Abstract:
A module or assembly is formed by interposing a polymer between a carrier and a semiconductor device to be secured to the carrier. The polymer has ionized metallic particles suspended in it. Before setting or curing the polymer, the module is exposed to an electric field which induces migration of the metallic particles to the opposing pads of the carrier and semiconductor device. Such migration ultimately forms metal dendrites extending between mating pad pairs. The dendrites establish a metallurgical bond and conductive paths between the carrier and the overlying semiconductor device. When the polymer is subsequently set, the carrier and device are not only adhered to each other, but the dendrite connections are fixed and structurally reinforced to provide the needed electrically conductive paths.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to electronic packaging and, more particularly, to a semiconductor module and related method for electrical interconnection between a chip and the substrate of the module.  
         BACKGROUND OF INVENTION  
         [0002]    One common method for electrically interconnecting a semiconductor device to an associated carrier or substrate is the flip chip attachment method. Flip chip attachment generally is accomplished by controlled collapse chip connection (“C4”). In the C4 method, small solder balls are reflowed to form the connection between matching metallized input/output (I/O) pads on both the chip and carrier mating surfaces.  
           [0003]    The C4 attachment technology, as practiced today, has several limitations: (1) sensitivity of the current practice to irregularities in the carrier surface; (2) the requirement for a joining temperature in excess of 300° C. to melt the high-lead-content solder generally used; (3) relatively complex and costly processing steps; and (4) the general need to use lead-based solder (which may be regulated in the future due to environmental legislation).  
           [0004]    Other electrical interconnect methods are available, but are either not well-suited for certain applications or suffer from their own drawbacks and disadvantages. For example, U.S. Pat. No. 5,045,249 issued to Jin et al. teaches electrical interconnection through a polymer medium in which electrically conductive magnetic particles are aligned using a strong magnetic field. The particles are made to penetrate the surfaces of the polymer film, offering conductive paths to mechanically connect to mating pads. One disadvantage to this method is that the connections formed by the method are merely mechanical (the connections are formed by contact of adjacent surfaces) rather than metallurgical (connections formed by fusing adjacent metal surfaces together at a molecular level). Such mechanical connections are generally not sufficiently reliable for many applications.  
           [0005]    U.S. Pat. No. 5,019,944 issued to Ishii et al. teaches attaching pre-existing, metallic contacts to the pads of one of the opposing surfaces to be joined, and then pressing these conductive contacts through an uncured polymer film and against corresponding mating pads of the other opposing surface to be joined. To maintain the contact, the polymer is cured. Again, this process relies on a less reliable mechanical bond rather than a metallurgical bond. Furthermore, the ability to make robust connections under this method is likely to be sensitive to irregularities in one or both of the mating surfaces.  
           [0006]    U.S. Pat. No. 4,548,862 issued to Hartman teaches a process for building a pressure-sensitive adhesive film. Anisotropic conductive paths are fabricated in the film through use of a magnetic field to align pre-existing particles embedded in the film before curing. Once again, this method disadvantageously provides only a mechanical rather than a metallurgical bond.  
           [0007]    Another electrical interconnection method is disclosed in the IEEE publication titled “Development of High Conductivity Lead (Pb)-Free Conducting Adhesives,” by Kang et al., in IEEE Transactions on Components, Packaging, and Manufacturing Technology-Part A, Vol. 21, No. 1, pages 18-22 (March 1998). The method disclosed uses tin-coated metal particles embedded in a thermoplastic polyimide-siloxane copolymer material that can be heated to allow the tin on the particles to melt and combine metallurgically with the mating I/O pads and each other. As the thermoplastic is cooled, the polymer solidifies, freezing in place the joined particles. This practice still requires relatively high-temperature reflow to melt the tin-based particles. Furthermore, the volume of particles available for electrical connection is limited because the process erects no barrier to lateral particle interconnect that can cause shorting between pads. The need to limit metal particle volume, in turn, limits the conductivity achievable by this approach.  
           [0008]    There is thus a need for a relatively low-cost semiconductor module and related manufacturing method for interconnecting a chip of such a module to the carrier of the module. There is a further need for the module to have robust, metallurgical connections as opposed to mechanical bonds or connections. There is a still further need for the interconnections to be tolerant of surface irregularities. There is yet a further need for the interconnections to be accomplished with less or without lead-based solder.  
