Abstract:
A polishing pad is described that has a polishing layer with a polishing surface, an adhesive layer on a side of the polishing layer opposite the polishing layer, and a solid light-transmitting window extending through and molded to the polishing layer. The window has a top surface coplanar with the polishing surface and a bottom surface coplanar with a lower surface of the adhesive layer. A method of making a polishing pad includes forming an aperture through a polishing layer and an adhesive layer, securing a backing piece to the adhesive layer on a side opposite a polishing surface of the polishing layer, dispensing a liquid polymer into the aperture, and curing the liquid polymer to form a window.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 60/942,956, filed on Jun. 8, 2007. 
     
    
     TECHNICAL FIELD 
       [0002]    A polishing pad with a window, a system containing such a polishing pad, and a process for making and using such a polishing pad are described. 
       BACKGROUND 
       [0003]    In the process of fabricating modern semiconductor integrated circuits (IC), it is often necessary planarize the outer surface of the substrate. For example, planarization may be needed to polish away a conductive filler layer until the top surface of an underlying layer is exposed, leaving the conductive material between the raised pattern of the insulative layer to form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate. In addition, planarization may be needed to flatten and thin an oxide layer to provide a flat surface suitable for photolithography. 
         [0004]    One method for achieving semiconductor substrate planarization or topography removal is chemical mechanical polishing (CMP). A conventional chemical mechanical polishing (CMP) process involves pressing a substrate against a rotating polishing pad in the presence of an abrasive slurry. 
         [0005]    In general, there is a need to detect when the desired surface planarity or layer thickness has been reached or when an underlying layer has been exposed in order to determine whether to stop polishing. Several techniques have been developed for the in-situ detection of endpoints during the CMP process. For example, an optical monitoring system for in-situ measuring of uniformity of a layer on a substrate during polishing of the layer has been employed. The optical monitoring system can include a light source that directs a light beam toward the substrate during polishing, a detector that measures light reflected from the substrate, and a computer that analyzes a signal from the detector and calculates whether the endpoint has been detected. In some CMP systems, the light beam is directed toward the substrate through a window in the polishing pad. 
       SUMMARY 
       [0006]    In one aspect, a polishing pad is described that has a polishing layer with a polishing surface, an adhesive layer on a side of the polishing layer opposite the polishing layer, and a solid light-transmitting window extending through and molded to the polishing layer. The window has a top surface coplanar with the polishing surface and a bottom surface coplanar with a lower surface of the adhesive layer. 
         [0007]    Implementations of the invention may include one or more of the following. The polishing layer may be a single layer. A removable liner may span the adhesive layer. The liner may have a hole aligned with the window. A removable window backing piece may be positioned in the hole in the liner and may abut the window. There may be grooves in the polishing surface, and a portion of the window may project into and be molded to the grooves. The perimeter of the window may follow a roughened path. The polishing pad may be circular, the window may extend along a radius of the polishing pad, and the window is longer along the radius than along a direction normal to the radius. The polishing pad may have a total thickness less than 1 mm. 
         [0008]    In another aspect, a method of making a polishing pad is described. The method includes forming an aperture through a polishing layer and an adhesive layer, securing a backing piece to the adhesive layer on a side opposite a polishing surface of the polishing layer, dispensing a liquid polymer into the aperture, and curing the liquid polymer to form a window. 
         [0009]    Implementations of the invention may include one or more of the following. A hole may be formed in a removable liner, and securing the backing piece may include installing the backing piece in the hole. A portion of the window may project above the polishing surface. The liquid polymer may flows into grooves in the polishing surface. The polishing layer may be a single layer. The aperture may be formed by stamping the polishing pad or cutting the polishing pad. A perimeter of the window may follow a roughened path. The polishing pad may be circular, the window may extend along a radius of the polishing pad, and the window is longer along the radius than along a direction normal to the radius. The polishing pad may have a total thickness less than 1 mm. 
