Patent Publication Number: US-2013252516-A1

Title: Polishing pad and polishing method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2012-067679, filed on Mar. 23, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate to a polishing pad and a polishing method. 
     BACKGROUND 
     In the manufacturing process of semiconductor devices, as the flattening technology, the chemical-mechanical polishing method (Chemical Mechanical Polishing, to be referred to as CMP) is mainly used. Usually, a groove is processed on the surface of the polishing pad used in the CMP method for feeding and exhausting the slurry to/from the polishing surface. 
     However, when the polishing pad is worn off due to dressing (conditioning of the polishing pad), the depth of the groove gradually becomes shallower and the shape of the groove changes, so that the polishing characteristics also change. That is, the groove formed on the surface of the polishing pad is a major factor in determining a lifetime of the polishing pad. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view illustrating an example of the constitution of the CMP apparatus for which the polishing pad of the embodiment of the present disclosure is used. 
         FIGS. 2A and 2B  are partial cross-sectional views illustrating an example of the constitution of the polishing pad in the embodiment. 
         FIG. 3  is a top view illustrating an example of the constitution of the polishing pad in the embodiment. 
         FIGS. 4A and 4B  are schematic cross-sectional views illustrating the state of the polishing pad after a certain time of use. 
         FIGS. 5A and 5B  are schematic partial cross-sectional views illustrating the state of use of the polishing pad after a certain time of use of the polishing pad having a conventional groove structure in a comparative example. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, the polishing pad and polishing method of the embodiment will be explained in detail with reference to attached figures. However, the present disclosure is not limited to the embodiment. 
     The embodiment has an aim to provide a polishing pad and a polishing method for reducing variation of polishing characteristics even when a polishing pad is polished. 
     According to the embodiment, there is provided a polishing pad having a polishing surface for polishing a workpiece. The polishing pad is made of a plate-shaped thermal shrinking material, and it has a half-cut portion cut to a particular depth from one principal surface. 
       FIG. 1  is a schematic side view illustrating an example of the constitution of the CMP apparatus for which the polishing pad of the embodiment is used.  FIGS. 2A and 2B  include schematic partial cross-sectional views illustrating the constitution of the polishing pad in the embodiment.  FIG. 2A  shows the normal state and  FIG. 2B  shows the state in the polishing operation.  FIG. 3  is a top view illustrating an example of the constitution of the polishing pad in the embodiment. 
     The CMP apparatus includes a rotatable polishing table  21 , a polishing pad  22  bonded via a bonding layer, not shown in the figure, on the polishing table  21 , a polishing head  23  arranged on the polishing pad  22  and that holds a semiconductor substrate or other polishing workpiece  20 , a chemical solution feeding nozzle  24  for feeding polishing slurry  241  or other chemical-solution in the polishing operation, and a dresser  25  made of, for example, a diamond disk or the like, arranged above the polishing pad  22  for dressing the polishing pad  22 . 
     The polishing head  23  holds the polishing workpiece  20  by a vacuum chuck holder or similar part so that the surface for polishing faces the polishing pad  22  on the polishing table  21 . The polishing head  23  and the dresser  25  have a structure such that they can be rotated in the same plane as the polishing table  21  and, at the same time, they can be driven to move in the direction perpendicular to the surface of the polishing table  21  so that the surface of the polishing head  23  or dresser  25  can make contact with the surface of the polishing pad  22 . In addition, although not shown in the figure, a polishing slurry feeding tank is connected with the chemical-solution feeding nozzle  24 . 
     The polishing pad  22  in this embodiment has a half-cut portion  31 . Here, the half-cut portion  31  is cut to a particular depth from the surface of the polishing pad  22 , and it does not go through to reach the back surface of the polishing pad  22 . Here, the half-cut portion  31  differs from the groove in that it does not have a particular width. In the normal state, not in the polishing operation, the two side surfaces with half-cut portion  31  held between them are in contact with each other. As shown in  FIG. 3 , the half-cut portion  31  is formed in, for example, a lattice shape on the surface of the polishing pad  22 . Also, the half-cut portion  31  may be formed in a vortex shape, or concentric circular shape or the like, instead of the lattice shape on the surface of the polishing pad  22 . The depth of the half-cut portion should be appropriate to ensure that it does not cut through to reach the bonding layer as the underlying layer of the polishing pad  22 . 
