the polishing pad within a polishing area. The method also includes retaining the polishing solution substantially within the polishing area with a polishing solution retaining mechanism, the polishing solution retaining mechanism being operable to press against a top surface of the polishing pad. The method also includes pressing a substrate against the polishing pad within the polishing area. The method also includes polishing the substrate.  
         [0009]     One or more of the following features may also be included. For example, the polishing solution retaining mechanism may be an enclosed ring, a partially open ring to allow a conditioning system to sweep across the polishing pad substantially unimpeded between a position near the center of the polishing pad and a position near an edge of the polishing pad, or a guide bar to guide polishing solution from a position near an edge of the polishing pad toward the center of the polishing pad during the polishing operation. The carrier head assembly may include a carrier head to engage a substrate and a carriage assembly to oscillate during the polishing operation. The polishing solution retaining mechanism may be attached to the base, the carrier head, or the carriage assembly.  
         [0010]     Additionally, the polishing solution dispenser may include a dispensing arm that extends above the polishing pad from an edge of the polishing pad toward the center of the polishing pad. The polishing solution dispenser may also dispense polishing solution upward through one or more perforations in the polishing pad. The apparatus and method may include a conditioning system that is attached to the base and operable to condition the polishing pad. The apparatus and method may also include a rinsing system to rinse the polishing pad during a rinsing operation. The rinsing operation may be used in conjunction with the guide bar polishing solution retaining mechanism. The guide bar may press against the polishing pad during the rinsing operation. The polishing pad may rotate in a first direction during the polishing operation and in a second direction during the rinsing operation, and the second direction may be the opposite of the first direction. The polishing solution retaining mechanism may withdraw from the polishing pad during a non-polishing operation. The apparatus may also include a substrate loss sensor attached to the polishing solution retaining mechanism.  
         [0011]     Certain implementations may have one or more of the following advantages. For example, a substantially uniform distribution of polishing solution can be maintained throughout the area in which the substrate is pressed against the polishing pad. This produces smoother substrate surfaces and increased substrate planarization efficiency and consistency from center to edge. In ECMP, angular velocity can be increased to approximately 7 to 30 rpm, e.g., 20 rpm, further enhancing those benefits. Moreover, polishing solution consumption may be reduced from approximately 2.5 liters per substrate to approximately 300 ml to 600 ml per substrate, e.g., 500 ml per substrate, which reduces fabrication cost.  
         [0012]     Different retainers can be used to accommodate different polishing machine configurations. For example, retainers can be attached to various components of the polishing machine, depending on which arrangement is most advantageous in a particular situation. Also, the retainer can be situated to allow a carrier head to travel substantially unimpeded through a full range of motion, achieving optimum planarization.  
         [0013]     Various other polishing functions can also be accommodated. For example, the polishing pad can be conditioned for improved polishing while still achieving other advantages related to retaining polishing solution. Similarly, the polishing pad can still be cleaned after polishing to remove excess solution and other residue. Additionally, the polishing machine can detect when a substrate has slipped from the carrier head without interference—in fact, with assistance—from the retainer.  
         [0014]     Other aspects, features, and advantages will be apparent from the following detailed description, including the drawings and the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  is an exploded perspective view of a polishing machine.  
         [0016]      FIG. 2  is a perspective view of a polishing solution retainer mounted to the base of the polishing machine.  
         [0017]      FIG. 3  is a perspective view of a polishing solution retainer mounted to a carrier head of the polishing machine.  
         [0018]      FIG. 4  is a perspective view of a polishing solution retainer mounted to a carriage assembly of the polishing machine.  
         [0019]      FIG. 5  is a top view of a polishing solution retainer in combination with a polishing pad conditioning system.  
         [0020]      FIG. 6  is a top view of another polishing solution retainer in combination with a polishing pad conditioning system.  
         [0021]      FIG. 7  is a side view of the polishing solution retainer shown in  FIG. 6 .  
         [0022]      FIG. 8  is a perspective view of a polishing machine with a substrate loss sensor. 
     
    
       [0023]     Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0024]      FIG. 1  shows a polishing machine  10 . The polishing machine  10  includes a base  15  connected to a carousel  20  by a center axis  25 . The base  15  includes three independently-operated polishing stations  27 ,  30 ,  33  and a substrate transfer station  35 .  
