Abstract:
A system and method for providing copper interconnect in a trench formed in a dielectric is disclosed. In one aspect, the method and system include providing a copper layer; removing a portion of the copper layer outside of the trench; annealing the copper layer; and providing a layer disposed above the copper layer. In another aspect, the method and system include providing a copper interconnect formed in a trench on a dielectric. The copper interconnect includes a copper layer disposed in the trench and a layer disposed above the copper layer. The copper layer has a bamboo structure at least one grain. The at least one grain has substantially one orientation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. Ser. No. 08/937,915, filed Sep. 25, 1997 now U.S. Pat. No. 6,043,153. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to formation of copper interconnects and more particularly to a method and system for providing a copper interconnect having higher resistance to electromigration. 
     BACKGROUND OF THE INVENTION 
     Copper interconnects can be utilized to carry electricity in microcircuits. Copper is, however, subject to electromigration. Electromigration can degrade the performance of copper interconnects, for example by aiding the growth of voids in the interconnect. As a result, copper interconnects may be more subject to failure. The resistance of copper to electromigration is strongly dependent upon the crystal structure of the copper interconnect. 
     Conventional processing of copper interconnects can include an annealing step used to control the crystal structure of the copper. The structure and grain size of the copper depends upon when annealing takes place. One conventional method for processing copper interconnects anneals the interconnect after the copper film is deposited and before polishing removes excess copper. This annealing step can result in the formation of grain boundaries which have grain boundary triple points through the interconnect. A grain boundary triple point is an intersection of grain boundaries. It is known that grain boundaries are paths of high diffusion for copper atoms. It is also known that the grain boundaries with grain boundary triple points are sites for high electromigration because the grain boundary triple points connect paths of high diffusion. As a result, a copper interconnect formed by this process is subject to relatively high electromigration. 
     A second conventional method for processing copper interconnects anneals the interconnect after the copper film has been polished and passivated. As a result, the copper interconnect may have grain boundaries with very few grain boundary triple points. When copper has this structure, it is said to have a bamboo structure. Consequently, electromigration due to grain boundaries having grain boundary triple points is reduced. However, the orientation of the grains is not controlled. As a result, some grain boundaries have a high defect density and are likely to be high diffusion paths. As a result, the copper interconnect is still subject to electromigration. 
     Accordingly, what is needed is a system and method for providing a copper interconnect having reduced electromigration. The present invention addresses such a need. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and system for providing copper interconnect. In one aspect, the method and system comprise providing a copper interconnect in a trench formed in a dielectric. In this aspect, the method and system comprise providing a copper layer; removing a portion of the copper layer outside of the trench; annealing the copper layer; and providing a layer disposed above the copper layer. In another aspect, the method and system comprise providing a copper interconnect formed in a trench in a dielectric. The copper interconnect comprises a copper layer disposed in the trench and a layer disposed above the copper layer. The copper layer has a bamboo structure and at least one grain. The at least one grain has substantially one orientation. 
     According to the system and method disclosed herein, the copper interconnect has higher resistance to electromigration, thereby increasing overall system performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a cross-section of a copper interconnect formed using a first conventional annealing process after annealing. 
     FIG. 1B is a longitudinal view of the copper interconnect formed using a first conventional annealing process after polishing. 
     FIG. 2A is a cross-section of a copper interconnect formed using a second conventional annealing process. 
     FIG. 2B is a longitudinal view of the copper interconnect formed using a second conventional annealing process. 
     FIG. 3 is a flow chart depicting one embodiment of a method for providing a copper interconnect in accordance with the present invention. 
     FIG. 4A is a cross-section of a copper interconnect formed in accordance with the method and system. 
     FIG. 4B is a longitudinal view of the copper interconnect formed in accordance with the method and system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to an improvement in the formation of copper interconnects. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     FIG. 1A is a cross-section of a conventional copper interconnect  10  after annealing has been performed. Typically, a trench  13  is provided in a dielectric  12 . A barrier metal layer  14  is then deposited. A copper film  16  is then deposited, for example by plating copper to the barrier metal layer  14 . The barrier metal layer  14  reduces the diffusion of copper in the copper film  16  into the dielectric  12 . 
     Copper is subject to electromigration. Electromigration can degrade the performance of copper interconnects, for example by aiding the growth of voids in the interconnect As a result, copper interconnects may be more subject to failure. The resistance of copper to electromigration is strongly dependent upon the crystal structure of the copper interconnect. The crystal structure, including the grain size, depends upon any annealing during processing. Consequently, annealing is performed to control the crystal structure of the copper film  16 . 
     In the interconnect  10  depicted in FIG. 1A, annealing has been performed after the copper film  16  is deposited and before polishing removes excess copper. Because the copper interconnect  10  is annealed prior to polishing, the copper layer  16  is very thick. As a result, grains  23  and  25  are formed during annealing. Refer now to FIG. 1B depicting a longitudinal view of the copper interconnect  10  taken from above. As shown in FIG. 1B, the interconnect  10  has grains  17 ,  19 ,  21 ,  23 ,  25 , and  27  which are separated by grain boundaries  18 ,  20 ,  22 ,  24 , and  26 . The grain boundary  24  intersects grain boundaries  22  and  26 . The grain boundary  18  also intersects the grain boundary  20 . These intersections are known as a grain boundary triple points. When copper has grain boundaries which have almost no grain boundary triple points, the copper is said to have a bamboo structure. Because the copper layer  16  has many grain boundary triple points, the copper layer  16  does not have a bamboo structure. 
