Abstract:
A method of forming a polishing pad with a polishing layer having a polishing surface and a back surface. A plurality of grooves are formed on the polishing surface, and an indentation is formed in the back surface of the polishing layer. A region on the polishing surface corresponding to the indentation in the back surface is free of grooves or has shallower grooves.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. application Ser. No. 10/444,921, filed May 23, 2003, which claims benefit of U.S. Provisional Application Ser. No. 60/398,632, filed Jul. 24, 2002. The entirety of each of the above applications is incorporated herein by reference. 
    
    
     BACKGROUND 
     This present invention relates to polishing pads used in during chemical mechanical polishing and methods and apparatus for monitoring a polishing process. 
     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 slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad. 
     One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness, or when a desired amount of material has been removed. Overpolishing (removing too much) of a conductive layer or film leads to increased circuit resistance. On the other hand, under-polishing (removing too little) of a conductive layer leads to electrical shorting. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time. 
     One way to determine the polishing endpoint is to monitor polishing of the substrate in-situ, e.g., with optical or electrical sensors. One monitoring technique is to induce an eddy current in the metal layer with a magnetic field, and detect changes in the magnetic flux as the metal layer is removed. In brief, the magnetic flux generated by the eddy current is in opposite direction to the excitation flux lines. This magnetic flux is proportional to the eddy current, which is proportional to the resistance of the metal layer, which is proportional to the layer thickness. Thus, a change in the metal layer thickness results in a change in the flux produced by the eddy current. This change in flux induces a change in current in the primary coil, which can be measured as change in impedance. Consequently, a change in coil impedance reflects a change in the metal layer thickness. 
     SUMMARY 
     In one aspect, the invention is directed to a polishing pad. The polishing pad has a polishing layer having a front surface for polishing and a back surface. A first plurality of grooves are formed on the front surface of the polishing layer, and an indentation is formed in the back surface of the polishing layer. A region on the polishing surface corresponding to the indentation in the back surface is either free of grooves or has a second plurality of grooves that are shallower than the first plurality of grooves. 
     Implementations of the invention may include one or more of the following features. The region on the polishing surface corresponding to the indentation may be substantially flat, e.g., it may free of grooves. Alternatively, the region on the polishing surface corresponding to the indentation may have the second plurality grooves. In addition, the region may be opaque or transparent. The polishing layer may be a unitary structure. The recess may be formed in a second portion of the polishing layer that is physically discrete from the first portion, and the second portion may be secure to the first portion. The first and second portions may have substantially the same material composition, and the second portion may have a top surface substantially flush with the polishing surface. An aperture may be formed in the first portion, and the second portion may be secured in the aperture. The second portion may have a top section with a first cross-sectional dimension and a bottom section with a second, different cross-sectional dimension. For example, the first cross-sectional dimension may be less than the second-cross-sectional dimension. The second plurality of grooves may extend past an inner surface of the indentation. 
     The pad may have a backing layer disposed on the back surface of the polishing layer. The backing layer may be softer than the polishing layer. The backing layer may have an aperture therethrough, and the aperture may be aligned with the indentation in the back surface of the polishing layer. The backing layer may be a thin non-compressible layer. The first plurality of grooves may be formed on a first portion of the polishing layer, and the recess may be formed in a second portion of the polishing layer that is physically discrete from the first portion. A second aperture may be formed in the polishing layer, and the second portion may be secured in the second aperture. The first aperture may have first cross-sectional dimension and the second aperture may have a second, different (e.g., larger or smaller) cross-sectional dimension. 
     In another aspect, the invention is directed to a polishing system. The polishing system has a carrier to hold a substrate, a polishing pad supported on the platen, and an eddy current monitoring system. The polishing pad includes a polishing layer having a front surface for polishing and a back surface, a first plurality of grooves formed in the front surface of the polishing layer, and an indentation formed in the back surface of the polishing layer. A region on the polishing surface corresponding to the indentation in the back surface is either free of grooves or has a second plurality of grooves that are shallower than the first plurality of grooves. The eddy current monitoring system has at least one of a coil and a core extending at least partially into the recess in the back surface of the polishing layer to monitor a metal layer on the substrate held by the carrier. 
