Abstract:
A foamed plastic is cut to form a single-layer polishing pad having a desired rigidity and compressibility. A polishing surface of the polishing pad has a higher density than a mounting surface of the polishing pad. The polishing surface and the mounting surface may have different areas having different densities for achieving desired rigidity and compressibility property. Furthermore, methods of making such single-layer polishing pads are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the priority benefit of U.S. Provisional Application Ser. No. 60/521,483, filed May 5, 2004, and U.S. Provisional Application Ser. No. 60/521,740, filed Jun. 29, 2004, the full disclosures of which are incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     1. Field of Invention  
         [0003]     The present invention relates to a polishing apparatus and manufacturing method thereof. More particularly, the present invention relates to a single-layer polishing pad and a method of producing the same.  
         [0004]     2. Description of Related Art  
         [0005]     During the manufacturing process of semiconductor integrated circuits, isolation structures, metal lines and dielectric layers are stacked layer by layer, and the surface of a wafer is thus less and less planar. Limited by the focus depth of an exposing machine, pattern transferal from a photomask to a photoresist layer is increasingly difficult, and the exposed pattern of the photoresist layer is increasingly distorted. Chemical mechanical polishing (CMP) is the only true global planarization process to resolve the problem mentioned above.  
         [0006]     In CMP, a wafer is pressed against on a polishing pad to allow movement of the wafer on the polishing pad having polishing slurry thereon. The polishing slurry contains fine abrasive particles and a chemical reagent. Both the wafer and the polishing pad are rotated automatically; hence the wafer is planarized by mechanical polishing by the abrasive particles and chemical reaction of the chemical reagent.  
         [0007]     An important goal of CMP is achieving uniform planarity of the wafer surface, and the uniform planarity also has to be achieved for a series of wafers processed in a batch. The rigidity (or stiffness) and the compressibility (or compliance) of a polishing pad have great influence on the planarity of the polished wafer. Generally speaking, a polishing pad with higher rigidity can increase the polishing planarity of the polished wafer, and a polishing pad with higher compressibility can increase the polishing uniformity of the polished wafer. Therefore, a wafer polished by a rigid polishing pad often needs to be further polished by a soft polishing pad to improve the polishing uniformity. The CMP process thus suffers from low throughput.  
         [0008]     Conventionally, to satisfy both the planarity and the uniformity requirements of the CMP process, at least a layer of rigid pad and at least a layer of soft pad are stacked to form a desired composite polishing pad, such as the polishing pads disclosed by U.S. Pat. No. 5,212,910 and U.S. Pat. No. 5,257,478. As stated in U.S. Pat. No. 6,217,426, although a composite polishing pad can partially satisfy both the planarity and the uniformity requirements of the CMP process, some other problems are also produced. For example, pressure transmission is different for a rigid pad and a soft pad, and the polishing uniformity can sometimes be poor. Furthermore, a greater number of layers stacked in a composite polishing pad creates more variables that can affect the rigidity and compressibility of the composite polishing pad. Hence, the polishing planarity and uniformity are more difficult to control.  
         [0009]     Besides, if the two pads in a composite polishing pad are not adhered well enough, the composite polishing pad may easily delaminate during the polishing process. Therefore, U.S. Pat. No. 6,217,426 discloses a polishing pad having a pattern of protrusions on the mounting surface of the polishing pad to limit the pressure transmission area and increase compressibility of the polishing pad.  
         [0010]     In the prior art described above, the cost and complexity in producing a polishing pad are unavoidably increased.  
       SUMMARY  
       [0011]     In one aspect, the present invention provides a single-layer polishing pad having desired rigidity and compressibility to meet the requirements of polishing planarity and uniformity.  
         [0012]     In another aspect, the present invention provides a method of producing a single-layer polishing pad having desired rigidity and compressibility. The method utilizes the pore-size-distribution property in a porous polymer to control the rigidity and compressibility of a polishing pad.  
         [0013]     In accordance with the foregoing and other aspects of the present invention, a single-layer polishing pad is provided. The single-layer polishing pad comprises a body, a polishing surface on one side of the body, and a mounting surface on the other side of the body. The body is made of a porous polymer, and the density of the polishing surface and the density of the mounting surface are different.  
         [0014]     In a preferred embodiment of the present invention, the density of the polishing surface is higher than the density of the mounting surface.  
         [0015]     In another preferred embodiment, the polishing surface has at least a first area and at least a second area, and the density of the first area is higher than the density of the second area.  
