Abstract:
A method and apparatus for detecting and obtaining a metric indicative of a polishing process is described. The apparatus includes a polishing pad having an optically transparent region adapted to obtain polishing metric from at least one substrate from at least two distinct radial positions of the polishing pad. The method includes obtaining a polishing metric from at least two substrates being polished simultaneously on a single polishing pad.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS: 
       [0001]    This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/059,622 (Attorney Docket 013037L), filed Jun. 6, 2008, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments described herein generally relate to a chemical mechanical polishing system suitable for use in semiconductor manufacturing. More specifically, to a pad assembly suitable for use in chemical mechanical polishing system. 
         [0004]    2. Description of the Related Art 
         [0005]    In the fabrication of electronic devices on semiconductor substrates, the use of chemical mechanical polishing, or CMP, has gained favor due to the widespread use of damascene interconnects structures during integrated circuit manufacturing. Although many commercially available CMP systems have demonstrated robust polishing performance, the move to smaller line widths requiring more precise fabrication techniques, along with a continual need for increased throughput and lower cost of consumables, drives an ongoing effort for polishing system improvements. Moreover, most conventional polishing systems have relatively limited flexibility for changes to processing routines, thereby limiting the diversity of processes that may be run through a single tool. Thus, new processing routines may require new or dedicated tools, or costly downtime for substantial tool configurational changes. 
         [0006]    During CMP, various methods and apparatus have been developed to signal an endpoint to the polishing process. One conventional endpoint detection apparatus uses an optical sensor adapted to obtain a metric from the substrate being polished, such as a reflection from a layer or layers on the substrate, to gauge material removal and/or prevent over polishing. The conventional CMP pads may include a single, small transparent region or window that is generally transmissive to light or an electromagnetic signal as shown in  FIG. 1 . 
         [0007]      FIG. 1  shows a conventional CMP station  1 , partly in cross-section, that may be a stand-alone unit or part of a larger system. The CMP station  1  generally includes a platen  20  with a polishing pad  10  mounted thereon, and a carrier head assembly  30  for holding a substrate to be polished, such as a semiconductor substrate  15 , against the upper surface of the polishing pad  10 . The polishing pad  10  has an upper surface serving as a polishing surface which is brought into sliding contact with the substrate  15  to be polished. A polishing liquid supply nozzle  22  is disposed above the platen  20  for supplying a polishing liquid  18  onto the polishing pad  10 . 
         [0008]    The polishing pad  10  may be a polymer material, which may be solely dielectric or, alternatively, at least partially conductive to facilitate electrochemical dissolution of material from the substrate  15  in an electrochemical mechanical polishing (ECMP) process. In another conventional application, the polishing pad  10  may contain fixed abrasives. Thus, the polishing liquid  18  may be a slurry or an electrolytic fluid depending on the polishing process used. 
         [0009]    The platen  20  is coupled to a shaft  42  and motor  21  to rotate the platen  20  and polishing pad  10  about an axis. The carrier head assembly  30  is coupled to a shaft, which is coupled to a motor and a lifting/lowering device (not shown) that is adapted to urge the held substrate  15  against the polishing surface of the polishing pad  10 . The carrier head assembly  30  may be rotatable to provide movement of the held substrate  15  relative to the polishing pad  10 . The carrier head assembly  30  may also be adapted to sweep the held substrate  15  in a linear or arcuate motion across the polishing surface of polishing pad  10 . The carrier head assembly  30  is typically configured to hold the substrate  15  and provide movement relative to the polishing pad  10  along a pre-determined radial position or area of the polishing pad  10  during processing. 
         [0010]    An optical sensor  40  is provided in the platen  20  for measuring a film thickness of an insulating film (or layer) or a metallic film (or layer) formed on the surface of the substrate  15 . The optical sensor  40  may be coupled in or on the platen  20  as shown, or coupled to a base  35  at a suitable radial location. The optical sensor  40  is typically fixed to the base  35  in that radial location. The optical sensor  40  may be a light-emitting element and a light-detecting element or an electromagnetic signal transmitter/receiver. The polishing pad  10  has a single transparent window  41  mounted therein and aligned with the optical sensor  40  for allowing signals from the optical sensor  40  to pass therethrough. The optical sensor  40  is electrically connected to a controller and the controller is connected to a display unit  45  for inspection by a user. Although the carrier head assembly  30  rotates and/or sweeps, the location of the window  41  and/or optical sensor are typically chosen based on the substrate  15  location in the pre-determined radial area or position of the polishing pad  10  during processing in order to facilitate access of the optical sensor  40  to the substantial geometric center of the substrate  15  during rotational and/or sweeping movement. 
