Abstract:
The present invention provides a composition and a method of polishing a surface that minimizes abrasive removal of material from the surface. To that end, the composition is formulated to maximize dissolution of the material from the surface.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The field of invention relates generally to the fabrication of integrated circuits. More particularly, the invention relates to compositions and methods for using polishing layers of material in furtherance of fabricating semiconductor circuits.  
         [0002]     The fabrication of modern semiconductor devices includes forming multiple layers of conductive and dielectric materials on substrates. To that end two various processes are employed to deposit and to remove material associated with the layer. Exemplary deposition techniques include electrochemical deposition, chemical vapor deposition (CVD), plasma enhanced CVD (PECVD), atomic layer deposition (ALD), physical vapor deposition (PVD) and the like. Exemplary removal techniques include etching, such as chemical or plasma etching, as well as polishing.  
         [0003]     Chemical-mechanical polishing (CMP) methods and polishing slurries are well known and widely used techniques for polishing layers to provide the same with a smooth, if not planar, shape. Often, however, the surface being polished has regions with differing materials present, e.g., materials with differing mechanical properties and chemical reactivity. As a result, the removal rate over the surface is not uniform, which makes obtaining the desired planarization of the surface difficult, while minimizing roughness over the area thereof. For example when polishing a surface having a metal region surrounded by dielectric, minimization of dishing is difficult. Dishing results from one of the regions, e.g., the metal region, being removed at a greater rate than the rate at which the other regions of the surface are removed. This results in a concave region in the metal area, which is often undesirable when a planar shape is desired. To avoid the deleterious effects of CMP of surfaces having regions of differing material properties, various CMP slurries have been developed to obtain desirable CMP characteristics: low polish induced damage, high polishing rate, process predictability, high polished surface uniformity, low polished surface roughness, and the use of non-hazardous, low-cost polish materials.  
         [0004]     Historically, polishing slurries for use in CMP contain fine, suspended abrasive particles to facilitate mechanical polishing of the surface, as well as acidic or basic chemical components to facilitate chemical polishing of the surface. The rate at which polishing occurs for a given material and operating conditions is related to the quantity of abrasive particles in the slurry. However, the damage to the surface being polished is also related to the size of the particles in the slurry. Chemical component selection may also dramatically affect polish rate and quality for a given material and operating conditions.  
         [0005]     Additionally, advanced integration schemes, e.g., stacks with ultra low dielectric (ULK) and air gap integration schemes, are often structurally compromised by CMP processes through interfacial stress created during the CMP process, erosion, as well as absorption of CMP slurry chemicals.  
         [0006]     Therefore, a need exists to provide improved techniques for polishing layers in furtherance of producing semiconductor circuits.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a cross-sectional view of a chemical-mechanical polishing machine known in the art but which can be used in practicing the present invention;  
         [0008]      FIG. 2  is a schematic cross-sectional view of a chemical-mechanical polishing machine known in the art but which can be used in practicing the present invention;  
         [0009]      FIG. 3  is a cross-sectional view showing an exemplary structure to undergo polishing in accordance with the present invention;  
         [0010]      FIG. 4  is a cross-sectional view showing a slurry composition in accordance with the present invention, being disposed between the exemplary structure of  FIG. 3  and a polishing pad of the polishing machine shown in  FIG. 1  in accordance with the present invention;  
         [0011]      FIG. 5  is a cross-sectional view showing the exemplary structure of  FIG. 3  having a surface undergoing polishing;  
         [0012]      FIG. 6  is a cross-sectional view showing the exemplary structure of  FIG. 3  after polishing in accordance with the present invention; and 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     Referring to  FIGS. 1 and 2 , a brief overview of a polishing machine  10  is depicted that may be employed in accordance with the present invention. Polishing machine  10  has a platen  12 , a wafer carrier  14 , a polishing pad  16 , and a slurry  18  on polishing pad  16 . An under-pad  20  is typically attached to the upper surface  22  of platen  12 , and polishing pad  16  is positioned on under-pad  20 . A drive assembly  24  rotates platen  12  as indicated by arrow A. In addition, drive assembly  24  may cause platen  12  to reciprocate as indicated by arrow B. The motion of platen  12  is imparted to polishing pad  16  through under-pad  20  because polishing pad  16  frictionally engages under-pad  20 . Wafer carrier  14  has a lower surface  26  to which a wafer  28  may be attached, or wafer  28  may be attached to a resilient pad  30  positioned between wafer  28  and lower surface  26 .  
