Abstract:
A system and method for forming a novel C4 solder bump for BLM (Ball Limiting Metallurgy) includes a novel damascene technique is implemented to eliminate the Cu undercut problem and improve the C4 pitch. In the process, a barrier layer metal stack is deposited above a metal pad layer. A top layer of the barrier layer metals (e.g., Cu) is patterned by CMP. Only bottom layers of the barrier metal stack are patterned by a wet etching. The wet etch time for the Cu-based metals is greatly reduced resulting in a reduced undercut. This allows the pitch of the C4 solder bumps to be reduced. An alternate method includes use of multiple vias at the solder bump terminal.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 10/710,562, filed Jul. 21, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the fabrication of semiconductor chips, and more particularly to a structure and novel methodology for forming solder bumps in Back-End-Of-Line (BEOL) semiconductor chip processing. 
     2. Description of the Prior Art 
     Controlled Collapse Chip Connection (C4) processes are well known in forming solder bumps in back-end-of line semiconductor fabrication, e.g., when chips are connected to their packaging. Typically, the formation of a C4 solder bump includes the conventional formation of a metallurgical system that includes the underlying final metal layer (pad), an Under Bump Metallurgy (UBM) and the solder ball. The UBM ideally should provide good adhesion to a wafer passivation and to the IC final metal pad, and, function as an effective solder diffusion barrier. 
     Current fabrication techniques implement Pb-free C4&#39;s using plating of the solder in a photoresist pattern, followed by wet etching of the UBM. In the prior art solder bump plating process, the UBM includes the deposition of an adhesion layer, e.g., a titanium-tungsten alloy (TiW), followed by wetting layers of Cr—Cu (chromium-copper alloy) and copper (Cu). The wetting layers ensure the solder completely covers the patterned Ti—W adhesion layer (thereby ensuring a large contact area between the solder ball and the chip, and providing high mechanical strength. In the solder bump plating process, the wafer is cleaned to remove oxides or organic residue prior to metal deposition and to roughen the wafer passivation and bond pad surface to promote better adhesion of the UBM. The UBM barrier layer metals such as TiW, Cr—Cu, and Cu may then be sequentially sputtered or evaporated over the entire wafer so that the UBM adheres to the wafer and passivation in addition to the bond pads. Next, a photoresist layer is applied and then metal layers (e.g., a Ni barrier layer followed by a Sn-based solder) are plated over the bond pad to a height as determined by the patterned photoresist. After the solder bump is formed, the photoresist is stripped, leaving the UBM exposed on the wafer. The UBM is subsequently removed from the wafer using a wet etch process (e.g., an H 2 O 2 -based wet etch). 
     As integrated circuits shrink in size, the pitch of the C4 solder bumps must also shrink. In the conventional C4 processing described, the barrier layer metals (typically TiW/CrCu/Cu) are wet etched after plating of the solder. Unfortunately, as C4 pitch decreases, the wet etch causes increasingly more undercut of the C4 solder bump (i.e., a greater percentage of area underneath the C4 is undercut), which degrades the mechanical reliability of the C4. 
     It would be highly desirable to provide a C4 fabrication technique that results in an improved C4 pitch and increases the mechanical stability of the formed solder bump. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to provide an improved method of forming a solder bump connection at a metallic bonding pad surface of a semiconductor chip. In accordance with this object, a novel damascene technique for patterning of the UBM layers is implemented to eliminate a copper (Cu) undercut problem and improve the C4 pitch. 
     According to a first embodiment, the method includes steps of forming a patterned passivation layer upon a metal bonding pad surface, the patterned passivation layer including an opening at the metallic bonding pad surface to define a location for the solder bump connection. Then a barrier material stack including top and bottom conductive material layers is formed upon the patterned passivation layer and conforming to a surface of the patterned passivation layer. A next step includes removing the top conductive material layer portion of the barrier material stack adjacent the solder bump connection location so that a top conductive material layer portion at the solder bump connection location provides a surface that is substantially coplanar with a surface of a remaining bottom conductive material layer adjacent the solder bump connection location. Then a patterned resist material layer is formed upon the substantially coplanar surface that includes an opening at the defined solder bump connection location. A diffusion barrier layer is then formed upon the substantially coplanar surface defined by said patterned resist material layer opening and a solder material is provided upon a surface of the diffusion barrier layer between patterned walls defining the patterned resist material layer opening. Finally, the patterned resist material layer is removed and the remaining bottom conductive material layer portions of the barrier material stack under the patterned resist layer are removed. The solder material may then be reflowed to form the solder bump connection. 
