Abstract:
The invention relates to disks for conditioning pads used in the chemical mechanical polishing of semiconductor wafers, and a method of fabricating the pads. In one embodiment, the conditioning pad includes multiple, pyramid-shaped, truncated protrusions which are cut or shaped in the surface of a typically stainless steel substrate. Each of the truncated protrusions includes a plateau in the top thereof. A seed layer, typically titanium nitride (TiN), is provided on the surface of the protrusions, and a contact layer such as diamond-like carbon (DLC) or other suitable film is provided over the seed layer. In another embodiment, each of the protrusions is pyramid-shaped and includes a pointed apex at the top thereof.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to disks used in the conditioning of polishing pads on chemical mechanical polishers for semiconductor wafers. More particularly, the present invention relates to a polishing pad conditioning disk having improved surface configurations for conditioning polishing pads in chemical mechanical polishers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Apparatus for polishing thin, flat semiconductor wafers are well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head, a wafer unload station, or a wafer load station.  
           [0003]    More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is “planarized” or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in deionized water.  
           [0004]    A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus  20  for chemical mechanical polishing consists of a rotating wafer holder  14  that holds the wafer  10 , the appropriate slurry  24 , and a polishing pad  12  which is normally mounted to a rotating table  26  by adhesive means. The polishing pad  12  is applied to the wafer surface  22  at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films.  
           [0005]    CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed separately.  
           [0006]    A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel.  
           [0007]    In a CMP head, large variations in the removal rate, or polishing rate, across the whole wafer area are frequently observed. A thickness variation across the wafer is therefore produced as a major cause for wafer non-uniformity. In the improved CMP head design, even though a pneumatic system for forcing the wafer surface onto a polishing pad is used, the system cannot selectively apply different pressures at different locations on the surface of the wafer. This effect is shown in FIG. 1C, i.e. in a profilometer trace obtained on an 8-inch wafer. The thickness difference between the highest point and the lowest point on the wafer is almost 2,000 angstroms, resulting in a standard deviation of 472 angstroms, or 6.26%. The curve shown in FIG. 1C is plotted with the removal rates in the vertical axis and the distance from the center of the wafer in the horizontal axis. It is seen that the removal rates obtained at the edge portions of the wafer are substantially higher than the removal rates at or near the center of the wafer. The thickness uniformity on the resulting wafer after the CMP process is poor.  
           [0008]    The polishing pad  12  is a consumable item used in a semiconductor wafer fabrication process. Under normal wafer fabrication conditions, the polishing pad is replaced after about 12 hours of usage. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard and stiffer pads are generally used to achieve planarity. Softer pads are generally used in other polishing processes to achieve improved uniformity and smooth surfaces. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.  
           [0009]    A problem frequently encountered in the use of polishing pads in oxide planarization is the rapid deterioration in oxide polishing rates with successive wafers. The cause for the deterioration is known as “pad glazing”, wherein the surface of a polishing pad becomes smooth such that slurry is no longer held in between the fibers of the pad. This physical phenomenon on the pad surface is not caused by any chemical reactions between the pad and the slurry.  
           [0010]    To remedy the pad glazing effect, numerous techniques of pad conditioning or scrubbing have been proposed to regenerate and restore the pad surface and thereby restore the polishing rates of the pad. The pad conditioning techniques include the use of silicon carbide particles, diamond emery paper, blade or knife for scraping or scoring the polishing pad surface. The goal of the conditioning process is to remove polishing debris from the pad surface and re-open pores in the pad by forming micro-scratches in the surface of the pad for improved pad lifetime. The pad conditioning process can be carried out either during a polishing process, i.e. known as concurrent conditioning, or after a polishing process.  
           [0011]    While the pad conditioning process improves the consistency and lifetime of a polishing pad, a conventional conditioning disk is frequently not effective in conditioning a pad surface after repeated usage. A conventional conditioning disk for use in pad conditioning is shown in FIGS. 2A, 2B and  2 C.  
           [0012]    Referring next to FIG. 2A, a conventional CMP apparatus  50  includes a conditioning head  52 , a polishing pad  56 , and a slurry delivery arm  54  positioned over the polishing pad. The conditioning head  52  is mounted on a conditioning arm  58  which is extended over the top of the polishing pad  56  for making a sweeping motion across the entire surface of the polishing pad  56 . The slurry delivery arm  54  is equipped with slurry dispensing nozzles  62  which are used for dispensing a slurry solution on the top surface  60  of the polishing pad  56 . Surface grooves  64  are further provided in the top surface  60  to facilitate even distribution of the slurry solution and to help entrapping undesirable particles that are generated by coagulated slurry solution or any other foreign particles which have fallen on top of the polishing pad  56  during a polishing process. The surface grooves  64 , while serving an important function of distributing the slurry, also presents a processing problem when the pad surface  60  gradually wears out after prolonged use.  
