Abstract:
A slurry for use in a chemical mechanical polishing process for planarizing copper-based metal structures on a substrate comprises an oxidizer, an organic complexing agent, surfactants, and a plurality of copper-based metal abrasive particles, wherein the copper in the copper-based metal is capable of dissolving into the slurry and forming copper ion complexes. During the chemical mechanical polishing process, the copper removal rate may be selectively increased by increasing the concentration of copper metal abrasive particles in the slurry, and the copper removal rate may be selectively decreased by decreasing the concentration of copper metal abrasive particles in the slurry.

Description:
BACKGROUND  
       [0001]     Chemical Mechanical Planarization (CMP), also known as chemical mechanical polishing, is one of the primary removal methods used in the manufacturing of integrated circuits because CMP is one of the most effective methods for achieving adequate local and global surface planarization. CMP uses a polishing pad and a slurry to planarize the wafer surface at a number of intermediate stages and as a final step after deposition of various features, interconnects, and coatings.  
         [0002]     CMP is used in dual damascene processes for producing final copper interconnects on a wafer. CMP slurries used for copper typically contain abrasive particles such as silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), or cerium oxide (CeO 2 ). CMP slurries for copper also tend to include an oxidizer species such as hydrogen peroxide (H 2 O 2 ), organic complexing agents, surfactants with both hydrophobic and hydrophilic chemical groups, and/or corrosion inhibitors such as benzotriazole.  FIG. 1  illustrates a conventional abrasive particle  100  with surfactants  102 . The abrasive particle  100  may be formed using silicon dioxide, aluminum dioxide, cerium oxide, or other conventional abrasive particle materials.  
         [0003]     A common problem that occurs during copper CMP is dishing and erosion of the copper surface. Dishing and erosion reduces the final thickness of the copper lines and interconnects and often leads to non-planarity of the copper surface, resulting in larger variations when multi-levels of metal or dielectric are added. It has been shown that dishing and erosion during copper CMP is dependent on geometry, slurry chemistry, the planarization process, and the thickness of the originally deposited copper layer.  
         [0004]     One conventional approach to customizing copper removal rates consists of making empirical modifications to the copper CMP process conditions, such as pressure of the polishing pad on the wafer, polishing pad velocity, slurry flow rate, slurry dilution, or other process conditions. Unfortunately, such modifications are time-consuming and limited in effectiveness due to the lack of direct control of the slurry chemical reactivity. Slurry chemical reactivity typically does not remain constant during carious CMP stages, which further complicates empirical modification efforts.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  illustrates a conventional abrasive particle used in a copper CMP slurry with surfactants.  
         [0006]      FIGS. 2A and 2B  illustrate an abrasive particle used in a copper CMP slurry with surfactants according to an implementation of the invention.  
         [0007]      FIGS. 3A and 3B  illustrate an abrasive particle used in a copper CMP slurry with surfactants according to another implementation of the invention.  
         [0008]      FIG. 4  illustrates the slurry chemistry provided using abrasive particles formed in accordance with the invention.  
         [0009]      FIG. 5  is a graph illustrating the improvement in copper removal initiation using abrasive particles formed in accordance with the invention.  
     
    
     DETAILED DESCRIPTION  
       [0010]     Described herein are systems and methods for a chemical mechanical polishing (CMP) slurry using novel abrasive particles that provide improved and controllable removal rates for copper. In the following description, various aspects of the illustrative implementations will be described using terms commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. However, it will be apparent to those skilled in the art that the present invention may be practiced with only some of the described aspects. For purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the illustrative implementations. However, it will be apparent to one skilled in the art that the present invention may be practiced without the specific details. In other instances, well-known features are omitted or simplified in order not to obscure the illustrative implementations.  
         [0011]     Various operations will be described as multiple discrete operations, in turn, in a manner that is most helpful in understanding the present invention, however, the order of description should not be construed to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation.  