     Although the copper interconnect  10  can function, one of ordinary skill in the art will readily realize that the grain boundaries  18 ,  20 ,  22 ,  24 , and  26  are high diffusion paths for copper atoms. Copper atoms preferentially diffuse along the grain boundaries  18 ,  20 ,  22 ,  24 , and  26 . A void is likely to be formed at the grain boundaries  18 ,  20 ,  22 ,  24 , and  26 . The grain boundary triple points connect these grain boundaries. Because of the connection between the grain boundaries, the diffusion can occur not only along an individual grain boundary  18 ,  20 ,  22 ,  24 , or  26 , but can also spread to other grain boundaries  18 ,  20 ,  22 ,  24 , and  26 . As a result, a copper interconnect  10  formed by this process is highly subject to electromigration. 
     FIGS. 2A and 2B depict a copper interconnect  30  made in accordance with a second conventional method. FIG. 2A is a cross-sectional view of the copper interconnect  30 . FIG. 2B is a longitudinal view from above of the copper interconnect  30 . After the trench  33  is formed in the dielectric  32 , a barrier metal layer  34  is deposited. A copper layer  38  is provided. The copper interconnect  30  is then polished to remove the portion of the copper layer  38  outside of the trench  33 . The copper layer  38  is then passivated by providing a dielectric layer  36 . The copper interconnect  38  is then annealed. Passivation is performed prior to annealing to ensure that annealing can easily be accomplished without concern over oxidation or other contamination of the copper layer  38 . The copper layer  38  may have a bamboo structure after annealing. The grain boundaries  42 ,  44  and  46  have almost no grain boundary triple points. The copper interconnect  30  may have reduced electromigration due to diffusion along connected grain boundaries. The electromigration is thereby reduced. 
     Although the electromigration along grain boundaries is reduced, one of ordinary skill in the art will readily realize that the orientation of the copper layer  38  is not controlled. Because neighboring grains may have differing orientations, the defect density on a grain boundary may be high. Grains having the same orientation are less subject to electromigration. For example, a copper crystal having a (111) orientation is more resistant to electromigration because there are fewer defects along the grain boundaries. Because of the presence of the dielectric layer  36 , the copper layer  38  will not preferentially grow with a particular orientation, such as the (111) orientation. The copper interconnect  30  is, therefore, still subject to electromigration. 
     The present invention provides for a method and system for providing a copper interconnect having greater resistance to electromigration. The present invention will be described in terms of a copper interconnect grown using a particular annealing temperature and annealing at a particular time in processing. However, one of ordinary skill in the art will readily recognize that this method and system will operate effectively for other annealing temperatures or annealing at another time in processing of the copper interconnect. In addition, the present invention will be described in terms of grains having a particular orientation. However, one of ordinary skill in the art will readily recognize that this method will operate effectively for other orientations. 
     To more particularly illustrate the method and system in accordance with the present invention, refer now to FIG. 3 depicting a method  50  for forming a copper interconnect in accordance with the present invention. First, a trench is formed in the dielectric via step  52 . In a preferred embodiment, the dielectric is silicon dioxide. A barrier metal layer is then provided via step  54  to prevent the diffusion of copper into the dielectric. A copper layer is then deposited in step  56 . The excess portion of the copper layer outside of the trench is then removed in step  58 . After removal of the excess copper, the interconnect is annealed in step  60 . In a preferred embodiment, annealing is performed at a temperature of about 350-400 degrees Celsius. Also in a preferred embodiment, the annealing is performed in hydrogen gas. In another embodiment, annealing is performed in a vacuum. However, any combination of annealing gases which preclude oxidation of the copper layer are sufficient. The copper is then passivated, if required, in step  62 . Note that only relevant steps in the method  50  have been described. Nothing prevents the method and system from use with a different number or different order of steps, as long as the desired structure is achieved. 
     Refer now to FIGS. 4A and 4B depicting one embodiment of a copper interconnect  100  formed in accordance with the method  100 . FIG. 4A is a cross-sectional view of the copper interconnect  100 . FIG. 4B is a longitudinal view of the copper interconnect  100  from above. The copper interconnect  100  is depicted after annealing step  60 . The copper interconnect  100  includes a trench  103  in the dielectric  102 . A barrier metal layer  104  prevents the diffusion of copper into the dielectric  102 . A copper layer  106  has been annealed as discussed with respect to FIG.  3 . 
     The copper layer  106  has a bamboo structure as shown by the grain boundaries  112 ,  114 , and  116 . The grains  111 ,  113 ,  115 , and  117  of the copper layer  106  have a (111) orientation because annealing was performed after removal of excess copper (not shown) outside of the trench  103 . The top surface of the copper layer  106  is free during annealing. Because a free surface exists during annealing, the surface energy of the copper layer  106  will be minimized during annealing. A (111) orientation minimizes the free surface energy. As a result, the copper layer  106  will have a (111) orientation. Thus, the copper interconnect  100  will have a copper layer  106  that has both a bamboo structure and a (111) orientation. 
     Because there are essentially no grain boundary triple points, diffusion starting on a grain boundary will be restricted to that grain boundary. In addition, the grain boundaries  112 ,  114 , and  116  have a very small defect density. The grain boundaries  112 ,  114 , and  116  have a small defect density because grains along the grain boundaries  112 ,  114 , and  116  share the same orientation. Consequently, the copper layer  106  is less subject to diffusion along a grain boundary. Thus, the copper interconnect  100  is more resistant to electromigration and more reliable. 
     A method and system has been disclosed for providing a copper interconnect having reduced susceptibility to electromigration. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.