Abstract:
A structure and method for determining barrier layer integrity for multi-level copper metallization structures in integrated circuit manufacturing. Novel testing structures prevent any conducting residues of the copper CMP from diffusing into the dielectric layer. Barrier layer integrity is tested by performing CV or IV measurements between the copper lines and the silicon wafer.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention deals with integrated circuit manufacturing processes and structures, and in particular to a process and structure for testing the integrity of a barrier layer for Cu metallization structures.  
         BACKGROUND OF THE INVENTION  
         [0002]    As integrated circuit devices shrink, with semiconductor device geometries approaching 0.18 micron minimum feature size, and as circuit speed and performance increase, copper has replaced aluminum as the preferred electrical interconnect material. The use of copper as an interconnect material in silicon integrated circuits has occurred in response to the need for lowered interconnect resistivity, good electromigration resistance, and good deposition characteristics which allow effective filling of vias and contacts.  
           [0003]    Copper metallization structures are often formed by a process known as Damascene, which is illustrated in FIG. 1 a . An insulating layer known as the Interlevel Dielectric (ILD) separates metal layers in a multilevel metallization structure. ILD dielectric layer  2 , which may be comprised of a bottom layer  4  and a top, low dielectric constant layer  6 , has Damascene line regions  8  etched therein into which the metal lines will be inlaid. A barrier layer  10  is deposited, which serves to prevent diffusion of copper from the metal lines into the dielectric. This barrier layer is generally comprised of Ta or Ta compounds. A copper seed layer is then generally deposited, followed by an electroplated copper layer. The excess copper is then removed by a process known as Chemical Mechanical Polishing (CMP), leaving embedded copper lines  18 ,  20 .  
           [0004]    The integrity of the barrier layer  10  is critical to preventing diffusion of Cu into nearby dielectric or silicon regions. Diffused Cu in Si can cause degradation of device characteristics, such as leakage currents in reverse biased junctions. Cu defects in dielectrics can cause threshold voltage shifts and parasitic leakage currents. It is therefore essential to utilize methods for testing and/or monitoring barrier layer integrity. Barrier layer integrity tests can be utilized during development of barrier layer deposition processes, and they may be incorporated into manufacturing processes for monitoring during production.  
           [0005]    In the prior art, barrier layer integrity has been evaluated by monitoring the line-to-line leakage current. In this method, unconnected, spaced apart copper lines are electrically stressed at higher electric field or temperature than would occur during normal circuit operation, and the leakage current between the lines is measured as an indication of copper diffusion through the barrier layer.  
           [0006]    This prior art method for testing barrier layer integrity has inherent inaccuracies. Under temperature or voltage stress, Cu ions will diffuse across the lowest resistance path. The lowest resistance path may be a path which passes through the barrier layer, but it may instead be a path across the top dielectric surface, and accordingly falsely indicate lack of barrier layer integrity . In this prior art, a nitride capping layer  16  is generally deposited atop the Cu and dielectric surface to prevent the copper surface from oxidizing or corroding, and to isolate the copper line from the dielectric. One potential Cu diffusion path under stress is the nitride/dielectric interface. This effect is greatly magnified if conducting residues remain on the dielectric surface after CMP, due to incomplete polish or ineffective or insufficient post-CMP clean. Other sources of residues include improper processing of the nitride cap layer.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of this invention to provide an improved structure and method for testing barrier integrity in an integrated circuit multilayer metallization structure.  
           [0008]    It is a further object of this invention to provide a structure and method for testing barrier integrity which does not utilize measurement of line-to-line leakage current.  
           [0009]    It is a further object ofthis invention to provide a structure and method for testing barrier integrity which is not subject to spurious leakage currents through paths other than through the barrier layer.  
           [0010]    These objects are met by providing novel testing structures which provide a layer atop the dielectric layer and abutting the barrier layer, which has low permeability to copper. This low permeability layer thereby inhibits any copper from the metal lines or from conducting residues of the copper CMP from diffusing into the dielectric layer. The barrier layer integrity is then tested by performing CV or IV measurements across the ILD between the copper lines and the back of the silicon wafer.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 a  shows a side cross sectional view of a typical Damascene structure.  
         [0012]    [0012]FIG. 1 b  shows a top view of metallization testing structures.  
         [0013]    [0013]FIG. 2 shows a first embodiment of an inventive barrier layer integrity test structure.  