     The polishing pad  22  is made of a material that shrinks under the heat generated in the polishing operation as to be explained later. An example of such a thermal shrinking material is the thermal shrinking polyurethane. Also, the polishing pad  22  may be made of either a foaming material or a non-foaming material. 
     In the following, a brief account will be given on the CMP processing method using the CMP apparatus. Here, as an example, a semiconductor substrate on which a silicon oxide film is formed will be taken as the polishing workpiece  20  for explanation. In this case, it is supposed that bumps/dips are formed on the surface of the silicon oxide film as the surface for polishing. 
     Before the polishing operation, the semiconductor substrate is held on the polishing head  23  so that the silicon oxide film faces the polishing pad  22 . In addition, a polishing slurry  241  containing, for example, cerium oxide grains and a surfactant is fed from the chemical-solution feeding nozzle  24  onto the polishing pad  22 . 
     As the polishing pad  23  is driven to move in the direction towards the polishing table  21 , the semiconductor substrate is pressed on the surface of the polishing pad  22  and, while the polishing table  21  and the polishing pad  23  are driven to rotate, the surface of the semiconductor substrate is subjected to a polishing operation. After the start of the polishing operation, while the polishing pad  22  and the polishing workpiece  20  are in contact with each other, they are driven to rotate in the in-plane direction, so that friction leads to a rise in the temperature near the surface of the polishing pad  22 . Depending on the types of polishing workpiece  20  and the polishing slurry  241  as well as the polishing conditions, the temperature of the surface of the polishing pad  22  may rise to about 60 to 80° C. in the polishing operation. 
     As shown in  FIG. 2B , because the polishing pad  22  is made of thermal shrinking polyurethane or other thermal shrinking material, due to the rise in the temperature on the surface of the polishing pad  22  in the polishing operation, the polishing pad  22  shrinks in the in-plane direction and in the direction perpendicular to the polishing surface on the surface of the polishing pad  22 . Also, as the position moves deeper from the surface of the polishing pad  22 , the temperature of the polishing pad  22  decreases, so that the shrinkage degree of the thermal shrinking material becomes smaller. As a result, at a particular depth d 1  position from the surface of the polishing pad  22  where no thermal shrinking takes place, the side surfaces that hold the half-cut portion  31  between them are in contact with each other. As the position becomes shallower (as the position moves towards the surface), the shrinkage degree in the in-plane direction increases, so that the side surfaces of the half-cut portion  31  are separated from each other. Consequently, a groove  32  is formed at the half-cut portion  31 . The polishing slurry  241  fed from the chemical solution feeding nozzle  24  is then fed into the formed groove  32 , and the polishing slurry  241  is exhausted from the groove  32  as the polishing operation is executed. 
     With the progress of polishing, cerium oxide grains generated in the polishing operation and the coagulated polishing grains formed due to coagulation by the surfactant as well as polishing-generated chaff, etc., are accumulated in a large quantity on the polishing pad  22  and the groove  32 . As a result, clogging of the polishing pad  22  takes place, so that the polishing speed falls. At this point, in order to eliminate this problem, a dressing treatment should be carried out. 
     In the dressing treatment, the surface of the dresser  25  is pressed on the surface of the polishing pad  22  and, as the polishing table  21  and the dresser  25  are driven to rotate, the surface of the polishing pad  22  is polished. As a result, while the coagulated polishing grains and polishing-generated chaff on the surface of the polishing pad  22  are removed, dressing is carried out in this treatment. As explained above, in the CMP treatment, the polishing treatment and dressing treatment are carried out. 
