Abstract:
Disclosed herein are a method of manufacturing a solder bump on a semiconductor device, a solder bump structure formed on a substrate, and an intermediate solder bump structure. In one embodiment, the method includes creating a bonding pad over a semiconductor substrate, and placing a mask layer over the substrate and the bonding pad. The method also includes forming an opening in the mask layer having a primary solder mold and at least one secondary solder mold joined with the primary mold, where the opening exposes a portion of the bonding pad. In this embodiment, the method further includes filling the primary solder mold and the at least one secondary solder mold with solder material to form corresponding primary and at least one secondary solder columns in electrical contact with the bonding pad. The method also includes removing the mask layer after the filling of the solder molds with the solder material. The method still further includes reflowing the solder material to form a primary solder bump from the solder material of the primary solder column and at least a portion of the solder material from the at least one secondary solder column through cohesion of the solder material from the at least one secondary solder column to the primary solder column when melted.

Description:
TECHNICAL FIELD 
     Disclosed embodiments herein relate generally to solder bumps for providing electrical and mechanical bonds between substrates, and more particularly to an intermediate IC chip solder bump structure, a finished IC chip solder bump structure, and method of manufacturing the same. 
     BACKGROUND 
     The packaging of integrated circuit (IC) chips is one of the most important steps in the manufacturing process, contributing significantly to the overall cost, performance and reliability of the packaged chip. As semiconductor devices reach higher levels of integration, packaging technologies, such as chip bonding, have become critical. Packaging of the IC chip accounts for a considerable portion of the cost of producing the device and failure of the package leads to costly yield reduction. 
     As semiconductor device sizes have decreased, the density of devices on a chip has increased, along with the size of the chip, thereby making chip bonding more challenging. One of the major problems leading to package failure as chip sizes increase is the increasingly difficult problem of thermal coefficient of expansion (TCE) mismatches between materials leading to stress buildup and consequent failure. For example, in flip-chip technology chip bonding is accomplished by means of solder bumps formed on under bump metallization (UBM) layers overlying an IC chip bonding pad where, frequently, improper wetting (bonding) between the solder and UBM layers may lead to a bond not sufficiently strong to withstand such stresses. 
     In many cases it is necessary to repackage the chip after a package failure, requiring costly detachment of the chip from the package and repeating the chip bonding process in a new package. Some chip bonding technologies use a solder bump attached to a contact pad (the bonding pad) on the chip to make an electrical (and somewhat structural) connection from the chip devices to the package substrate. For example, C4 (Controlled-Collapse Chip Connection) is a means of connecting semiconductor chips to substrates in electronic packages. C4 is a flip-chip technology in which the interconnections are small solder balls (bumps) on the chip bonding pads. Since the solder balls form an area array (a “ball grid array” (BGA)), C4 technology can achieve a very high-density scheme for chip interconnections. The flip-chip method has the advantage of achieving a very high density of interconnection to the device with a very low parasitic inductance. 
     Solder bumps may be formed by, for example, vapor deposition of solder material over layers of under bump metallization (UBM) layers formed on the bonding pad. In another method, the layers of solder material may deposited by electro-deposition onto a seed layer material deposited over UBM layers formed on the bonding pad. In yet another method, solder bumps may be formed by a solder-paste screen-printing method using a mask (stencil) to guide the placement of the solder-paste. Typically, after deposition of the solder materials, for example, in layers or as a homogeneous mixture, the solder bump (ball) is formed after removing a photoresist mask defining the solder material location by heating the solder material to a melting point (a “reflow” process) such that a solder ball is formed with the aid of surface tension. Alternatively, a solder bump may be formed within a permanent mask made of photoresist or some other organic resinous material defining the solder bump area over the bonding pad. Because of the importance of the solder bumps/balls in such flip-chip techniques, improvements in processes used to form the solder balls on the IC chips are continuously being pursued. 
     BRIEF SUMMARY 
     Disclosed herein is a method of manufacturing a solder bump on a semiconductor device. In one embodiment, the method includes creating a bonding pad over a semiconductor substrate, and placing a mask layer over the substrate and the bonding pad. The method also includes forming an opening in the mask layer having a primary solder mold and at least one secondary solder mold joined with the primary mold, where the opening exposes a portion of the bonding pad. In this embodiment, the method further includes filling the primary solder mold and the at least one secondary solder mold with solder material to form corresponding primary and at least one secondary solder columns in electrical contact with the bonding pad. The method also includes removing the mask layer after the filling of the solder molds with the solder material. The method still further includes reflowing the solder material to form a primary solder bump from the solder material of the primary solder column and at least a portion of the solder material from the at least one secondary solder column through cohesion of the solder material from the at least one secondary solder column to the primary solder column when melted. 
