Patent Publication Number: US-2020276685-A1

Title: Controlling Chemical Mechanical Polishing Pad Stiffness By Adjusting Wetting in the Backing Layer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 62/841,769, filed May 1, 2019, and claims priority to U.S. Provisional Application Ser. No. 62/812,212, filed Feb. 28, 2019, both of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to chemical mechanical polishing of substrates. 
     BACKGROUND 
     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. For certain applications, the filler layer is planarized until the top surface of a patterned layer is exposed. A conductive filler layer, for example, can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. 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. For other applications, such as oxide polishing, the filler layer is planarized until a predetermined thickness is left over the non planar surface. In addition, planarization of the substrate surface is usually required 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 typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. An abrasive polishing slurry is typically supplied to the surface of the polishing pad. 
     SUMMARY 
     In one aspect, a polishing pad for a chemical mechanical polishing apparatus includes a polishing layer having a polishing surface a backing layer formed of a fluid-permeable material and having a lower surface configured to be secured to a platen and an upper surface secured to the polishing layer, and a plurality of seals including a first seal that circumferentially seals an edge of the backing layer and a second seal that seals and separates the backing layer into a first region and a second region. 
     In another aspect, a chemical mechanical polishing system includes a platen, a polishing pad that includes a polishing layer having a polishing surface and a backing layer formed of a fluid-permeable material and having a lower surface secured to the platen and an upper surface secured to the polishing layer, a plurality of seals including a first seal that circumferentially seals an edge of the backing layer, and a second seal that seals and separates the backing layer into a first region and a second region, and a fluid source coupled to the backing layer to direct fluid into the first region and second region of the backing layer. 
     In another aspect, a method of controlling stiffness of a backing layer of a polishing pad in a chemical mechanical polishing system includes controlling flow of liquid into first and second regions of a fluid-permeable backing layer of the polishing pad that are separated by a seal. 
     Implementations may include one or more of the following features. 
     The backing layer may have an open-cell structure. The backing layer may include a polymer matrix having interconnected voids therein. 
     At least some of the plurality of seals may be provided by portions of the backing layer that are impregnated with a sealant material. At least some of the plurality of seals may be provided by crimped portions of the backing layer. The first region may surround the second region. The first region and the second region may be concentric. 
     The fluid source may be configured to independently control fluid flow into the first region and the second region. The fluid source may include a plurality of independently controllable pumps. 
     A plurality of passages may extend through the platen and a plurality of vents may permit fluid flow into the first region and second region from the plurality of passages. The plurality of vents may project from the platen into the backing layer. The plurality of vents may include a first multiplicity of vents in the first region and a second multiplicity of vents in the second region. The first multiplicity of vents may be spaced at equidistant intervals within the first region and the second multiplicity of vents may be spaced at equidistant intervals within the second region. The liquid can be water. 
     Controlling flow of liquid into the first region and second region can include flowing liquid through vents that project from a platen into the backing layer. 
     Implementations may optionally include, but are not limited to, one or more of the following advantages. Polishing non-uniformity, e.g., caused by variations in stiffness across the backing layer due to wetting of the backing layer can be controlled and corrected. Another advantage to controlling the polishing pad stiffness is that different zones with varying stiffness can be created to control polishing rates at multiple regions of the wafer, e.g., to perform edge-correction or to correct for slow or fast removal zones caused by differences in slurry distribution. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic cross-sectional view of a chemical mechanical polishing system. 
         FIG. 2  shows a schematic close-up cross-sectional view of a pinched polishing pad. 
         FIG. 3A  shows a schematic top view of an exemplary backing layer. 
         FIG. 3B  shows a schematic top view of an exemplary backing layer. 
         FIG. 4A  shows a schematic cross-sectional view of a polishing layer and a backing layer with an impregnated seal. 
         FIG. 4B  shows a schematic cross-sectional view of a polishing layer and a backing layer with a crimped seal. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Fluids, such as a polishing fluid, can be retained in and spread through the backing layer (e.g., by capillary action). Accumulation of fluid in the backing layer can result in uneven stiffness of the pad, which can result in uneven polishing rates between wetter regions and dryer regions of the backing layer. Additionally, as fluid seeps into the backing layer, over time the accumulation of fluid can result changes in the size of the wetted region, which can lead to wafer to wafer variation. However, the stiffness of a polishing pad can be controlled by pumping fluid into sealed regions of a backing layer. 
       FIG. 1  shows a polishing system  20  operable to polish a substrate  10 . The polishing system  20  includes a disk-shaped platen  22  on which a polishing pad  30  with a polishing surface  36  is situated. The platen  22  is operable to rotate about an axis  25 . A motor  26  can turn a drive shaft  24  to rotate the platen  22 . 
     The polishing pad  30  can be secured to the upper surface  28  of the platen  22 , for example, by a layer of adhesive  66  (described in more detail below). When worn, the polishing pad  30  can be detached and replaced. 
     The polishing system  20  can include a polishing liquid delivery arm  84  and/or a pad cleaning system such as a rinse fluid delivery arm. During polishing, the arm  84  is operable to dispense a polishing liquid  82 , e.g., slurry with abrasive particles, onto the polishing pad  30 . In some implementations, the polishing system  20  include a combined slurry/rinse arm. 