         [0014]    [0014]FIG. 3 shows a second embodiment of an inventive barrier layer integrity test structure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIG. 1 a  illustrates the typical Damascene structure which has been used in prior barrier layer integrity tests. Leakage current is measured between first metal line  18  and second metal line  20 . Leakage can occur through barrier layer  10  and dielectric  2 , via leakage path  22 . Alternately, leakage can occur along nitride/dielectric interface  24 , via leakage path  26 . Path  26  would have lower resistance, and thereby would be a more likely leakage path if conducting residue from Cu CMP were present at interface  24 . Line-to-line leakage currents may be detected in this case, even though the barrier layer is intact. FIG. 1 b  shows a top view of a typical leakage test structure, showing lines  18  and  20  in an interleaving comb pattern to increase the available area for leakage to occur.  
         [0016]    According to our invention, barrier layer integrity can be tested by measuring one of: leakage current, Capacitance-Voltage (C-V) characteristics, or Current-Voltage (I-V) characteristics vertically across the layers, between the copper line and the back of the silicon wafer, rather than measuring line-to-line leakage currents horizontally as in the prior art. Our invention provides test structures which prevent any conducting residues of the copper CMP from diffusing into the dielectric layer and which also inhibit diffusion of copper from the copper Damascene lines into the dielectric by any paths other than through the barrier layer. This is accomplished by providing a “low permeability” layer which has low permeability to copper diffusion atop the dielectric layer and abutting the barrier layer in the Damascene line region. Electrical characteristics are then measured vertically through the wafer rather than across its surface, which provides a better measure of copper ion diffusion through the barrier material into the dielectric.  
         [0017]    [0017]FIG. 2 illustrates a first embodiment of our inventive method and our inventive structure for barrier layer integrity testing. The embodiment as shown is utilized with a single Damascene structure, but can be extended to a dual Damascene structure. The Damascene structure is formed according to known methods, as described hereinafter. Thin layer  28  of dielectric such as PETEOS, 2000 Angstroms thick by way of example with an expected acceptable range of 1000-5000 Angstroms, is deposited on silicon wafer  30 . Thin nitride layer  32  with a thickness of 250-1000 Angstroms is deposited as an etch stop. Dielectric layer  34 , 4000-5000 Angstrom thick PETEOS by way of example with an expected acceptable range of 1000-15000 Angstroms, is deposited atop nitride layer  32 . Dielectric  34  is patterned and etched to provide Damascene lines  36  into which copper will be deposited, with the dielectric etch stopping at nitride etch stop layer  32 . Nitride layer  32  is then removed from the Damascene line regions  36  with a second etching step. Barrier layer  38 , generally with a thickness between 25-400 Angstroms, is next deposited. Barrier layer  38  is generally comprised of Ta, but may also be comprised of Ti, TiN, TaN, WN, WSiN, TaSiN, or TiSiN, deposited either by CVD or PVD. A copper seed layer is then deposited, followed by an electroplated copper layer which fills Damascene line regions  36  and which also is deposited atop barrier layer  38  atop dielectric  34 . Post plating anneal to approximately 100 C to 450 C is performed. Copper CMP is then performed to removed excess copper from atop dielectric  34 , leaving copper lines  43 .  
         [0018]    In conventional Damascene processing, the copper CMP polishes off both copper layer  42  and barrier layer  38  from the dielectric surface. In contrast, in the first embodiment of our inventive process, CMP is stopped on barrier layer  38 , leaving the barrier intact atop the dielectric surface  45  in the field regions. Post-CMP capping layer  44 , nitride by way of example, is deposited to prevent copper surface  46  from oxidizing. Metal  50  is deposited onto backside  48  of wafer  30  to enhance the substrate contact for C-V or leakage measurements. In order to test for diffusion of copper into the dielectric, copper lines  43  are subjected to bias temperature stressing while monitoring the leakage current or intermediate C-V or I-V characteristics between copper lines  43  and silicon wafer  30 . The copper lines may be contacted in standard ways, such as by providing a large contact pad on the metal level having the aforementioned copper lines therein, the copper pad being electrically connected to the copper lines in question, then masking and opening a window in nitride capping layer  44  to expose the metal contact pad. Electrical testing is performed using such standard measurement hardware as the S900 Tester made by Keithley, and the HP 4071 Tester made by Hewlett-Packard.  
         [0019]    According to this embodiment, the presence of the remaining intact barrier layer  38  atop dielectric surface  45  provides the aforementioned low permeability layer, and inhibits diffusion of copper into dielectric  34  except through barrier layer  38 . There is no alternate low resistance diffusion path for copper which would affect the electrical measurements, in contrast to the prior art method of measuring line-to-line leakage. Therefore, the vertical electrical characteristics measured through the wafer rather than across its surface provide a better measure of copper ion diffusion through the barrier material into the dielectric.  