     In another aspect, a solder bump structure is disclosed that is formed on a bonding pad of a first substrate for electrically and mechanically coupling the first substrate to a bonding pad of a second substrate. In one embodiment, the structure includes a primary solder bump comprising a volume of solder material and having a first height and a base perimeter defined by a nadir. In addition, in this embodiment, the solder bump structure further includes at least one secondary solder bump comprising a volume of solder material having a second height less than the first height, the secondary solder bump adjacent the primary solder bump and metallurgically adjoined thereto at the nadir. 
     In yet another aspect, an intermediate structure is disclosed. In one embodiment, the intermediate structure includes a primary solder column comprising primary solder material and configured to electrically contact a bonding pad on a semiconductor substrate. Also in this embodiment, the intermediate structure includes at least one secondary solder column comprising secondary solder material in electrical contact with the primary solder column, where the at least one secondary column has a height and volume less than a height and volume of the primary solder column. In addition, in this embodiment, the primary solder column is further configured to form a primary solder bump comprising the primary solder material and at least a portion of the secondary solder material through cohesion from the at least one secondary solder column when the intermediate structure undergoes a reflow process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the principles disclosure herein, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A–1E  illustrate an exemplary conventional process for forming a solder bump on a semiconductor chip shown through cross section views of a IC chip bonding pad area; and 
         FIGS. 2A–2F  illustrate one embodiment of an exemplary process for forming solder bump on a semiconductor chip in accordance with the disclosed principles. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring initially to  FIGS. 1A–1E , illustrated is an exemplary conventional process for forming a solder bump on a semiconductor chip shown through cross section views of an IC chip bonding pad area. With reference to  FIG. 1A , the process of creating the solder bumps begins after the chip bonding pad  10 , for example, Cu or Al formed by vapor deposition, has been formed on the surface of a semiconductor wafer  8 . After the bonding pad  10  is formed, a passivation layer  12  of, for example, silicon dioxide (SiO 2 ) is formed over the semiconductor device surface excluding a portion overlying the bonding pad  10 . Typically, one or more under-bump metallization (UBM) layers, e.g., layer  14 A, of from about 500 Å to about 5000 Å are then deposited over chip bonding pad  10  and a layer of photoresist  16  formed thereover, as shown in  FIG. 1B . 
     The UBM layer  14 A may be, for example, a layer of titanium. The photoresist layer  16  is typically from about 10 to about 25 microns high. As shown in  FIG. 1B , the photoresist layer  16  is photolithographically patterned and developed to form an opening  17  above the bonding pad  10  to expose a UBM layer, e.g.,  14 A. Additional UBM layers may be formed within the mask opening  17  by, for example, an electroplating process or vapor deposition process forming, for instance, UBM layers  14 B and  14 C in  FIG. 1C . Layers  14 B and  14 C may be, for example, layers of copper and nickel, respectively. UBM layers are typically formed over the bonding pad  10  to allow for better bonding and wetting of the solder material to the uppermost UBM layer  14 C adjacent to the solder material, and for protection of the bonding pad  10  by the lowermost UBM layer  14 A. A column of solder material  18 A may either be deposited in layers, for example, a layer of lead followed by a layer of tin, where the solder material layers are later formed into a homogeneous solder during a reflow (e.g., temporary melting) process for solder material. In other embodiments, the solder material may be deposited as a homogeneous solder material by vapor deposition or electroplating onto a “seed layer,” such as UBM layer  14 C. 
     Looking at  FIG. 1D , after removal of the photoresist layer  16 , the UBM layer  14 A is etched through by an etching process, such as a reactive ion etch (RIE) process, to the underlying passivation layer  12  using the solder column  18 A as an etching mask to protect the underlying UBM layers  14 A,  14 B, and  14 C. The solder column  18  is then temporarily heated to a melting point (“reflow”) to form a solder bump  18 B over the UBM layer  14 C, as shown in  FIG. 1E . After the reflow process, a homogeneous lead/tin solder bump is formed, for example, with composition ratios indicating weight percent, high lead alloys including 95 Pb/5 Sn (95/5) or 90 Pb/10 Sn (95/10) with melting temperatures in excess of 300° C. or eutectic 63 Pb/37 Sn (63/37) with a melting temperature of 183° C. The resulting solder bump  18 B is composed of a homogeneous material and has a well-defined melting temperature. For example, the high melting Pb/Sn alloys are reliable bump metallurgies that are particularly resistant to material fatigue. 
