Abstract:
An article for polishing a surface of a semiconductor workpiece is provided. The article includes a polishing layer and a plurality of protrusions repeating across the polishing layer. The protrusions include abrasive particles and can be elastically deformed while polishing the surface of the wafer.

Description:
FIELD  
       [0001]     The present invention generally relates to semiconductor integrated circuit technology and, more particularly, to an electrochemical polishing process and apparatus.  
       BACKGROUND  
       [0002]     Conventional semiconductor devices generally include a semiconductor substrate, usually a silicon substrate, and a plurality of sequentially formed dielectric layers and conductive paths or interconnects made of conductive materials. Interconnects are usually formed by filling a conductive material in trenches etched into the dielectric layers. In an integrated circuit, multiple levels of interconnect networks laterally extend with respect to the substrate surface. Interconnects formed in different layers can be electrically connected using vias or contacts.  
         [0003]     The filling of a conductive material into features such as vias, trenches, pads or contacts, can be carried out by electrodeposition or electroplating. In electrodeposition method, a conductive material, such as copper is deposited over the substrate surface including into such features. Then, a material removal technique is employed to planarize and remove the excess metal from the top surface, leaving conductors only in the features or cavities. Currently, chemical mechanical polishing (CMP) and electropolishing or electrochemical mechanical polishing (ECMP) are employed to planarize and remove excess metal layers deposited on semiconductor wafers.  
         [0004]     Both CMP and ECMP processes involve placing the metal plated surface of the wafer on a polishing pad and establishing a relative motion between the surface and the polishing pad to planarize or remove the metal layer while a polishing solution is supplied to the polishing pad. For the case of electropolishing or electrochemical mechanical polishing, an anodic potential is applied to the metal plated surface of the wafer with respect to an electrode that makes contact to the polishing solution, which also wets the surface of the wafer. Both CMP and ECMP methods require that the wafer be held by a wafer carrier which provides a controllable load on the wafer surface to press it against the polishing pad. Alternately, a force may be applied behind the polishing pad pushing it onto the wafer surface. In general, among other parameters, the type of the polishing pad used and the force pressing the wafer against the polishing pad are important parameters that determine the surface finish and flatness as well as polishing rate of the polished metal layer. Other important parameters are polishing solution or slurry, relative speed between the polishing pad and the wafer, and the applied potential or polishing current for the electropolishing case.  
         [0005]     Copper removal processes such as CMP traditionally use polymeric pad materials such as IC-1000™ pad supplied by Rodel. The copper removal solution typically contains abrasive particles to improve removal rate and surface quality. An alternative way of chemical mechanical polishing of copper has been recently proposed and it involves use of a fixed abrasive pad and an-abrasive free or low-abrasive-content solution. This set of consumables offer better dishing and erosion behavior on the wafer after the polishing step.  
         [0006]     Fixed abrasive polishing pads include a polishing surface having three-dimensional abrasive protrusions. The polishing surface including the abrasive protrusions are made of a high modulus composite material, such as a material having a modulus of elasticity E in the range of 3-6 GPa. The high modulus material is comprised of hard abrasive particles disposed in a binder material. Size of the abrasive particles may be in the 0.1-0.5 microns. Mechanical polishing of a wafer surface is performed by contact with the abrasive protrusions.  
         [0007]      FIG. 1A  illustrates an exemplary portion of a standard fixed abrasive pad  10 . A plurality of abrasive protrusions  12  are formed on the surface of the fixed abrasive pad  10 . The abrasive protrusions  12  may be shaped as three-dimensional geometrical shapes such as pyramids, cylinders, hemispheres, etc., and distributed on the surface to form an array of abrasive protrusions  12 . As described before, abrasive protrusions contain small abrasive particles distributed in a binder material. During the polishing operation, surface of the wafer first comes into contact with upper ends  16  of the plurality of abrasive protrusions. In terms of quality of the final surface finish, it is important that the upper ends  16  of the abrasive protrusions  14  be in the same contact plane P c , so that the upper ends of the abrasive protrusions contact the wafer surface at the same time and polish the wafer surface uniformly. The contact plane P c  is an ideal plane that all the upper ends  16  are expected to meet the wafer surface at the same time, as the wafer surface is pushed onto the fixed abrasive pad. As the same polishing pad is repeatedly used for polishing, the abrasive protrusions begin to uniformly wear against the conductive wafer surface and their height gets shorter.  FIG. 1B  shows as a surface  18  of a wafer  20  is polished by the abrasive protrusions  12 , which are worn after a series of polishing operations.  