     The polishing system  20  can include a conditioner system  40  with a rotatable conditioner head  42  to maintain the surface roughness of the polishing surface  36  of the polishing pad  30 . The conditioner head  42  can be a removable conditioning disk. A drive shaft  46  can connect the conditioner head  42  to a motor  44  which can drive the conditioner head  42 . The conditioner head  42  can also be positioned at the end of an arm  48  that can swing so as to sweep the conditioner head  42  radially across the polishing pad  30 . 
     A carrier head  70  is operable to hold the substrate  10  against the polishing pad  30 . The carrier head  70  is suspended from a support structure  72 , for example, a carousel or track, and is connected by a carrier drive shaft  74  to a carrier head rotation motor  76  so that the carrier head can rotate about an axis  75 . In addition, the carrier head  70  can oscillate laterally across the polishing pad, e.g., by moving in a radial slot in the carousel as driven by an actuator, by rotation of the carousel as driven by a motor, or movement back and forth along the track as driven by an actuator. 
     The polishing pad  30  is a two-layer polishing pad with a polishing layer  32  and a backing layer  34 . The backing layer  34  has an edge seal  52  and one or more internal seals  54 . The backing layer  34  can have an open-cell structure (e.g., a solid foam having interconnected pores that extend through the backing layer) that is fluid permeable. In particular, the backing layer can be formed of a polymer matrix material with voids in the matrix providing the interconnected pores. The pores can occupy about 10-50%, e.g., 30% of the volume of the backing layer. The backing layer can be microporous, e.g., the pores can have an average diameter of about 10 to 100 microns. In contrast, the edge seal  52  and the internal seals  54  are fluid impermeable. 
     During polishing, some of the polishing fluid, e.g., the slurry, can flow over the sides of the platen  24 . However, as shown in the example of  FIG. 1 , the perimeter of the backing layer  34  is sealed by an edge seal  52 . The edge seal  52  prevents fluid, e.g., the polishing fluid that flows over the side platen  24 , from seeping into the backing layer  34 . 
     The backing layer  34  also has a plurality of internal seals  54  positioned within the backing layer  34 . The internal seals  54  divide the backing layer  34  into multiple regions  50  (see  FIGS. 3A, 3B, 4A, 4B ). For example, assuming the seals  52 ,  54  are annular, a first annular region can defined by the area between the edge seal  52  and the outermost internal seal  54 , a second annular region can be defined by the area between the outermost internal seal  54  and the next outermost internal seal  54 , etc. The internal seals  54  provide a barrier to prevent fluid flow between the regions  50  of the backing layer  34 . 
     The edge seal  52  and the internal seals  54  can be annular, e.g., circular. Moreover, the edge seal  52  and the internal seals  54  can be concentric with the center of the backing layer  34 . The internal seals  54  need not form circular arcs, but have other shapes (e.g., wavy, straight lines, etc.). In addition, the internal seals  54  can form other shapes within the backing layer  34  (e.g., polygons, a cross-hatched pattern, etc.) and divide the backing layer  34  into regions of other shapes, e.g., concentric polygons, a rectangular array, etc.). 
     The edge seal  52  and the internal seals  54  can be formed, for example, by impregnating the backing layer  34  with a sealant material (see  FIG. 4A ), or by crimping the backing layer  34  (see  FIG. 4B ). 
     One or more passages extend through the platen, and one or more vents  56  permit fluid flow into and/or out of the regions  50  of the backing layer  34  from the one or more passages. The vents  56  can project upward from the platen  22 . The vents can be formed from a body that is more rigid than the backing layer  34 , and that has an internal conduit for fluid flow. For example, the vents  56  can be needles or other similar injection devices. Assuming that the vents project up from platen  20 , when the polishing pad  30  is lowered onto the platen  20 , the vents  56  can puncture and extend into the backing layer  34 . 
     The vents  56  can inject fluid (e.g., water, air) into the separate regions  50  of the backing layer  34 . The stiffness of the polishing pad  30  be controlled by controlling fluid flow into the regions  50  of backing layer  34  via the vents  56 . Wetting and drying of the regions  50  of backing layer  34  can be accomplished by pumping liquid, e.g., water, into the regions  50  and pumping liquid out the regions  50  via the vents  56 . For example, a vent  56  can be used for wetting of the associated region  50  of the backing layer  34  by pumping liquid into the region  50  of the backing layer  34 . In another example, a vent  56  can be used for drying the associated region  50  of the backing layer  34  by pumping liquid out of the regions  50  of the backing layer  34 . In another example, a vent  56  can be used for drying the associated region  50  of the backing layer  34  by pumping air into the region  50  of the backing layer  34 . For example, a region  50  of the backing layer  34  that is wet with water can have air injected into it to replace the water and dry the region  50  (e.g., effectively “push out” the water with the air). 
     The fluids can be urged into the regions  50  of the backing layer  34 , e.g., using a fluid pump  68 , e.g., a centrifugal pump, peristaltic pump, etc. The fluids can be drawn out of the regions  50  of the backing layer  34 , e.g., using a vacuum source  69 , e.g., a pump or facilities vacuum line. Assuming a single pump were to be used to pump fluid into the entirety of a 30-inch diameter backing layer, the maximum fluid flowrate of the pump should be around 100 ml to 1 liter per minute. Similarly, assuming a single pump were to be used to pump fluid out of a 30-inch diameter backing layer, the maximum fluid flowrate should be around 100 ml to 1 liter per minute. However, where there are multiple pumps used for multiple regions of the backing layer, the maximum flow rate of the pump can be correspondingly reduced. 
     The optimal flow rate for an individual vent can depend on the number of vents  56  within a region  50  and the size of the region. Exemplary fluid flow rates, e.g., for a 30-inch diameter backing layer  34 , are described in Table 1 below: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 exemplary fluid flowrates for a 30-inch diameter backing layer 
               