         [0011]    While the conventional CMP station  1  is suitable for performing a polishing process and endpoint detection on a single substrate  15  at a pre-determined radial area of the polishing pad  10 , it may not be suitable for performing a polishing process and endpoint detection on more than one substrate and/or on a system where the pre-determined radial position of the substrate may change. In one example, one or more substrates held in respective carrier head assemblies may be caused to contact the polishing pad at the same time. In another example, a first substrate may be caused to contact the polishing pad  10  at or near a perimeter of the polishing pad  10 , while a second substrate may be caused to contact the polishing pad  10  at or near a center of the polishing pad  10 . In both of these examples, the optical sensor  40  may not be in a suitable position to access the substrate for a reliable endpoint detection metric to be obtained. 
         [0012]    What is needed is a CMP polishing station and polishing pad that is configured to adapt to various polishing motions and positional changes of the substrate being polished, which will provide enhanced flexibility and/or provide a more reliable endpoint detection metric. 
       SUMMARY OF THE INVENTION 
       [0013]    Embodiments described herein provide a method and apparatus for detecting and obtaining a metric indicative of a polishing process. The apparatus includes a polishing pad having an optically transparent region adapted to obtain polishing metric from at least one substrate from at least two distinct radial positions of the polishing pad. The method includes obtaining a polishing metric from at least two substrates being polished simultaneously on a single polishing pad. 
         [0014]    In one embodiment, a polishing pad assembly is described. The polishing pad includes a circular body comprising a polishing material having a polishing surface, and at least two portions formed in the polishing surface that are transparent to light and electromagnetic radiation, each of the at least two portions comprising a transmissive window disposed below a plane defined by the polishing surface and the transmissive window is coupled to the body by a bonding agent. 
         [0015]    In another embodiment, a polishing pad assembly is described. The polishing pad assembly includes a circular body comprising a polishing material having a polishing surface, and a linear strip positioned along a portion of the radius of the polishing surface that is transparent to light and electromagnetic radiation, the linear strip occupying at least one half of the radius of the polishing surface and comprising a transmissive window coupled to the body by a bonding agent and disposed below a plane defined by the polishing surface. 
         [0016]    In another embodiment, a method for obtaining a polishing metric while polishing at least two substrates simultaneously on a single circular polishing pad is described. The method includes providing rotational movement of the polishing pad relative to each of the at least two substrates, providing at least one optical sensor in selective communication with a surface of both of the at least two substrates at different radial positions, emitting light or an electromagnetic signal toward each substrate at intervals corresponding with the rotational movement of the polishing pad, and receiving a reflected signal from a surface of at least one of the substrates indicative of polishing progress. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    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. 
           [0018]      FIG. 1  is cross-sectional view of a conventional chemical mechanical polishing station according to the prior art. 
           [0019]      FIG. 2  is a cross-sectional view of one embodiment of a chemical mechanical polishing station. 
           [0020]      FIG. 3  is a cross-sectional view of a portion of one embodiment of polishing pad. 
           [0021]      FIG. 4  is a top view of one embodiment of a polishing pad. 
           [0022]      FIG. 5  is a top view of another embodiment of a polishing pad. 
           [0023]      FIG. 6  is a top view of another embodiment of a polishing pad. 
           [0024]      FIG. 7  is a top view of another embodiment of a polishing pad. 
       
    
    
       [0025]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. 
       DETAILED DESCRIPTION 
       [0026]    Embodiments described herein provide a polishing system and pad assembly suitable for use in a polishing process. The polishing system and pad assembly is capable of providing a metric indicative of polishing performance at any radial location or radial area of the polishing pad surface and/or facilitate endpoint detection of a polishing process at multiple locations of the polishing pad surface. In one embodiment, a polishing system is described that is adapted to perform a polishing process on at least two substrates simultaneously while providing a polishing metric, such as endpoint data, on each of the at least two substrates. 
         [0027]      FIG. 2  shows a simplified view of a chemical mechanical polishing (CMP) station  200 , partly in cross-section, that may be a stand-alone unit or part of a larger polishing system. The CMP station  200  includes a platen  220  with a polishing pad  210  mounted thereon. In this embodiment, the CMP station  200  includes two carrier head assemblies shown as  230 A and  230 B. Each carrier head assembly  230 A,  230 B is adapted to hold substrates, such as semiconductor substrates  215 A,  215 B, respectively, to be polished. Each carrier head assembly  230 A,  230 B is configured to urge the respective substrate against the upper surface of the polishing pad  210 . The polishing pad  210  has an upper surface serving as a polishing surface  218 , which is brought into sliding contact with each substrate  215 A,  215 B to be polished. A polishing liquid supply nozzle  222  is disposed above the platen  220  for supplying a polishing liquid  219  onto the polishing pad  210 . 