         [0025]     Each polishing station  27 ,  30 ,  33  includes a rotatable platen  40 . The platen  40  supports a polishing pad  45 . In ECMP, the polishing machine  10  applies an electrical bias to the substrate. A variety of techniques are available to apply this electrical bias. In one implementation, the bias is applied by electrodes that extend through apertures in a non-conductive dielectric polishing layer to contact the substrate. The polishing pad  45  may include a non-conductive polishing layer having a polishing surface, a non-conductive backing layer that can be softer than the polishing layer, and a counter-electrode layer which abuts the surface of platen  40 . A more detailed description of applying an electrical bias to a substrate can be found in U.S. patent application Ser. No. 10/773,868, filed on Feb. 4, 2004, the entirety of which is incorporated by reference.  
         [0026]     Each polishing station  27 ,  30 ,  33  also includes a dispensing arm  50  mounted to the base  15 . The dispensing arm  50  may serve two purposes. First, it may spread a thin layer of polishing solution across the surface of the polishing pad  45 . The polishing solution may contain abrasive particles, reactive agents, catalyzers, oxidizers, and other appropriate components. For an ECMP process, the polishing solution may contain an electrolyte, such as copper plating and/or copper anodic dissolution are available from Shipley Leonel, in Philadelphia, Pa., under the tradename Ultrafill 2000, and from Praxair, in Danbury, Conn., under the tradename EP3.1. The abrasive particles may include silicon dioxide in the form of colloidal silica or fumed silica. The appropriate polishing solution should be selected for each operation. Second, the dispensing arm  50  may serve to dispense a cleaning liquid across the surface of the polishing pad  45 . The cleaning liquid may be, for example, de-ionized water. The dispensing arm  50  is supported by a dispenser base  52 , which may serve as a hinge, allowing the dispensing arm  50  to sweep across the surface of the polishing pad  45 .  
         [0027]     Each polishing station  27 ,  30 ,  33 , also includes a polishing solution retaining mechanism  54  mounted to the base  15 . The polishing solution retaining mechanism  54  serves to maintain the polishing solution in a process area during polishing. The polishing solution retaining mechanism  54  includes a retaining base  55 , which is connected to the base  15  and supports a retaining arm  57 . The retaining arm  57  is connected to a retaining ring  58 . The retaining base  55  may serve as a hinge, permitting the retaining arm  57  to raise and lower with respect to the polishing pad  45 , thus raising and lowering the retaining ring  58 . When the retaining arm  57  is lowered, the retaining ring  58  presses against the polishing pad  45  in such a way that the dispensing arm  50  is enabled to dispense polishing solution within the retaining ring  58 . The retaining base  55  may also permit the retaining arm  57  to pivot across the surface of the polishing pad  45 , causing the retaining ring  58  to pivot. Operators should try to seek an optimal balance between maintaining enough pressure to retain enough polishing solution within the process area while not maintaining so much pressure that the retaining ring  58  inhibits the polishing pad&#39;s  45  rotation.  
         [0028]     Each polishing station  27 ,  30 ,  33  may also include a conditioning system  60  mounted to the base  15 . The conditioning system  60  serves to maintain the condition of the polishing pad  45  so that it will provide uniform polishing. The conditioning system  60  maintains the condition of the polishing pad  45  by removing contaminants that may be imbedded in the polishing pad  45  and by refreshing the surface of the polishing pad  45 , which can become deformed during operation. The conditioning system  60  includes a conditioner base  65 , which is attached to the base  15  and supports a conditioner arm  70 . The conditioner arm  70  extends across the surface of the polishing pad  45  and supports a conditioner head  75 . The conditioner head  75  may also contact a conditioner reservoir  80 , which contains a conditioning liquid for rinsing the conditioner head  75 . The conditioner base  70  may permit the conditioner arm  70  to sweep across the surface of the polishing pad  45  and also to raise and lower with respect to the polishing pad  45 .  
         [0029]     The carousel  20  includes four independent carrier head assemblies  85 . Each carrier head assembly  85  is partially encased by a support plate  90  and sidewalls  95 . Each carrier head assembly  85  may include a carrier head  107 ,  109 ,  111 ,  113 , which may attach to a drive shaft  105 . The drive shaft  105  may be coupled to a carriage assembly  100 . In  FIG. 1 , one of the sidewalls  95  is removed, exposing the carriage assembly  100  that corresponds to the first carrier head  107 . The carriage assembly  100  may include a motor for rotating the drive shaft  105 , causing the first carrier head  107  to rotate. The carriage assembly  100  may also have a mechanism for oscillating the first carrier head  107  back and forth. The oscillating mechanism could include a motor in conjunction with a trolley-and-rail assembly, a chain assembly, a pneumatic or computerized system, or any other assembly that would enable oscillation. Each of the carriage assemblies  100  may include the same features. Carrier head assemblies are described in further detail in U.S. patent application Ser. No. 10/810,784, filed on Mar. 26, 2004, the entirety of which is incorporated by reference.  
