Abstract:
A polishing pad has an opaque polishing layer with an aperture therethrough and a polishing surface, and a solid light-transmissive window in the aperture. The solid light-transmissive window includes an outer portion secured to the polishing layer and an inner portion secured to the outer portion. The outer portion has a upper surface recessed relative to the polishing surface, whereas the inner portion has an upper surface that is substantially co-planar with the polishing surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Application Ser. No. 60/818,423, filed on Jul. 3, 2006. 
     
     BACKGROUND 
       [0002]    This invention relates a polishing pad for use in chemical mechanical polishing (CMP). 
         [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, the invention is directed to a polishing pad. The polishing pad has an opaque polishing layer with an aperture therethrough and a polishing surface, and a solid light-transmissive window in the aperture. The solid light-transmissive window includes an outer portion secured to the polishing layer and an inner portion secured to the outer portion. The outer portion has a upper surface recessed relative to the polishing surface, whereas the inner portion has an upper surface that is substantially co-planar with the polishing surface. 
         [0007]    Implementations of the inventions may include one or more of the following features. The outer portion can surround the inner portion. The outer portion can be rectangular and the inner portion can be square. The inner portion and the polishing layer can have substantially the same hardness. The outer portion can be harder than the inner portion. The outer portion can have substantially the same hardness as the polishing layer. Corners of the inner portion that project above the upper surface of the outer portion can be smoothed, e.g., beveled or rounded. Corners of an inner edge of the polishing layer that project above the upper surface of the outer portion can be smoothed. The inner portion can be molded to the outer portion. Bottom surfaces of the polishing layer, the first portion and the second portion can be substantially coplanar. 
         [0008]    In another implementation, the invention is directed to a method of fabrication a polishing pad. The method includes forming a first light-transmissive layer in an aperture in an opaque polishing layer, and forming a second light-transmissive layer in an aperture in the first light-transmissive layer. The first light-transmissive layer has an upper surface recessed relative to a polishing surface of the polishing layer, and the second light-transmissive layer has an upper surface that is substantially co-planar with the polishing surface. 
         [0009]    Implementations of the inventions may include one or more of the following features. Forming the second light-transmissive layer in the aperture in the first light-transmissive layer can include cutting a hole in the first light-transmissive layer. Forming the second light-transmissive layer in an aperture in the first light-transmissive layer can include filling a hole in the first light-transmissive layer with a liquid precursor and curing the precursor. Curing the precursor can create a transparent body that projects above the polishing surface. The body can be ground until an upper surface of the second portion is substantially co-planar with the polishing surface. Filling the hole with the liquid precursor can create a meniscus that projects above the polishing surface. Corners of the second portion that project above the upper surface of the first portion can be smoothed. 
         [0010]    Potential advantages of the invention may include one or more of the following. The window is relative soft (e.g., as compared to a conventional window for an IC1000 type polishing pad. Thus, the window can be used with a softer polishing pad, e.g., a Politex polishing pad, with low danger of scratching the substrate. 
         [0011]    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. 
     
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a schematic across-sectional side view of a chemical mechanical polishing apparatus with an optical monitoring system for endpoint detection. 
           [0013]      FIG. 2  is a simplified top view of a polishing pad with a window. 
           [0014]      FIG. 3  is a simplified schematic cross-sectional view of the polishing pad of  FIG. 2  along line  3 - 3 . 
           [0015]      FIG. 4  is a simplified schematic cross-sectional view of a polishing pad with a pressure sensitive adhesive and liner. 
           [0016]      FIGS. 5-8  are cross-sectional views illustrating assembly of a polishing pad. 
       
    
    