         [0028]    The polishing pad  210  also includes one or more windows, such as a window W 1  and W 2  that are formed or disposed in the polishing pad  210 . Each of the one or more windows W 1  and W 2  are positioned in the polishing pad  210  to provide enhanced access to an optical sensor at more than one location on the polishing pad  210 . Additionally, if a single window is used, the single window provides a larger opening in the polishing pad  210  to provide a large access area to the optical sensor for endpoint detection and process monitoring. 
         [0029]    The polishing pad  210  may be a polymer material, which may be solely dielectric to facilitate removal of materials from each of the substrates  215 A,  215 B. Alternatively, the polishing pad  210  may be at least partially conductive to facilitate electrochemical dissolution of material from the each of the substrates  215 A,  215 B in an electrochemical mechanical polishing (ECMP) process. Suitable polymeric materials that may be used include polyurethane, polycarbonate, fluoropolymers, PTFE, PTFA, polyphenylene sulfide (PPS), or combinations thereof, and other polishing materials used in polishing substrate surfaces. In one embodiment, polishing pad  210  includes at least a polishing surface made of a polymeric material, such as open-pored or closed-pored polyurethane material typically used in the fabrication of polishing pads for service in the polishing of semiconductor wafers. In another application, the polishing pad  210  may contain fixed abrasives. Thus, the polishing liquid  219  may be a slurry or an electrolytic fluid depending on the polishing process used. 
         [0030]    While only two carrier head assemblies  230 A,  230 B are shown, more carrier heads may be provided to hold additional substrates so that the surface area of polishing pad  210  may be used efficiently. Thus, the number of carrier head assemblies adapted to hold substrates for a simultaneous polishing process may be based, at least in part, on the surface area of the polishing pad  210 . While only one polishing liquid supply nozzle  222  is shown, additional nozzles, such as one or more dedicated polishing liquid supply nozzles per carrier head assembly may be used. 
         [0031]    The platen  220  is coupled to a shaft  242  and a motor  221  to rotate the platen  220  and the polishing pad  210  about an axis. Each of the carrier head assemblies  230 A,  230 B are coupled to a shaft  232 A,  232 B, which may be coupled to a motor and a lifting/lowering device (not shown) that is adapted to urge the respective substrates  215 A,  215 B against the polishing surface  218  of the polishing pad  210 . The carrier head assemblies  230 A,  230 B may be rotatable to provide movement of the respective substrates  215 A,  215 B relative to the polishing pad  210 . The carrier head assemblies  230 A,  230 B may also be adapted to sweep the respective substrates  215 A,  215 B in a linear or arcuate motion across the polishing surface  218  of polishing pad  210 . In one embodiment, the carrier head assemblies  230 A,  230 B are disposed on a circular track (not shown) mounted above the polishing pad  210 . 
         [0032]    At least one optical sensor  240 A is provided on the CMP station  200  for measuring a thickness of an insulating film (or layer) or a metallic film (or layer) formed on the surface of the substrates  215 A,  215 B. As an option, an additional optical sensor  240 B may be coupled to the CMP station  200 . In one embodiment, the optical sensor  240 A is coupled to a base  235  and is stationary relative to any rotational movement of the platen  220  and/or the polishing pad  210 . 
         [0033]    In one embodiment, the optical sensor  240 A is movable, at least in a linear direction, relative to the platen  220  and/or the polishing pad  210 . One or both of the platen  220  and the base  235  may include a recessed portion and a track  205 A disposed therein allowing linear movement of the optical sensor  240 A (and  240 B). In one embodiment, the optical sensor  240 A may be coupled to the track  205 A disposed on the base  235  and movable along the track by a linear actuator  250 , such as a servo or stepper motor, or a magnetic actuator. In one application, the optical sensor  240 A is configured to be selectively positioned at different positions along at least one half of a dimension of the base  235  or the polishing pad  210 , as chosen by the user or determined by the polishing parameters, in order to align with one or more windows formed in the polishing pad  210 . 