         [0014]     Wafer carrier  14  may be a weighted, free-floating wafer carrier, or an actuator assembly  32  may be attached to wafer carrier  14  to impart axial and rotational motion, as indicated by arrows C and D, respectively. Polishing pad  16  may be embodied as a conventional polishing pad, a web-type polishing pad, a belt-type polishing pad, or any other polishing pad format known in the art. Polishing pad  16  may also be employed as a fixed-abrasive polishing pad. Such a fixed-abrasive polishing pad  16  may be impregnated with particulate abrasives including, but not limited to, alumina, titanium dioxide, silicon dioxide, and cerium dioxide. The abrasives in a fixed-abrasive polishing pad  16  are typically leached therefrom during polishing of wafer  28 .  
         [0015]     Referring to  FIGS. 1 and 3 , an exemplary wafer  28  that undergoes polishing in accordance with the present invention includes a substrate  40  having a recess  42  disposed within a surface  44 . In a preferred embodiment, substrate  40  includes a dielectric layer and recess  42  is formed within the dielectric layer. A metal layer  48 , such as copper, is disposed on surface  44  and substantially fills recess  42 . A liner  50  is disposed between substrate  40  and metal layer  48 , and is located on surface  44  and surfaces  46  of recess  42 . Wafer  28  may comprise various other layers adjacent to recess  42 , surface  44 , liner  50 , and/or metal layer  48 , but for the purposes of simplicity of discussion, no other such structures are depicted.  
         [0016]     Referring to  FIGS. 4 and 5 , to polish metal layer  48  in accordance with the present invention, slurry  18  is disposed between metal layer  48  and polishing pad  16 . Polishing pad  16  is placed in close proximity to metal layer  48 . Subsequently, polishing pad  16  is brought in frictional contact with metal layer  48  and, in combination with slurry  18 , removes portions of metal layer  48 . To attenuate, if not prevent, “dishing,” portions  52  of metal layer  48  are removed before portions  54  of metal layer  48 , which are more distant from polishing pad  16 . Once metal layer  48  is substantially removed outside the trench region, liner  50  is subsequently removed from surface  44  outside the trench region by continued polishing with slurry  18  and polishing pad  16 .  
         [0017]     Referring to  FIG. 6 , upon removal of metal layer  48  and liner  50  from outside the trench region, a new surface  144  is defined having first and second regions  146  and  148 . First region  146  is comprised of metal from the remaining portions of metal layer  48 , with second region  148  comprising substrate  40  and liner  50 . As a result, surface  144  has varying material properties across an area thereof, with region  146  typically being harder than region  148 , e.g. when region  148  is a dielectric material. As a result, were polishing pad  16  to impart a uniform force against surface  144  for a given slurry composition, the polish rate of region  146  may be greater than the polish rate of region  148 . This may present as “dishing” in which region  146  has a concave shape.  
         [0018]     The present invention, however, significantly attenuates dishing by changing the rate limiting step of the polishing operation. In the present invention, metal removal is controlled more by dissolution rather than kinetics during polishing of metal layer  48 . Specifically, it was recognized that by controlling or limiting the removal rate by dissolution from surface  144 , dishing may be avoided while at the same time minimizing roughness. For purposes of understanding the present invention, the polishing operation can be understood to have two principle operating mechanisms or steps, dissolution and kinetics. The kinetic step of removal can be defined as the reaction to form soluble metal oxides, while dissolution can be defined as the removal of the metal oxide by dissolving the same in a solvent. In the context of polishing copper, copper itself does not dissolve in solvents but copper oxide does. Thus, to effectively remove copper using solvent containing slurries, one has to first react the copper to form copper oxide. In the present invention, the removal process is principally governed or controlled by the removal of the oxides from the surface by dissolution, and not by kinetics at the metal interface.  