           [0013]    The conventional conditioning disk  68  may be of several different types, two of which are shown in cross-section in FIGS. 2B and 2C. A conventional brazed grid-type conditioning disk  68 , shown in FIG. 2B, is formed by embedding or encapsulating diamond particles  72  in random spacings with each other in the surface of a stainless steel substrate  70 . A conventional dia grid-type conditioning disk  68 , shown in FIG. 2C, is formed by embedding cut diamonds  80  at regular spacings in a nickel film  82  coated onto the surface of a stainless steel substrate  78 . The diamonds  80  are typically coated with a diamond-like carbon (DLC) layer  83 . One of the problems associated with the conventional conditioning disks  68  is that the polishing slurry tends to easily damage the nickel film holding the diamonds onto the substrate, and this causes the diamonds to drop from the disk onto the polishing pad and scratch the surface of the wafer during the CMP process. Furthermore, use of diamonds in the disk is excessively costly, as the diamonds are lost from the polishing pads over a typical pad lifetime of from 10-50 hours.  
           [0014]    Accordingly, an object of the present invention is to provide new and improved conditioning disks for conditioning polishing pads used in the chemical mechanical polishing (CMP) of semiconductor wafers.  
           [0015]    Another object of the present invention is to provide a CMP conditioning disks which are characterized by increased lifetime and durability.  
           [0016]    Still another object of the present invention is to provide CMP conditioning disks which are inexpensive to manufacture and use.  
           [0017]    Yet another object of the present invention is to provide CMP conditioning disks which are capable of effectively conditioning CMP pads while preventing or minimizing particle contamination of wafers polished by the pads.  
           [0018]    Another object of the present invention is to provide CMP conditioning disks which do not provide a source of potential particulate contaminants for a polishing pad or semiconductor wafer during a CMP process.  
           [0019]    A still further object of the present invention is to provide CMP conditioning disks which enable fine-tuning chemical mechanical polishing process parameters in order to optimize chemical mechanical polishing of semiconductor wafers.  
           [0020]    Still another object of the present invention is to provide CMP conditioning disks having multiple protrusions arranged in a uniformly-spaced pattern on the surface of each disk and which protrusions are substantially uniform in shape, size and quality.  
           [0021]    Yet another object of the present invention is to provide a method of fabricating a new and improved CMP conditioning disk.  
         SUMMARY OF THE INVENTION  
         [0022]    In accordance with these and other objects and advantages, the present invention comprises new and improved disks for conditioning pads used in the chemical mechanical polishing of semiconductor wafers, and a method of fabricating the pads. In one embodiment, the conditioning pad includes multiple, pyramid-shaped, truncated protrusions which are cut or shaped in the surface of a typically stainless steel substrate. Each of the truncated protrusions includes a plateau in the top thereof. A seed layer typically of titanium nitride (TiN) is provided on the surface of the protrusions, and a contact layer of diamond-like carbon (DLC) or other suitable film is provided over the seed layer. In another embodiment, each of the protrusions is pyramid-shaped and includes a pointed apex at the top thereof. When mounted on a conditioning head of a chemical mechanical polisher, the protrusions are effective in scoring or scratching a CMP polishing pad to enhance retention of slurry in the polishing pad during a CMP operation.  
           [0023]    In both embodiments of the present invention, the pyramidal protrusions are separated by a network of grooves cut or shaped in the typically stainless steel substrate. The depth of the grooves typically ranges from about 0.1 mm to about 3 mm, whereas the height of the protrusions typically ranges from about 0.2 mm to about 5 mm. The width of the top or extending portion of each pyramid-shaped protrusion ranges from about 0 (in the case of the protrusions having an apex) to about 5 mm (in the case of the truncated protrusions having the plateau shaped in the upper end thereof). The thickness of the seed layer on the protrusions and grooves ranges from typically about 10 μm to about 2000 μm, whereas the thickness of the contact layer on the seed layer ranges from typically about 5 μm to about 500 μm.  
