Abstract:
In the back end of an integrated circuit employing dual-damascene interconnects, the interconnect members have a first non-conformal liner that has a thicker portion at the top of the trench level of the interconnect; and a conformal second liner that combines with the first liner to block diffusion of the metal fill material.

Description:
TECHNICAL FIELD  
       [0001]     The field of the invention is that of forming connections between circuit elements in the back end of line (BEOL) integrated circuits, in particular forming damascene lines that are resistant to reliability failures that develop over time.  
       BACKGROUND OF THE INVENTION  
       [0002]     The damascene process is a process in which metal interconnect lines are delineated and isolated in dielectrics by means of chemical mechanical polishing (CMP). A dual damascene process is a similar process in which metal interconnect lines and vias (i.e., conductor-filled channels) are defined independently in photolithography and etch but metallized simultaneously.  
         [0003]     Time Dependent Dielectric Breakdown (TDDB), in which a weakness in an insulator develops over time into a failure under a bias condition, is a significant source of reliability fails in the interconnection of devices in integrated circuits.  
         [0004]     In the particular case of copper interconnect structures, the majority of TDDB fails occur in the upper portion of the copper line, often caused by a weak liner near the interface between the copper fill and the dielectric cap over the line.  
         [0005]     Often times TDDB fails occur in the metal interconnections. For example, short circuits may be caused by residual metal (e.g., copper (Cu)) or liner materials (e.g., titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), tungsten (W), etc.) that is not removed from the dielectric material by the CMP during the damascene or dual damascene process.  
         [0006]     It is known that TDDB reliability can be improved by increasing the thickness of the liner material that confines the copper within the interconnect structure, but a uniform liner thickness increase on all covered surfaces reduces the amount of copper in the structure for a given interconnect dimension and is not acceptable.  
         [0007]     One known failure mode is the presence of Cu (and/or Ta) particles between adjacent metal lines. It is known that such particles can be left by incomplete chemical-mechanical polishing (CMP) of the damascene interconnect. If the interconnect diffusion barrier liner were thicker in the transverse dimension (perpendicular to the interconnect member axis and parallel to the surface of the interlevel dielectric (ILD) that contains the interconnect) the metal particles would have to travel a larger distance before they could cause a short circuit between adjacent lines and reliability would be improved.  
         [0008]     As noted above, the simple solution of thickening the liner uniformly on all covered surfaces is not acceptable because of the constant need to reduce the dimension of circuit elements.  
         [0009]     The art could benefit from an interconnect structure that maintained an acceptable conductivity by supplying a sufficient cross section of copper material while also maintaining sufficient thickness of liner to reduce this failure mode.  
       SUMMARY OF THE INVENTION  
       [0010]     The invention relates to an interconnect member in the BEOL of an integrated circuit that has a liner with a non-uniform thickness, the liner being sufficiently thick on the sides of the member to contain the interconnect metal and being thicker at the top corners of the member to suppress metal diffusion out of liners along a transverse axis through an interface.  
         [0011]     A feature of the invention is a dual liner comprising a non-conformal first layer and a conformal second layer, the thicknesses of the two layers being selected such that the combined thickness on the aperture sidewalls is sufficient to confine the metal fill material and the combined thickness on the top corners is sufficient to suppress diffusion of metals out of liners. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows a cross section of an aperture in an interlevel dielectric for forming a damascene interconnect member.  
         [0013]      FIG. 2  shows a cross section after the deposition of the non-conformal liner layer.  
         [0014]      FIG. 3  shows the same cross section after on optional step of sputtering the first layer of liner to selectively reduce its thickness.  
         [0015]      FIG. 4  shows the cross section after the deposition of the second conformal liner layer and the deposition of a copper seed layer by atomic layer deposition (ALD).  
         [0016]      FIG. 5  shows the cross section after deposition of the copper fill material and CMP. 
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 1  shows an initial step in forming a damascene interconnect conductive line, referred to generally as an interconnect member, extending perpendicular to the plane of the paper. An aperture  100  is formed in an interlevel dielectric  10 , the aperture having top corners  110  intersecting the top surface  12  of dielectric  10 , a bottom surface  102  and sidewalls  104 . The slope of the sidewall is defined by the etch process during the trench opening in the dielectric layer. A steeper slope will yield thicker liner at the top corners relative to that on the sidewall. A damascene trench with vertical walls would also work. The particular etching process will depend on the material of the dielectric and is conventional, well known to those skilled in the art.  
         [0018]     A non-conformal first liner  22  is deposited, e.g. by physical vapor deposition. The parameters of the deposition are chosen such that material is deposited preferentially on the top corners  110 , which is an aspect of deposition that is ordinarily a drawback, since it tends to cause voids in later deposition steps. Illustratively, the non-conformal liner is TiN, Ti, Ta or TaN, deposited in conventional physical deposition chambers, e.g. as manufactured by Novellus Systems and Applied Materials, according to processes well known to those skilled in the art, but typically excluding process features such as an in-situ sputtering which enhance conformality.  
         [0019]      FIG. 2  shows the result of this process, with the additional material on corners  110  indicated.  