     A series of layers may be advantageously used to form the UBM layers. The uppermost UBM layer adjacent the solder bump supplies a wettable layer during reflow for the solder bump subsequently formed over the layers. For example, to form the plurality of UBM layers, some UBM systems may include, reciting the lowermost layer adjacent the bonding pad  10  first, chromium and copper (Cr/Cu), titanium and copper (Ti/Cu), and titanium-tungsten and copper (Ti:W/Cu), and titanium, copper, nickel (Ti/Cu/Ni). Since conventional bumps melt completely in the reflow soldering process of the flip-chip bonding technique to intimately contact the UBM layer, the UBM layer must be able to withstand thermal and mechanical stresses, and resist intermetallic phase formations. Thus, the quality of the UBM layers and wettability during reflow is critical to the reliability of the complete assembly. In addition, the UBM layers help define the size of the solder bump  18 B after reflow, and provide a surface that is wettable by the solder and that reacts with the solder to provide an adhesion bond with mechanical integrity and thereby acceptable reliability under mechanical and heat stresses. Furthermore, the UBM layers act as a barrier between the semiconductor device and the metals in the interconnections. 
     Turning now to  FIGS. 2A–2F , illustrated is one embodiment of an exemplary process for forming a solder bump on a semiconductor chip in accordance with the disclosed principles. Looking first at  FIG. 2A , illustrated is a solder bump area  200  early in the process for forming a solder bump to provide an electrical, and mechanical, bond between an IC chip and another component such as a printed circuit board. As shown, a typical solder bump area  200  includes a semiconductor substrate  205  with a bonding pad  210  formed on a portion thereof. Also often included is a passivation layer  215  typically constructed from dielectric material. If a passivation layer  215  is included, a portion of the layer  215  over the bonding pad  210  is removed, perhaps using conventional etching techniques, to expose a part of the bonding pad  210 . One or more UBM layers  220  may then be formed over the passivation layer  215  and in electrical contact with the bonding pad  210 . Although not required, a UBM layer  220 , provides a larger footprint on which to form the solder bump, and often using materials, such as titanium, that provide a stronger bond with the solder bump when formed. 
     Referring now to  FIG. 2B , the same solder bump area  200  discussed above is shown, a little further into the bump formation process. Specifically, a masking layer  225  is placed over the surface of the solder bump area  200  so that certain portions of layers in the area may be removed, while others will remain. In an advantageous embodiment, the mask layer is a photoresist layer  225  that has been deposited over the solder bump area  200 . The photoresist layer  225  is then patterned and developed, typically using conventional photolithography techniques. The portions of the solder bump area  200  no longer masked by the photoresist layer  215  may then be removed, usually through etching. In the illustrated embodiment, a width of the UBM layer  220  is defined using the photoresist layer  225  and etching process. 
     Turning now to  FIG. 2C , a top view of a different pattern is illustrated in the photoresist layer  225 , although in alternative embodiments this may be a different photoresist layer  225  than the layer illustrated in  FIG. 2B . As shown, the photoresist layer  225  is patterned and developed so as to create distinct, but interconnected, openings (or “molds”) to be filled with solder material later in the manufacturing process. More specifically, a primary solder mold  230  is formed in the photoresist layer  225  proximate to the center of the solder bump area  200 , typically immediately over the actual bonding pad  210  (and UBM layer  220 , if present). Adjacent to the primary solder mold  230 , two secondary solder molds  235   a ,  235   b  are also formed in the photoresist layer  225 . These secondary molds  235   a ,  235   b  may be beneficially formed near the outer edges of the defined UBM layer  220 , and will also be filled with solder material later in the manufacturing process. While the illustrated embodiment shows molds  230 ,  235   a ,  235   b  having an octagonal shape, other various shapes, including circular or teardrop, may also be employed without departing from the broad scope of the disclosed principles. 
     With reference now to  FIG. 2D , illustrated is a top view of the solder bump area  200  after solder material has been deposited. After the patterning and developing of the photoresist layer  225  done with reference to  FIG. 2C , solder material is deposited in the primary and secondary solder molds  230 ,  235   a ,  235   b . Although any appropriate technique may be employed, exemplary embodiments of the disclosed process employs a vapor deposition process or electroplating to deposit the solder material. In addition, any appropriate type of solder material, including alloys of different metals, may be used as the solder material. Examples of solder materials includes, but are not limited to, lead, gold, silver, copper, and tin. In some specific embodiments, the solder material comprises over 90% lead, however this is not required. Embodiments with lead-based alloys may also be eutectic to assist in the reflow process, but again this is not required. 