           [0024]    Each of the conditioning disks of the present invention may be fabricated by initially providing a circular stainless steel substrate which is typically about 4 inches in diameter. Next, multiple grooves are etched into the surface of the substrate using conventional mechanical means. This step forms the multiple pyramid-shaped protrusions on the substrate, with the network of grooves separating the protrusions. In the case of the truncated protrusions, the upper portion of each protrusion is next removed. Next, the seed film is deposited on the substrate and provides a continuous coating on both the grooves and the protrusions on the substrate surface. The seed layer enhances adhesion of the substrate on the contact layer, which is then deposited on the seed layer as a final step in the fabrication process. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The invention will now be described, by way of example, with reference to the accompanying drawings, wherein:  
         [0026]    [0026]FIG. 1A is a cross-sectional view of a typical conventional chemical mechanical polishing (CMP) apparatus;  
         [0027]    [0027]FIG. 1B is an enlarged, cross-sectional view of a section of a wafer and the polishing pad of a conventional CMP apparatus, with a slurry solution there between;  
         [0028]    [0028]FIG. 1C is a graph illustrating the changes in removal rates as a function of distance on a wafer after a polishing pad is repeatedly used;  
         [0029]    [0029]FIG. 2A is a perspective view of a conventional CMP polishing pad with a slurry dispensing arm and a conditioning disk positioned on top;  
         [0030]    [0030]FIG. 2B is a cross-sectional view of a conventional, brazed-type diamond conditioning disk;  
         [0031]    [0031]FIG. 2C is a cross-sectional view of a conventional, dia grid diamond conditioning disk;  
         [0032]    [0032]FIG. 3 is a cross-sectional view of a first embodiment of the conditioning disk of the present invention;  
         [0033]    [0033]FIG. 4 is a cross-sectional view of a protrusion component of the conditioning disk illustrated in FIG. 3;  
         [0034]    [0034]FIG. 5 is a top view, partially in section, of the conditioning disk illustrated in FIG. 3;  
         [0035]    [0035]FIG. 6 is a cross-sectional view of a second embodiment of the conditioning disk of the present invention;  
         [0036]    [0036]FIG. 7 is a cross-sectional view of a protrusion of the conditioning disk illustrated in FIG. 6;  
         [0037]    [0037]FIG. 8 is a top view, partially in section, of the conditioning disk illustrated in FIG. 6;  
         [0038]    [0038]FIG. 9 is a flow diagram illustrating an illustrative method of fabricating a conditioning disk of the present invention; and  
         [0039]    [0039]FIG. 10 is a side view, partially in section, of a conditioning head, with a conditioning disk of the present invention mounted on the conditioning head in use of the conditioning disk to condition a CMP polishing pad (in section). 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]    Referring initially to FIGS.  3 - 5 , a first illustrative embodiment of a conditioning disk of the present invention is generally indicated by reference numeral  84  and includes a substrate  86  which is typically a circular plate of stainless steel #316. It is understood, however, that other metals or grades of steel may be used as the substrate  86 . Multiple pyramid-shaped protrusions  88  are cut or otherwise formed in the upper surface of the substrate  86 . As illustrated in FIG. 5, each protrusion  88  typically includes four sides  90  and a plateau  92 . A network of intersecting perpendicular grooves  89  separate adjacent protrusions  88  from each other. As particularly illustrated in FIG. 4, a seed layer  94 , such as titanium nitride (TiN), is deposited on the substrate  86  and coats the surface of the sides  90  and plateau  92  of each protrusion  88 , as well as the surfaces of the grooves  89 . The seed layer  94  provides adhesion for a contact layer  96  subsequently deposited on the seed layer  94 , which contact layer  96  is typically diamond-like carbon (DLC) or alternatively, a CVD diamond film layer.  
         [0041]    In a typical embodiment, each groove  89  has a depth, designated by the letter “A” in FIG. 3, typically in the range of from about 0.1 mm to about 3 mm. Each protrusion  88  has a height, designated by the letter “B” in FIG. 4, in the range of from about 0.2 mm to about 5 mm. The plateau  92  of each protrusion  88  has a width of up to about 5 mm. The seed layer  94  has a thickness of from about 10 μm to about 2000 μm, whereas the thickness of the contact layer  96  on the seed layer  94  ranges from typically about 5 μm to about 500 μm.  
         [0042]    Referring next to FIG. 10, a CMP apparatus  42  is shown in typical application of the conditioning disk  84 , which is mounted face-down, according to methods which are well-known by those skilled in the art, on a conditioning head  46  provided on the end of a conditioning arm  44  of the CMP apparatus  42 . A polishing pad  48  is supported on a platen  47  of the CMP apparatus  42 , and the platen  47  and conditioning head  46  are simultaneously rotated according to parameters which are known by those skilled in the art. Accordingly, as the platen  47  rotates the polishing pad  48 , the conditioning head  46  simultaneously rotates the conditioning disk  84  mounted thereon, and the protrusions  88  (FIG. 3) extending downwardly from the surface of the conditioning disk  84  scratch and score the upper surface of the polishing pad  48 . This may be carried out as a wafer (not illustrated) rests on the polishing pad  48  during a CMP operation, or alternatively, during a conditioning process in which a CMP operation is not being carried out on the polishing pad  48 . The scratches made in the surface of the polishing pad  48  facilitate enhanced retention of polishing slurry (not illustrated) on the polishing pad  48  during CMP operations. Furthermore, the typically stainless steel protrusions  88  are incapable of inadvertently breaking off from the substrate  86  of the conditioning disk  84  and dropping on the polishing pad  48  and potentially contaminating a wafer (not illustrated) on the polishing pad  48  during a CMP operation.  