         [0016]     In yet another preferred embodiment, the mounting surface has at least a third area and at least a fourth area, and the density of the third area is higher than the density of the fourth area.  
         [0017]     In yet another preferred embodiment, the single-layer polishing pad comprises a polishing surface, a mounting surface, and a central part between the polishing surface and the mounting surface, and the porosity of the central part is higher than the porosity of the polishing surface.  
         [0018]     In accordance with the foregoing and other aspects of the present invention, a method of producing a single-layer polishing pad is provided. A foamed pad is formed by a foam process, and the foamed pad has a first planar surface and a second planar surface. A third planar surface is formed by removing the first planar surface. The density of the second planar surface, which serves as a polishing surface, is higher than the density of the third planar surface, which serves as a mounting surface.  
         [0019]     In accordance with the foregoing and other aspects of the present invention, a method of producing a single-layer polishing pad is provided. A foamed pad is formed by a foam process. The foamed pad has at least a first region and at least a second region, and the thickness of the first region is larger than the thickness of the second region. A first planar surface is formed by removing a surface of the foamed pad. The density of the first planar surface on the first region is lower than the density of the first planar surface on the second region.  
         [0020]     In a preferred embodiment, a second planar surface is formed by removing the other surface of the foamed pad.  
         [0021]     In the foregoing, a pore-size-distribution property in a porous polymer is utilized to produce a single-layer polishing pad having two surfaces with uniform rigidity or various rigidities. Therefore, not only can the requirements for lower cost and higher CMP process throughput be achieved, but also the polishing planarity and uniformity can be achieved.  
         [0022]     It is to be understood that both the foregoing general description and the following detailed description are made by use of examples and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,  
         [0024]      FIG. 1  is a cross-sectional diagram illustrating foamed cells distributed in a foamed polymer;  
         [0025]      FIGS. 2A-2B  are cross-sectional diagrams showing polishing pads according to a first preferred embodiment of this invention;  
         [0026]      FIG. 3  is a cross-sectional diagram showing a mold according to a second preferred embodiment of this invention;  
         [0027]      FIG. 4  is a cross-sectional diagram showing a foamed polymer formed by using the mold shown in  FIG. 3 ;  
         [0028]      FIG. 5  is a cross-sectional diagram showing a polishing pad formed by cutting the foamed polymer shown in  FIG. 4 ; and  
         [0029]      FIGS. 6A-6D  are cross-sectional diagrams showing the distribution of soft regions and rigid regions on the polishing surface of the polishing pad shown in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
         [0031]     This invention provides a single-layer polishing pad having desired rigidity and compressibility and a method producing the same. In a preferred embodiment, a pore-size-distribution property in a foamed polymer is used to produce a polishing pad with optimum rigidity to solve problems of conventional polishing pads.  
         [0032]     After adding a foaming agent or a gas into a polymer, a foamed polymer can be formed by conventional foam processes. There are many foamed cells with various sizes in the foamed polymer. Generally, small foamed cells are distributed near the surfaces of the foamed polymer, and large foamed cells are distributed in the central part of the foamed polymer, as shown in  FIG. 1 .  
         [0033]      FIG. 1  is a cross-sectional diagram illustrating foamed cells distributed in a foamed polymer. In  FIG. 1 , the foamed polymer  100  is composed of a polymer  102  having foamed cells  104 . In the surface regions  108  of the foamed polymer  100 , the foamed cells  104  are smaller. In the interior regions  106  of the foamed polymer  100 , the foamed cells  104  are larger. Therefore, the density of the foamed polymer  100  in the interior region  106  is lower than that in the surface regions  108 . That is, the porosity, i.e. pore volume in a unit volume, of the interior region  106  is higher than the porosity of the surface regions  108 . Moreover, the porosity gradually increases from the surface regions  108  to the center of the interior region  106 . Therefore, the rigidity of the surface regions  108  in the foamed polymer  100  is greater, and the compressibility of the interior region  106  is better.  
       Embodiment 1  
       [0034]     According to a preferred embodiment, a polishing pad is formed by the foam process described above. Reference is made to  FIG. 1 ; the polymer  102  of the foamed polymer  100  is preferably polyurethane, epoxy resin, phenol formaldehyde resin, melamine resin or other suitable thermosetting resins. The foamed polymer  100  can be made by any suitable foam process, such as injection molding. The material of the polymer  102  and the porosity of the foamed polymer  100  can affect the rigidity of the foamed polymer  100 . Since any one skilled in the art can adjust the relevant factors affecting the rigidity of the foamed polymer  100 , a detailed discussion of the same is omitted here.  