     Advantageously, the bottom conductive material layer adjacent the solder bump connection exhibits decreased amount of undercut under the diffusion barrier layer to enable reduced pitch and increased mechanical stability of the formed solder bump connection. This is because the step of removing the remaining bottom conductive material layer portions of the barrier material stack includes implementing a wet etch, wherein a total etch time for removing the remaining bottom conductive material layer portions is reduced due to prior removal of the top conductive material layer portion adjacent the solder bump connection location. Thus, for example, a top layer of the barrier material layer stack (e.g., Cu) is patterned by CMP. Only bottom two layers (e.g., Cr—Cu, Ti—W) are patterned by wet etching (or dry etching in the case of TiW). The wet etch time for the Cu-based metals is greatly reduced (by about 66%, because Cu is two (2) times the thickness of the Cr—Cu in the bottom layers), so the undercut is also reduced. This allows the pitch of the C4 solder bumps to be reduced. In a further embodiment the formed patterned passivation layer may include two or more via openings at the metallic bonding pad surface that defines a location for the solder bump connection. This facilitates the CMP process, by reducing dishing of the top conductive material layer during a CMP step that removes the top conductive material layer of the barrier layer stack. 
     Alternately, or in addition, the formed patterned passivation layer upon may include a first via opening at the metallic bonding pad surface to define a location for said solder bump connection, and include a second via opening adjacent the solder bump connection location to define a dummy via location. As the method of forming a solder bump connection includes implementing an electroplating technique for depositing the solder material, the remaining top conductive material layer portion at the dummy via location enabling a plating of solder with increased uniformity. Using this option, it is possible to remove a greater amount of the wetting layers (Cr—Cu) by CMP, thereby allowing a further reduction in the UBM removal wet etch time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, aspects and advantages of the structures and methods of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1(   a ) illustrates a last Cu wire level and patterned passivation layer according to the first embodiment of the invention; 
         FIG. 1(   b ) depicts a deposition of barrier layer metal stack upon structure of  FIG. 1(   a ) including an optional polish stop layer; 
         FIG. 1(   c ) illustrates the resulting structure after the top upper conductive material layer polish step; 
         FIG. 1(   d ) illustrates the step of forming a resist pattern for the C4 solder bumps; 
         FIG. 1(   e ) illustrates the resulting structure after diffusion barrier deposition and solder deposition; 
         FIG. 1(   f ) illustrates the resulting structure after wet etch of the lower barrier metal layers (e.g., Cr—Cu and Ti—W layers) of the stack and the resist removal; 
         FIG. 1(   g ) illustrates the resulting structure after the reflow solder step; 
         FIG. 2  illustrates the resulting structure according to a second embodiment of the invention whereby two or more small vias are provided at the solder bump location to prevent Cu dishing during a Cu polish step; and, 
         FIGS. 3(   a )- 3 ( c ) illustrate additional steps for forming a solder joint according to another embodiment of the invention that includes forming a dummy via bar adjacent the Cu barrier layer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The process flow for forming a novel solder bump metallurgy according to the invention is now described with reference to  FIGS. 1(   a )- 1 ( g ). As shown in  FIG. 1(   a ), a first process step  12  is depicted that includes the step of forming a passivation layer  30  upon the chip surface that includes a last conducting metal layer  20 , e.g., a metallic bonding pad layer comprising Cu, Al, or other conducting metal layer, in a low-k interconnect dielectric material layer  15  such as an oxide such as FSG (i.e., fluoro-silicate glass or like fluorinated silicon oxide,) an organic dielectric, such as SiLK or a hybrid dielectric, such as SiCOH. The passivation layer  30  is deposited using conventional deposition techniques, such as plasma-enhanced chemical vapor deposition (PECVD)and may comprise one or more passivation material layers, such as SiO2 and SiN. In one embodiment depicted in  FIG. 1(   a ), the passivation layer  30  comprises a stack of deposited passivation material layers including a lower layer such as SiN, an intermediate layer such as SiO 2  and an upper passivation layer including SiN formed using conventional processing. It is understood that the passivation material layer stack  30  is patterned to outline a location for the solder bump terminal  40 . For example, as shown in  FIG. 1(   a ), a single via opening  45  is etched at a location above the surface of the metallic pad layer  20  to define the location for the solder bump terminal  40 . Next, as shown in  FIG. 1(   b ), a further process step  22  includes depositing underbump metallurgy UBM which, according to the embodiments described herein, comprise a stack of barrier layer metals  50  that conform to the shape of the underlying passivation layer  30 . In one embodiment depicted in  FIG. 1(   b ), the barrier layer metals  50  comprises a stack of deposited material layers including a bottom layer of Ti—W (Titanium-Tungsten diffusion barrier  52 ), an intermediate layer of Cr—Cu (chromium copper wetting layer  54 ) and, a top layer of Cu  56  all deposited utilizing physical vapor deposition techniques (PVD). It is understood however, that the barrier metal layer may comprise other materials in a variety of stack configurations. Other metal layers that can be used for the under bump metallurgy include Ta, TaN, W, Ti, Al, Ni, Ni alloys, and Au. Some other UBM stacks (in addition to TiW/CrCu/Cu) include TiW/Cu, Ti/Cu, Ti/Ni—V/Cu, Al/Ni—V/Cu. It is further understood that other deposition techniques may be utilized to form the barrier metal layer stack including CVD, electroless plating, and electroplating. 
