Abstract:
Apparatus and methods of polishing substrates are disclosed. A retaining ring for a polishing apparatus includes an inner surface exposed to contact a peripheral edge of a substrate to be polished against a polishing surface, a bottom surface exposed to contact the polishing surface while the substrate is being polished, and a wear marker indicative of a preselected amount of wear of the bottom surface. The inner surface, bottom surface and wear marker may form part of a retaining ring used in chemical mechanical polishing operations. In one method, one or more substrates may be polished against a polishing surface using the retaining ring, and at least a portion of the retainer may be replaced when the bottom surface has been worn away by the preselected amount indicated by the wear marker. In another method, one or more substrate may be polished against a polishing surface with a substrate carrier that includes a substrate retaining ring with a wear marker indicative of a preselected amount of wear of the retaining ring, and a warning signal may be generated upon detection of the wear marker.

Description:
BACKGROUND 
     This invention relates to apparatus and methods for determining when to replace a retaining ring used in substrate polishing operations (e.g., chemical-mechanical polishing). 
     Chemical mechanical polishing (CMP) is a process for planarizing the surface of a substrate (e.g., a semiconductor wafer). In a typical CMP process, a polishing surface of a polishing sheet (or pad) is covered with a slurry solution containing abrasive particles and one or more reactive chemicals. A substrate to be polished is held against the polishing surface by a carrier head in a recess defined by a substrate support surface and a retaining ring. The polishing surface and the carrier head are moved relative to one another causing the slurry to mechanically and chemically remove portions of the substrate surface. 
     The retaining ring serves to hold the substrate in position on the carrier head and improves the uniformity of the polishing process. During this process, however, the retaining ring is exposed to the polishing action of the slurry and, after a period of time, a significant portion of the retaining ring will have been worn away. After a certain amount of material has worn away, its ability to retain the substrate in place and its beneficial impact on polishing uniformity diminishes. Eventually, the retaining ring must be replaced to avoid detrimental impact on the quality and yield of the polishing process. 
     SUMMARY 
     In one aspect, the invention features a retaining ring. The retaining ring has an inner surface exposed to contact a peripheral edge of a substrate to be polished against a polishing surface, a bottom surface exposed to contact the polishing surface while the substrate is being polished, and a wear marker indicative of a preselected amount of wear of the bottom surface. 
     In another aspect, the invention features a substrate polishing apparatus that includes a carrier head configured to hold a substrate against a polishing surface, and a retaining ring having an inner surface exposed to contact a peripheral edge of the substrate, a bottom surface exposed to contact the polishing surface while the substrate is being polished, and a wear marker indicative of a preselected amount of wear of the bottom surface. 
     Embodiments may include one or more of the following features. 
     The wear marker may comprise a visual indicator that is located at the outer surface of the polishing apparatus and is exposed for visual inspection while the substrate is being polished. The visual indicator may comprise a color change. The color change may result from a change in material composition between the bottom surface and the location of the visual indicator, or from a colorant applied to the outer surface. The wear marker may comprise a change in a structural feature of the outer surface. The structural feature change may comprise a hole extending from the outer surface to the inner surface; the hole preferably extends in a linear direction oriented at an acute angle relative to the bottom surface. Alternatively, the structural feature change may comprise a continuous groove that defines a plane that is substantially parallel to the bottom surface. 
     The wear marker may be exposed for detection at the bottom surface after the bottom surface has been worn away by a preselected amount. The wear marker and the bottom surface may be formed from different material compositions. The wear marker may be formed from a polymeric material, or a metal. The material compositions of the wear marker and the bottom surface may have different reflectivity characteristics. For example, in one embodiment, the wear marker is formed from a metal and the bottom surface is formed from a polymeric material. 
     The bottom surface may include a groove having a characteristic depth, and the wear marker may be exposed for detection after the depth of the groove has been reduced sufficiently by wearing. The wear marker may comprise a metallic surface disposed in the groove, an annular ring, or one or more spaced-apart wear marker plugs. 