         [0034]    In another embodiment, the optical sensor  240 B is coupled in or on the platen  220  and configured to rotate with the platen  220  and polishing pad  210 . In this embodiment, the optical sensor  240 B may be coupled to a linear track  205 B disposed on a lower surface of the platen  220  and is movable along the track by a linear actuator  250 , such as a servo or stepper motor, or a magnetic actuator. In one application, the optical sensor  240 B is configured to be selectively positioned at differing radial positions along at least one half of a dimension of the platen  220  and/or polishing pad  210 , as chosen by the user or determined by the polishing parameters, in order to align with one or more windows, such as window W 1  and window W 2  formed in the polishing pad  210 . In this embodiment, electrical connections or wires may be disposed through the shaft  242 , or a wireless electrical and signal connection may be provided to control the optical sensor  240 B. The optical sensor  240 B may be a light-emitting element and a light-detecting element or an electromagnetic signal transmitter/receiver. 
         [0035]      FIG. 3  is a cross-sectional view of a portion of one embodiment of polishing pad  210 . The polishing pad  210  includes a body  350 , which includes a polishing layer  360 , a supporting layer  370  and an adhesive layer  380  enabling adhesion to the platen. The polishing pad  210  also includes one or more optical windows  341  (only one is shown in the cross-sectional portion of polishing pad  210  shown in  FIG. 3 ). The polishing layer  360  can include a compressible material, such as a polymeric foam, and has a polishing surface  362 . The polishing layer  360  can be grown on the supporting layer  370  or a pressure sensitive adhesive (PSA) layer may be disposed between the polishing layer  360  and the supporting layer  370 . For example, a polymer layer can be grown on supporting layer  370  to form the polishing layer  360  so that a PSA layer is not needed between the supporting layer  370  and polishing layer  360 . 
         [0036]    An opening  322  extends through polishing layer  360  to house a transmissive window  390 , which is transparent to light or electromagnetic radiation. The transmissive window  390  can be formed of one or more polymeric materials, such as, a polyurethane or a halogenated polymer (e.g., polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), or polytetra-fluoroethylene (PTFE)). In some embodiments, the transmissive window  390  can be formed of a material having a Shore D hardness of from about 20-80. If the hardness for the material for transmissive window  390  is not within a desired range, two materials having two different hardness can be combined to provide a material with hardness in the desired range. For example, liquid forms of two materials having two different hardness can be combined in a ratio calculated to achieve the desired hardness, then the combined material can be cured and cut to size to form transmissive window  390 . 
         [0037]    A window recess  396  may be defined between the plane in which polishing surface  362  lies and the plane in which the upper surface of the transmissive window  390  lies. The window recess  396  is designed to be of a predetermined depth D to ensure that when the compressible material forming the polishing layer  360  is compressed, the transmissive window  390  does not extend beyond the polishing layer  360  and contact the substrate that is being polished. The predetermined depth of the window recess  396  is also designed to be small enough so that air bubbles do not form in any chemical polishing solution that leaks between transmissive window  390  and a substrate during polishing. For example, the window recess  396  can be 3-4 mils deep. Selection of a specific depth to ensure that the transmissive window  390  does not contact a substrate may be determined by the compressibility of the polishing layer  360  and the load applied to the substrate during polishing. In some applications, the upper surface of the transmissive window  390  may at least incidentally contact the substrate being polished. In this application, the transmissive window  390  may be adapted to provide a polishing surface. 
         [0038]    In some embodiments, an opening is formed through the supporting layer  370  to allow the optical sensor  240 A (and/or  240 B) ( FIG. 2 )) to monitor the substrate. However, in the embodiment shown in  FIG. 3 , the supporting layer  370  does not include an opening below the transmissive window  390 . In this embodiment, the supporting layer  370  may be formed from a transparent material to allow monitoring of polishing progress through the material. The supporting layer  370  can be formed of an incompressible and fluid-impermeable polymer. For example, supporting layer  370  can be formed of polyethylene terephthalate (“PET”) or a MYLAR® material. 
         [0039]    The transmissive window  390  is secured to supporting layer  370  by a window bonding adhesive  394 . The transmissive window  390  can be bonded using window bonding adhesive  394  directly to the supporting layer  370  (as shown in  FIG. 3 ), or to an optional adhesive or PSA layer between supporting layer  370  and polishing layer  360  (not shown). Alternatively, the transmissive window  390  could be adhered directly to the adhesive layer disposed between the supporting layer  370  and the polishing layer  360  (without the window bonding layer). The window bonding adhesive  394  is composed of a material that seals any gap between the transmissive window  390  support layer, such as supporting layer  370  or a PSA layer, and transmissive window  390 . The window bonding adhesive  394  also supports the transmissive window  390  against shear stress during polishing. The window bonding adhesive  394  can include an adhesive sealant, such as a viscous rubber-like glue. For example, for some PSA layers, the window bonding adhesive  394  can include one-part room temperature vulcanizing (“RTV”) silicone TSE399™ or TSE397™ materials, which are distributed by GE Silicones of Waterford, N.Y. 