     After the solder material is deposited, the photoresist layer  225  is removed from the solder bump area  200 . Once the photoresist layer  225  is removed, a primary solder column  240  remains where the primary solder mold  230  was filled with solder material, while secondary solder columns  245   a ,  245   b  are present where the secondary solder molds  235   a ,  235   b  were filled. Moreover, the primary solder column  240  is also substantially larger than the secondary solder columns  245   a ,  245   b , for example, where the secondary columns  245   a ,  245   b  have a volume of solder material anywhere between about 10% to 90% of the volume of the primary column  240 . In addition, solder joining regions  250  are also present now in the solder bump area  200  where solder material filled openings in the photoresist layer  225  that adjoined the primary solder mold  230  and the secondary solder molds  235   a ,  235   b . Typically, these joining regions  250  are substantially smaller in overall size and volume than either the primary or secondary solder molds  230 ,  235   a ,  235   b . In other embodiments, the secondary solder columns  245   a ,  245   b  simply adjoin directly to the primary solder column  240 . Moreover, the solder columns  240 ,  245   a ,  245   b  shown in  FIG. 2D  are octagonal shaped, corresponding to the octagonal shape of the solder molds  230 ,  235   a ,  235   b  in the photoresist layer  225 , but any other corresponding shapes are possible. 
     Looking now at  FIG. 2E , illustrated is the solder bump area  200  after a reflow process used to form the final shape of the solder bump. Specifically, the entire assembly, typically having dozens if not hundreds of solder bump areas, is heated to a point where the solder columns  240 ,  245   a ,  245   b  melt. During the reflow process, the primary solder column  240  melts into the primary solder bump  255 , which typically has a spherical shape around its upper half. In addition to the creation of the primary solder bump  255 , secondary solder bumps  260   a ,  260   b  are also created adjacent to, and adjoined with, the primary solder bump  255  at the nadir defining the base perimeter of the primary solder bump  255 . Furthermore, in accordance with the principles disclosed herein, adjoining of the solder columns  245   a ,  245   b  along side the primary column  240  results in cohesion between these columns during the reflow process. As a result, solder material originally deposited as part of the secondary solder columns  245   a ,  245   b  moves towards and into the primary solder column  240  during reflow, as indicated by arrows A 1  and A 2 , thus increasing the volume of the primary solder bump  255  with solder material flowed from the secondary solder columns  245   a ,  245   b.    
     Thus, as all the solder material melts and then is allowed to cool and re-harden during the reflow process to form the finished solder bumps  255 ,  260   a ,  260   b , the solder material added to the primary bump  255  from the secondary bumps  260   a ,  260   b  increases the overall volume and size of the primary solder bump  255  such that it is larger than it would have been had only the primary solder column  240  been formed (as is done in the prior art). Therefore, the size of the finished primary solder bump  255  is larger than it would have been if made using only conventional techniques. Additionally, the height of the primary solder bump  255  is substantially taller than the height of each of the secondary solder bumps  260   a ,  260   b  not only because of the original size of the solder columns, but also because of the movement of material towards the primary solder bump  255  through cohesion. In many embodiments, the height of each of the secondary solder bumps  260   a ,  260   b  is about 10% to 90% of the height of the primary solder bump  255 , but no specific height ratio is required. Specifically, the volume of the solder material in the primary solder bump  255  and/or its height is sufficient to electrically and mechanically couple the bonding pad  210  of the first substrate  205  to another, corresponding bonding pad of a second substrate, and the volume of solder material of each of the at least one secondary solder bumps  260   a ,  260   b  is not sufficient and does not reach height enough to contact the second substrate. This is especially beneficial in bonding techniques such as flip-chip techniques. Furthermore, although two secondary solder bumps  260   a ,  260   b  (and two secondary solder columns  245   a ,  245   b ) have been illustrated, the process disclosed herein is not limited to any particular number of secondary columns or bumps, and therefore as few as one may be employed. 
     Turning finally to  FIG. 2F , illustrated is a top view of a finished solder bump area  200  constructed using the principles and processes set forth in this disclosure. This view further demonstrates the spherical shape taken by both the primary and secondary solder bumps  255 ,  260   a ,  260   b  after the reflow process. In addition, the direction of the cohesion that occurs between the primary and secondary solder bumps  255 ,  260   a ,  260   b  is illustrated again using arrows A 1  and A 2 . Moreover, the adjoining of the secondary solder bumps  260   a ,  260   b  to the primary solder bump  255  at its nadir is also shown. 
     While various embodiments of forming a unique solder bump for a semiconductor substrate according to the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the invention(s) should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages. 
     Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” such claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Brief Summary” to be considered as a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.