         [0012]      FIG. 2A  illustrates a novel abrasive particle  200  formed in accordance with an implementation of the invention. In the implementation shown, the abrasive particle  200  is formed entirely from copper metal or a copper metal alloy. The copper metal abrasive particle  200  replaces conventional abrasive particles used in CMP slurries made from materials such as silicon dioxide, aluminum oxide, or cerium oxide. In some implementations, the diameter of the abrasive particle  200  may be similar to the diameter of those conventional abrasive particles used in CMP slurries. In some implementations, the diameter of the abrasive particle  200  may range from 3 to 500 nanometers (nm). In the implementation shown, the abrasive particle  200  is substantially spherical, while in other implementations the abrasive particle  200  may be formed using other known shapes for particles.  
         [0013]     As the abrasive particle  200  is suspended in the CMP slurry, the copper metal in the abrasive particle  200  may oxidize and dissolve into solution. The copper metal may have an oxidized outer layer of CU 2 O and/or CuO in solution. The size of the abrasive particle  200  is reduced as the copper metal dissolves, as shown in  FIG. 2B . As will be explained below, the copper metal that dissolves into the solution improves the reactivity and provides better control of the CMP slurry.  
         [0014]     A CMP slurry to polish a copper-based film or layer may be formed in accordance with the invention using the abrasive particles  200 . The CMP slurry of the invention may include surfactants  202  to surround the abrasive particles  200  while they are suspended in the CMP slurry, as shown in  FIG. 2A . Each surfactant molecule may include hydrophilic groups  202   a  and hydrophobic groups  202   b . The surfactants  202  may be used to prevent some of the abrasive particles  200  from clustering together and/or from settling out of solution. The CMP slurry of the invention may also include an oxidizer such as hydrogen peroxide (H 2 O 2 ) and a copper complexing agent such as glycine. The abrasive particles may interact with surfactant molecules that contain both hydrophilic ( 202   a ) and hydrophobic ( 202   b ) ends. The hydrophilic groups  202   a  may preferentially interact with the surface of the copper coated abrasive particle, in addition to being solvated in the slurry solution. In some implementations, a corrosion inhibitor such as benzotriazole (BTA), and an organic complexing agent may be introduced into the slurry. The organic complexing agent may be an amino acid and its ions, such as glycine, or an organic acid and its ions, such as citric acid.  
         [0015]      FIG. 3A  illustrates another implementation of the abrasive particle  200  that consists of a conventional abrasive particle covered by a copper metal shell. An interior portion  300  of the abrasive particle  200  may be silicon dioxide, aluminum oxide, cerium oxide, or any other material that is generally used to form abrasive particles in CMP slurries. A copper shell  302  of the abrasive particle  200  consists of copper metal. In some implementations, the diameter of this abrasive particle  200  may also range from 3 to 500 nm. As with the copper abrasive particle  200  of  FIG. 2B , the size of the abrasive particle  200  of  FIG. 3A  is reduced as the copper metal dissolves, as shown in  FIG. 3B .  
         [0016]     In some implementations, the copper shell  302  may be formed over the interior portion  300  using a deposition process such as chemical vapor deposition, atomic layer deposition, or a sputtering process. In some implementations, depending on the material chosen for the interior portion  300 , an electroless plating process may be used to form the copper shell  302  over the interior portion  300 .  
         [0017]     A CMP slurry made in accordance with the invention introduces copper ions that dissolve into the CMP slurry to form copper ion complexes. Detailed quantum chemistry calculations have shown that the presence of copper ion complexes lowers the activation energy barrier necessary for the formation of reactive radicals such as hydroxyl (OH) and hydroperoxyl (OOH) radicals, and thereby increases the probability and rates of formation of these radicals. An increase in reactive radical concentration would generally lead to a corresponding increase in the reactivity of the CMP slurry and hence an increase in the copper removal rate.  
         [0018]      FIG. 4  illustrates quantum chemistry simulation results showing how radicals are formed in both conventional CMP slurries and CMP slurries made in accordance with the invention. In conventional CMP slurries, the formation of reactive radicals requires high activation energy barriers. For instance, the formation of hydroxyl radicals from H 2 O 2  (see conventional reaction  400 ) has an activation barrier of around 46 kcal/mol, while the formation of hydroperoxyl radicals from H 2 O 2  (see conventional reaction  402 ) has an activation barrier of around 83 kcal/mol. These high activation barriers tend to prevent the formation of reactive radicals under typical copper CMP conditions.  