         [0030]     In operation, a substrate is manually or automatically positioned at the substrate transfer station  35 . The carousel  20  positions the first carrier head  107  directly above the substrate, and the first carrier head  107  engages the substrate. The carousel  20  rotates one-quarter turn, positioning the first carrier head  107  directly above the first polishing station  27 .  
         [0031]     The carrier head  107  then presses the substrate against the first polishing station&#39;s  27  polishing pad  45  inside the retaining ring  58 . Meanwhile, the carousel  20  has positioned the second carrier head  109  directly above the substrate transfer station  35 , readying the corresponding carriage assembly  100  to engage a second substrate. Once the first substrate is brought into contact with the polishing pad  45  within the retaining ring  58 , the dispensing arm  50  begins dispensing polishing solution on the surface of the polishing pad  45 . Both the first substrate and the polishing solution are within the retaining ring  58 . This occurs at substantially the same time as when the polishing pad  45  and the first carrier head  107  begin to rotate independently. Also, during this polishing process, the conditioning system  60  conditions the polishing pad  45 . The polishing solution dispensing process will be described in more detail in conjunction with  FIGS. 2-4 , and the conditioning process will be described in more detail in conjunction with  FIGS. 5-6 .  
         [0032]     Once the substrate is polished to a desired smoothness, the polishing pad  45  and the first carrier head  107  stop rotating, the dispensing arm  50  stops dispensing polishing solution, and the conditioning system  60  stops conditioning the polishing pad  45 . The carriage assembly  100  raises the substrates and the carousel  20  rotates another quarter turn. This positions the first carrier head  107  directly above the second polishing station  30 , the second carrier head  109  directly above the first polishing station  27 , and the third carrier head  111  directly above the substrate transfer station  35 . Meanwhile, the retaining ring  58  either raises from the polishing pad  45  or pivots off the polishing pad  45 , and the dispensing arm  50  of the first polishing station  27  begins dispensing cleaning liquid on the surface of the polishing pad  45  as the polishing pad  45  begins to rotate again. The cleaning process will be described in more detail in conjunction with  FIGS. 5-6 .  
         [0033]     Each substrate is polished according to the above process at three different polishing stations  27 ,  30 ,  33 . The objective of each polishing station  27 ,  30 ,  33  is described in U.S. patent application Ser. No. 10/773,868, filed on Feb. 4, 2004, the entirety of which is incorporated by reference. Once a substrate passes through all three polishing stations  27 ,  30 ,  33 , the carousel  20  returns it to the substrate transfer station  35  where it may be removed manually or automatically.  
         [0034]     Many additional implementations are possible. For example, the polishing machine  10  need not have three polishing stations  27 ,  30 ,  33 , one substrate transfer station  35 , and four carrier heads  107 ,  109 ,  111 ,  113 . It could have a greater or lesser number of any of these. Also, the base  15  need not be connected to the carousel  20  by a central axis  25 . The carousel  20  may be controlled by an arm that attaches to the top of the carousel  20  on one end and to the base  15  on the other end. A substrate need not be polished at each polishing station  27 ,  30 ,  33 . Also, polishing may be performed in parallel in some implementations.  
         [0035]     Another different implementation involves the dispensing arm  50  and the dispenser base  52 . The two components may be integrally molded of the same material. Also, no dispensing arm  50  and dispenser base  52  need be used at all. Instead, the polishing solution and the cleaner can be pumped from underneath the polishing pad  45  to the surface of the polishing pad  45  (see  FIGS. 2-4 ). In such an implementation, the liquids generally enter through the center of the rotating polishing pad  45 , and centrifugal forces cause the liquids to disperse across the surface of the polishing pad  45  until they contact the polishing solution retaining mechanism  54  or spill over the edge of the polishing pad  45 , depending on the configuration of the polishing solution retaining mechanism  54 .  
         [0036]     The polishing solution retaining mechanism  54  may take various shapes. The retaining ring  58  may be attached to the carrier head  107 ,  109 ,  111 ,  113  (see  FIG. 3  and corresponding discussion for more detail) or to the carriage assembly  100  (see  FIG. 4  and corresponding discussion for more detail). In either case, the retaining ring  58  would be positioned to contact the polishing pad  45  as the substrate contacts the polishing pad  45 . Further, the polishing solution retaining mechanism  54  need not involve a retaining ring  58  at all. A ring with a section removed (see  FIG. 5  and corresponding discussion for more detail) or a crescent-shaped guide (see  FIG. 6  and corresponding discussion for more detail) may be substituted for the retaining ring  58 .  
