Abstract:
A polishing pad, polishing system, method of making a polishing pad and method of using a polishing pad. The polishing pad includes a polishing layer having a polishing surface, a backing layer with an aperture and a first portion that is permeable to liquid, and a sealant that penetrates a second portion of the backing layer adjacent to and surrounding the aperture such that the second portion is substantially impermeable to liquid. The aperture is positioned below a substantially fluid-impermeable element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional (and claims the benefit of priority under 35 USC 120) of U.S. patent application Ser. No. 11/213,623, filed on Aug. 26, 2005. The disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application. 

   BACKGROUND 
   This present invention relates to chemical mechanical polishing. 
   An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface, and planarizing the filler layer until the non-planar surface is exposed. For example, a conductive filler layer can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. The filler layer is then polished until the raised pattern of the insulative layer is exposed. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate. In addition, planarization is needed to planarize the substrate surface for photolithography. 
   Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing disk pad or belt pad. The polishing pad can be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing liquid, which can include abrasive particles, is supplied to the surface of the polishing pad. 
   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 
   The invention provides methods and apparatus for sealing a portion of a polishing pad to prevent liquid from collecting on a bottom surface of a window. 
   In one aspect, the invention is directed to a polishing pad for use in a chemical mechanical polishing system. The polishing pad includes a polishing layer having a polishing surface. a backing layer including a first portion that is permeable to liquid, a window from the polishing surface to a bottom surface of the polishing pad, and a sealant. The window includes a transparent portion that is substantially impermeable to liquid secured to the polishing pad and an aperture in the backing layer aligned with the transparent portion and positioned on a side of the transparent portion opposite the polishing surface. The sealant penetrate a second portion of the backing layer adjacent to and surrounding the aperture such that the second portion is substantially impermeable to liquid. 
   Implementations of the invention may include one or more of the following features. The backing layer may be a foam. The sealant may be silicone. The polishing layer may be generally impermeable to liquid. A top surface of the transparent portion may be coplanar with the polishing surface. A bottom surface of the transparent portion may be coplanar with a lower surface of the polishing layer. The first portion may extends adjacent to an outer peripheral edge of the backing layer. A recess may be formed in the lower surface of the transparent portion. 
   In one aspect, the invention is directed to a polishing system. The polishing system includes a polishing pad, a platen, and a monitoring module. The polishing pad includes a polishing layer having a polishing surface and a backing layer with an aperture and a first portion that is permeable to liquid. The aperture is positioned below a substantially fluid-impermeable element, and a sealant that penetrates a second portion of the backing layer adjacent to and surrounding the aperture such that the second portion is substantially impermeable to liquid. The platen supports the polishing pad and includes a second recess, and the monitoring module is positioned in the recess. A volume is formed at least in part between a lower surface of the fluid-impermeable element and an upper surface of the optical monitoring module. 
   Implementations of the invention may include one or more of the following features. The monitoring module may be an optical monitoring module and fluid-impermeable element may be transparent. A purge system may direct a purge gas into the volume and/or draw fluid out of the volume. The purge gas may include clean dry air, nitrogen, or inert gas. The purge system may include an exit passage connected to an external environment. A portion of the monitoring module may extend into the polishing pad. The sealant may be silicone. The fluid-impermeable element may be the polishing layer. 
   In another aspect, the invention is directed to a method of making a polishing pad. The method includes securing a polishing layer with a polishing surface to a backing layer with a first portion that is permeable to liquid, forming a window from the polishing surface to a bottom surface of the polishing pad, and applying a sealant. The window includes a transparent portion that is substantially impermeable to liquid secured to the polishing pad and an aperture in the backing layer aligned with the transparent portion and positioned on a side of the transparent portion opposite the polishing surface. The sealant penetrates a second portion of the backing layer adjacent to and surrounding the aperture such that the second portion is substantially impermeable to liquid. 
   Implementations of the invention may include one or more of the following features. The sealant may be applied after the aperture in the backing layer is formed, after the window is formed, or after the polishing layer is secured to the backing layer. 
   The invention can provide one or more of the following advantages. Collection of liquid on the bottom surface of the window, such as by condensation or fogging, can be reduced. This can improve optical signal strength, thus reducing noise, and thereby improve endpoint detection reliability. 
   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. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic side view, partially cross-sectional, of a chemical mechanical polishing station with a polishing pad according to the present invention. 
       FIG. 2  is an enlarged cross-sectional view of a portion of the polishing pad on a platen. 