         [0019]     In accordance with the invention, reaction  404  shows the end reaction that forms the hydroperoxyl radical using copper ion complexes. The reaction  404  has a low activation energy barrier of around 11 kcal/mol, which is much lower than the activation energy barriers for direct scission of HO—OH (46 kcal/mol) and H—OOH (83 kcal/mol), thus indicating the effects of complexed copper ions in the formation of reactive radicals such as hydroperoxyl. Table 1 shows exemplary reaction mechanisms that may occur in a CMP slurry made in accordance with the invention.  
                   TABLE 1                           Cu(H 2 O) 4   2+  + glycine →   Cu-glycine-(H 2 O) 2   2+  + 2H 2 O       H 2 O 2  + H 2 O              OOH −  + H 3 O +         Cu-glycine-(H 2 O) 2   2+  + OOH −  →   Cu-glycine-H 2 O—OOH +  + H 2 O       Cu-glycine-H 2 O—OOH +  →   Cu-glycine-H 2 O + +  OOH •                    
 
         [0020]     In implementations of the invention, a CMP process to polish copper on a substrate can be modified through the selective addition and removal of the copper abrasive particles  200  in the slurry. The addition of the copper abrasive particles  200  into the slurry will enhance the copper removal rate of the CMP process. The removal of the copper abrasive particles  200  from the slurry will reduce the copper removal rate of the CMP process. Accordingly, the addition and/or removal of the copper abrasive particles  200  of the invention during various CMP stages enables the copper removal rate to be increased or decreased depending on what is required. This provides improved control of slurry reactivity and copper removal rate, and provides an effective chemical control strategy to optimize CMP performance and minimize copper loss during clearing. The amount of copper abrasive particles  200  to be added to the slurry may be pre-determined for each particular wafer to be polished, or it may be determined during the CMP process itself and adjusted using a suitable process control strategy.  
         [0021]     For instance, the addition of the copper abrasive particles  200  into the slurry at the beginning of the CMP process will increase the copper removal rate, thereby overcoming the typical low removal rate initiation period that occurs in conventional CMP processes for copper. Furthermore, the removal of the copper abrasive particles  200  from the slurry (e.g., by diluting the slurry with a more conventional, copper-free slurry) may be used in stages where a decreased copper removal rate is required, such as during the copper clear or end-pointing stage.  
         [0022]     In implementations of the invention, the abrasive particles used in a copper CMP slurry may be only the copper abrasive particles  200 . In some implementations, the abrasive particles used in a copper CMP slurry may consist of both the copper abrasive particles  200  as well as conventional abrasive particles formed from materials such as silicon dioxide, aluminum oxide, or cerium oxide. The amounts used for each of these abrasive particles may be modified depending on the wafer characteristics and the CMP process needs.  
         [0023]     In one implementation of the invention, the abrasive particles  200  shown in  FIGS. 3A and 3B  may be used in a CMP process to provide a copper removal rate that begins at a high level and then steadily decreases over the span of the CMP process. The thickness of the copper shell  302  may be fixed such that the copper shell  302  completely dissolves by the time the CMP process reaches a stage where the lowest copper removal rate is required. The CMP process will therefore have a high copper removal rate at the beginning of the process when the copper begins dissolving off the abrasive particle  200 , and the copper removal rate will steadily decrease as the amount of copper dissolving off the abrasive particle  200  decreases until all that is left is the interior portion  300 . The interior portion  300  will then provide the same abrasive properties as conventional abrasive particles.  
         [0024]      FIG. 5  is a graph showing experimental results of adding a copper salt into a representative CMP slurry formulation for copper. As shown by the graph, an increase in the copper removal rate occurs when the copper salt is included in the copper slurry. For each of the polishing times tested, the addition of the abrasive particles  200  resulted in improved copper polishing rates.  
         [0025]     The addition of copper therefore provides an improved and controlled method to modify the copper removal rate of a CMP process using similar process conditions and equipment configurations. The copper abrasive particles  200  provide improved and consistent copper removal that is generally not attainable by simply altering process conditions alone.  
         [0026]     The above description of illustrated implementations of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific implementations of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.  
         [0027]     These modifications may be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific implementations disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.