         [0037]     Also with respect to the polishing solution retaining mechanism  54 , the entire polishing solution retaining mechanism  54  may be integrally molded from the same material. Possible materials include polyurethane, polyphenlyene sulfide, polytetrafluoroethylene, stainless steel, or other appropriate polymers and metals.  
         [0038]     Other examples of alternative implementations involve the conditioning system  60 . The conditioner arm  70 , and the conditioner head  75  may be integrally molded of the same material. Also, some implementations do not require a conditioning system  60  at all. In such systems, conditioning may be accomplished through the same mechanism that processes the substrate.  
         [0039]     Other carousel  20  implementations are also possible. For instance, the components partially encasing the carriage assemblies  100 —the support plate  90  and the sidewalls  95 —may be integrally molded of the same material. Also, the motor that rotates the drive shaft  105  may be independent of the carriage assembly  100 .  
         [0040]     Other operational implementations are also possible. For instance, when a substrate is polished to the degree specified for a particular polishing station  27 ,  30 ,  33  and is raised from the polishing pad, the polishing pad  45 , the carrier head  107 ,  109 ,  111 ,  113 , or both may continue rotating or begin rotating at a different speed or in the opposite direction. Rotation speed and direction generally depend on the type of polishing process involved and the degree of complexity of the particular polishing machine  10 .  
         [0041]      FIG. 2  shows a polishing solution retaining mechanism  54  in which a retaining ring  58  is attached to the base  15  of a polishing machine  10  by a retaining arm  57  and a retaining base  55 . As shown, liquid reaches the surface of the polishing pad  45  through perforations  115 . The combination of the dispensing arm  50  and the dispenser  52  shown in  FIG. 1  may generally be used interchangeably with the perforation  115  configuration. During polishing, a pump dispenses polishing solution to the surface of the polishing pad  45  through perforations  115 . Centrifugal forces cause the polishing solution to move toward the edge of the polishing pad  45 . As the polishing solution moves toward the edge of the polishing pad  45 , it contacts retaining ring  58 , which remains substantially stationary during polishing. Some of the polishing solution may escape the retaining ring  58  by, for example, seeping underneath the retaining ring. Most of the polishing solution, however, is retained within the retaining ring  58 .  
         [0042]     As the polishing solution is dispensed, a carrier head assembly  85  brings a substrate into contact with the polishing pad  45  inside the retaining ring  58 . As shown, the retaining ring  58  is an oval shape, which allows the carrier head assembly  85  to oscillate between the center and the edge of the polishing pad  45 . The retaining ring  58  may be of non-uniform height to allow the carrier head assembly  85  to move unimpeded through a complete range of motion. The height of the retaining ring  58  is typically about one-half inch. If a non-uniform height is used, a notched retaining ring  58  may be used. The notch may be positioned along the rotary path traveled by the carrier when the carousel rotates, permitting unimpeded motion. The height of the notched portion may generally coincide with the height of the platen&#39;s  40  edge. Many different height configurations are possible, depending generally on the range of motion and physical configuration of the carrier head assembly  85 .  
         [0043]     During cleaning, the polishing solution retaining mechanism  54  may be removed from the surface of the polishing pad  45  for cleaning. The cleaning liquid is dispensed to the surface of the polishing pad through perforations  115 . Like in the polishing process, the polishing pad  45  rotates, producing centrifugal force, which causes the cleaning liquid to disperse over the surface of the polishing pad. As the cleaning liquid disperses, it collects residues and contaminants—such as residual liquid polishing solution, dust, dried polishing solution, abraded polishing pad material, and abraded substrate—and spills over the edge of the polishing pad  45 . The polishing solution retaining mechanism  54  may be removed by either rotating horizontally away from the polishing pad&#39;s  45  surface or raising away from the surface. Some combination of rotating and raising is also possible.  
         [0044]      FIG. 3  shows a polishing solution retaining mechanism  54  in which a retaining ring  58  is attached to a carrier head  107  by connectors  120 . In this implementation, the carrier head assembly  85  is lowered such that both the substrate, which is carried by the carrier head  107 , and the retaining ring  58  are pressed against the polishing pad  45 . The retaining ring  58  rotates and oscillates with the carrier head  107 , while remaining situated such that polishing solution is dispensed through the perforations  115  within the retaining ring  58 . The retaining ring  58  retains the majority of the polishing solution as centrifugal forces cause the polishing solution to disperse across the surface of the polishing pad  45 . During cleaning, the polishing solution retaining mechanism  54  is raised from the polishing pad  45 , allowing the cleaning liquid to collect residues and contaminants—such as residual liquid polishing solution, dust, dried polishing solution, abraded polishing pad material, and abraded substrate—and spill over the edge of the polishing pad  45 .  