         [0043]    Referring next to FIGS.  6 - 8 , a second illustrative embodiment of the conditioning disk of the present invention is generally indicated by reference numeral  1  and includes a substrate  2  which is typically a circular plate of stainless steel #316. It is understood, however, that other metals or grades of steel may be used as the substrate  2 . Multiple pyramid-shaped protrusions  3  are cut or otherwise formed in the upper surface of the substrate  2 . As illustrated in FIG. 8, each protrusion  3  typically includes four sides  5  which meet at an apex  6 . A network of perpendicular intersecting grooves  4  separate adjacent protrusions  3  from each other. As particularly illustrated in FIG. 7, a seed layer  7 , typically titanium nitride (TiN), coats the surface of the sides  5  and apex  6  of each protrusion  3 , as well as the surfaces of the grooves  4 . The seed layer  7  provides adhesion for a contact layer  8  subsequently deposited on the seed layer  7 , which seed layer  7  is typically diamond-like carbon (DLC) or alternatively, a CVD diamond film layer.  
         [0044]    In a typical embodiment, each groove  4  of the conditioning disk  1  has a depth, designated by the letter “D” in FIG. 6, typically in the range of from about 0.1 mm to about 3 mm. Each protrusion  3  has a height, designated by the letter “E” in FIG. 7, in the range of from typically about 0.2 mm to about 5 mm. The apex  6  of each protrusion  3  has a width of typically from about 0 mm to about 5 mm. The seed layer  7  has a thickness of from about 10 μm to about 2000 μm, whereas the thickness of the contact layer  8  on the seed layer  7  ranges from typically about 5 μm to about 500 μm.  
         [0045]    In application, and referring again to FIG. 10, the conditioning disk  1  is mounted on the conditioning head  46  of the CMP apparatus  42  and rotated in conjunction with the polishing pad  48  on the platen  47  in the same manner as heretofore described with respect to the conditioning disk  84 . Accordingly, as the platen  47  rotates the polishing pad  48 , the conditioning head  46  simultaneously rotates the conditioning disk  1  mounted thereon, and the protrusions  3  (FIG. 6) extending downwardly from the surface of the conditioning disk  1  scratch and score the upper surface of the polishing pad  48 . This may be carried out as a wafer (not illustrated) rests on the polishing pad  48  during a CMP operation, or alternatively, during a conditioning process in which a CMP operation is not being carried out on the polishing pad  48 . The scratches made in the surface of the polishing pad  48  facilitate enhanced retention of polishing slurry (not illustrated) on the polishing pad  48  during CMP operations. Furthermore, the typically stainless steel protrusions  3  are incapable of inadvertently breaking off from the substrate  2  of the conditioning disk  1  and dropping on the polishing pad  48  and potentially contaminating a wafer (not illustrated) on the polishing pad  48  during a CMP operation.  
         [0046]    A typical method of manufacturing a conditioning disk  1  of the present invention is outlined in FIG. 9. First, the protrusions  3  and grooves  4  are cut into the upper surface  9  of the blank substrate  2  using conventional mechanical techniques. The substrate  2  is typically a circular plate of stainless steel #316 or other steel grade or suitable metal, and is typically about 4 inches in diameter. After the protrusions  3  and grooves  4  have been cut in the substrate  2 , the protrusions  3  and grooves  4  are coated with the seed layer  7 , typically a film of TiN having a thickness of from about 10 μm to about 2000 μm, using conventional chemical vapor deposition (CVD) techniques. Next, the contact layer  8 , typically a layer of diamond-like carbon (DLC) or CVD diamond film, is deposited on the seed layer  7  typically using conventional CVD techniques. The contact layer  8  typically has a thickness of from about 5 μm to about 500 μm.  
         [0047]    While the fabricating method outlined in FIG. 9 describes a typical process for fabricating the conditioning disk  1  having the protrusions  3  with the respective apices  6 , it is understood that the method heretofore described with respect to FIG. 9 is equally applicable to fabricating the conditioning disk  84  having the protrusions  88  with the plateaus  92  instead of the apices  6 . In fabricating the conditioning disk  84 , however, the plateaus  92  are formed in the respective protrusions  88  prior to depositing the seed layer  94  and contact layer  96  (FIG. 4) on the conditioning disk  84 .  
         [0048]    While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications may be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.