     In another aspect, the invention is directed to a method of manufacturing a polishing pad. The method includes forming a first plurality of grooves in a polishing layer of the polishing pad, forming an indentation in a back surface of the polishing layer, and forming a region on the polishing surface corresponding to the indentation that is either free of grooves or has a second plurality of grooves that are shallower than the first plurality of grooves. 
     Implementations of the invention may include one or more of the following features. The polishing layer may be secured to a backing layer. Forming the recess may include machining the recess or molding the recess. Forming the indentation in the back surface may include securing a physically discrete first portion of the polishing pad having the indentation in an aperture in a second portion of the polishing pad having the grooves. 
     In another aspect, the invention is directed to a method of polishing. In the method, a substrate is brought into contact with a front surface of a polishing layer of a polishing pad, the polishing layer having a first plurality of grooves formed in a first portion of the front surface of the polishing layer and an indentation formed in a back surface of the polishing layer. A region on the polishing surface corresponding to the indentation in the back surface is either free of grooves or has a second plurality of grooves that are shallower than the first plurality of grooves. A polishing liquid is supplied to the front surface of the polishing layer, and relative motion is created between the substrate and the front surface. 
     Implementations of the invention may include one or more of the following features. A metal layer on the substrate may be monitored with an eddy current monitoring system that has at least one of a coil and a core extending at least partially into the recess in the back surface of the polishing layer. 
     In another aspect, the invention is directed to a polishing pad with a polishing layer having a front surface and a back surface. The front surface has a first portion with a plurality of grooves and a second portion that is substantially flat, and the back surface has a recess aligned with the second portion of the front surface. 
     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 
         FIG. 1  is a schematic side view, partially cross-sectional, of a chemical mechanical polishing station that includes an eddy current monitoring system. 
         FIG. 2  is a schematic top view illustrating the polishing pad of  FIG. 1 . 
         FIG. 3  is a schematic cross-sectional side view illustrating the polishing pad of  FIG. 2  along line  3 - 3 . 
         FIG. 4  is a schematic cross-sectional side view illustrating a polishing pad having multiple indentations in the bottom surface of the covering layer. 
         FIG. 5  is schematic cross-sectional side view illustrating a polishing pad in which a grooveless insert is secured to a grooved polishing pad. 
         FIG. 6  is schematic cross-sectional side view of another implementation of a polishing pad in which the backing layer is a thin sheet. 
         FIGS. 7A and 7B  are schematic cross-sectional side views of another implementation of a polishing pad in which an insert is secured to a bottom surface of the covering layer. 
         FIG. 8  is a schematic cross-sectional side view illustrating a polishing pad having shallow grooves over the recess. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , one or more substrates  14  can be polished at a polishing station  10  of a CMP apparatus. A description of a suitable polishing apparatus can be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. 
     The polishing station  10  includes a rotatable platen  16  on which is placed a polishing pad  18 . The polishing pad  18  can be a two-layer polishing pad with a soft backing layer  20  and a hard durable outer layer  22  with a substantially uniform composition. The durable outer layer  22  provides a polishing surface  24 . At least a portion of the polishing surface  24  can have grooves  28  for carrying slurry. 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 slurry  30  containing a liquid and a pH adjuster can be supplied to the surface of polishing pad  18  by a slurry supply port or combined slurry/rinse arm  32 . Slurry  30  can also include abrasive particles. 
     The substrate  10  is held against the polishing pad  18  by a carrier head  34 . The carrier head  34  is suspended from a support structure, such as a carousel, and is connected by a carrier drive shaft  36  to a carrier head rotation motor so that the carrier head can rotate about an axis  38 . 
     A recess  40  is formed in platen  16 , and an in-situ monitoring module  42  fits into the recess  40 . The in-situ monitoring module  42  can includes an situ eddy current monitoring system with a core  44  positioned in the recess  26  to rotate with the platen. Drive and sense coils  46  are wound the core  44  and are connected to a controller  50 . In operation, an oscillator energizes the drive coil to generate an oscillating magnetic field  48  that extends through the body of core  44 . At least a portion of magnetic field  48  extends through the polishing pad  18  toward the substrate  12 . If a metal layer is present on the substrate  10 , the oscillating magnetic field  48  will generate eddy currents. The eddy current produces a magnetic flux in the opposite direction to the induced field, and this magnetic flux induces a back current in the primary or sense coil in a direction opposite to the drive current. The resulting change in current can be measured as change in impedance of the coil. As the thickness of the metal layer changes, the resistance of the metal layer changes. Therefore, the strength of the eddy current and the magnetic flux induced by eddy current also change, resulting in a change to the impedance of the primary coil. By monitoring these changes, e.g., by measuring the amplitude of the coil current or the phase of the coil current with respect to the phase of the driving coil current, the eddy current sensor monitor can detect the change in thickness of the metal layer. 
