Abstract:
A method of coupling an integrated circuit to a substrate includes providing the substrate, forming a contact pad in the substrate, contacting the contact pad with a solder ball, and repeatedly exposing the solder ball to a thermal process to cause intermetallics based on a metal in the contact pad to be formed in the thermal ball.

Description:
BACKGROUND 
     The present invention relates to integrated circuits and, more specifically, to joining integrated circuits together. 
     Controlled-collapse chip connection (C4) is a means of connecting integrated circuit (IC) chips to substrates in electronic packages. C4 is known as a flip-chip technology, in which the interconnections are small solder balls on the bottom side chip surface. C4 technology represents one of the highest density schemes known in the art for chip interconnections. Historically, the PbSn (lead-tin) solder for the formation of the solder ball was evaporated through a metal mask. In the 1990&#39;s, electrochemical fabrication of C4 interconnections was introduced. Electroplating is more extendible than evaporation to small C4-pad dimensions, closer pad spacing, larger wafers, and lower-melting solders (which have a higher content of tin (Sn)). 
     In general, the top layers of an integrated circuit (IC) chip are wiring levels, separated by insulating layers of dielectric material that provide input/output for the device. In C4 structures, the chip wiring is terminated by a plurality of metal films that form the ball-limiting metallurgy (BLM), which is also referred to as under-bump metallurgy (UBM). The ball-limiting metallurgy defines the size of the solder bump after reflow, provides a surface that is wettable by the solder, and that reacts with the solder to provide good adhesion and acceptable reliability under mechanical and heat stress. The BLM also serves as a barrier between the integrated-circuit device and the metals in the interconnection. 
       FIGS. 1A and 1B  are a typical implementation of the C4 manufacturing process. In  FIG. 1A  an integrated circuit (IC)  100  formed on a base material  102  (for example, silicon) has a solder ball  108  formed for subsequent attachment to a contact pad  112  (see  FIG. 1B ) on a carrier  114 . A BLM  106  constricts the solder flow and aids in the formation of the solder ball  108  (which is formed by reflowing a deposit of solder paste), and serves as a wettable surface and contact for an underlying contact  110  for the IC  100 . A passivation layer  104 , typically a polymer dielectric, insulates the IC  100 , and supports the BLM  106 . In  FIG. 1B  the IC  100  is attached to the contact pad  112  on the carrier  114 , by reflowing the solder ball  108 . Solder flow is restricted on the carrier  114  by solder dams  116 , which outline and define the contact pad  112 . A secondary reflow is employed to attach the IC  100  to the contact pad  112  on the carrier  114 . 
     Certain state and federal regulations have limited or eliminated the use of lead based solder. One approach to complying with these regulations includes utilizing tin (Sn) based lead-free solders. Sn has a tetragonal crystal symmetry that exhibits anisotropic properties such related to elastic constants and diffusion of solute atoms through the Sn. In first orientation of the Sn particles, elements that form the contact pad  112  (e.g., copper (Cu) or nickel (Ni) diffuse under an electric field at a rate thousands of times slower than when the particles in a second orientation perpendicular to that of the first orientation. Solder balls  108  having the first orientation (e.g., with low diffusion) exhibit a slow/controllable failure of joint known as Mode 1 electromigration failure herein. In this mode, the failure has a formation of voids near the surface of the BLM  106 . For solder balls having the second orientation (e.g., perpendicular to the orientation for Mode 1), a failure known as Mode 2 electromigration failure herein occurs. Mode 2 failure is characterized by the movement of intermetallics from the carrier  114  to the BLM  106  or vice-versa. The possibility of mode 2 failures make tin based lead-free solder balls  108  unfit for the high-end applications requiring long life (e.g., 100 k Hr) at elevated temperatures (e.g. 100 C). 
     Several efforts are in progress to make the ball  108  an agglomerate of large number of randomly oriented grains to reduce the effective mass flow of Cu or Ni perpendicular to the BLM  106 . However, due to the small size of the ball  108 , the number of grains per ball  108  is limited to less than 5. 
     SUMMARY 
     According to one embodiment of the present invention, a method of coupling an integrated circuit to a substrate is disclosed. The method of this embodiment includes providing the substrate; forming a contact pad in the substrate; contacting the contact pad with a solder ball; and repeatedly exposing the solder ball to a thermal process to cause intermetallics based on a metal in the contact pad to be formed in the thermal ball. 
     According to another embodiment of the present invention another method of coupling an integrated circuit to a substrate is disclosed. This method includes providing the substrate; forming a contact pad in the substrate; contacting the contact pad with a solder ball; and exposing the solder ball to a thermal process until a desired amount of intermetallics based on a metal in the contact pad are formed in the solder ball. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIGS. 1   a  and  1   b  show a typical implementation of the C4 manufacturing process; 
         FIG. 2  shows a solder ball interspersed with intermetallics according to one embodiment; 
         FIG. 3  shows a method according to one embodiment; 
         FIG. 4  shows an example of a contact pad according to one embodiment; and 
         FIG. 5  shows a different example of a contact pad according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment of the present invention, Mode 2 failure may be reduced or eliminated by slowing down the diffusion of the carrier  114  atoms (e.g. Cu and Ni) through the tin grains in the ball  108  in the case of the second orientation. 