         [0020]    Due to the presence of the conducting barrier layer material across the field regions, this embodiment is designed to be used for test or development wafers only, since line-to-line shorting would occur if this structure were formed on product wafers. As test structures, the copper lines  43  may be formed into long serpentine structures to increase the capacitance for ease of measurement.  
         [0021]    [0021]FIG. 3 illustrates a second embodiment of our inventive method and structure for Ta barrier layer integrity testing. The embodiment as shown is utilized with a single Damascene structure, but can be extended to a dual Damascene structure.  
         [0022]    The Damascene structure is formed similarly to the method of the first embodiment. Thin layer  28  of dielectric such as PETEOS, 2000 Angstroms thick by way of example with an expected acceptable range of 1000-5000 Angstroms, is deposited on silicon wafer  30 . Thin nitride layer  32  with a thickness of 250-1000 Angstroms is deposited as an etch stop. Dielectric layer  34 , 4000-5000 Angstrom thick PETEOS by way of example with an expected acceptable range of 1000-15000 Angstroms, is deposited atop nitride layer  32 . A novel feature of this embodiment is that, subsequent to deposition of the thick dielectric layer  34 , and prior to patterning of the dielectric, a thin nitride cap layer  52  of thickness 500-1000 Angstroms is deposited. Dielectric  34  is then patterned and etched to provide Damascene lines  36  into which copper will be deposited. Barrier layer  38  is next deposited. Barrier layer  38 , generally with a thickness between 25-400 Angstroms, is generally comprised of Ta, but may also be comprised of Ti, TiN, TaN, WN, WSiN, TaSiN, TiSiN, or a combination of any of the aforementioned, deposited either by CVD or PVD. A copper seed layer is then deposited, followed by an electroplated copper layer which fills Damascene line regions  36  and which also is deposited atop barrier layer  38  atop dielectric  34 . Post plating anneal to approximately 100 C to 450 C is performed. Copper CMP is then performed to removed excess copper and the barrier layer from atop dielectric  34 , leaving copper lines  43 . In this embodiment, in contrast to the first embodiment, the barrier layer  38  is removed by the CMP, and the CMP stops on nitride cap layer  52 . A second, post-CMP cap layer  44 , 500-1000 Angstroms of silicon nitride by way of example, is deposited to prevent copper surface  46  from oxidizing. Metal  50  is deposited onto backside  48  of wafer  30  to enhance the substrate contact for C-V or leakage measurements. Copper lines  43  are subjected to bias temperature stressing while monitoring the leakage current or intermediate C-V or I-V characteristics between copper lines  43  and silicon wafer  30 .  
         [0023]    According to this embodiment, the deposition of nitride capping layer  52  atop dielectric surface  45  prior to copper deposition and CMP provides the aforementioned low permeability layer, and inhibits diffusion of copper from copper lines  43  into dielectric  34 , due to the very low diffusion rate of copper through silicon nitride. Similarly, the copper CMP residues and other impurities from copper deposition and anneal on the surface of nitride capping layer  52  are inhibited from diffusing into dielectric  34 . There is no likely alternate lower resistance diffusion path for copper into the dielectric which would affect the vertical electrical barrier integrity measurements, in contrast to the prior art method of measuring line-to-line leakage. Therefore, the vertical electrical characteristics measured through the wafer rather than across its surface provide a better measure of copper ion diffusion through the barrier material into the dielectric.  
         [0024]    In contrast to the structure of the first embodiment, in this embodiment there is no electrically conducting layer overlying the dielectric between metal lines. As a result, this embodiment may be utilized on product wafers as a structure that enables monitoring of barrier layer integrity. If it were desired to avoid having the nitride capping layer present on product circuit regions, a masking step could be employed to cover the test structures, and then the nitride capping layer could be etched off of the product regions. As test structures, the copper lines  43  may be formed into long serpentine structures to increase the capacitance for ease of measurement.  
         [0025]    By utilizing our inventive structure and method, a much more reliable and unambiguous indication of barrier layer integrity can be obtained. This can assist both in development of barrier layer materials and deposition methods, but can also be used in production to monitor barrier layer integrity. The use of our inventive structure can also indirectly detect the presence of conduction CMP residues, by comparison of vertical electrical characteristics and line-to-line leakage.  
         [0026]    It is not intended that our invention be restricted to the exact embodiments described herein. For example, different materials may be used for the dielectric or the barrier layer without altering the inventive concept. The scope of the invention should be construed in view of the claims.