     In one embodiment depicted in  FIG. 1(   b ), the first layer of Ti—W  52  may be deposited to a thickness ranging between about 50 nm to 300 nm, with a typical thickness of about 150 nm, for instance; the CrCu layer  54  may be deposited to a thickness ranging between about 50 nm to 500 nm, with a typical thickness of about 200 nm, for instance; and, the Cu layer  56  may be deposited to a thickness ranging between about 100 nm to 1000 nm, with a typical thickness of about 400 nm, for instance. An optional sacrificial Ta layer  58 , deposited to a thickness ranging between about 10 nm to 200 nm, with a preferred thickness of about 50 nm, can be further deposited on top of the barrier layer metal stack  50  to function as a polish stop thereby preventing dishing of Cu during a subsequent CMP step. 
     In the next process step  32 , shown in  FIG. 1(   c ), the top Cu barrier metal layer  56  is polished (and the optional Ta layer  58  is polished) stopping on the intermediate CrCu layer  54  to leave the structure where the top metal layer (e.g., Cu) of the barrier stack is substantially coplanar with the intermediate layer  54  (e.g., CrCu) and forms a substantially flat horizontal surface. This is accomplished utilizing well known CMP (chemical mechanical polishing) steps. Then, in a next process step  42  as shown in  FIG. 1(   d ), using conventional processing, a resist material layer  70  is patterned to include an opening  71  that defines the subsequent formation of the solder bump at solder bump terminal  40 . 
     The next process step  52  depicted in  FIG. 1(   e ) includes the deposition of a diffusion barrier layer  75  over the coplanar surface defined at the opening between walls of the patterned resist  70 . In the embodiments described, diffusion barrier layer may include a nickel (Ni) material or alloy, such as Ni—V or Ni—P, that is deposited over the flat coplanar surface  60  at the solder bump terminal  40 . Further depicted in  FIG. 1(   e ) is the formation of solder bump material  80  within the walls of the patterned resist layer  70  at the solder bump terminal. Preferably, the solder material  80  is deposited using a well-known electroplating technique. In one embodiment depicted in  FIG. 1(   e ), the Ni diffusion layer  75  may be deposited to a thickness ranging from about 500 nm to 5000 nm, for instance, with a typical thickness of about 1000 nm. 
     Next, as shown as process step  62  depicted in  FIG. 1(   f ), the patterned resist material layer  70  is removed, e.g., stripped, and a wet etch process is implemented to remove the CrCu  54  and TiW  53  barrier metal layers adjacent, the terminal  40 . Finally, the solder is reflowed to form a sphere or solder ball  90  such as shown in  FIG. 1(   g ). 
     The methodology described herein with respect to  FIGS. 1(   a )- 1 ( g ) enables the pitch of the C4 solder bumps to be reduced. That is, the polishing of the Cu layer  56  as shown in  FIG. 1(   c ) effectively removes the Cu material underneath the patterned resist  70  and thus, reduces the total etch time for the underlying Cr—Cu and Ti—W barrier metal layers. For example, when only the bottom two layers (e.g., Cr—Cu layer  54 , and Ti—W layer  52 ) are patterned by wet etching as shown in  FIG. 1(   f ), the wet etch time for the Cu-based metals is greatly reduced (e.g., by about 66%, because Cu is about two (2) times the thickness of the Cr—Cu in the bottom layers. This reduction of the total etch time enables a reduced undercut of the barrier material layers formed under the solder bump (i.e., under the Ni barrier layer  75 ) during the wet etch process, and consequently increases the mechanical stability of the formed solder bump. 