     A detection system may be provided. The detection system may be configured to detect the wear marker and to generate a warning signal upon detection of the wear marker. 
     In another aspect, the invention features a substrate polishing method, in which one or more substrates are polishing against a polishing surface with a retaining ring having an inner surface exposed to contact a peripheral edge of the substrate, a bottom surface exposed to contact the polishing surface while the substrate is being polished, and a wear marker indicative of a preselected amount of wear of the bottom surface. At least a portion of the retaining apparatus is replaced when the bottom surface has been worn away by the preselected amount indicated by the wear marker. 
     The invention also features a substrate polishing method, in which one or more substrates are polished against a polishing surface with a substrate carrier that includes a substrate retaining ring with a wear marker indicative of a preselected amount of wear of the retaining ring, and a warning signal is generated upon detection of the wear marker. 
     The wear marker may be detected optically. 
     Among the advantages of the invention are the following. The invention enables CMP operators to determine when a retaining ring should be replaced based upon a simple visual inspection of the retaining ring before, during or after a CMP process. The invention also provides a system for automatically determining when a retaining ring should be replaced. The invention allows retaining rings to be efficiently used without risk of the detrimental impact on process quality and yield that might be caused by using overly worn retaining rings. The invention reduces processing costs by reducing materials costs (in the form of reducing premature disposal of retaining rings) and by reducing labor costs (in the form of reducing CMP operator time required to monitor retaining ring life). 
     Other features and advantages will become apparent from the following description, including the drawings and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic side view, in partial cross-section, of a substrate polishing apparatus that includes a substrate carrier head, and a polishing pad mounted on a rotatable platen. 
     FIG. 2 is a diagrammatic perspective view of a retaining ring having a bottom layer and a top layer formed from different material compositions. 
     FIG. 3 is a diagrammatic perspective view of a retaining ring with a wear marker consisting of a colored ring disposed around the outer surface of the retaining ring. 
     FIG. 4A is a diagrammatic perspective view of a retaining ring with a wear marker consisting of a groove disposed around the outer surface of the retaining ring. 
     FIG. 4B is a diagrammatic cross-sectional side view of the retaining ring of FIG. 4A taken along the line  4 B— 4 B. 
     FIG. 5A is a diagrammatic perspective view of a retaining ring with a wear marker consisting of a plurality of vent holes extending from the outer surface to the inner surface of the retaining ring. 
     FIG. 5B is a diagrammatic cross-sectional side view of the retaining ring of FIG. 5A taken along the line  5 B— 5 B. 
     FIG. 6A is a diagrammatic perspective view of a retaining ring having an internal annular ring of one material (or color) embedded in a retaining ring of a different material (or color). 
     FIG. 6B is a diagrammatic cross-sectional side view of the retaining ring of FIG. 6A taken along the line  6 B— 6 B. 
     FIG. 7A is a diagrammatic bottom view of a grooved retaining ring with a wear marker consisting of a plurality of cylindrical wear markers disposed in the grooves of the retaining ring. 
     FIG. 7B is a diagrammatic cross-sectional side view of the retaining ring of FIG. 7A taken along the line  7 B— 7 B. 
     FIG. 7C is a diagrammatic cross-sectional side view of an alternative grooved retaining ring with a wear marker consisting of a plurality of cylindrical wear markers disposed in the grooves of the retaining ring. 
     FIG. 8 is a diagrammatic side view of a substrate polishing system that includes a substrate carrier head, a polishing pad mounted on a rotatable platen, and an optical detection system. 
     FIGS. 9A and 9B are graphs of the intensity of light detected by the optical detection system of FIG. 8 plotted against the distance across the width dimension of the substrate carrier head. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a CMP polishing system  10  includes a carrier head  12  which is mounted to a rotatable shaft  14 , a polishing pad  16 , and a rotatable platen  18  which is mounted to a rotatable shaft  20 . Carrier head  12  includes a retaining ring  22  configured to hold a substrate  24  in place on carrier head  12 . In operation, carrier head  12  holds substrate  24  against a polishing pad  26  of polishing pad  16 , while carrier head  12  and rotatable platen  18  independently rotate relative to one another. Carrier head  12  also may be moved back-and-forth across polishing pad  26  over a linear or nonlinear polishing path. A reactive slurry solution may be deposited on polishing pad  26  to enhance the polishing process. 