         SUMMARY OF THE INVENTION  
         [0009]    To meet these and other needs, and in view of its purposes, the present invention provides a method for assembling a carrier and a semiconductor device to each other. A die-attach polymer is used to adhere the device and carrier and electrically interconnect those components. The die-attach polymer includes a low density of ionized metallic particles and is applied to one or both of the surfaces to be mated to each other.  
           [0010]    The resulting assembly is exposed to an electric field of sufficient strength to produce a controlled migration of the metallic particles to the pads of the semiconductor device, the carrier, or both components. Such migration forms anisotropic metal dendrites extending from the pads. The dendrites establish a conductive path and a metallurgical bond between the carrier and the device. Once sufficient conductive paths have been formed by the dendrites, the polymer is caused to set so as to structurally reinforce and electrically insulate from each other the metal dendrites of adjacent pads.  
           [0011]    In one preferred embodiment of the present invention, the die-attach polymer is formed with ionic metallic particles in a size range of about 1 to about 25 micrometers, and in a volume fraction of about 5% to about 20%, which has been found to minimize instances of lateral conduction. The metallic particles may be silver, copper, or nickel, although other metals are also suitable. The polymer is advantageously applied as a film to the surface or surfaces to be joined.  
           [0012]    In accordance with another aspect of the present invention, a small concentration of a metallic salt is also added to the polymer, preferably in a concentration of about 0.1% to about 1.0% by weight of the metallic particles in the polymer. In accordance with still another aspect of the invention, a structure is added to inhibit certain undesired migrations of the metallic particles during the application of the electric field. One form of this inhibiting structure is a planar spacer of insulating material with passages through the mater al at locations corresponding to the pads. The spacer is placed between the carrier and the semiconductor device, and the polymer is applied in a manner to enter the passages of the spacer.  
           [0013]    In accordance with yet another aspect of the present invention, some or all of the pads to be joined to each other have portions extending outwardly from the plane of the corresponding mating surface. The dendrites which are formed by the electric field complete the required conductive paths more quickly, or can be formed with less field intensity, when they are formed on such protruding pads.  
           [0014]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]    The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:  
         [0016]    [0016]FIG. 1 is a side sectional view of a module or assembly in accordance with the present invention at an intermediate stage of completion;  
         [0017]    [0017]FIG. 2 is a side sectional view of the assembly of FIG. 1 after further processing according to the present invention; and  
         [0018]    [0018]FIG. 3 is a top, plan, partial cut-away view of an alternative embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    Referring now to the drawing in general, and to FIGS. 1 and 2 in particular, a semiconductor module or assembly  21  includes a carrier  23 , such as a ceramic substrate, and a semiconductor device  25 , such as a chip, secured to carrier  23 . Carrier  23  has a carrier mating surface  35  with an array of associated carrier pads  37  defined on the carrier mating surface  35 . The semiconductor device  25 , in turn, has a semiconductor mating surface  39  and a corresponding array of device pads  41 . For ease of illustration, only two mating pairs  42  of carrier pads  37  and device pads  41  are shown in FIGS. 1 and 2.  
         [0020]    Semiconductor mating surface  39  is positioned or placed in an opposing relationship to carrier mating surface  35 , with polymer  27  interposed between the semiconductor mating surface  39  and the carrier mating surface  35 . More specifically, carrier  23  and device  25  are positioned relative to each other so that device pads  41  oppose corresponding carrier pads  37 . In this embodiment, device pads  41  are substantially vertically aligned, as shown in the drawing, with corresponding carrier pads  37 .  
         [0021]    Significantly, polymer  27  has ionized metallic particles suspended in it. As shown in FIG. 1, polymer  27  has not been cured or set, and the metallic particles are suspended substantially randomly in the polymer  27 . An electric field  45 , shown schematically by the indication of applied voltage, is applied to assembly  21 . As detailed below, the suspension of metallic particles, the characteristics of the electric field  45 , and the time of exposure of the assembly  21  to the electric field  45  have been formulated or devised so as to cause certain of the metallic particles to migrate within the polymer  27  toward carrier pads  37  and device pads  41 , referred to collectively as pads  33 .  
         [0022]    The exposure to the electric field  45  causes the metallic particles to migrate into concentrated regions  29  (FIG. 1), and to disassociate from uniform suspension sufficiently to form multiple dendrites  31  extending anisotropically between mating pairs  42  of pads  33 . Continued exposure causes continued particle migration and disassociation until dendrites  31  grow sufficiently to electrically connect pads  33  of the mating pad pairs  42  to each other. The dendrites  31  thus formed between mating pad pairs  42  constitute metallurgical bonds  43  between the mating pad pairs  42 . Bonds  43  are “metallurgical” in the sense that metal particles have been concentrated and fused together at a microscopic level to form a seamless or boundary-less connection between the two pads of pad pairs  42 . Bonds  43  also create corresponding conductive paths between the carrier  23  and device  25 .  
