Abstract:
Disclosed is a system for fabricating a semiconductor device ( 100 ). An interconnect structure ( 110 ) is formed on the semiconductor device ( 100 ) and a cap ( 112 ) is deposited over the interconnect structure ( 110 ). The interconnect structure ( 110 ) is annealed with the overlying cap ( 112 ) in place. The cap ( 112 ) is then removed after the interconnect structure ( 110 ) is annealed.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention is related generally to the field of fabricating interconnect structures for integrated circuits and, more specifically, to improving the thermal stability of copper damascene interconnect structures.  
       BACKGROUND OF THE INVENTION  
       [0002]     This application claims priority from Provisional Application Ser. No. 60/344,465, filed on Dec. 28, 2001.  
         [0003]     Since the invention of integrated circuits, the number of devices on a chip has grown at a near-exponential rate. The fabrication methods of the semiconductor industry have been modified and improved continuously for almost four decades. With each improved method, the capacity of a single semiconductor chip has increased from several thousand devices to hundreds of million devices. Future improvements will require integrated circuit devices such as transistors, capacitors, and connections between devices to become even smaller and more densely populated on the chip.  
         [0004]     The increased packing density of the integrated circuit generates numerous challenges to the semiconductor manufacturing process. Every device must be smaller without damaging the operating characteristics of the integrated circuit devices. High packing density, low heat generation, and low power consumption, with good reliability and long operation life must be maintained without any functional device degradation. Increased packing density of integrated circuits is usually accompanied by smaller feature size.  
         [0005]     As integrated circuits become denser, the widths of interconnect layers that connect transistors and other semiconductor devices of the integrated circuit are reduced. As the widths of interconnect layers and semiconductor devices decrease, their resistance increases. As a result, semiconductor manufacturers seek to create smaller and faster devices by using, for example, a copper interconnect instead of a traditional aluminum interconnect. Unfortunately, copper is very difficult to etch in a semiconductor process flow. Therefore, damascene processes have been proposed to form copper interconnects.  
         [0006]     The damascene method involves forming a trench and/or an opening in a dielectric layer that lies beneath and on either side of the copper-containing structures. Once the trenches and/or openings are formed, a blanket layer of the copper-containing material is formed over the entire device. Electrochemical deposition (ECD) is typically the only practical method to form a blanket layer of copper. The thickness of such a layer must be at least as thick as the deepest trench and/or opening. After the trenches and/or the openings are filled with the copper-containing material, the copper-containing material over the trenches/openings is removed, e.g., by chemical-mechanical polishing (CMP), so as to leave the copper-containing material in the trenches and openings but not over the dielectric or over the uppermost portion of the trench/opening.  
         [0007]     Copper deposited by ECD, however, has fine grains and will re-crystallize during subsequent processing steps. During anneal steps, deposited copper interconnects frequently form voids at via bottoms and other interfaces, which may ultimately cause device failure. One solution is to anneal the copper at low (below 200° C.) temperatures. A low temperature anneal, however, will not completely stabilize the deposited copper and also result in device failure. Finally, the copper may be annealed at high temperatures. Stresses within the copper interconnect structure may cause the interconnect to fail during the high temperature annealing process, which also causes via open failures.  
         [0008]     Copper via and interconnect fabrication processes are growing in use. One example of a conventional interconnect fabrication process is depicted in  FIGS. 1A-1E . In particular,  FIG. 1A  depicts a typical damascene interconnect process in which an interlevel dielectric (ILD)  12  is formed over a semiconductor body  10 . The interlevel dielectric  12  is then patterned and etched to remove the dielectric material from the areas  14  where the interconnect lines are desired, as depicted in  FIG. 1B . Referring now to  FIG. 1C , a barrier layer  16  is then deposited over the structure including over the dielectric  12  and in the areas  14  where the dielectric has been removed. A copper seed layer  18  is then formed over the barrier layer  16 . The copper layer  20  is then formed from the seed layer  18  using, for example, electrochemical deposition (ECD), which is also known as an electroplating process, as depicted in  FIG. 1D . Chemical-mechanical polishing (CMP) is then used to remove the excess copper and planarize the copper  20  with the top of the interlevel dielectric layer  12 , as depicted in  FIG. 1E .  
       SUMMARY OF THE INVENTION  
       [0009]     As should now be apparent, a method of forming copper interconnect structures that does not add excessive costs or procedures to the fabrication process is now needed, providing for fabrication of more reliable semiconductor devices while overcoming the aforementioned limitations of conventional methods.  
         [0010]     The present invention provides a system for fabricating a semiconductor device. An interconnect structure is formed on the semiconductor device and a cap is deposited over the interconnect structure. The interconnect structure is annealed with the overlying cap in place. The cap is then removed after the interconnect structure is annealed.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     For a more complete understanding of the present invention, including its features and advantages, reference is made to the following detailed description, taken in conjunction with the accompanying drawings. Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated.  
         [0012]      FIGS. 1A-1E  depict a damascene interconnect fabrication process in accordance with the prior art;  
         [0013]      FIGS. 2A-2D  depict an embodiment of an interconnect fabrication process in accordance with the present invention;  
         [0014]      FIG. 3  depicts a chart of stress reduction results in accordance with the present invention; and  
         [0015]      FIGS. 4A-4D  depict another embodiment of an interconnect fabrication process in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     It should be understood that the principles and applications disclosed herein may be applied to a wide range of semiconductor device fabrication processes. For purposes of explanation and illustration, the present invention is hereafter described in reference to several specific embodiments of methods of semiconductor device fabrication. The present invention, however, is equally applicable in any number of fabrication processes that might benefit from the present invention.  