         [0035]     The ratio of the porosity of the interior region  106  (Pi) over the porosity of the surface region (Ps), i.e. Pi/Ps, is preferably larger than 1.3, and more preferably greater than 1.5. The thickness of the foamed polymer  100  is preferably about 2 mm to about 8 mm.  
         [0036]     Therefore, a suitable cutting position can be chosen along the thickness direction t of the foamed polymer  100 . For example, cutting the foamed polymer  100  along the cutting lines A-A′ or B-B′ can obtain the polishing pads  200  or  250  as shown in  FIGS. 2A  or  2 B, respectively. Comparing the polishing pad  200  in  FIG. 2A  and the polishing pad  250  in  FIG. 2B , since the density of bottom surface  204  of the polishing pad  200  is less than the density of the bottom surface  254  of the polishing pad  250 , the bottom surface  204  is softer than the bottom surface  254 ; i.e. the bottom surface  204  is more compressible than the bottom surface  254 .  
         [0037]     Generally speaking, in the polishing pad  200  in  FIG. 2A  and the polishing pad  250  in  FIG. 2B , since the density of the top surfaces  202  and  252  are denser, and thus less porous, the top surfaces  202  and  252  are more rigid. Therefore, the top surfaces  202  and  252  usually serve as polishing surfaces to contact directly with, for example, wafers. The bottom surfaces  204  and  254  are less dense, therefore more porous, and usually serve as mounting surfaces for mounting the polishing pads  200  and  250  on a polishing device.  
         [0038]     Moreover, for the polishing pad  200  and  250 , the central parts  203  and  253  are more porous than the top surfaces  202  and  252 , respectively, and the porosity thereof gradually increases from the top surfaces  202  and  252  to the central parts  203  and  253 , respectively. The porosity ratios of the central part  203  and  253  to the top surfaces  202  and  252 , respectively, are preferably greater than 1.3, and more preferably greater than 1.5. For example, when the porosity of the top surface  202  or  252  is 20%, the porosity of the central part  203  or  253  is greater than 30%.  
         [0039]     A polishing pad with higher rigidity can achieve better polishing planarity; a polishing pad with higher compressibility can achieve better polishing uniformity. Accordingly, a polishing pad having desired rigidity and compressibility can be obtained by choosing a suitable cutting position along the thickness direction of a foamed polymer.  
       Embodiment 2  
       [0040]     According to another embodiment, a mold, as shown in  FIG. 3 , can be used to produce a desired polishing pad by a foam process.  FIG. 3  is a cross-sectional diagram showing a mold according to a second preferred embodiment of this invention. In  FIG. 3 , the mold  300  has a cavity  302 , the interior bottom surface  306  is planar, and the interior top surface  304  is non-planar. Hence, the cavity  302  can be divided into at least two regions having different spacing. That is, a region  310  has a larger spacing and a region  320  has a smaller spacing.  
         [0041]     For example, a polymer is injected into the mold cavity  302  of the mold  300  in an injection molding process with a foaming agent, a gas, or a combination thereof added. The polymer is foamed in the cavity  302  of the mold  300  to form a foamed polymer  400 , as shown in  FIG. 4 .  FIG. 4  is a cross-sectional diagram showing a foamed polymer formed by using the mold shown in  FIG. 3 . The polymer  401  of the foamed polymer  400  is preferably polyurethane, epoxy resin, phenol formaldehyde resin, melamine resin, or other suitable thermosetting resins. The material of the polymer  401  and the porosity of the foamed polymer  400  can affect the rigidity of the foamed polymer  400 . Since anyone skilled in the art can adjust the relevant factors affecting the rigidity of the foamed polymer  400 , detailed discussion of the same is omitted here.  
         [0042]     In  FIG. 4 , the pore size distribution of the foamed cells  402  in the foamed polymer  400  is similar to the pore size distribution as discussed above. That is, smaller foamed cells  402  are distributed near the surfaces of the foamed polymer  400 , and larger foamed cells are distributed in the interior region of the foamed polymer. Moreover, the foamed polymer  400  has two regions with different thicknesses, i.e. a thicker region  410  and a thinner region  420 . Therefore, even at the same level, such as level  405 , of the foamed polymer  400 , the porosity is different in different regions. For example, the density of the region  415  is less than the density of the region  425 , and both the regions  415  and  425  are near the level  405 . The reason is that the region  415  is in the thicker region  410 , and the region  415  is thus located in the interior region of the foamed polymer  400 . In contrast, the region  425  is in the thinner region  420 , and the region  425  is thus located in the surface region of the foamed polymer  400 .  