         [0040]      FIG. 4  is a top view of one embodiment of a polishing pad  210  having two optical windows  441 A and  441 B that may be similar to the optical window  341  described in  FIG. 3 . The first optical window  441 A is positioned at a first radial location along a first radial path R 1  of substrate  415 A and the second optical window  441 B is positioned along a second radial path R 2  of substrate  415 B. In one embodiment, R 1  and R 2  are equal such that both optical windows  441 A,  441 B are positioned the same distance from the center C of the polishing pad  210 . In this embodiment, the substrates  415 A and  415 B are rotated relative to the rotating polishing pad  210  and additionally sweep relative to the rotating polishing pad  210  in a direction generally indicated as sweep paths  405 A and  405 B, and each optical window is positioned to allow light or an electromagnetic signal from the optical sensor ( FIG. 2 ) to impinge the center of the respective sweep path  405 A and  405 B. While each of the sweep paths  405 A and  405 B are shown as linear, it is understood that the sweep path may be arcuate as well. 
         [0041]    While not shown, a single, movable optical sensor ( 240 A or  240 B of  FIG. 2 ) is aligned with the first optical window  441 A and second optical window  441 B during each revolution of the polishing pad  210 . Alternatively, the optical sensor is aligned with the first optical window  441 A and second optical window  441 B at a specified interval or intervals based on one or more revolutions of the polishing pad  210 . For example, the controller ( FIG. 2 ) contains an algorithm allowing the optical sensor to move between the two optical windows  441 A and  441 B corresponding with each revolution of the polishing pad  210 . Alternatively, the single optical sensor may move between the two optical windows  441 A and  441 B at alternating intervals based on a fraction of one revolution of the polishing pad  210 . In one application, the optical sensor may alternate between optical windows  441 A,  441 B at every other revolution. In another application, the optical sensor may move between the two optical windows at or about each half revolution of the polishing pad  210 . Alternatively, two optical sensors ( 240 A and  240 B of  FIG. 2 ) may be dedicated to and positioned to align with each optical window  441 A,  441 B. In this manner, an endpoint metric may be provided for each substrate  415 A and  415 B as the respective substrate passes an optical window at each revolution, or at any specified periodicity. 
         [0042]    In another application, the optical windows  441 A,  441 B may be positioned at a specific radius or distance from the center C to overlap with a sweep path of either or both of the substrates  415 A,  415 B in a manner that provides an endpoint metric at different locations on a single substrate. For example, substrate  415 A and its respective sweep path  405 A may be centered at R 1 , which is the same radial position of optical window  441 A. While an algorithm may be provided to provide a signal and metric as the substantial geometric center of substrate  415 A passes optical window  441 A, the optical window  441 B may provide a signal and a metric from another portion of substrate  415 A. Alternatively, an algorithm may be provided to provide a signal and metric as any portion of the substrate  415 A passes optical window  441 A and the optical window  441 B may provide a signal and a metric from a different portion of substrate  415 A. This allows more accurate endpoint determination as the periodicity is increased and/or a greater surface of the substrate is surveyed. In one example, an endpoint metric may be obtained from a center of the substrate and another endpoint metric may be obtained from a periphery of the same substrate at each revolution of the polishing pad  210  (or at any specified periodicity), which in this example is substrate  415 A. 
         [0043]      FIG. 5  is a top view of another embodiment of a polishing pad  210  having four optical windows  541 A- 541 D, wherein each optical window  541 A- 541 D may be similar to the optical window  341  described in  FIG. 3 . In this embodiment, each optical window  541 A- 541 D is positioned at four specific radial locations on the polishing pad  210 . While the pattern of optical windows  541 A- 541 D are shown in an arcuate or spiral pattern on the polishing pad  210 , the optical windows  541 A- 541 D may be any pattern, such as in a cross-shape or X shape, or a linear pattern. Additionally, while four optical windows  541 A- 541 D are shown, the polishing pad  210  may have any number of windows. 