       FIG. 3  is a schematic bottom view of the polishing pad. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
   As shown in  FIG. 1 , one or more substrates  10  can be polished by a CMP apparatus  20 . A description of a suitable polishing apparatus  20  can be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. 
   The polishing apparatus  20  includes a rotatable disk-shaped platen  24  on which is placed a polishing pad  30 . The polishing pad  30  can be secured to the platen  24 , e.g., by a layer of adhesive. The polishing pad  30  can be a two-layer polishing pad with an outer cover layer or polishing layer  32  that provides a polishing surface  36 , and a backing layer  34 . In general, although the outer polishing layer is roughened and can transport slurry, it is generally fluid-impermeable. The outer polishing layer  32  may be a cast polyurethane with fillers, such as a layer of IC-1000 from Rodel. In addition, the polishing The backing layer  34  is typically softer than the polishing layer  32 , and may be formed from a foam or fibrous mat, such as a layer of PORON, e.g., PORON 4701-30 from Rogers Corporation, or Suba-IV from Rodel, that can be fluid-permeable. Slurry transport grooves may be formed in the polishing surface by a milling or molding process. 
   The polishing station can also include a pad conditioner apparatus to maintain the condition of the polishing pad so that it will effectively polish substrates. During a polishing step, a polishing liquid  38 , e.g., a slurry, containing a liquid and a pH adjuster can be supplied to the surface of polishing pad  30  by a slurry supply port or combined slurry/rinse arm  39 . The polishing liquid  38  can also include abrasive particles. 
   A carrier head  70  can hold the substrate  10  against the polishing pad  30 . The carrier head  70  is suspended from a support structure, for example, a carousel, and is connected by a carrier drive shaft  74  to a carrier head rotation motor so that the carrier head can rotate about an axis  71 . In addition, the carrier head  70  can oscillate laterally in a radial slot formed the support structure  72 . In operation, the platen is rotated about its central axis  25 , and the carrier head is rotated about its central axis  71  and translated laterally across the top surface of the polishing pad. A description of a suitable carrier head  70  can be found in U.S. patent application Ser. Nos. 09/470,820, 09/535,575 and 10/810,784, filed Dec. 23, 1999, Mar. 27, 2000, and Mar. 26, 2004, the entire disclosures of which are incorporated by reference. 
   A recess  26  is formed in the platen  24 , and an in-situ monitoring module  50  of an in-situ monitoring system fits into the recess  26 . The in-situ monitoring system can be an optical monitoring system, or a combination of an optical monitoring system with another type of monitoring system such as an eddy current monitoring system. The in-situ monitoring module  50  can include one or more sensor elements, which provide better resolution when they are situated close to the substrate being polished. Examples of a sensor element include but are not limited to an optical fiber and a ferromagnetic core. A suitable in-situ modules is further described in commonly owned U.S. patent application Ser. No. 09/847,867, filed on May 2, 2001, Ser. No. 10/124,507, filed on Apr. 16, 2002, Ser. No. 10/123,917, also filed on Apr. 16, 2002, and Ser. No. 10/633,276, filed on Jul. 31, 2003, which are hereby incorporated by reference in their entireties. In some implementations, the monitoring system might not include an optical monitoring system. In this case, the pad need not include a transparent portion, although the monitoring module should be positioned below a fluid-impermeable element, e.g., an opaque plug or the polishing layer itself. 
   Referring to  FIGS. 2 and 3 , the polishing pad can include a solid transparent portion  42  that provides a window  40 . The transparent portion  42  can be an integral portion of the polishing pad, or it can be an element secured, e.g., molded or adhesively attached, to the polishing pad. In particular, the window  40  can include a transparent portion  42  positioned in the polishing layer  32  with generally the same thickness as the polishing layer, and an aperture  44  in the backing layer  34  that is aligned with the transparent portion  42 . A top surface of the transparent portion  42  can be co-planar with the polishing surface  36 . In addition, one or more optional recesses can be formed in the bottom surface  46  of the transparent portion  42  that extend partially but not entirely through the transparent portion. In general, the material of the transparent portion  42  should be non-magnetic and non-conductive. The plug can be a relatively pure polymer or polyurethane, for example, formed without fillers, or the plug can be formed of a fluorocarbon, such as Teflon, or a polycarbonate. In an implementation in which the window includes a rigid crystalline portion or glass-like portion and the recess is formed in the bottom surface of this portion by machining, the recess can be polished so as to remove scratches caused by the machining. Alternatively, a solvent and/or a liquid polymer can be applied to the surfaces of the recess to remove scratches caused by machining. The removal of scratches usually caused by machining reduces scattering and can improve the transmittance of light through the window. 
