Abstract:
Solder bump structures for semiconductor device packaging is provided. In one embodiment, a solder bump structure comprises a semiconductor substrate, the substrate has at least one contact pad and an upper passivation layer having at least one opening formed therein exposing a portion of the contact pad. At least one patterned and etched polymer layer is formed on a portion of the contact pad. At least one patterned and etched conductive metal layer is formed above the polymer layer and is aligned therewith. And at least one layer of solder material having a solder height is provided above the conductive metal layer, the layer of solder is aligned with the conductive metal layer, the layer of solder is thereafter reflown thereby creating a solder ball.

Description:
BACKGROUND 
   The present invention relates generally to the fabrication of semiconductor devices, and more particularly, to solder bump structures in the packaging of semiconductor devices. 
   Faster, reliable, and higher-density circuits at lower costs are the goals for integrated circuit (IC) packaging. Conventional wirebond technology, the most common method for electrically connecting aluminum bonding pads on a chip surface to the package inner lead terminals on the lead-frame or substrate has proven to be low cost and reliable. But for the future, packaging goals will be met by increasing the density of chips and reducing the number of internal interconnections. Packages with fewer interconnecting links lower potential failure points, reduce the circuit resistance, and reduce interconnect capacitance, which affects electrical performance. The need to reduce the IC package to fit end-user applications (e.g., smart cards, palmtop computers, camcorders, and so on) is driving the new packaging designs that reduce size and overall profile. This reduction is offset by the need for handling larger amounts of parallel data lines, therefore driving the need to increase package input/output requirements with more leads. 
   Advanced packaging designs are regularly introduced to solve packaging challenges. One such advanced package design is flip chip. Flip chip is a packaging method of mounting the active side of a chip (with the surface bonding pads) toward the substrate (i.e., upside down placement of the bumped die relative to the wirebonding approach—thus the reason for the term “flip” chip). It provides the shortest path from the chip devices to the substrate and low cost interconnection for high volume automated production. There is also a reduction in weight and profile since leadframes or plastic packages are often not used. Flip chip technology uses solder bumps—usually formed from tin/lead solder in a 5% Sn and 95% Pb ratio—to interconnect the chip bonding pads to the substrate. 
   There are several methods known to those skilled in the art for producing solder bumps on a semiconductor device.  FIGS. 1A–1E  illustrate a prior art method of forming a bump on a substrate such as a semiconductor wafer. As shown in  FIG. 1A , a semiconductor wafer  10  is provided having a base silicon substrate  12  with metal interconnect layers (not shown) overlying substrate  12  and an upper passivation layer  14 , which may be one or more layers, that extends partially over a bond pad or contact pad  15  located on the upper surface of the semiconductor wafer  10 . Passivation layer  14  has an opening overlying contact pad  15  so that electrical contact to an external circuit may be made from the semiconductor wafer  10 . Contact pad  15  may be made from any of a variety of metals, such as aluminum, aluminum alloys, copper, and copper alloys. Typically, an under bump metallurgy (UBM)  16  is provided over the entire upper surface of semiconductor wafer  10  and over the upper surface of contact pad  15 . UBM  16  may be composed of a plurality of individual layers of a variety of different metals and may be deposited by any of a variety of methods including electroless plating, sputtering, or electroplating. As shown in  FIG. 1B , thereafter, a photoresist layer  22  is thereafter deposited over UBM  16  and patterned to provide an opening  24  overlying contact pad  15  on semiconductor wafer  10 . Thereafter, a seed layer  26  may be deposited by conventional methods such as electroplating over UBM  16 . An electrically conductive material  30  may then be deposited on top of seed layer  26  as shown in  FIG. 1C  and the electrically conductive material  30  includes solder, for example in a 63 weight percent Sn, 37 weight percent Pb eutectic composition. As shown in  FIG. 1D , photoresist  22  is removed by plasma etching.  FIG. 1E  illustrates the step of reflowing the solder to provide a bump or ball  32  on semiconductor wafer  10 . 
   After the solder bumps on a semiconductor device have been formed, typically an epoxy underfill is used in flip chip packaging. The underfill is typically an adhesive, such as an epoxy resin, that serves to reinforce the physical and mechanical properties of the solder joints between the IC chip and the substrate. The underfill improves the fatigue life of the packaged system, and also serves to protect the chip and interconnections from corrosion by sealing the electrical interconnections of the IC chip from moisture. 
   While the use of underfills has presented a solution to the problems associated with flip chip packaging, it has created new challenges for the semiconductor manufacturing process. In traditional solder bump structures, the new manufacturing steps required to apply the underfill, and to bake the assembly to harden the underfill, substantially complicate and lengthen the manufacturing process. An additional disadvantage to traditional solder bump structures in flip chip packaging has been that the use of an adhesive underfill can make it difficult, if not impossible, to disassemble the chip components when a defect is discovered after assembly of an electrical component. Because the solder assembly and underfill steps may occur simultaneously during the heating process, it is difficult to test the electronic assembly until the assembly is complete. Thus, if a defect is discovered, the underfill has already hardened, making removal and disassembly impractical. This results in increased production costs due to the waste of otherwise usable components. 