         [0043]     If the foamed polymer  400  is cut along the level  405 , a polishing pad as shown in  FIG. 5  is obtained. In  FIG. 5 , the polishing surface  550  of the polishing pad  500  has at least a soft area  510  and at least a rigid area  520  corresponding to the thicker region  410  and the thinner region  420 , respectively. As discussed in  FIG. 4 , the density of the soft area  510  is smaller, and the density of the rigid area  520  is larger. Hence, the compressibility of the soft area  510  is better, and the soft area  510  can provide better polishing uniformity. The rigidity of the rigid area  520  is larger, and the rigid area  520  can provide better polishing planarity.  
         [0044]     In addition, the polishing pad  500  can be further cut along the line  505 . Hence, the compressibility of the mounting surface of the polishing pad can be further adjusted.  
         [0045]      FIGS. 6A-6D  are cross-sectional diagrams showing the distribution of soft regions and rigid regions on the polishing surface of the polishing pad shown in  FIG. 5 . In  FIG. 6A , a circular polishing pad  500  is divided into several sectors, and the soft areas  510  and the rigid areas  520  are arranged alternately. The ratio of the surface area of the soft areas  510  over the surface area of the rigid areas  520  can be adjusted according to the desired polishing planarity and uniformity. When a wafer  600  moves around on the polishing pad  500 , the wafer  600  passes the soft area  510  and the rigid areas  520  orderly. Hence, both the polishing uniformity and the polishing planarity can be achieved.  
         [0046]     In  FIG. 6B , the soft area  510  is located at the center of the passing area of the wafer  600 . That is, the shape of the soft area  510  is like a ring located between the center and the circular edge of the polishing pad  500  to provide better polishing uniformity for the center region of the wafer  600 . In  FIG. 6C , the soft area  510  is located at the perimeter of the polishing pad  500  to provide better polishing uniformity for the edge region of the wafer  600 . In  FIG. 6D , the soft area  510  is circle and located at the central region of the polishing pad  500  to provide better polishing uniformity for the edge region of the wafer  600 .  
         [0047]     The allocation of the soft area  510  and rigid area  520  on the polishing pad  500 , as illustrated in  FIG. 5  and  FIGS. 6A-6D , can also be applied on the bottom surface of the polishing pad  500 . Therefore, the rigidity of the polishing pad  500  can be further adjusted to provide better polishing planarity and uniformity. Besides, the shape of the polishing pad  500  is not limited to a circle, and the shape also can be, for example, a square or a rectangle. The allocation of the soft area and rigid area also can be varied according to the shape of the polishing pad and the desired polishing planarity and uniformity. Since anyone skilled in the art can adjust the relevant factors, a detailed discussion of the same is omitted here.  
         [0048]     In light of the foregoing, the pore-size-distribution property in a foamed polymer is used to foam a polymer in a mold having a cavity with variable spacing between the interior top surface and the interior bottom surface in different regions. A polishing pad having a desired allocation of soft areas and rigid areas can be obtained through adjusting the level difference of the interior top and/or bottom surface in different regions of the mold&#39;s cavity, the allocation of different regions of the mold&#39;s cavity, and removing the top and/or the bottom surface of the foamed polymer. Therefore, the polishing planarity and uniformity can be easily achieved.  
         [0049]     From the preferred embodiments described above, a pore-size-distribution property in a porous polymer is utilized to produce a single-layer polishing pad having two surfaces with uniform rigidity or various rigidities. The polishing pad described above is a single-layer polishing pad, and the rigidity and compressibility of the top and bottom surfaces of the polishing pad can be easily controlled by various factors, such as the cavity&#39;s shape of the mold used to foam a polymer, the material of the polymer, the foamed level of the foamed polymer, and the cutting process of the foamed polymer. Therefore, not only can the requirements of lower cost and higher CMP process throughput be easily achieved, but also the polishing planarity and uniformity can be easily achieved.  
         [0050]     The method of producing a porous polymer is not limited to a foam process; other suitable methods, such as embedded polymeric microelement, sintered polymer particles, or fiber coating, can also be used. Moreover, the usage of the single-layer polishing pad, according to the preferred embodiments of this invention, is not limited to CMP applied on a wafer; other polishing process applied on glass or other substrates can also use the single-layer polishing pad provided by the preferred embodiments of this invention.  
         [0051]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.