       FIGS. 4A and 4B  include schematic cross-sectional views illustrating the state of use after a certain time of use from the start of use of the polishing pad.  FIG. 4A  is a schematic cross-sectional view illustrating the initial state before use.  FIG. 4B  is a schematic cross-sectional view illustrating the state during the polishing treatment after a certain time of polishing. After use of the polishing pad  22 , the polishing pad  22  with the initial state shown in  FIG. 4A  becomes the state shown in  FIG. 4B , with a reduced thickness of the polishing pad  22  due to the thermal shrinking and dressing treatment. When the polishing treatment is carried out in such a state, as explained above, due to the heat generated in the polishing, the portion of the polishing pad  22  from the top surface to a particular depth is deformed due to thermal shrinking, and groove  32  is formed on the half-cut portion  31 . The depth d 2  of the groove  32  formed in this case is similar to the depth dl of the groove  32  formed using the polishing pad  22  in the initial state shown in  FIG. 2B . The shape and size of the groove  32  as shown in  FIG. 4B  are similar to those shown in  FIG. 2B . This remains true and is independent of the thickness of the polishing pad  22  until the position of height d 1  from the end point  311  of the half-cut portion  31  (d 2 ) becomes the top surface of the polishing pad  22 . 
     In this way, by using the thermal shrinking material, it is possible to ensure that the shape and size of the groove  32  formed in the half-cut portion  31  are kept the same even when the thickness of the polishing pad  22  changes. As a result, it is possible to ensure a constant quantity of the polishing slurry  241  fed to the groove  32  in the polishing operation, and it is possible to ensure stable polishing characteristics independent of the remaining thickness of the polishing pad  22 . 
       FIGS. 5A and 5B  include partial cross-sectional views schematically illustrating the state of variation over time of the conventional polishing pad as a comparative example due to the polishing operation.  FIG. 5A  is a partial cross-sectional view illustrating the state of the polishing pad in the initial state.  FIG. 5B  is a partial cross-sectional view illustrating the state of the polishing pad after use for a certain time. As shown in  FIG. 5A , for the conventional polishing pad  22 , a groove  35  with a width of W (&gt;0) and depth of d 3  is formed on it. As shown in  FIG. 5B , after use for a certain time, because the polishing pad  22  is worn off due to the dressing treatment or the like, the groove  35  on polishing pad  22  has a width of W and a depth of d 4 . 
     When the polishing pad  22  is in use, the polishing slurry  241  enters the groove  35  while the polishing operation is carried out. However, the groove  35  shown in  FIG. 5A  is deeper than the groove  35  shown in  FIG. 5B , so that the polishing slurry  241  is fed into the groove  35 . As a result, in the polishing operation, the quantity of the polishing slurry  241  changes corresponding to the difference in depth (d 3 −d 4 ) of the two grooves  35 , so that a significant difference takes place in the polishing characteristics between them. That is, because the polishing characteristics depend on the thickness of the polishing pad  22 , it is necessary to carry out the polishing operation with the thickness of the polishing pad  22  taken into consideration. 
     On the other hand, for the polishing pad  22  in the embodiment, the shape and size of the groove  32  formed for the half-cut portion  31  in the polishing operation are kept constant and independent of the thickness of the polishing pad  22 . Consequently, the quantity of the polishing slurry  241  enclosed in the groove  32  during the polishing operation is kept constant. As a result, there is no difference in the polishing characteristics depending on the thickness of the polishing pad  22 , and it is possible to realize stable polishing characteristics over the entire lifetime. 
     Also, it is possible to carryout the polishing operation under conditions without considering the thickness of the polishing pad  22 . That is, there is no need to determine the optimum polishing conditions for each thickness value of the polishing pad  22 . More specifically, for the conventional polishing pad  22  shown in  FIGS. 5A and 5B , the lifetime of the polishing pad  22  is taken as the time when the depth of the groove  35  reaches a certain level, so that the lifetime of the polishing pad  22  depends on the polishing characteristics that vary depending on the depth and shape of the groove  35 . On the other hand, according to the present embodiment, the position of the end point  311  of the half-cut portion  31  is selected as the depth without going through the polishing pad  22 . Consequently, compared with the case of the polishing pad  22  having the groove  35  shown in  FIGS. 5A and 5B , it is possible to prolong the lifetime. This is an effect of the present embodiment. During the period until the lifetime of the polishing pad  22  is reached, the polishing rate and evenness can be kept constant independent of the thickness of the polishing pad  22 . This is another effect of the present embodiment. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein maybe made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.