         [0008]     Although an ideal fixed abrasive pad has abrasive protrusions with perfectly lined up upper ends, in practice fixed abrasive pads often include some abrasive protrusions which are taller than the majority of the abrasive protrusions. As exemplified in  FIG. 2A , upper end  16 T of a tall abrasive protrusion  12 T extends beyond the imaginary contact plane P c  of upper ends  16 R of regular abrasive protrusions  12 R that establish the majority of the abrasive protrusions. Although, tall abrasive protrusions are defects in the fixed abrasive pads, during a chemical mechanical polishing process, their height is reduced down to the height of the regular abrasive protrusions during polishing of the first few wafers. As illustrated in  FIG. 2B , as the surface  18  of the wafer  20  is forced against the abrasive protrusions  12 T,  12 R, with the downward pressure applied by a wafer carrier (not shown), upper end  16 T of the tall abrasive protrusion is eroded down. During a CMP process, typically a process pressure of more than  1  psi is necessary. With a force at this level, height of the tall abrasive protrusions can be quickly reduced to the height of the regular abrasive protrusions during the process of a wafer without causing excessive scratching on the wafer surface. Therefore, tall abrasive protrusions do not cause any visible defects or scratches on the surface of the wafer during a CMP process. However, the same is not true for an electropolishing process using the same fixed abrasive pad at very low pressure levels.  
         [0009]     As exemplified in  FIG. 2C , owing to the combined affect of high modulus of the abrasive features and the low downward pressure (&lt;0.5 psi) applied to the wafer  20  during the electropolishing or electrochemical mechanical polishing, the height of the tall abrasive protrusion  12 T cannot be quickly reduced. Doted line  21  in the protrusion  12 T shows where the tip of the protrusion is supposed to be located. Even after processing a number of wafers, the tall abrasive protrusions touch the surface  18  at specific locations causing high local pressure application on the surface at those locations and causing scratching. This is because such fixed abrasive pads are manufactured to be utilized at relatively high pressures commonly used in CMP processes. During electropolishing, generally, a low downward pressure, such as a pressure of less than 0.5 psi is used. With a force at this level, the tall abrasive protrusions may scratch as many as 20 to 50 wafers before their height can be reduced to the height level of regular abrasive protrusions. In addition, some of the high modulus materials used in the standard fixed abrasive pads are fragile and brittle and may break off upon impact instead of worn down uniformly. The pieces of materials falling off may then cause further surface scratches during a low force ECMP process.  
         [0010]     To this end, there is need for high performance abrasive polishing pads that can be safely used in the electropolishing technology field.  
       SUMMARY  
       [0011]     Present invention provides a high elasticity fixed abrasive pad for polishing a surface of a semiconductor workpiece. The fixed abrasive pad includes a polishing layer having a plurality of protrusions repeating across the polishing layer. The protrusions include abrasive particles and being capable of elastically deforming while polishing the surface of the wafer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1A  is a schematic side view of a prior art fixed abrasive polishing pad;  
         [0013]      FIG. 1B  is a schematic side view of the prior art fixed abrasive polishing pad in use while polishing a wafer;  
         [0014]      FIGS. 2A-2B  are schematic side views of a prior art fixed abrasive polishing pad having a defective protrusion;  
         [0015]      FIG. 3  is a schematic illustration of an electropolishing system employing polishing article of the present invention;  
         [0016]      FIG. 4  is a schematic side view of the fixed abrasive polishing pad of the present invention;  
         [0017]      FIGS. 5A-5B  are schematic side views of a fixed abrasive pad of the present invention with an exemplary tall protrusion and its elastic deformation at the top; and  
         [0018]      FIGS. 6A-6B  are schematic side views of a fixed abrasive protrusion and its elastic deformation and stretch. 
     
    
     DETAILED DESCRIPTION  
       [0019]     The present invention provides a high elasticity fixed abrasive pad for electrochemical mechanical polishing or planarization of conductive surfaces. The high elasticity fixed abrasive pad is a fault tolerant polishing pad so that any tall abrasive protrusions on the high elasticity fixed abrasive pad of the present invention elastically comply with the conductive surface of the wafer that is pushed against the high elasticity fixed abrasive pad. As the conductive surface is pressed against the polishing surface including tall abrasive protrusions, upper ends of the abrasive protrusions are bent and level with the contact plane of the high elasticity fixed abrasive pad allowing scratch-free processing even for the first wafer that is processed with a new pad.  
         [0020]     Reference will now be made to the drawings wherein like numerals refer to like parts throughout.  FIG. 3  shows an electrochemical mechanical processing system (ECMPR)  100  using a high elasticity fixed abrasive pad  102  having abrasive protrusions  104 . The high elasticity fixed abrasive pad  102  of the present invention will be referred to as the pad hereinafter. A wafer  106  having a surface  108  is held by a wafer carrier  110  which can rotate and move the wafer laterally or vertically. The surface of the wafer includes a conductive layer, preferably a copper layer, to be electropolished by the system  100 . A process solution  112 , such as an electropolishing solution, is delivered between the pad and the surface of the wafer. The process solution can be delivered onto the pad  102  from a supply line (not shown) or through openings  112  in the pad  102 . Openings  112  may also allow electric field to reach the surface  108  which is connected to a power supply  114 . An electrode  116  of the system  100  is also connected to the power supply  114 . The electrode, which is cathode for electropolishing, may be placed in proximity of the pad or placed right under the pad to support it. If the pad is directly mounted on the electrode, the solution may be directly supplied to the top of the pad. Alternately, the pad may be mounted on and supported by a porous support plate through which the solution may flow to the pad and flows through the openings of the pad to its top surface. The support plate may be made of any material that has resistance to the chemical environment of the system such as a hard polymer, stainless steel, etc. The electropolishing pad may move with the support plate, or a relative motion may be established between the pad and the support plate using a moving mechanism. In the latter case, the pad may be shaped as a belt pad.  