         [0010]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a cross-sectional view of a CMP apparatus containing a polishing pad. 
           [0012]      FIG. 2  is a top view of an embodiment of a polishing pad with a window. 
           [0013]      FIG. 3  is a cross-sectional view of the polishing pad of  FIG. 2 . 
           [0014]      FIGS. 4-9  illustrate a method of forming a polishing pad. 
       
    
    
       [0015]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0016]    As shown in  FIG. 1 , the CMP apparatus  10  includes a polishing head  12  for holding a semiconductor substrate  14  against a polishing pad  18  on a platen  16 . The CMP apparatus may be constructed as described in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. 
         [0017]    The substrate can be, for example, a product substrate (e.g., which includes multiple memory or processor dies), a test substrate, a bare substrate, and a gating substrate. The substrate can be at various stages of integrated circuit fabrication, e.g., the substrate can be a bare wafer, or it can include one or more deposited and/or patterned layers. The term substrate can include circular disks and rectangular sheets. 
         [0018]    The effective portion of the polishing pad  18  can include a polishing layer  20  with a polishing surface  24  to contact the substrate and a bottom surface  22  to secured to the platen  16  by an adhesive  28 . The polishing pad can be a single-layer pad with the polishing layer  20  formed of a thin durable material suitable for a chemical mechanical polishing process. Such a polishing pad is available under the trade name H7000HN from Fujibo in Tokyo, Japan. 
         [0019]    Referring to  FIG. 2 , in some implementations the polishing pad  18  has a radius R of 15.0 inches (381.00 mm), with a corresponding diameter of 30 inches. In other implementations, the polishing pad  18  can have a radius of 15.25 inches (387.35 mm) or 15.5 inches (393.70 mm), with corresponding diameter of 30.5 inches or 31 inches. 
         [0020]    Referring to  FIG. 3 , in some implementations, grooves  26  can be formed in the polishing surface  24 . The grooves can be in a “waffle” pattern, e.g., a cross-hatched pattern of perpendicular grooves with sloped side walls that divide the polishing surface into rectangular, e.g., square, areas. 
         [0021]    Returning to  FIG. 1 , typically the polishing pad material is wetted with the chemical polishing liquid  30 , which can include abrasive particles. For example, the slurry can include KOH (potassium hydroxide) and fumed-silica particles. However, some polishing processes are “abrasive-free”. 
         [0022]    The polishing head  12  applies pressure to the substrate  14  against the polishing pad  18  as the platen rotates about its central axis. In addition, the polishing head  12  is usually rotated about its central axis, and translated across the surface of the platen  16  via a drive shaft or translation arm  32 . The pressure and relative motion between the substrate and the polishing surface, in conjunction with the polishing solution, result in polishing of the substrate. 
         [0023]    An optical aperture  34  is formed in the top surface of the platen  16 . An optical monitoring system, including a light source  36 , such as a laser, and a detector  38 , such as a photodetector, can be located below the top surface of the platen  16 . For example, the optical monitoring system can be located in a chamber inside the platen  16  that is in optical communication with the optical aperture  34 , and can rotate with the platen. The optical aperture  34  can be filled with a transparent solid piece, such as a quartz block, or it can be an empty hole. In one implementation, the optical monitoring system and optical aperture are be formed as part of a module that fits into a corresponding recess in the platen. Alternatively, the optical monitoring system could be a stationary system located below the platen, and the optical aperture could extend through the platen. The light source can employ a wavelength anywhere from the far infrared to ultraviolet, such as red light, although a broadband spectrum, e.g., white light, can also be used, and the detector can be a spectrometer. 