     In one embodiment, Cu—Sn or Ni—Sn intermetallics are nearly uniformly distributed through the solder ball  108 . These intermetallic particles act as barrier boulders to the movement of the solute atoms (Cu or Ni) under the influence of the electric field. For example, when the electric field is applied such that electron flow occurs from BLM  106  to contact pad  112 , the solute atoms tend to move from top to bottom. However, upon impingement of a moving atom (under the influence of the electric field) with intermetallic particles, either the movement of the atom comes to a complete halt or the moving atom must change the direction. If the tin grain is oriented in the first orientation, the impingement of the solute atom results into change of direction. Any change of direction will slow down the moving atom thousands of times. The net result is that the mode 2 type of catastrophic electromigration failure is eliminated. To that end, embodiments of the present invention are directed forming intermetallics in the solder ball  108 . Indeed, controlling the amount of intermetallics in the ball  108  may also allow for the control of the hardness of the ball  108 . 
     In particular, one embodiment includes introducing intermetallics into the ball  108  via thermal processing. In one embodiment, this may be accomplished by exposing the ball  108  to heat (either repeatedly or for a particular amount of time) to control the amount of intermetallics disposed therein. The intermetallics may come from particles in the contact pad  112  in one embodiment. 
       FIG. 2  shows an example of a ball  108  having intermetallics  202  formed therein. The ball  108  is shown coupled to a BLM  106  and to a contact pad  112 . The contact  112  is disposed within a carrier  114 . In one embodiment, the carrier  114  is formed of a ceramic material. 
     In one embodiment, the intermetallics  202  are caused to form in the ball  108  due to repeated exposure of the ball  108  to heat while the ball  108  is coupled to one or both the BLM  106  or the contact pad  112 . In one embodiment, the ball  108  is coupled to the contact back  112  and then exposed to one or more thermal processes that cause the intermetallics  202  to form in the ball  108  before being coupled to the BLM  106 . In such an embodiment, the intermetallics  202  are composed, at least partially, of the materials that form the contact pad  112 . The thermal processes may include exposing the ball to  108  to heat that exceeds a reflow temperature for the ball  108  multiple times or for an extended period of time. For example, the ball  108  may be exposed to heat equal to or exceeding the reflow temperature for greater than 3 minutes. 
       FIG. 3  shows a method according to one embodiment of the present invention. At block  302 , a substrate (e.g., carrier  114 ) is provided. In one embodiment, the substrate is formed of a ceramic material. After the substrate is provided, a contact pad is formed in the substrate at block  304 . In one embodiment, the contact pad is formed of a single layer. In another embodiment, the contact pad is formed of multiple layers of different materials. For example, the contact pad may be formed of sintered Cu or Ni overlaid by a layer of electroless gold. 
     At block  306  a solder ball is brought into contact with the contact pad. The solder ball is a Ti based solder ball in one embodiment. Of course, the solder ball could be formed of any suitable solder material. The solder ball is connected to a BLM at some point before block  306  in one embodiment. Of course, the solder ball could be formed directly on the contact pad before being coupled to a BLM in one embodiment. 
     Regardless, at block  308  the solder ball and the contact pad are exposed to heat sufficient to cause materials in the contact pad to migrate into the solder ball. The exposure to heat may include exposing the contact pad and solder ball to heat multiple times in one embodiment. Of course, in another embodiment, varying the time of single exposure may achieve the same results. In one embodiment, the solder ball is exposed to 10 or more reflow conditions as part of block  308 . In the prior art, only two reflows were typically performed, one to couple the ball of the BLM and another to couple the ball to the contact pad. It will be understood that varying the number of thermal exposures or time of thermal exposure will vary the amount of intermetallic migration and, thus, the hardness of the resultant solder ball. 
       FIG. 4  shows an example of a contact pad  112  disposed in a carrier  114 . In the embodiment, the carrier  114  is formed of a ceramic material. The contact pad  112  of this embodiment is formed of a base sintered layer  402  covered by an electroless gold layer  404 . The base sintered layer  402  is formed of either sintered copper or sintered nickel and is at least partially disposed within the carrier  114 . Of course the electroless gold layer  404  could be omitted in one embodiment. To the extent the electroless gold layer  404  is included it may provide for protection against corrosion and, in one embodiment, has a thickness of about 1000 angstroms. 
       FIG. 5  shows another example of contact pad  112  formed in a substrate  114 . The contact pad  112  of this embodiment includes the base sintered layer  402  covered by an electroless gold layer  404  of  FIG. 4 . In addition, the contact pad  112  includes an electroless copper layer  502  over the electroless gold layer  404  and an additional electroless gold layer  504  over the electroless copper layer  502 . As shown, layers  502  and  504  are above an upper surface  506  of the substrate  114 . Of course, these layers could be even with or below the upper surface  506  in an alternative embodiment. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one ore more other features, integers, steps, operations, element components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 
     The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.