         [0021]     There are several patents and patent applications describing the electropolishing process carried out with the assistance of the mechanical action provided by a pad or WSID. Details of such processes are given in the following patents and patent applications; U.S. Pat. No. 6,402,925; U.S. patent application Ser. No. 10/238,665, entitled Method and apparatus for electroplating and electropolishing, filed Sep. 20, 2002, U.S. patent application Ser. No. 09/671,800 entitled, Method to minimize/eliminate metal coating over the top surface of a patterned substrate and layer structure made thereby, filed Sep. 28, 2000; U.S. patent application Ser. No. 09/841,622 entitled Electroetching system and method, filed Apr. 23, 2001; U.S. patent application Ser. No. 10/201,604 entitled, Multi-step electrodeposition process, filed Jul. 22, 2002; U.S. application Ser. No. 10/238,665, entitled Method and apparatus for electroplating and electropolishing, filed Sep. 20, 2002 all commonly owned by the assignee of the present invention and all incorporated herein by reference.  
         [0022]     As shown in  FIG. 4 , the high elasticity fixed abrasive pad of the present invention may include a multiplayer structure. In this embodiment, an abrasive polishing layer  120  is attached on a base layer  122 . The abrasive polishing layer  120  includes the abrasive protrusions  104 . The polishing layer and hence the abrasive protrusions are made of abrasive particles dispersed into a binder matrix, and therefore material forming them is a composite material. Exemplary, abrasive particles may be ceramic particles such as ceria, alumina, zircon or the like. Particles may have a particle size in the range of 50-500 nm. Exemplary, binder materials may be polymeric materials such as polyester, acrylated polyesters etc. An exemplary height and width for the pyramidal shape protrusions may be 100-200 um and 100-500 um, respectively. In this respect, upper ends  124  of the abrasive protrusions  104  are ideally leveled across a contact plane P c , where the surface of the wafer contacts and is polished uniformly by the abrasive protrusions  104 . The abrasive polishing layer comprises a composite layer having abrasive particles dispersed in a binder matrix. As opposed to high modulus fixed abrasive pads used for CMP, the modulus of elasticity E of the abrasive polishing layer is very low, in the range of 0.5-1.5 GPa. In this embodiment, abrasive protrusions have triangular or pointed top shapes in side views. The protrusions may have any three dimensional shape such as pyramidal, cubic or the like, and this is within the scope of this invention.  
         [0023]     The fault tolerant nature of the high elasticity fixed abrasive pad of the present invention may be seen in  FIGS. 5A-5B .  FIG. 5A  exemplifies an abrasive polishing layer  104  of the present invention including tall abrasive protrusions  104 T and regular abrasive protrusions  104 R. Upper ends  124 R of the regular abrasive protrusions  104 R define the contact plane P c . The upper end  124 T of the tall abrasive protrusion  104 T extends beyond the imaginary contact plane P c . As shown in  FIG. 5B , as the surface  108  of the wafer applies a pressure less that 0.5 psi onto the abrasive polishing layer  120 , upper ends  124 T of the tall abrasive protrusions  104 T flexes and are moved into contact plane of the regular abrasive protrusions  104 R. As the polishing of the surface continues the upper end of the tall abrasive protrusion erodes gradually along with the upper ends  124 R of the regular abrasive protrusions  104 R without causing any defects such as deep scratches on the surface  108 .  
         [0024]     Additionally, the material of abrasive polishing layer stretches upon impact of a moving wafer. Stretching occurs within the plastic deformation limits of the material and thus it is temporary. Protrusions recover back their original shape once the impact of the wafer is over. As a result, the abrasive protrusions would not break and fall off, which situation causes scratching as is the case for brittle and hard abrasive protrusions.  FIG. 6A  illustrates in side view an abrasive protrusion  200  having a pyramidal shape without any impact that causes stretching. At this state, a bottom width of the abrasive protrusion is A.  FIG. 6B  shows the abrasive protrusion  200  as it is elastically deformed with application of a force in the direction of arrow I. The arrow I is represents the impact of wafer surface to the protrusion during the process. Applied force moves the upper part of the pyramid with respect to bottom with a distance D. In this respect, maximum elastic deformation without changing the shape of the abrasive protrusion may be between 5%-50%. In other words yield point may be reached beyond 50% deformation.  
         [0025]     Although various preferred embodiments and the best mode have been described in detail above, those skilled in the art will readily appreciate that many modifications of the exemplary embodiment are possible without materially departing from the novel teachings and advantages of this invention.