       [0017]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0018]    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. 
         [0019]    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. 
         [0020]    The effective portion of the polishing pad  18  can include a polishing layer  20  with a bottom surface  22  to secured to the platen  16  and a polishing surface  24  to contact the substrate. The polishing layer can be a relatively soft material suitable for a buffing process. Such polishing pads can have a hardness in the Shore A range, e.g., 50 to 80 Shore A. In one implementation, the polishing pad includes a poromeric coating with large vertically oriented pores disposed over a microporous felt substrate. Such a polishing pad is available under the trade name Politex from Rohm &amp; Hass. An example of soft polishing pad is described in U.S. Pat. No. 4,841,680. In some implementations, grooves can be formed in the polishing surface  24 . 
         [0021]    Typically the polishing pad material is wetted with a chemical polishing liquid solution  30  with abrasive particles. The liquid can be a solution including a chemically reactive components. 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 conduction 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 . 
         [0025]    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. 
         [0026]    Referring to  FIG. 2 , in one implementation 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. The optical monitoring system can use an area about 0.5 inches (12.70 mm) wide and 0.75 inches (19.05 mm) long centered a distance D of 7.5 inches (190.50 mm) from the center of the polishing pad  18 . Thus, the window should cover at least this area. 
         [0027]    Referring to  FIG. 2-3 , the window  40  can include two portions, a thin outer portion  50  and a thicker central portion  60 . Both portions of the window can formed from a polymer material, e.g. polyurethane. 
         [0028]    The thin outer portion  50  can have a top surface  54  that is recessed relative the uncompressed polishing surface  24 . The outer portion  50  can be secured to the inner edges  26  of the polishing layer  20 . Alternatively, if the polishing pad  18  includes a backing layer, e.g., a compressible subpad or an incompressible backing film, then the outer portion can be secured to the backing layer. In addition, the outer portion  50  of the window  40  can be formed of a material that is harder than the polishing layer  20 , e.g., a relatively pure polyurethane without fillers, e.g., JR111 or Calthan 3200. The polishing layer  20  itself does not extend over the outer portion  50  of the window  40 , so that the top surface  54  is exposed to the polishing environment and can transmit light. 
         [0029]    The outer portion  50  of the window  40  can have a 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 outer portion  50  can have other shapes, such as circular or oval, and the center of the window need not be located at the center of the area used by the optical monitoring system. The outer portion  62  can have a length of about 2.25 (57.15 mm) inches and a width of about 0.75 inches (19.05 mm). 
         [0030]    The thick central portion  60  of the window  40  can have a top surface  64  that is substantially coplanar with the polishing surface  24 . The bottom surface of the central portion  60  can be coplanar with both the bottom surface of the thin portion  50  and the polishing layer  20 . The central portion  60  can be secured to the inner edges  56  of the outer portion  50 , e.g., by being cured in place in an aperture in the outer portion and thus molded to the outer portion. The outer portion  50  can completely surround the central portion  60 . 
         [0031]    The thick central portion  60  can be formed of the same material as the thin outer portion  50 , e.g., a relatively pure polyurethane without fillers, but with about the same hardness as the polishing layer  20  (the thick portion can be formed using a different ratio of precursors, e.g., polyol and diisocyanate, than the thin portion in order to achieve the different hardness). Thus, the thick portion  60  is softer than the thin portion  50 . Because the central portion  60  has about the same hardness as the polishing layer  20 , the likelihood of scratching the substrate can be reduced, thus increasing yield. 
         [0032]    The central portion  60  of the window  40  can be square and be positioned in the center of the outer portion  50 . However, the central portion  60  can have other shapes, such as circular. A circular central portion may be less likely to scratch the substrate. The central portion can be about 0.5 inches across, e.g., a 0.5 by 0.5 inch square. 
         [0033]    In one implementation of the polishing pad, the outer portion  50  is rectangular whereas the central portion  60  is square. In another implementation, the outer portion  50  rectangular whereas the central portion  60  is circular. In another implementation, the outer portion  50  and the central portion  60  are generally congruent shapes, e.g., both rectangular or both circular. 
         [0034]    The corners  68  of the thick central portion  60  that project above the thin outer portion  50  can be smoothed, e.g., rounded or beveled, to further reduce the likelihood of scratching the substrate. The inner corners  28  of the polishing layer  20  can also be smoothed, e.g., rounded or beveled. 
         [0035]    Referring to  FIG. 4 , before installation on a platen, the polishing pad  18  can also include a pressure sensitive adhesive  70  and a liner  72  that spans the bottom surface  22  of the polishing pad. In use, the liner is peeled from the polishing layer  20 , and the polishing layer  20  is applied to the platen with the pressure sensitive adhesive  70 . The pressure sensitive adhesive  70  and liner  72  can span the window  40 , or either or both can be removed in and immediately around the region of the window  40 . 
         [0036]    To manufacture the polishing pad, initially a thin window layer (which will become thin portion  50 ) can be installed in the polishing layer  20 , as shown by  FIG. 5 . Then, the region in which the thick central portion will be formed is removed from window layer, as shown by  FIG. 6 . One or more liquid polyurethane precursors are poured into the hole. Surface tension of the precursor liquid is such that a meniscus is formed so that the liquid protrudes above the polishing surface  24 , as shown by  FIG. 7 . Then the liquid polyurethane is cured to form a solid plastic, and the solid plastic is flattened, e.g., by abrasion with a diamond conditioning disk, to form the thick central portion of the window, as shown by  FIG. 8 . The corners of the thick central portion and the polishing layer can then be smoothed, if necessary. 
         [0037]    In another implementation, both the thin outer portion and the thick inner portion of the window are formed of a soft material and have substantially the same hardness. Thus, both the thin outer portion and the thick inner portion have about the same hardness as the polishing layer  20 . 
         [0038]    In general, polishing pads used for buffing, e.g., Politex, are softer than polishing pads used for polishing, e.g., cast polyurethane with fillers, such as IC-1000 material from Rohm &amp; Hass. Thus, in a multi-station polishing system in which the substrate is polishing in sequence by different polishing pads at the different stations, the polishing pad  18  can be the last polishing pad in the sequence and can be the softest polishing in the sequence. 
         [0039]    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. For example, the invention may be applicable to polishing pads made of other materials, e.g., a polyester fiber felt, or to multilayer polishing pads. Accordingly, other embodiments are within the scope of the following claims.