         [0023]    Further processing is done to set, “freeze”, or otherwise cure polymer  27 . Once set, polymer  27  structurally reinforces and electrically insulates the plurality of metal dendrites  31  forming the metallurgical bond and the electrical connection between carrier pads  37  and device pads  41 .  
         [0024]    One preferred method for assembling carrier  23  and semiconductor device  25  in accordance with the present invention is now further detailed. An appropriate polymer  27  is formulated using any of a large number of commercially available die attach polymer materials as a basic matrix, including epoxies, the cyanide esters, and thermoplastics such as a polyimide-siloxane copolymer. Metallic particles, preferably in a size range of about 1 to about 25 micrometers, are added in a volume fraction sufficiently low to avoid formation of lateral conduction paths. Such lateral conduction paths between adjacent ones of either carrier pads  37  or device pads  41  may cause “shorting” of the assembly  21 .  
         [0025]    A suitable fraction of particles ranges between about 5% to about 20% by volume. Silver is one metal suitable for the metallic particles, but other metals known to migrate under electrical fields are likewise suitable. Copper and nickel are two alternate choices.  
         [0026]    In the preferred embodiment, a small concentration of an organic salt of the chosen metal is also added to improve the kinetics of migration. One suitable salt is silver acetate when using silver particles. Another is silver pyrophosphate, selected for having low solubility in water but high solubility in alcohol and organic media. The salt assists in ionic mobility of the polymer particles before setting the polymer, and concentrations ranging from about 0.1% to about 1.0% by weight of the metal particle filler fraction are preferred.  
         [0027]    The polymer  27  formulated as described above is then dispensed onto one of the two mating surfaces: carrier mating  35  or semiconductor mating surface  39 . In this embodiment, polymer  27  is applied as a film to carrier mating surface  35 , which has processing advantages when carrier  23  is to receive multiple semiconductor devices  25  on it, such as in a multi-chip version of assembly  21 . A volume of polimer  27  sufficient to give a final film thickness of 1-5 mils is preferred. Although thicknesses beyond the preferred thickness are also suitable, thicker films may require extensive processing time to grow the dendrites  31 ; films too thin may be of insufficient compliance to support the mismatched strains generated by different thermal coefficients of expansion in chip and carrier materials. Film application can be achieved in any number of ways, from a droplet or pattern that is flattened during chip placement to a doctor-bladed film.  
         [0028]    Once the carrier  23  is prepared as above, semiconductor device  25 , with matching metallized I/O device pads  41 , is aligned over the carrier pads  37  of carrier  23 . Semiconductor device  25  is placed with a slight compressive load (0.1 to 1 gram per I/O pad is recommended) to assure good contact of the device pads  41  with the die attach polymer  27 . Any of a number of commercially available chip placement tools can be used in this align-and-place operation of device  25 .  
         [0029]    The module or assembly  21  described above is then placed in the uniform electric field  45 , typically created by two parallel plates  49  at different voltages, such that field lines  47  (FIG. 1) will occur substantially parallel to the carrier mating surface  35  and carrier semiconductor mating surface  39 . Field strengths on the order of 2-10 Volts/mil are appropriate to induce sufficiently rapid metal migration in the pre-set die attach polymer  27  described above. One preferred way to generate and apply the electric field  45  is as part of a conveyor belt system for high manufacturing throughput. Electric field  45  is effective whether formed from direct current or alternating current.  
         [0030]    In this process, metal particles will migrate in such a manner as to complete the conductive path between the mating pad pairs  42 . This migration will occur by metal disassociating from the matrix of the polymer and bridging the gaps both between metallic particles and between metallized pad and the particles. In particular, after a suitably long exposure to the electric field  45 , the metal particle disassociation and bridging form dendrites  31  of sufficient length so that they extend between opposing pairs of pads  42 . The dendrites  31  extend generally perpendicular to electric field lines  47 , that is, the dendrites grow anisotropically. Dendrites  31  formed by this process comprise both electrical and metallurgical connections to mating chip and carrier pad pairs  42 . The process is continued such that many substantially parallel conductive dendrites  31  form conductive paths and metallurgical bonds for each I/O pad pair  42 , as shown schematically in FIG. 2.  