         [0044]      FIG. 6  is a top view of another embodiment of a polishing pad  210 . In this embodiment, the polishing pad includes five optical windows  641 A- 641 E that may be similar to the optical window  341  shown in  FIG. 3 . The optical windows  641 A- 641 E are arranged in a linear fashion, such as in a radial line. In this embodiment, the optical windows  641 A- 641 E are separated by polishing material  662 , which is part of the polishing surface of the polishing pad  210 . Although five optical windows  641 A- 641 E are shown, any number of optical windows may be used. 
         [0045]    Collectively, the optical windows  641 A- 641 E and polishing material  662  between each optical window covers at least about one half of the radius of the polishing pad  210 . For example, if the polishing pad  210  has a diameter of about 42 inches, which corresponds to a radius of 21 inches, the collective length of the optical windows  641 A- 641 E and polishing material  662  between each optical window covers at least about 10.5 inches. In another embodiment, the optical windows  641 A- 641 E and polishing material  662  between each optical window collectively covers at least about 75% to about 90% of the radius of the polishing pad  210 . For example, if the polishing pad  210  has a diameter of about 42 inches, which corresponds to a radius of 21 inches, the collective length of the optical windows  641 A- 641 E and polishing material  662  between each optical window covers about 15.7 inches to about 18.9 inches. 
         [0046]    In another embodiment, the optical windows  641 A- 641 E may be spaced along the radial line in a manner that spaces each optical window at specific radial distances from center C and/or an outside diameter of the polishing pad  210 . In one example, if the polishing pad  210  has a diameter of about 42 inches, which corresponds to a radius of 21 inches, the center of each optical window  641 A- 641 E may be spaced at about 4 inch increments. In another embodiment, the perimeter optical window  641 E may be spaced about 1-2 inches from the outer diameter of the polishing pad  210 , and the remaining optical windows may be spaced, either equally or unequally, towards the center C. 
         [0047]      FIG. 7  is a top view of another embodiment of a polishing pad  210 . In this embodiment, the polishing pad includes a single radially elongated optical window  741  that may be similar to the optical window  341  shown in  FIG. 3 , with the exception of occupying a greater portion of the area of the polishing pad  210 . In this embodiment, the optical window  741  is arranged in a linear fashion, such as in a line corresponding to a radius, and includes no polishing material therebetween. While the optical window  741  is disposed along a radius of the polishing pad  210 , the optical window  741  may be disposed on the polishing pad  210  as a chord, or a segment thereof (not shown). In another embodiment (not shown), optical window  741  may be formed as an arc. 
         [0048]    In the embodiments shown in  FIGS. 5-7 , a single, movable optical sensor, such as optical sensor  240 A or  240 B ( FIG. 2 ) is aligned with the optical windows  541 A- 541 D or  641 A- 641 E, or at various points along the single optical window  741  during each revolution of the polishing pad  210 , or at specified intervals, such as every half revolution or every other revolution of the polishing pad  210 . For example, the controller ( FIG. 2 ) contains an algorithm allowing the optical sensor to move between various positions corresponding with each revolution of the polishing pad  210 , or the optical sensor may move between positions to align with the optical window(s) at alternating revolutionary intervals, such as half revolutions, one-quarter revolutions, among other intervals. Alternatively, more than one optical sensor may be dedicated to and positioned to align with one or more of the optical windows  541 A- 541 D or  641 A- 641 E. In another alternative, more than one optical sensor may be positioned, either fixed or movable relative to the polishing pad  210  and/or platen (not shown), to be aligned with various points along the single optical window  741  at each revolution or at any specified periodicity. In this manner, an endpoint metric may be provided for one or more substrates as the respective substrate passes an optical window and/or optical sensor at each revolution, or at any specified periodicity. In yet another application, at least one optical sensor may be configured to align with one or more optical sensors to provide an endpoint metric at more than one location on a single substrate. 
         [0049]    The inventive polishing pad is configured to adapt to various polishing motions and/or positional changes of a substrate or substrates being polished on the pad. The polishing pad as described herein provides enhanced flexibility and/or a more reliable endpoint detection metric of the substrate or substrates being polished. Modification of a polishing pad as described herein minimizes the available polishing surface of the polishing pad as additional sections of the polishing surface are removed for installation of additional or larger windows. Further, properties of the pad, such as flatness, hardness and other properties are changed when additional or larger windows are added. Thus, modification of a polishing pad meets general reluctance in the field due to the aforementioned problems. However, the inventive polishing pad with multiple and/or larger windows has provided a more precise endpoint metric and enables an endpoint metric at a greater frequency. 
         [0050]    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.