     Retaining ring  22  includes an inner surface  28  which is exposed to contact a peripheral edge  30  of substrate  24 , and a bottom surface  32  which is exposed to contact polishing pad  26  while substrate  24  is being polished. As mentioned above, after retaining ring  22  has been exposed to the combined polishing action of polishing pad  26  and the reactive slurry solution for a period of time, retaining ring  22  will have become sufficiently worn that it no longer is able to adequately perform the functions of holding substrate  24  in place and improving the uniformity of the polishing process. At this point, retaining ring  22  should be replaced. As described in detail below, retaining ring  22  includes a wear marker that facilitates the determination of when retaining ring  22  should be replaced. 
     Referring to FIG. 2, in one embodiment, retaining ring  22  includes a bottom (or wear) layer  34  and a top (or support) layer  36 . A wear marker  38  includes a visual indicator that is located at the outer surface of retaining ring  22  and is exposed for visual inspection while substrate  24  is being polished. In this embodiment, wear marker  38  consists of a color change between bottom layer  34  and top layer  36 ; these layers are formed from different material compositions. Bottom layer  22  is formed from a material, such as a fiber-reinforced TEFLON® matrix (e.g., a ZYMAXX® component available from DuPont) which may contact polishing pad  26  of polishing pad  16  without detrimental impact. Top layer  36  preferably also is formed from a material, such as a fiber-reinforced TEFLON® matrix, which may contact polishing pad  26  without a detrimental impact. The different colors of bottom layer  34  and top layer  36  may be achieved by embedding different fibers in the TEFLON® matrix. For example, layer  34  may appear black as a result of carbon fibers embedded in the matrix, and top layer  36  may appear beige as a result of KEVLAR® fibers embedded in the matrix. 
     Other combinations of materials may be used to form top and bottom layers  36 ,  34 . Also, more than two layers of different color may be used. For example, there may be one or more intermediate layers disposed between bottom layer  34  and top layer  36 ; the intermediate layers may be of different color to provide multiple warning indications to a CMP operator. Each layer may correspond to a preselected amount by which retaining ring  22  has been worn away, and each layer may indicate a time when retaining ring  22  should be replaced for a particular set of CMP process tolerance parameters. For example, an intermediate layer that is close to bottom layer  34  may indicate when retaining ring  22  should be replaced when used in a CMP process that has relatively demanding tolerance parameters, whereas an intermediate layer that is closer to top layer  36  may indicate when retaining ring  22  should be replaced when used in a CMP process that has less demanding tolerance parameters. 
     The thickness of bottom layer  34  (and therefore the location of wear marker  38 ) is selected to correspond to the amount by which bottom surface  32  of retaining ring  22  may be worn away before the performance of retaining ring  22  is significantly degraded. This amount may vary depending upon the nature of the polishing process (e.g., the rate at which retaining ring  22  is worn away), polishing tolerances, and the particular impact of the gradual thinning of retaining ring  22  on the polishing process. 
     In operation, a CMP operator may visually inspect the outer surface of retaining ring  22  before, during or after a CMP process, and when the color of bottom layer  34  is no longer visible, the operator may replace retaining ring  22 . Alternatively, when used in substrate polishing system  87  (described below in connection with FIG.  8 ), the polishing system automatically may detect when retaining ring  22  should be replaced. For example, if the reflectivity of bottom layer  34  is different from the reflectivity of top layer  36 , the polishing system would detect when bottom layer  34  has been worn away based upon a change in detected signal intensity at the location of retaining ring  22 . To achieve a difference in reflectivity, for example, bottom layer  34  may be formed from a thermoplastic, such as PPS® mechanical plastic (available from Interstate Plastic, Inc. of Sacramento, Calif., U.S.A.) or a polyurethane composition, and top layer  36  may be formed from a metal (e.g., aluminum or stainless steel). Alternatively, bottom layer  34  and top layer  36  may be formed from different color materials. 