   In general, the transparent portion  42  is secured to the polishing pad so as to prevent fluid from flowing from the polishing surface  36  into the region below the transparent surface. In one implementation, forming the window  40  includes cutting a hole in the polishing layer  32  and securing the transparent portion  42  in the hole. For example, the transparent portion  42  may secured by an adhesive to the backing layer  34  and/or to the polishing layer  32 . The adhesive can form a slurry-tight seal between the transparent potion  42  and the polishing layer  32  and/or backing layer  34 . As another example, the transparent portion  42  can be secured by dispensing a liquid window material into the hole and curing the liquid to mold the transparent portion  42  in place. In another implementation, forming the window  40  includes forming the transparent portion  42  during fabrication of the polishing layer  32 . For example, a transparent plug can be positioned in a liquid pad material, and the liquid pad material can be cured to solidify the polishing layer  32  around the transparent portion  42 . In either case where the transparent portion is molded to the polishing layer, the window may be formed in a cast block of pad material from which the polishing layer (including transparent portion) is then cut. Where the transparent portion  42  is to be secured directly to the polishing layer  32 , the securing step can occur before or after the polishing layer  32  is attached to the backing layer  34 . 
   The window  40  is situated over at least a portion of the recess  26  and the module  50 . The module  50  and window  40  are positioned such that they pass beneath substrate  10  during a portion of the platen&#39;s rotation. In some implementations, a portion of the module  50 , such as a ferromagnetic core, extends into and partially (but not entirely) through the polishing pad  30 . 
   Optionally, the module  50  can include a purge system to purge liquids and gases from a volume  64  between the top surface of the module  50  and the bottom surface of the transparent portion  42 . The purge system can include a fluid inlet line  60  coupled to a purge gas source, and a fluid outlet line  62  that can be coupled to a vacuum source. In general, the fluid lines  60  and  62  will extend through the platen and through a rotary coupling to the purge gas source and vacuum source. Although illustrated as extending through the module  50 , the fluid lines  60  and  62  can be connected directly to the volume  64  without passing through the module  50 . Alternatively, the fluid outlet line  62  can simply extend to the external environment, in which case the outline line may simply pass through the platen (and not through the rotary coupling). 
   In operation, the purge gas can flow continuously through the volume  64 , preventing water vapor from accumulating in the volume and thus preventing condensation or fogging on the bottom surface of the transparent portion  42 . The purge gas can be a composition, e.g., clean dry air, nitrogen, or an inert gas, that does not interfere with the polishing process, does not damage the polishing pad, and does not include vapor which might condense. 
   One potential problem is that, if the backing layer  34  is fluid-permeable, the suction generated by the fluid outline line  62  can draw liquid from the edge of the backing layer into the volume  64 . This can result in condensation or fogging, even if a purge gas is flowing through the volume  64 . 
   To address this issue, a portion  48  of the backing layer  34  can be made substantially impermeable to liquid so that liquid will not reach the volume  64 . In particular, a portion  48  of the backing layer immediately adjacent the aperture  44  can have a permeability much lower than that of the remaining portion  54  of the backing layer  34 . The remaining portion  54  can include a portion at the peripheral edge  56  of the backing layer  34 . 
   To create the impermeable portion, a sealant can be applied to the backing layer so that the sealant permeates the backing layer. The sealant penetrates the backing layer to plug pores, thus providing the fluid-impermeable portion  48  of the backing layer. The sealant can be, for example, silicone, or another polymer sealant. The sealant may be applied in liquid form and then harden, e.g., be cured. The sealant can be applied before or after the backing layer is attached to polishing layer, and can be applied before or after the window is completed. 
   The above described apparatus and methods can be applied in a variety of polishing systems. Either the polishing pad, or the carrier head, or both can move to provide relative motion between the polishing surface and the substrate. 
   The polishing pad can be a circular (or some other shape) pad secured to the platen. Terms of relative positioning are used; it should be understood that the polishing surface and substrate can be held in a vertical orientation or some other orientation. The polishing layer can be a standard (for example, polyurethane with or without fillers) polishing material, a soft material, or a fixed-abrasive material. The entire polishing layer can be transparent, and a portion of the opaque backing layer can be removed to provide the window. There may be additional layers between the backing layer and the polishing layer, or below the polishing layer. A portion of the transparent portion may project into the aperture in the backing layer. The aperture in the backing layer may be larger than the aperture in the polishing layer, and the transparent portion may be secured to a lip on the underside of the polishing layer. 
   A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the specification.