     In a second embodiment of the invention depicted in  FIG. 2 , a series of multiple small vias  81 ,  82  are fabricated over the last metal pad layer at the solder bump terminal  40  rather than the single large via depicted in  FIGS. 1(   a )- 1 ( g ). In the example shown in  FIG. 2 , two small via openings  81 ,  82  are manufactured using conventional via opening formation techniques instead of the one larger via opening depicted in  FIG. 1(   a ). The advantage to this is that there is less dishing of the Cu during the CMP step depicted in  FIG. 1(   c ). Thus, in this second embodiment, as shown in the example depicted in  FIG. 2 , two smaller via openings are formed above the last metal pad layer surface. All other fabrication steps for the C4 solder connection terminal according to the second embodiment of the invention are the same as in the first embodiment depicted in  FIGS. 1(   a )- 1 ( g ), however, the upper Cu barrier metal layer  56  of the barrier material stack between each of said two via openings and adjacent the solder bump connection location are removed, stopping on the adjacent CrCu layer  54 , so that respective remaining upper conductive material layer portions  83 ,  84  at the via openings  81 ,  82  of said solder bump terminal defines a surface that is substantially coplanar with a surface of said remaining lower conductive material layers  54  of the barrier material stack adjacent the solder bump terminal location. This is accomplished utilizing well known CMP (chemical mechanical polishing) steps. Then, using conventional processing, a resist material layer is patterned to outline the subsequent formation of the solder bump at the solder bump terminal  40 , in the manner as described herein with respect to  FIGS. 1(   d )- 1 ( g ), to result in the structure  100  as shown in  FIG. 2 . 
     In a third embodiment of the invention depicted in  FIGS. 3(   a )- 3 ( c ), rather than removing only the top barrier metal layer  56  (e.g., Cu) by CMP (and the optional Ta layer) as in the prior embodiments, both the Cu  56  and intermediate CrCu layers  54  are polished to leave the structure  200  shown in  FIG. 3(   a ). This further reduces the wet etch time and undercut, allowing further reduction in solder ball pitch. It is noted that the undercut can be completely eliminated if the Ti—W is patterned by a reactive ion etch (RIE) process. However, a disadvantage is that the solder ball electroplating process will be more difficult, due to the relatively high resistance of the remaining Ti—W barrier stack layer  52 , which serves as an electrode during electroplating. To counter the phenomena of increased resistance as shown in  FIG. 3(   a )- 3 ( c ), solder bump plating is optimized to achieve uniform plating by fabricating a dummy via bar  95 , adjacent the solder bump terminal, that provide low resistance Cu  96  in between solder bumps to allow uniform plating. 
     Thus, in a third embodiment of the invention depicted in  FIGS. 3(   a )- 3 ( c ), the processing is the same as in the prior two embodiments of the method for forming a solder bump connection at a metallic bonding pad surface of a semiconductor chip. However, the formed patterned passivation layer upon the metal bonding pad surface includes a first via opening at the metallic bonding pad surface to define a location for the solder bump connection, and the second via bar opening adjacent the solder bump connection location to define a dummy via bar location. After forming the barrier material layer stack  50  including top and bottom conductive material layers, the top conductive material layer portions of the barrier material stack adjacent the solder bump connection location and adjacent the dummy via location are removed (e.g., by CMP) so that a remaining top conductive material layer portion at the solder bump connection location and at the dummy via bar location defines a surface that is substantially coplanar with a surface of remaining underlying conductive material layer  52 ,  54  adjacent the solder bump connection and dummy via bar locations. The remaining method steps depicted in  FIGS. 3(   a ) and  3 ( b ) of forming the solder bump connection according to the third embodiment are the same as described herein with respect to  FIGS. 1(   c )- 1 ( d ), with the advantage that the added conductive material layer portion at the dummy via  95  enables a more uniform plating of the solder bump. That is, dummy via bars provide a low resistance seed layer for the solder material electroplating step. The resultant structure for the solder bump terminal  200  according to the third embodiment of the invention is shown in  FIG. 3(   c ). 
     While the invention has been particularly shown and described with respect to illustrative and preformed embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention which should be limited only by the scope of the appended claims.