         [0017]     Turning now to the present invention as depicted in  FIGS. 2A-2D , a copper interconnect structure may be formed, for example, generally according to the procedures depicted in and described with reference to  FIGS. 1A-1D  above. As depicted in  FIG. 2A , the interlevel dielectric  102  is formed over the semiconductor body  100 . The interlevel dielectric  102  is then patterned and etched to remove the dielectric material from the areas  118  (not shown) where interconnect lines are desired. The barrier layer  104  is then deposited over the structure including over the dielectric  102  and in the areas  118  (not shown) where the dielectric has been removed. The copper layer  106  is then formed from the seed layer  108  (not shown) using, for example, an ECD or electroplating process.  
         [0018]     Turning now to  FIG. 2B , chemical-mechanical polishing may be used to remove most of the excess copper from the copper layer  106 . Chemical-mechanical polishing stops at the barrier layer  104  and may leave a thin layer of copper in seed layer  108  over the dielectric  102  and copper interconnect  110 .  
         [0019]      FIG. 2C  depicts a low temperature deposition process that forms a cap  112  over the barrier layer  104  and any remaining seed layer  108 . The cap  112  maintains compressive forces on the copper interconnect  110  during subsequent annealing or elevated temperature processes. The process that forms the cap  112  may occur at temperatures that are less than 200° C. The cap  112  may be formed from silicon nitride, silicon oxide, silicon dioxide or organic silicon glass (OSG), for example, by conventional chemical vapor deposition (CVD) or spin-on tools, and may be easily implemented in manufacturing processes. Thickness of the cap  112  may be in the range of about 10 nm to about 200 nm, although the stress reduction in the copper interconnect  110  is relatively independent of the thickness of the cap  112 , as will be described with reference to  FIG. 3 .  
         [0020]     In  FIG. 2D , an annealing process is performed to stabilize the copper interconnect  110 . The annealing process may be performed at or near the interlevel dielectric deposition temperature. Compressive forces from the initial chemical-mechanical polishing and the cap  112  suppress the effects of residual tensile stress, which result from the annealing process, in the copper interconnect  110 . As a result, the copper interconnect  110  is less likely to detach from the dielectric  102 , thereby creating open failures, during the annealing process and subsequent high temperature processes. Production yield consequently increases because open failures are reduced. After the annealing process, the cap  112  and the barrier layer  108  may be removed in a single chemical-mechanical polishing process. Deposition of the cap  112 , therefore, adds minimal fabrication steps that may be easily implemented into many semiconductor manufacturing processes.  
         [0021]     As depicted in  FIG. 3 , the internal stress reduction benefit of cap  112  is relatively independent of the thickness of cap  112 . For example, after annealing, the hydrostatic stress in the copper interconnect  110  remains between about 300 Mpa and 325 Mpa if the thickness of cap  112  is between about 50 nm and 200 nm. For comparison, stress in an uncapped copper layer  106  after annealing is approximately 425 Mpa. Therefore, less material may be used to form the cap  112  while still gaining the stress reducing benefits of the cap  112 . Process costs and time are consequently saved.  
         [0022]     Another embodiment of the present invention is depicted in  FIGS. 4A-4D . A copper interconnect structure may be formed, for example, generally according to the procedures depicted in and described with reference to  FIGS. 1A-1D  above. As depicted in  FIG. 4A , the interlevel dielectric  102  is formed over the semiconductor body  100 . The interlevel dielectric  102  is then patterned and etched to remove the dielectric material from the areas  118  (not shown) where interconnect lines are desired. The barrier layer  104  is then deposited over the structure including over the dielectric  102  and in the areas  118  (not shown) where the dielectric has been removed. The copper layer  106  is then formed from the seed layer  108  (not shown) using, for example, an electrochemical deposition/electroplating process.  
         [0023]     As depicted in  FIG. 4B , chemical-mechanical polishing may be used to remove the excess copper from the copper layer  106  and also remove the barrier layer  104 .  
         [0024]      FIG. 4C  depicts a low temperature deposition process that forms a cap  114  over the copper interconnect  110  and the dielectric  102 . The cap  112  maintains compressive forces on the copper interconnect  110  during subsequent annealing or elevated temperature processes. In this particular embodiment, the cap  114  may also serve as an etch stop. The process that forms the cap  114  may occur at temperatures that are less than 200° C. The cap  114  may be formed from silicon nitride, silicon oxide, silicon dioxide or OSG, for example, by conventional chemical vapor deposition (CVD) or spin-on tools, and may be easily implemented in manufacturing processes. Thickness of the cap  114  may be in the range of about 10 nm to about 200 nm, although the stress reduction in the copper interconnect  110  is relatively independent of the thickness of the cap  114 , as described with reference to  FIG. 3 .  
         [0025]     In  FIG. 4D , an annealing process is performed to stabilize the copper interconnect  110 . The annealing process may be performed at or near the interlevel dielectric deposition temperature. Compressive forces from the initial chemical-mechanical polishing and the cap  114  suppress the effects of residual tensile stress, which result from the annealing process, in the copper interconnect  110 . As a result, the copper interconnect  110  is less likely to detach from the dielectric  102 , thereby creating open failures, during the annealing process and subsequent high temperature processes. Production yield consequently increases because open failures are reduced. After the annealing process, the cap  114  is used as an etch stop layer and an interlevel dielectric  116  may be deposited on top of the cap  114 . In this particular embodiment, deposition of the cap  114  saves the time and cost of an additional chemical-mechanical polishing by acting as an etch stop.  
         [0026]     Although this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Upon reference to the description, it will be apparent to persons skilled in the art that various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention can be made without departing from the spirit and scope of the invention. It is therefore intended that the appended claims encompass any such modifications or embodiments.