     The drive system and sense system for the eddy current monitoring system will not be described in detail, as descriptions of suitable systems can be found in U.S. patent application Ser. Nos. 09/574,008, 09/847,867, and 09/918,591, filed Feb. 16, 2000, May 2, 2001, and Jul. 27, 2001, respectively, the entire disclosures of which are incorporated by reference. 
     Various electrical components of the eddy-current monitoring systems can be located on a printed circuit board in the controller  50 . The controller can include circuitry, such as a general purpose microprocessor or an application-specific integrated circuit, to convert the signals from the eddy current sensing system into digital data. 
     As previously noted, the monitoring system  42  includes a core  44  positioned in the recess  26 . 
     Referring to  FIGS. 2 and 3 , the covering layer  22  of the polishing pad  18  includes one or more recesses or indentations  52  formed in the bottom surface of the covering layer. These indentations create one or more thin sections  54  in the covering layer of the polishing pad. The core  44  and/or coils  46  can extend into the indentations  52  so that they pass partially through the polishing pad. By positioning the core or coils close to the substrate, the spatial resolution of the eddy current monitoring system can be improved. These recesses  52  can extend through at least 50% of the thickness of the covering layer  22 , e.g., through 75-80%. For example, in a polishing pad having an covering layer  22  that is 100 mils thick, the recess  52  can have a depth D 1  of about 80 mils, leaving the thin section  54  with a thickness of about 20 mils. 
     As previously mentioned, the covering layer  22  can also include a plurality of grooves  28  formed therein. The grooves may be of nearly any pattern, such as concentric circles, straight lines, spirals, and the like. However, the grooves do not extend over the thin section  54  in the covering layer  22 . Thus, the polishing surface  24  of the polishing pad includes portions with and without grooves, and the indentation is located in one of the portions without grooves. The grooves  28  can be at least 10 mils deep, e.g., about 20 mils deep. The grooves  28  can extend through about 20-25% of the thickness of the covering layer  22 . For example, in a polishing pad having an covering layer  22  that is 80 mils thick, the grooves  28  can have a depth D 2  of about 20 mils. The grooves can be sufficiently deep that they extend to or past the plane defined by the inner surface  58  of the recess. 
     In addition, the backing layer  20 , if present, includes one or more apertures  56  positioned to provide access of the core  44  and/or coils  46  to the indentations  52 . Thus, the core  44  and/or coils  46  can also extend through the backing layer  20 . As illustrated in  FIG. 2 , a single aperture  52  can extend across all of the indentations  52 . However, as illustrated in  FIG. 4 , in another implementation there is one aperture  56  aligned with each recess  52 . However, for some polishing operations, only a single-layer polishing pad is used, and there is not backing layer. 
     Referring to  FIGS. 1 and 4 , when the polishing pad  18  is secured to the platen, the thin section  54  fits over the recess  26  in the plate and over a portion of the core and/or coil that projects beyond the plane of the top surface of the platen  16 . By positioning the core  42  closer to the substrate, there is less spread of the magnetic fields, and spatial resolution can be improved. Assuming that the polishing pad is not being used with an optical endpoint monitoring system, then the entire polishing layer, including the portion over the recess, can be opaque. 
     In one implementation (shown in  FIG. 3 ), the covering layer  22  can be manufactured, e.g., by a molding process, with grooves and recesses preformed in the upper and lower surfaces of the covering layer, respectively. Thus, the cover layer  22 , including the grooved portion and the thin section, can be a single unitary body. The covering layer  22  can be manufactured by a molding process, e.g., by injection molding or compression molding, so that the pad material cures or sets in mold with indentations that form the grooves recess. Alternatively, the covering layer  22  can be manufactured by a more conventional technique, e.g., by scything a thin sheet of pad material from a block. The grooves and recess can then be formed by machining or milling the top and bottom surfaces of the covering layer, respectively. Once the covering layer  22  has been manufactured, it can then be secured to the backing layer  20 , e.g., with an adhesive, with the recess  52  in the covering layer  22  aligned with the aperture  56  in the backing layer  20 . 