         [0020]      FIG. 3  shows the result of an optional step of sputter cleaning that tapers the corners and allows a thicker film to be deposited in that region without increasing the number of voids in the interior fill in the conductor deposition step. This is done to reduce the thickness of the liner material on the sidewalls and the bottom of aperture  100 . Those skilled in the art are aware that it is desirable to have the thickness of the liner on the sidewalls the minimum amount that will act as a bulk copper diffusion barrier blocking diffusion of the copper metal fill material in order to have the maximum amount of copper, and thus the minimum resistivity, in the interconnect, for a given cross section.  
         [0021]     The dimensions of the liner will need to satisfy two constraints. The thickness on the sidewalls must be sufficient to act as a bulk copper diffusion barrier and the thickness along a transverse axis parallel to the surface  12  of the dielectric and extending left-right in the Figures must also be sufficient to provide an effective surface diffusion barrier for copper atoms, ions and particulates after the CMP step and prior to passivation.  
         [0022]     Referring now to  FIG. 4 , there is shown the result of a second liner deposition, this time with parameters set for a conformal liner  30 . Illustratively, the deposition is performed in conventional chemical or physical vapor deposition chambers, e.g. as manufactured by Novellus Systems and Applied Materials, according to processes well known to those skilled in the art but, in the case of physical vapor deposition, typically employing process features such an in-situ sputtering which enhance conformality. The conformal liner thickness is in the range 5 to 100 nm depending on the dimensions of the interconnect, with smaller interconnects typically having thinner liners.  
         [0023]     The conformal liner may be the same or a different material or sequence of materials from that of the first material. For example the non-conformal material may be physical vapor deposition (PVD) TaN and the conformal liner IPVD (ionized PVD) Ta. TaN is superior to Ta as a Cu diffusion barrier but has higher resistivity. By preferentially depositing TaN in the corners where the TDDB concern is greatest but utilizing Ta as the primary barrier along the remainder of conductor surfaces, an optimized combined liner is obtained.  
         [0024]     Also shown in  FIG. 4  is copper seed layer  42 , denoted by a thicker line on the outer surface of the conformal liner  30 . Preferably, layer  42  is deposited by a conformal deposition process, typically based on PVD processes but also including atomic layer deposition and chemical vapor deposition (CVD) processes, such as to provide coverage on the lower sides of corners  110 .  
         [0025]      FIG. 5  shows the cross section after the completion of the copper fill and CMP to bring the top of the interconnect in line with the top surface  12  of the ILD. The copper deposition step is conventional, well known to those skilled in the art.  
         [0026]     Bracket  24  in  FIG. 5  illustrates the increased thickness of the material on the top corner, which is the parameter that prevents metal particles from diffusing outside the interconnect line, potentially causing short circuits to adjacent lines.  
         [0027]     The structure shown in  FIG. 5  may be described as having a pair of thickened upper corners that together extend over a range of 10%-25% of the total width of the distance between the outer corners of liner  22 . For example, in a BEOL using ground rules for the 90 nm node of the technology roadmap, the combined width included in both brackets  24  is about 10-25% of 120 nm. Those skilled in the art will appreciate that a greater width on the corners will provide more protection against TDDB, subject to an increased danger of forming voids in the metal line. The preceding range is based on current technology and may be changed as the technology changes.  
         [0028]     An alternative embodiment of the invention may be practiced with the non-conformal layer  22  being an insulator, such as oxide or nitride. In that case, the non-conformal layer, e.g. nitride, would be deposited in conventional chemical or physical vapor deposition chambers, e.g. as manufactured by Novellus Systems and Applied Materials, according to processes well known to those skilled in the art.  
         [0029]     In this embodiment, non-conformal liner  22  is an insulator such as oxide or nitride, which also provides the required diffusion barrier to confine the copper fill. The conformal liner  30  would then preferably be selected from the group of conductive refractory materials such as Ti, TiN, Ta or TaN and deposited with appropriate parameters to provide the required conformal property as is known in the art.  
         [0030]     The optional steps of sputter cleaning and depositing a seed layer by ALD may be used or not, as in the previous embodiment.  
         [0031]     Other embodiments of the invention may include a sacrificial hardmask, which is used to protect against damage on the top of the dielectric surface during CMP. It is put down on top of dielectric ( 10 ) before trench opening. The mask is then patterned to define the trench locations, serving as a mask during a subsequent etch of the dielectric to open the trench. Layers of liners are then deposited with the same steps, followed by seed and metal (e.g. Copper) deposition. After CMP to define the metal line, the sacrificial mask is then removed.  
         [0032]     Another alternative would be used if the ILD is porous. In that case, it is necessary to have the liner stay in the trench and avoid having the liner leak into the porous dielectric, so that the material of the non-conformal liner must be one that does not leak into the ILD and the deposition process must be such that the non-conformal profile is achieved.  
         [0033]     For porous ILD, it is important to keep conductive liner material from getting into the pores and potentially bridging adjacent interconnects. With this invention, the first liner is deposited with non-conformal process, and thus the liner will not get into pores but only seals the pore openings. The second liner (conformal process) can then be deposited safely.  
         [0034]     It should be understood that although the invention is primarily intended for copper (Cu) as well as Cu alloys for the metallization, the method can be implemented with other interconnect materials, such as aluminum (Al), gold (Au), silver (Ag) and alloys thereof.  
         [0035]     While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.