         [0024]    A window  40  is formed in the overlying polishing pad  18  and aligned with the optical aperture  34  in the platen. The window  40  and aperture  34  can be positioned such that they have a view of the substrate  14  held by the polishing head  12  during at least a portion of the platen&#39;s rotation, regardless of the translational position of the head  12 . The light source  36  projects a light beam through the aperture  34  and the window  40  to impinge the surface of the overlying substrate  14  at least during a time when the window  40  is adjacent the substrate  14 . Light reflected from the substrate forms a resultant beam that is detected by the detector  38 . The light source and the detector are coupled to an unillustrated computer that receives the measured light intensity from the detector and uses it to determine the polishing endpoint, e.g., by detecting a sudden change in the reflectivity of the substrate that indicates the exposure of a new layer, by calculating the thickness removed from of the outer layer (such as a transparent oxide layer) using interferometric principles, or by monitoring the signal for predetermined endpoint criteria. 
         [0025]    One problem with placement of a normal large rectangular window (e.g., a 2.25 by 0.75 inch window) into a very thin polishing layer is delamination during polishing. In particular, the lateral frictional force from the substrate during polishing can be greater than the adhesive force of the molding of the window to the sidewall of the pad. 
         [0026]    Returning to  FIG. 2 , the window  40  is thin along the direction of the frictional force applied by the substrate during polishing (tangential to a radius in the case of a rotating a polishing pad) and wide in the direction perpendicular direction (along a radius in the case of a rotating a polishing pad). For example, the window  40  can use an area about 4 mm wide and 9.5 mm long centered a distance D of about 7.5 inches (190.50 mm) from the center of the polishing pad  18 . 
         [0027]    The window  40  can have an approximately rectangular shape with its longer dimension substantially parallel to the radius of the polishing pad that passes through the center of the window. However, the window  40  can have a ragged perimeter  42 , e.g., the perimeter can be longer than a perimeter of a similarly shaped rectangle. This increases the surface area for contact of the window to the sidewall of the polishing pad, and can thereby improve adhesion of the window to the polishing pad. In some implementations, the window  40  includes three generally circular portions  50 ,  52  and  54 , with the center circular portion  52  connected to the outer circular portions  50  and  54  by linear segments  56  and  58 , respective. Each circular portion can have about the same diameter, and the linear segments can be narrower than the diameter of the circular portions. Each circular portion  50 ,  52  and  54  can have a diameter of about 4 mm. 
         [0028]    Referring to  FIG. 3 , the window  40  is as deep as the combination of the polishing layer  20  and the adhesive layer  28 , so that a top surface  44  of the window  40  is coplanar with the polishing surface  24  and a bottom surface  46  of the window is coplanar with a bottom surface of the adhesive layer  28 . The perimeter of the window  40  can be secured, e.g., molded, to the inner sidewall edges  48  of the polishing layer  20 . 
         [0029]    Referring to  FIG. 4 , before installation on a platen, the polishing pad  18  can also include a liner  70  that spans the adhesive layer  28  on the bottom surface  22  of the polishing pad. The liner can be an incompressible and generally fluid-impermeable layer, for example, polyethylene terephthalate (PET), e.g., Mylar™. In use, the liner is manually peeled from the polishing pad, and the polishing layer  20  is applied to the platen with the pressure sensitive adhesive  28 . The liner, however, does not span the window  40 , but is removed in and immediately around the region of the window  40  to form a hole  72 . 
         [0030]    The polishing pad  40  is very thin, e.g., less than 2 mm, e.g., less than 1 mm. For example, the total thickness of the polishing layer  20 , adhesive  28  and liner  70  can be about 0.9 mm. The polishing layer  20  can be about 0.8 mm thick, with the adhesive  28  and the liner  70  providing the remaining 0.1 mm. The grooves  26  can be about half the depth of the polishing pad, e.g., roughly 0.5 mm. 