   For these reasons and other reasons that will become apparent upon reading the following detailed description, there is a need for an improved solder bump structure in advanced IC packaging such as flip chip that avoids the cost and reduced throughput concerns associated with conventional solder bump structures. 
   SUMMARY 
   The present invention is directed to solder bump structures, particularly, but not by way of limitation, for flip chip packaging of semiconductor devices. In one embodiment, a solder bump structure comprises a semiconductor substrate, the substrate has at least one contact pad and an upper passivation layer having at least one opening formed therein exposing a portion of the contact pad. At least one patterned and etched polymer layer is formed on a portion of the contact pad. At least one patterned and etched conductive metal layer is formed above the polymer layer and is aligned therewith. And at least one layer of solder material having a solder height is provided above the conductive metal layer, the layer of solder is aligned with the conductive metal layer, the layer of solder is thereafter reflown thereby creating a solder ball. 
   In another embodiment, a solder bump structure comprises a semiconductor substrate, the substrate has at least one contact pad and an upper passivation layer having at least one opening formed therein exposing a portion of the contact pad. At least one patterned and etched polymer layer is formed on a portion of the contact pad. At least one patterned and etched conductive metal layer is formed above the polymer layer and is aligned therewith. And at least one layer of solder material having a solder height is provided above the conductive metal layer, the layer of solder is aligned with the conductive metal layer, and the layer of solder is thereafter reflown thereby creating a solder ball. 
   In yet another embodiment, a solder bump structure comprises a semiconductor substrate, the substrate has at least one contact pad and an upper passivation layer having at least one opening formed therein exposing a portion of the contact pad. At least one patterned and etched polymer layer is formed on a portion of the contact pad. At least one patterned and etched conductive metal layer is formed above the surfaces of the polymer layer and the contact pad and portions of the passivation layer, the conductive metal layer is aligned with the polymer layer. And at least one layer of solder material having a solder height is provided above the conductive metal layer, the layer of solder is aligned with the conductive metal layer, the layer of solder is thereafter reflown thereby creating a solder ball. 
   In still another embodiment, a solder bump structure comprises a semiconductor substrate, the substrate has at least one contact pad and an upper passivation layer having at least one opening formed therein exposing a portion of the contact pad. At least one patterned and etched polymer layer is formed on the exposed surface of the contact pad. At least one patterned and etched conductive metal layer is formed above the surfaces of the polymer layer and portions of the passivation layer, the conductive metal layer is aligned with the polymer layer. And at least one layer of solder material having a solder height is provided above the conductive metal layer, the layer of solder is aligned with the conductive metal layer, the layer of solder is thereafter reflown thereby creating a solder ball. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, aspects, and advantages of the present invention will become more fully apparent from the following detailed description, appended claims, and accompanying drawings in which: 
       FIGS. 1A–1E  are cross-sectional views of a semiconductor device depicting a prior art method of forming a solder bump structure. 
       FIG. 2  is a cross-sectional view of a solder bump structure according to one embodiment of the present invention. 
       FIG. 3  is a cross-sectional view of a solder bump structure according to one embodiment of the present invention. 
       FIG. 4  is a cross-sectional view of a solder bump structure according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, one having an ordinary skill in the art will recognize that the invention can be practiced without these specific details. In some instances, well-known structures and processes have not been described in detail to avoid unnecessarily obscuring the present invention. 
   Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 2  is a cross-sectional view of a solder bump structure according to one embodiment of the present invention. The solder bump structure has a semiconductor substrate  12  and an upper passivation layer  14  overlying portions of a bond pad or contact pad  15 . Semiconductor substrate  12  is understood to include active and passive devices, conductive layers and dielectric layers and the type of the substrate is a design choice dependent on the fabrication process being employed. Upper passivation layer  14  has an opening therein exposing a portion of contact pad  15  and may be comprised of a material such as for example, silicon nitride (SiN), silicon dioxide (SiO2), and silicon oxynitride (SiON). Contact pad  15  establishes electrical contact between the electrical interconnects in semiconductor substrate  12  and a later to be formed overlying solder bump. Contact pad  15  may be comprised of any of a variety of metals, such as for example, aluminum, aluminum alloys, copper, and copper alloys. Thereafter, a patterned and etched polymer body or polymer layer  17  is provided over a portion of contact pad  15 . Polymer layer  17  may be deposited by any of a variety of methods, such as for example chemical vapor deposition and sputtering. The choice of materials for polymer layer  17  is important as polymer layer  17  must withstand temperatures encountered during bonding. Examples of polymers that may be used are silicons, carbons, fluoride, chlorides, parylene or teflon, polycarbonate (PC), polysterene (PS), polyoxide (PO), poly polooxide (PPO), benzocyclobutene (BCB). In one embodiment, polymer layer  17  may be polyimide and have a thickness of from about 5 microns to about 100 microns. Polyimide films can tolerate temperatures of up to 500 degrees C. without degradation of their dielectric film characteristics. In one embodiment, the polyimide may be polyamic acid polyimide such as “PROBIMIDE 7010” or “PROBIMIDE 514” produced by OCG Microelectronic Materials, Inc., Tempe, Ariz. In another embodiment, polymer layer  17  may be an epoxy having a thickness of from about 5 microns to about 100 microns. As polymer layer  17  relieves mechanical strain between the IC chip and the substrate, the solder bump structure of the present invention prevents the premature failures of the solder bumps due to solder cracks often associated with conventional solder bump structures. 