         [0019]     Kinetic removal of material from surface  144  in accordance with the invention has less of an influence in the polishing rate in large part as a result of providing a neutral pH environment. In a preferred embodiment, this is accomplished using a reactive liquid (RL) slurry having a neutral pH. RL slurries are generally characterized by containing little or no abrasives, i.e., particles. Removal of materials is achieved primarily through chemical reaction of the material being polished with the RL slurry components.  
         [0020]     More specifically, a composition in accordance with an embodiment of the present invention is provided with a pH that is generally in the range of 5 to 8. Optimal results were achieved using a pH of approximately 7.5. If present at all, particles in the slurry are generally no greater than 250 parts per million of the slurry composition or 0.0025 weight percent. Also included in the composition is a corrosion inhibitor that further minimizes kinetic removal of material from surface  144  during polishing. Other components of the composition may include an oxidizing agent, as well as a complexing agent that controls the rate of dissolution of the material from surface  144 . An exemplary material from which region  146  is formed is copper. As a result, it is desired that the RL composition facilitate removal of copper. An exemplary corrosion inhibitor for the slurry composition may be a triazole-based compound, such as 1,2,4-triazole, C 2 H 3 N 3 , and benzotriazole. Other suitable inhibitors may include imidazole, polyvinylimidazole, theophiline, bipyridyl, mercapto benzothizole,phenyl marcapto tetrazole, or pyrazole compounds. An exemplary oxidizing agent may be hydrogen peroxide, H 2 O 2 . An exemplary complexing agent may be dibasic ammonium citrate, (NH 4 ) 2 HC 6 H 5 O 7 , or more generally ammonium salts of citric, oxalic, tartaric, succinic, or actetic acids.  
         [0021]     A first embodiment of the present invention may be as follows:  
                                         COMPOSIITON 1                                    hydrogen peroxide           dibasic ammonium citrate           1,2,4-triazole           water                      
 
 Hydrogen peroxide consists of approximately 0.1% to 3%, and more preferably 1% to 3%, by weight of COMPOSITION 1, and dibasic ammonium citrate consists of approximately 0.1% to 12% by weight of COMPOSITION 1. 1,2,4-Triazole consists of approximately 1% to 6% by weight of COMPOSITION 1, with the remaining portion of the COMPOSITION 1 consisting of a carrier including water. 
 
         [0022]     A second embodiment of the present invention may be as follows:  
                                         COMPOSITION 2                                    hydrogen peroxide           dibasic ammonium citrate           benzotriazole           water                      
 
 Hydrogen peroxide consists of approximately 0.1% to 3%, and more preferably 1% to 3%, by weight of COMPOSITION 2, and dibasic ammonium citrate consists of approximately 0.1% to 12% by weight of COMPOSITION 2. Benzotriazole consists of approximately 0.0001% to 3% by weight of COMPOSITION 2, with the remaining portion of COMPOSITION 2 consisting of a carrier including water. 
 
         [0023]     A neutral pH RL slurry of the present invention offers many advantages over conventional slurries, including improved planarity. Specifically, copper is passivated when exposed to neutral pH compositions. It is believed that the passivation of copper during polishing provides improved planarization. Additionally, the neutral pH slurry of the present invention reduces the corrosion of the copper during polishing, thereby minimizing the formation of micro-trenches and minimizing roughness. As a result, the present neutral pH RL slurry provides wider process windows, lower defects, and ease of integration into present copper low-K dielectric layers.  
         [0024]     The embodiments of the present invention described above are exemplary. Many changes and modifications may be made to the disclosure recited above, while remaining within the scope of the invention. For example, the components of COMPOSITIONS 1 and 2 are selected to facilitate planarization of surfaces having copper-containing materials and dielectric-containing materials. However, other components may be employed dependent upon the materials contained in the layer being polished. Therefore, this invention is not limited to the particular forms illustrated above. Nor is the invention limited or restricted to the particular theories, advantages, or perceived properties disclosed above. Rather, the invention should be defined as set forth in the appended claims and will cover all modifications that do not depart from the scope of this invention.