         [0045]      FIG. 4  shows a polishing solution retaining mechanism  54  in which a retaining ring  58  is attached to a carriage assembly  100  by a retaining arm  57 . In this implementation, the carriage assembly  100  lowers both the retaining ring  58  and the substrate, which is carried by the carrier head  107 , to press against the polishing pad  45 . The carriage assembly  100  could lower both the retaining ring  58  and the carrier head  107  with the same motion at the same time or it could do so with separate motions at different times. The retaining ring  58  oscillates with the carriage assembly  100 . Polishing solution is dispensed through perforations  115  within the retaining ring  58 , which prevents a majority of the polishing solution from spilling over the edge of the polishing pad  45 . During cleaning, the polishing solution retaining mechanism  54  may be raised from the polishing pad  45 , allowing the cleaning liquid to collect residues and contaminants and spill over the edge of the polishing pad  45 .  
         [0046]      FIG. 5  shows a polishing solution retaining mechanism  54  in which a section of the retaining ring  58  has been removed to allow a conditioning system  60  to proceed through its full range of motion unimpeded. While the substrate is being polished by the polishing pad  45 , the conditioning system  60 , which includes a conditioner base  65 , a conditioner arm  70 , a conditioner head  75 , and a conditioner reservoir  80 , conditions the polishing pad  45 . Conditioning maintains the condition of the polishing pad  45  so that it provides uniform polishing. The conditioner head  75  may sweep across the polishing pad  45  with a motion that is synchronized with the motion of the carrier head  107  to avoid collision. Such synchronization may be controlled, for example, by a general purpose computer.  
         [0047]      FIG. 6  shows a polishing solution retaining mechanism  54  in the form of a crescent-shaped retaining guide  59 . During polishing, a carrier head  107  presses a substrate against a polishing pad  45  while polishing solution is dispensed through perforations  115  onto the surface of the polishing pad  45 . As centrifugal forces cause the polishing solution to disperse across the surface of the polishing pad  45 , some contacts the guide  59 . The polishing solution may also contact a ledge protruding upward from the edge of the platen  40 . The polishing solution may then travel along that ledge before contacting the guide  59 . The guide&#39;s  59  crescent shape directs the polishing solution that contacts it back to the center of the polishing pad  45 . Once the polishing solution returns to the center of the polishing pad  45 , centrifugal forces again distribute it across the surface of the polishing pad  45 . This process continues until polishing is complete. Like in  FIG. 5 , this implementation allows the conditioning system  60  to sweep across the polishing pad  45  without interference from the polishing solution retaining mechanism  54 .  
         [0048]     During cleaning, the polishing solution retaining mechanism  54  may be raised or pivoted away from the polishing pad  45 . The polishing solution retaining mechanism  54 , however, may also remain pressed against the polishing pad  45 . In such a situation, if the polishing pad rotates in the same direction, the contaminated cleaning liquid may be guided back to the center of the polishing pad  45  just as the polishing solution was during polishing. That may not be desirable. To avoid such a situation, the polishing pad  45  may be rotated in the opposite direction, causing the contaminated cleaning liquid to contact the back side of the guide  59 . The back side of the guide  59  may be shaped in such a way as to direct the contaminated cleaning liquid toward the edge of the polishing pad  45 .  FIG. 7  provides a cross-sectional view of what such a guide  59  might look like.  
         [0049]      FIG. 8  shows another possible implementation the polishing solution retaining mechanism  54 . This implementation involves a substrate loss sensor  125 . Substrates slipping from the carrier head  107  is a common problem in CMP and ECMP. If the substrate is not being pressed against the polishing pad  45  by the carrier head  107 , it is not being polished, resulting in wasted time, wasted polishing solution, and possible damage to the substrate. Further, as a substrate slips from the carrier head  107 , the carrier head  107  may crush the wayward substrate. Substrate loss sensors  125  that alert an operator or a machine when a substrate has slipped may reduce these adverse effects. Such substrate loss sensors  125  may include devices that measure changes in light such as a laser or a optic sensor. As shown, the substrate loss sensor  125  is mounted within a cutout of the base of the retaining ring  58 , allowing substrate loss sensor  125  to sit flush with the retaining ring  58 . This arrangement provides that when the retaining ring  58  is in a position to retain polishing solution during polishing, the substrate loss sensor  125  is also in position to sense substrate loss. The substrate loss sensor  125  may be used in conjunction with any of the various retaining ring configurations discussed in this document or with any other similar configurations that allow retaining of polishing solution during CMP or ECMP.  
         [0050]     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.