   Next, a patterned and etched conductive metal layer  40  is provided above the upper surface of the semiconductor wafer and polymer layer  17  and is aligned with polymer layer  17 . One skilled in the art will understand that ideally, conductive metal layer  40  and polymer layer  17  need to be well chosen to be compatible with the temperatures used during the bonding process. Further, conductive metal layer  40  covering polymer layer  17  must be chosen to provide good adhesion to polymer layer  17 . Conductive metal layer  40  may comprise of a BLM (ball limiting metallurgy) or UBM (under bump metallurgy). To prepare the semiconductor wafer for solder bumping, typically a cleaning step is provided. Further, the preparation may include preparing a pad metallurgy that will protect the integrated circuits while making good mechanical and electrical contact with a to be formed solder bump. Accordingly, protective metallurgy layers may be provided over the bond pad. UBM may comprise of successive layers of metal and in one embodiment, UBM may comprise of an adhesion layer  18 , a wetting layer ( 19 ), and a protection layer ( 20 ). Adhesion layer  18  must adhere well to the polymer layer  17 , contact pad  15  and the surrounding passivation layer(s), while providing a strong, low-stress mechanical and electrical connection. Wetting layer  19  provides a wettable surface for the molten solder during the solder bumping process, for good bonding of the solder to the underlying metal. Protection layer  20  may be provided to add reliable protection to the underlying layers and polymer layer  17 . 
   UBM may be deposited by any of a variety of methods including, for example electroless plating, sputtering, or electroplating. After deposition of UBM, an electrically conductive material may be deposited over the conductive metal layer  40  and the deposition may be by evaporation, electroplating, electroless plating, and screen printing. The electrically conductive material may be any of a variety of metals, metal alloys or metals and mixtures of other materials, but preferably, the electrically conductive material is a solder. The solder may be any of a variety of compositions and in one embodiment the solder is in a 63 weight percent Sn, 37 weight percent Pb composition. Finally, the electrically conductive material (solder) is reflown by heating to form a ball or bump  32  on the semiconductor wafer as shown in  FIG. 2 . 
   The bonding of the IC chip and the substrate may be formed by conventional processes such as for example, thermocompression bonding, ultrasonic bonding, tape automated bonding, application of heat energy, or application of light energy. During the bonding process between the IC chip and the substrate, the polymer body or layer  17  in each bump structure may deform as electrical connection is formed. This deformation is important in forming a good electrical contact and the deformation requires a very small bonding force and produces little or no tendency to separate the connection after it has been made. 
   Unlike in conventional solder bump structures that require an epoxy underfill, the employment of the solder bump structure of the present invention in advanced IC packaging such as flip chip does not require the use of underfill. It is understood that negating the use of underfill simplifies and lessens the manufacturing process. Furthermore, a major challenge with the use of epoxy underfill is that the flip chip cannot be removed once the epoxy is applied. This creates problems of rework if the chip is found to be defective during test. The present invention solves this problem and saves production costs due to the waste of otherwise usable components. While the solder bump structure of the present invention in chip packaging does not require the introduction of underfill in the spaces or gaps remaining between the IC chip and the substrate, it is understood by those skilled in the art that such underfill may nevertheless be optionally used. 
     FIG. 3  is a cross-sectional view of a solder bump structure according to another embodiment of the present invention.  FIG. 3  is identical to  FIG. 2  except that in this embodiment, the patterned and etched conductive metal layer  40  is formed on the surfaces of polymer layer  17  and contact pad  15  and on portions of passivation layer  14 . Other than the conductive metal layer  40 , the solder bump structure is as described above with reference to  FIG. 2 . 
     FIG. 4  is a cross-sectional view of a solder bump structure according to yet another embodiment of the present invention.  FIG. 4  is identical to  FIG. 2  except that in this embodiment, the patterned and etched polymer layer  17  is formed on the exposed surface of the contact pad  15 . Further, the patterned and etched conductive metal layer  40  is formed on the surfaces of polymer layer  17  and on portions of the passivation layer  14 . Other than the polymer layer  17  and the conductive metal layer  40 , the solder bump structure is as described above with reference to  FIG. 2 . 
   In the preceding detailed description, the present invention is described with reference to specifically exemplary embodiments thereof. It will, however, be evident that various modifications, structures, processes, and changes may be made thereto without departing from the broader spirit and scope of the present invention, as set forth in the claims. The specification and drawings are, accordingly, to be regarded as illustrative and not restrictive. It is understood that the present invention is capable of using various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.