     Alternatively, as shown in  FIG. 5 , the polishing pad can be manufactured in two parts. For example, the main body  60  of the pad can be manufactured with grooves  28  (either by molding or machining). A grooveless insert  62  having the recess  52  in its bottom surface can be manufactured separately. The main portion  60  and the insert  62  can be formed from the same material. An aperture  64  is cut in the main portion  60  of the covering layer  22 , and the insert  64  is secured in the aperture  64 , e.g., by an adhesive that bonds the insert  64  to the upper surface of the backing layer  20 . The thickness D 4  of the insert  62  can be equal to the thickness D 3  of the covering layer  22 , so that the top surface of the insert  62  is flush with respect to the polishing surface  24 , or the thickness D 4  of the insert  62  can be slightly less than the thickness D 3  of the covering layer  22 , so that the top surface of the insert  62  is slightly recessed with respect to the polishing surface  24 . 
     In another implementation, illustrated in  FIG. 6 , the backing layer  20  is a thin sheet of non-compressible, tear-resistant material, such as Mylar (this implementation could be considered to function as a single-layer polishing pad). The Mylar sheet can be applied to the back of the covering layer  22 , and then the insert  62  can be placed into the aperture  64  in the covering layer  22  and adhesively secured to the top surface of the Mylar sheet  20 . A portion of the Mylar sheet is then removed to expose the recess  52 . 
     In another implementation, illustrated in  FIG. 7A , the insert  62  is secured to the underside of the covering layer  22 . In this implementation, the insert  62  includes a narrow upper portion  70  that fits into an aperture  72  in the covering layer  22 , and a wide lower portion  74  that fits into an aperture  76  in the backing layer  20 . The top surface  78  of the wide portion  74  can be adhesively secured to the bottom surface  79  of the portion of the covering layer  22  that projects beyond the backing layer  20 . The upper portion  70  can have the same thickness as the covering layer  22  so that the top surface of the insert is flush with the polishing surface  24 , whereas the lower portion  74  can be thinner than the backing layer  20  to provide a gap between the platen and the insert. 
     Referring to  FIG. 7B , a two-part insert can also be secured to a single layer polishing pad. In this implementation, a two-part aperture  80  with an upper section  82  and a lower section  84  of different cross-sectional dimensions is formed in the covering layer  22 . Assuming that the covering layer and insert have the same rigidity, the lower portion  74  can have the same thickness as the lower section  84  of the aperture. 
     Referring to  FIG. 8 , in another implementation, the portion of the polishing surface  24  corresponding to the recess  52 , i.e., the thin section  54 , can have very shallow grooves  28   a , whereas the remainder of the polishing surface can have deep grooves  28   b . The deep grooves  28   b  can be at least 10 mils deep, e.g., about 20 mils deep. In contrast, the shallow grooves  28   a  must have a depth that is less than (e.g., less than 25% of) the thickness of the thin section  54 . For example, if the thin section  52  has a thickness of 20 mils, the shallow grooves  28   a  can have a depth of about 5 mils. 
     The eddy current monitoring system can be used 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, a tape extending between supply and take-up rollers, or a continuous belt. The polishing pad can be affixed on a platen, incrementally advanced over a platen between polishing operations, or driven continuously over the platen during polishing. The pad can be secured to the platen during polishing, or there could be a fluid bearing between the platen and polishing pad during polishing. The polishing pad can be a standard (e.g., polyurethane with or without fillers) rough pad, a soft pad, or a fixed-abrasive pad. 
     In addition, although terms of vertical positioning are used, it should be understood that the polishing surface and substrate could be held upside down, in a vertical orientation, or in some other orientation. 
     The eddy current monitoring system can include separate drive and sense coils, or a single combined drive and sense coil. In a single coil system, both the oscillator and the sense capacitor (and other sensor circuitry) are connected to the same coil. 
     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 following claims.