            
           
           
               
               
               
            
               
                   
                 Number of vents 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Unit = cc/min 
                 4 
                 8 
                 16 
                 32 
                 64 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Number 
                 1 
                 145 
                 72.50 
                 36.25 
                 18.13 
                 9.06 
               
               
                 of regions 
                 2 
                 72.50 
                 36.25 
                 18.13 
                 9.06 
                 4.53 
               
               
                   
                 4 
                 36.25 
                 18.13 
                 9.06 
                 4.53 
                 2.27 
               
               
                   
                 8 
                 18.13 
                 9.06 
                 4.53 
                 2.27 
                 1.13 
               
               
                   
                 16 
                 9.06 
                 4.53 
                 2.27 
                 1.13 
                 0.57 
               
               
                   
               
            
           
         
       
     
     The fluid pumped into the regions  50  of the backing layer  34  can be provided by a fluid source  58 . For example, the fluid source  58  can be a reservoir that is connected to the vents  56 . The fluid source  58  can be located within the platen  22 . The fluid source  58  can be connected to the vents  56  using the passages through the platen  22 . 
     The fluid can be pumped into the regions  50  of the backing layer  34  using the pump  68 . In some implementations, multiple pumps can be used to can independently control fluid flow along each conduit connecting the fluid source  58  to each vent  56 . 
     The fluids can be drawn out of the regions  50  of the backing layer  34  using the vacuum source  69 . If each conduit is connected to a separate vacuum source  69 , e.g., a different pump, then fluid flow along each conduit can be independently controlled. The vacuum source  69  can be located within the platen  22 . The vacuum source  69  can be connected to the vents  56  using conduits through the platen  22 . 
     Fluid flow can be independently controlled in each region  50 . For example, the pump  68  can provide fluid into the first region (e.g., the region of the backing layer  34  that is defined by the edge seal  42  and the outermost internal seal  54 ) using the vents  56  corresponding to the first region, and at the same time provide fluid into the second region (e.g., the region of the backing layer  34  that is defined by the outermost internal seal  54  and the second outermost internal seal  54 ) using the vents  56  corresponding to the second region. The amount of fluid pumped into the first region can be more than the amount of fluid pumped into the second region. In some implementations, the amount of fluid pumped into the first region can be less than the amount of fluid pumped into the second region. In some implementations, the amount of fluid pumped into the first region can be the same as the amount of fluid pumped into the second region. Similarly, the pump  69  can remove more fluid the first region than the second region, less fluid from the first region than the second region, or the same amount of fluid from the first region and the second region. 
     Varying the fluid flow into a regions  50  of the backing layer  34  can control the stiffness of the polishing pad  30  corresponding to that regions  50 , which ultimately affects the polishing characteristics the substrate  10  for that region  50  of the backing layer  34 . In general, increased stiffness results in an increased polishing rate, although there can be secondary effects, such as a reduction in polishing uniformity. 
     For example, if the second region of the backing layer  34  is wetter than a first region of the backing layer  34 , the portion of the polishing pad  30  corresponding to the second region will be stiffer relative to the portion of the polishing pad  30  corresponding to the first region. Thus, if the carrier head  70  positions a center portion of the substrate  10  over a portion of the polishing pad  30  that corresponds to the second region, and positions an edge portion of the substrate  10  over a portion of the polishing pad  30  that corresponds to the first region, the polishing system  10  can establish different polishing rates in different portions of the substrate  10 . 
     By controlling the wetness and dryness of the regions  50  of the backing layer  34 , the regions  50  can be configured to provide the polishing pad  30  with a substantially uniform stiffness, thus reducing the wear and tear on the polishing pad  30  and increasing the lifespan of the polishing pad  30 . Additionally, different polishing rates in different portions of the substrate  10  can provide correction of the substrate  10 , e.g., by reducing the polishing of the edge of the substrate  10  to result in a more uniform polishing of the substrate  10 . 