     As shown in FIG. 3, in another embodiment, a wear marker  40  consists of a ring of a substance that defines a plane that is substantially parallel to bottom surface  32  and has a different color than the substance forming retaining ring  22 . For example, retaining ring  22  may be formed from a light-colored polyurethane material and wear marker  40  may be formed from a dark colorant (e.g., a paint or a dye) applied (or injected into) the outer surface of retaining ring  22 . In operation, a CMP operator may visually inspect the outer surface of retaining ring  22  before, during or after a CMP process, and when wear marker  40  is no longer visible, the operator may replace retaining ring  22 . 
     Referring to FIGS. 4A and 4B, in another embodiment, a wear marker  42  consists of an annular groove  44  that is formed in the outer surface of retaining ring  22  and defines a plane that is substantially parallel to bottom surface  32 . The location of annular groove  44  (and therefore the location of wear marker  42 ) is selected to correspond to the amount by which bottom surface  32  of retaining ring  22  may be worn away before the performance of retaining ring  22  is significantly degraded. Retaining ring  22  may be replaced when a CMP operator visually observes that bottom surface  32  of retaining ring  22  has been worn away up to groove  44 . 
     As shown in FIGS. 5A and 5B, in another embodiment, a wear marker  46  may consist of one or more angled vent holes  48 ,  50 ,  52  and  54 , that extend from the outer surface of retaining ring  22  to inner surface  28 . Vent holes  48 - 54  preferably extend in a linear direction from the outer surface of retaining ring  22  to inner surface  28  and are oriented at an acute angle relative to bottom surface  32 , as shown. The locations where vent holes  48 - 52  appear in the outer surface of retaining ring  22  (and therefore the location of wear marker  46 ) is selected to correspond to the amount by which bottom surface  32  of retaining ring  22  may be worn away before the performance of retaining ring  22  is significantly degraded. Retaining ring  22  may be replaced when a CMP operator visually observes that bottom surface  32  of retaining ring  22  has been worn away up to the locations where vent holes  48 - 54  are formed in the outer surface of retaining ring  22 . Because the vent holes are angled, the polishing pad is exposed to only a portion of the groove that is formed after bottom surface  32  of retaining ring  22  has been worn away up to the locations where vent holes  48 - 54  are formed in the outer surface of retaining ring  22 . 
     Referring to FIGS. 6A and 6B, another retaining ring embodiment includes a wear marker  56  that is exposed for detection at bottom surface  32  of retaining ring  22  after bottom surface  32  has been worn away by a preselected amount. In this embodiment, wear marker  56  is formed from an internal ring  58  embedded within retaining ring  22 . Internal ring  58  is formed from material that is different from the material composition of retaining ring  22 . Retaining ring  22  may formed from a material, such as a fiber-reinforced TEFLON® matrix (e.g., a ZYMAXX® component available from DuPont) which may contact polishing pad  26  of polishing pad  16  without detrimental impact. Internal ring  58  preferably also is formed from a material, such as a fiber-reinforced TEFLON® matrix, which may contact polishing pad  26  without a detrimental impact. The different detection characteristics of internal ring  58  and retaining ring  22  may be achieved by embedding different fibers in the TEFLON® matrices. For example, internal ring  58  may appear black as a result of carbon fibers embedded in the matrix, and retaining ring  22  may appear beige as a result of KEVLAR® fibers embedded in the matrix. In an alternative embodiment, retaining ring  22  may be formed from a polymeric material (e.g., polyurethane) and internal ring  58  may be formed from a metal (e.g., aluminum or stainless steel). 
     Internal ring  58  extends into retaining ring  22  a depth  60  that is selected to correspond to the amount by which bottom surface  32  of retaining ring  22  may be worn away before the performance of retaining ring  22  is significantly degraded. As mentioned above, this amount may vary depending upon the nature of the polishing process (e.g., the polishing rate of retaining ring  22 ), polishing tolerances, and the particular impact of the gradual thinning of retaining ring  22  on the polishing process. 