         [0031]    In addition to the liner  70 , an optional window backing piece  74  can be span the window  40  and be secured to a portion of the pressure sensitive adhesive  28  immediately around the window  40 . The window backing piece  74  can be slightly smaller than the hole  72  so the backing piece is separated from the liner  70  by a gap. The gap can have a width of, for example, a couple millimeters, e.g., 2 mm. The hole  72  and the backing piece  74  can cover an area about twice the maximum dimension of the window  40 . For example, the hole can be a circular area about 24 mm diameter, and the backing piece  72  can be a disk of about 20 mm diameter. The backing piece  72  can be the same thickness as the liner  70 , or thinner than the liner  70 . The backing piece  72  can be polytetrafluoroethylene (PTFE), or another non-stick material. 
         [0032]    To manufacture the polishing pad, initially the polishing layer  20  is formed and the bottom surface of the polishing layer  20  is covered with the pressure sensitive adhesive  28  and a liner layer  70 , as shown by  FIG. 5 . Grooves  26  can be formed in the polishing layer  20  as part of a pad molding process before attachment of the pressure sensitive adhesive  28  and a liner layer  70 , or cut into the polishing layer  20  after the pad is formed and after the liner is attached. 
         [0033]    An aperture  80  is formed through the entire pad, including the polishing layer  20 , the adhesive  28  and the liner  70 , as shown by  FIG. 6 . In particular, to form the window shape shown in  FIG. 2 , three separate holes, e.g., of four mm diameter, can be punched through the pad. Then channels are cut between the holes to form a continuous aperture having a “dumbbell” shape. 
         [0034]    A portion of the liner  70  is removed from the region around the aperture  80  to form the hole  72  in the liner  70 , as shown in  FIG. 7 . For example, the liner  70  can be peeled of the polishing pad entirely, a hole can be punched through the liner around the aperture  80 , and the liner  70  can be placed back on the polishing layer  20  with the hole  72  aligned with the aperture  80 . Alternatively, the hole  72  could be punched in the liner  70  before or during initial assembly of the polishing pad. 
         [0035]    The window backing piece  74 , such as a Teflon™ disk, is then installed in the hole  72  with the edges of the window backing piece  74  abutting the adhesive  28 , as shown in  FIG. 8 . The window backing piece should be cleaned, e.g., wiped with ethanol. The window backing piece  74  will serve as the bottom of the mold for the window. 
         [0036]    A liquid polymer is prepared and transferred into the aperture  80 , and then cured to form the window  40 , as shown in  FIG. 9 . The polymer can be polyurethane, and can be formed from a mixture of several components. In one implementation, the polymer is a mixture of 2 parts Calthane A 2300 and 3 parts Calthane B 2300 (available from Cal Polymers, Inc. of Long Beach, Calif.). The liquid polymer mixture can be degassed, e.g., for 15-30 minutes, before being placed into the aperture. The polymer can be cured at room temperature for about 24 hours, or a heat lamp or oven can be used to decrease cure time. If the cured window  40  projects above the polishing surface then the window can be leveled to be coplanar with the polishing surface, e.g., by abrasion with a diamond conditioning disk. 
         [0037]    The window backing piece  74  can be removed from the aperture  72  by the manufacturer after the cure is complete before shipment of the pad to the customer, or the customer can remove the window backing piece before installation of the polishing pad on the platen. 
         [0038]    If the grooves  24  intersect the aperture  80 , then when the liquid polymer is transferred into the aperture, a portion of the liquid polymer can flow along the grooves  24 . Thus, some of the polymer can extend past the edge of the aperture  80  to form projections into the grooves. When cured, these projections further increase the bonding of the window to the polishing pad. In addition, if sufficient liquid polymer is provided, then some of the liquid polymer can flow over the top surface of the polishing layer. Again, when cured, the portion of the polymer over the polishing surface can increase the bonding of the window to the polishing pad, although as discussed above the portion of the window  40  projecting above the polishing surface can be removed so that the top of the window is flush with the polishing surface. 
         [0039]    While certain embodiments have been described, the invention is not so limited. For example, although a window with a ragged edge is described, the window could be a simpler shape, such as a rectangle or oval. It will be understood that various other 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.