     Additionally, the amount of fluid in the regions  50  of the backing layer  34  can be controlled to reduce the effect of “pinching” of the polishing pad  30 , particularly in-between the substrate  10  and a retaining ring of the carrier head  70 . As illustrated in  FIG. 2 , pinching  38  can occur when the substrate  10  and/or the carrier head  70  press on the polishing pad  30  to “pinch” or “squeeze” a portion of the polishing pad  30 . Pinching  38  of the polishing pad  30  can result in an increased polishing rate. To reduce the effect of pinching  38 , the vents  56  can inject fluid into a regions  50  of the backing layer  34  that underlies the pinching  38  to stiffen the polishing pad  30  (e.g., reduce the flexibility of the polishing pad  30 ). In some implementations, the vents  56  can reduce the fluid in a regions  50  of the backing layer  34  that is underlying the pinching  38  to soften the polishing pad  30  (e.g., reduce how stiff the polishing pad is and reduce the polishing rate of the polishing pad  30 ). 
     As illustrated in  FIGS. 3A and 3B , the vents  56  can include inlet vents  56   a  (e.g., to pump fluids into a region  50 ) and outlet vents  56   b  (e.g., to pump out fluids out of a region  50 ). Referring to  FIG. 3A , the inlet vents  56   a  and the outlet vents  56   b  can be arranged in a radial pattern, e.g., rows of vents extending along the radii of the polishing pad  30 . The rows of inlet vents  56   a  and outlet vents  56   b  can alternate within each regions  50  of backing layer  34 . Further, the vents within the row of vents  56  extending along the radii of the polishing pad  30  can be equidistant from one another. Referring to  FIG. 3B , the inlet vents  56   a  and the outlet vents  56   b  can be arranged such that the space between each of the inlet vents  56   a  and the outlet vents  56   b  within one region  50  is approximately the same as the space between each of the inlet vents  56   a  and the outlet vents  56   b  of another region  50 . While not expressly illustrated, other arrangements, patterns, and numbers of inlet vents and outlet vents are possible. Further, the number, width, shape (e.g., circular, polygonal, or other shape), and concentricity (e.g., concentric regions or non-concentric regions) of regions  50  are also possible. 
     As illustrated in  FIGS. 4A and 4B , a fluid-impermeable film  64  (e.g., a plastic film or a wax film) can be located between the top pad  32  and the upper surface of the backing layer  34 . The film  64  can be a thin plastic layer. The film  64  can prevent fluids from passing from a first region  50  and into a second region  50 . In some implementations, the film  64  is secured to the top pad  32  and/or the backing layer  34  using an adhesive  66  (e.g., pressure sensitive adhesive, tape, or glue). In some implementations, the film  64  is located on the lower surface of the backing layer  34 . The film  64  is secured to the lower surface of the backing layer  34  using the adhesive  66 , and secured to the platen  22  (not illustrated) using the adhesive  66 . 
     Referring now to  FIG. 4A , the edge seal  52  and the internal seals  54  can be provided by portions of the backing layer  34  that are impregnated with a sealant material. For example, the impregnated edge seal  52   a  and impregnated seals  54   a  can be composed of a polyurethane or an epoxy resin, or other polymers, such as polyethylene terephthalate (PET), polyvinyl chloride (PVC), or polypropylene (PP). This sealant material fills the pores in the polymer matrix in the area of the seal, thus preventing fluid flow. 
     Referring now to  FIG. 4B , the edge seal  52  and the internal seals  54  can be provided by crimped portions of the backing layer  34 . The crimping can be done, for example, by crimping or embossing the backing layer  34 . The crimping collapses the pores in the area of the seal, thus preventing fluid flow. 
     In some implementations, the edge seal  52  and the seals  54  can be composed of a combination of impregnated seals (e.g.,  52   a  and  54   a ) and crimped seals (e.g.,  52   b  and  54   b ). 
     If a window or aperture extends through the polishing pad, then an additional seal can be used to seal the inner edge of the backing layer  34  adjacent the window or aperture. 
     As used in the instant specification, the term substrate can include, 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. 
     The above described polishing system 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. The polishing layer can be a standard (for example, polyurethane with or without fillers) polishing material, a soft material, or a fixed-abrasive material. 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. 
     Particular embodiments of the invention have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results.