     In operation, a CMP operator may visually inspect bottom surface  32  of retaining ring  22  before or after a CMP process, and when the bottom surface of internal ring  58  appears, the operator may replace retaining ring  22 . Alternatively, when used in substrate polishing system  87  (described below in connection with FIG.  8 ), the polishing system automatically may detect when retaining ring  22  should be replaced. For example, if the reflectivity of internal ring  58  is different from the reflectivity of retaining ring  22  (e.g., when internal ring  58  is formed from a metal and retaining ring  22  is formed from a polymeric material), the polishing system would detect when bottom surface  32  has been sufficiently worn away based upon a change in detected signal intensity at the location of retaining ring  22 . 
     In an alternative embodiment, internal ring  58  may be replaced by one or more spaced-apart wear marker plugs (or pins) that are formed from a material that is different from the material composition of retaining ring  22 . These wear marker plugs may be distributed along an annular path corresponding to the location of internal ring  58  and may extend into retaining ring  22  the same depth as internal ring  58 . 
     Referring to FIGS. 7A and 7B, in another embodiment, retaining ring  22  includes a plurality of angled grooves  62 ,  64 ,  66  and  68 , each having a characteristic depth  70 , and a wear marker consisting of a plurality of spaced-apart wear marker plugs (or pins)  72 ,  74 ,  76  and  78  that are formed from a material that is different from the material composition of retaining ring  22 . Plugs  72 - 78  are exposed for detection (e.g., by automatic optical detection) after the depth of the groove has been reduced sufficiently by wearing—this depth may be less than characteristic depth  70 . Plugs  72 - 78  are formed from material that is different from the material composition of retaining ring  22 . Retaining ring  22  may formed from a material, such as a fiber-reinforced TEFLON® matrix (e.g., a ZYMAXX® component available from DuPont), which may contact polishing pad  26  of polishing pad  16  without detrimental impact. Plugs  72 - 78  preferably also are formed from a material, such as a fiber-reinforced TEFLON® matrix, which may contact polishing pad  26  without a detrimental impact. The different detection characteristics of plugs  72 - 78  and retaining ring  22  may be achieved by embedding different fibers in the TEFLON® matrices. For example, plugs  72 - 78  may appear black as a result of carbon fibers embedded in the matrix, and retaining ring  22  may appear biege as a result of KEVLAR® fibers embedded in the matrix. In an alternative embodiment, retaining ring  22  may be formed from a polymeric material (e.g., polyurethane) and plugs  72 - 78  may be formed from a metal (e.g., aluminum or stainless steel). 
     In an alternative embodiment, wear marker plugs  72 - 78  may be replaced by layers (or coatings) of a material that is of a different material composition than retaining ring  22  and is disposed along the bottom surface of grooves  62 - 68 . Suitable layers include layers that produce an initial optical response at a time before bottom surface  32  has been worn away that is different from the optical response produced when retaining ring  22  has been sufficiently worn that it should be replaced. For example, these layers may be formed from a reflective material (e.g., aluminum or stainless steel). 
     In operation, when used in the substrate polishing system described below in connection with FIG. 8, the polishing system automatically may detect when retaining ring  22  should be replaced. For example, if the optical characteristics of plugs  72 - 78  are different from the optical characteristics of retaining ring  22  (e.g., when plugs  72 - 78  are formed from a metal and retaining ring  22  is formed from a polymeric material), the polishing system would detect when bottom surface  32  has been sufficiently worn away based upon a change in detected optical signal intensity at the location of plugs  72 - 78 . Specifically, as the bottom surface of retaining ring  22  is worn away, plugs  72 - 78  move closer to window  90 , resulting in a change in the detected optical signal intensity. 
     As shown in FIG. 7C, plugs  72 - 78  may be sunk into respective recesses  80 ,  82 ,  84  and  86  in grooves  62 - 68  in order to improve the signal to noise ratio of the detected optical signal, or to prevent plugs  72 - 78  from contacting polishing pad  26  of polishing pad  16 , or both. 
     Referring to FIG. 8, in one embodiment, platen  18  of substrate polishing system  10  includes a light passage  88  and polishing pad  16  includes a window  90  formed from a material (e.g., polyurethane) that is at least semi-transparent (substantially transmissive) with respect to the light produced by a monitoring system  92 . In operation, monitoring system  92  produces a laser beam  94 , e.g., at least a portion of which passes through light passage  88  and window  90 . A portion of beam  94  is partially reflected from one or more layers of substrate  24  and retaining ring  22  to produce a beam  96  which has an intensity that varies as layers are removed from substrate  24  and varies as the optical characteristics (e.g., reflectivity) of retaining ring  22  change over time. For example, if the surface layer of substrate  24  is partially reflective and partially transmissive, beam  96  will be formed from at least two beams reflecting from different surfaces and the intensity of beam  96  will vary depending on whether the constituent beams interfere constructively or destructively, a characteristic which is primarily a function of the thickness of the surface layer of substrate  24 . If the surface layer is substantially reflective, the intensity of beam  96  will be significantly reduced when the surface layer has been polished away. Monitoring system  92  monitors the variation in the intensity of beam  96  during a polishing process to determine the amount of material that has been removed from the surface of substrate  24 , to determine the end point of the polishing process, and to determine when retaining ring  22  should be replaced. The operation of monitoring system  92  is coordinated with the movement of carrier head  12  to enable monitoring system  92  to periodically probe substrate  24 . In particular, monitoring system  92  is configured to trigger the laser when substrate  24  is positioned over window  90 ; alternatively, monitoring system  92  may be configured to open a shutter over the detector when substrate  24  is positioned over window  90 . 
     Referring to FIGS. 9A and 9B, monitoring system  92  automatically may determine when to replace retaining ring  22  as follows. At an initial time T 0  (before retaining ring  22  should be replaced), monitoring system  92  detects the intensity of beam  96  across the width dimension of carrier head  12 . The resulting intensity distribution  100  is characterized by a relatively low (or high) intensity at the locations  102 ,  104  corresponding to retaining ring  22  and by a relatively high (or low) intensity at the locations  106  corresponding to substrate  24 . At a later time T 1  (after retaining ring  22  has been sufficiently worn away that it should be replaced, usually after 1,500-4,000 substrates have been polished), the resulting intensity distribution  108  detected by monitoring system  92  is characterized by a higher +|ΔI| (or lower −|ΔI|) detected intensity at the locations corresponding to retaining ring  22  relative to the intensity detected at time T 0 . Once the detected intensity ( 110 ,  112 ) of the light received from retaining ring  22  exceeds (or is lower than) the initial intensity ( 102 ,  104 ) by more than a selected threshold (i.e., |ΔI|&gt;I Threshold ), monitoring system  92  generates a warning signal indicating that retaining ring  22  should be replaced. It should be noted that the threshold (I Threshold ) selected will depend upon the characteristics of retaining ring  22 , the type and composition of wear marker used, and the characteristics of monitoring system  92 . Also, it should be noted that the detected intensity of light received from retaining ring  22  may exceed (or be lower than) the detected intensity of light received from substrate  24  depending upon the optical characteristics of substrate  24 , retaining ring  22  and the wear marker used. 
     Other embodiments are within the scope of the claims. The invention may be implemented with other substrate polishing designs. For example, rotatable platen  18  and polishing pad  16  may be implemented with a different rotating polishing system design, or may be replaced by a linear drive mechanism and a linear polishing pad. 
     Monitoring system  92  may be configured to direct beam  94  at the outer surface of retaining ring  22 . This configuration may be used in combination with the embodiments of FIGS. 2-5B to detect changes in the optical characteristics of the outer surface of retaining ring  22  as bottom surface  32  is being worn away. 
     Still other embodiments are within the scope of the claims.