         [0031]    The process of dispensing polymer  27  and inducing sufficient migration of metallic particles in polymer  27  is preferably done at temperatures in which polymer  27  is not set, that is, in a “pre-set” form, as shown in FIG. 1. For epoxy-based polymers, temperatures below curing temperatures are used. If a thermoplastic is used as the polymer  27 , with metal particles suspended in polymer  27 , the process of dispensing such thermoplastic polymer and inducing migration of metallic particles is done at a temperature above the set point so the material is fluid, typically in the range of about 125° C. to about 175° C.  
         [0032]    The electric field lines  47  are modified by the presence of the conductive metal pads  33  in such a fashion as to concentrate dendrite growth between mating pad pairs  42 , rather than laterally between adjacent carrier pads  37  or device pads  41 . Although the process time will be a function of the specific materials, metal fraction, salt fraction, film thickness, and electric field characteristics, metal migration kinetics indicate that process time ranges from 30-1,000 seconds.  
         [0033]    Once sufficient metallurgical and conductive bonds have been formed between mating pad pairs  42 , polymer  27  is caused to set or cure by appropriate application of light, heat, cold, or like processing. The setting step is a cure, as for an epoxy, or simply cooling the material to room temperature, in the case of a thermoplastic resin. Typical epoxy cures are in the 130° C. to 150° C. ranges for 0.5 to 3 hours.  
         [0034]    Once set, the polymer  27  structurally reinforces and electrically insulates the metal dendrites  31  to preserve the required connections between carrier  23  and semiconductor device  25 . The polymer  27  thus offers a compliant and robust interface similar in strength to die attach and underfill adhesives currently in use in the microelectronics industry, but with the various advantages over current interconnections apparent from the description set out in this document.  
         [0035]    The metallized pads  33  on the carrier  23  and device  25  to be mounted on the carrier  23  are generally comparable in structure and composition to those used for soldering or wire bonding. For example, an overcoating of copper, aluminum, nickel, or gold is suitable. A preferred embodiment is copper-gold on the device  25  when in the form of a chip, and copper-gold or nickel-aluminum on the carrier  23 .  
         [0036]    [0036]FIG. 3 shows an alternative embodiment of the present invention, in which a cured polymer sheet or spacer  128  has been interposed between carrier  123  and device  125 . Cured polymer sheet or spacer  128  is formed with passages or holes  130 , such as by punching. Holes  130  match the array of mating chip and carrier pads  133 . The array of passages or holes  130  is aligned with the array of pads  133 . Instead of a uniform film of polymer  27  as in the embodiment shown in FIGS. 1 and 2, polymer  127  with metal particles suspended in polymer  127  is placed into holes  130  at a suitable point during processing so as to fill holes  130 .  
         [0037]    Holes  130  are defined by walls  131  which extend between the opposing mating surfaces of carrier  123  and device  125 . Carrier mating surface is shown at  135 . Preferably, holes  130  are sized so that walls  131  are outside of the perimeters  134  of corresponding pads  133 . At a minimum, walls  131  are coextensive with perimeters  134 . Walls  131  are thereby positioned relative to perimeters  134  so that portions of walls  131  extend between the perimeters  134  of adjacent pads  133  on either carrier  123  or device  125 .  
         [0038]    As such, when the assembly  121  is subjected to electric field  145 , walls  131  inhibit lateral migrations of metallic particles between adjacent pads  133  on carrier  123  and device  125 . As discussed previously, such lateral migrations are undesirable because they risk forming conductive paths between adjacent pads  133  which, in turn, may cause electrical “shorts.” 
         [0039]    Holes  130  are shown as substantially circular in cross-section but may have any cross section, so long as the corresponding walls  131  are coextensive with or outside of the outer perimeters  134  of corresponding pads  133 . The formulation of polymer  127  is similar to that of polymer  27 , except that the structures inhibiting lateral migration allow greater concentrations of suspended particles without risk of generating shorts. Such increased fraction of metallic particles is in the range of about 50% to about 70% by volume. The length of exposure to the electric field  145  is also correspondingly shortened, because sufficient conductive paths are formed more quickly from the higher concentrations of metallic particles.  
         [0040]    The polymer spacer  128  is preferably formed from polyimide material. Spacer  128  also preferably includes a contact adhesive on the surfaces of the spacer to be adhered to the opposing, mating surfaces of carrier  123  and device  125 . In this embodiment, spacer  128  is placed upon carrier mating surface  135  of carrier  123 , and the polymer  127  is then doctor-bladed over the spacer  128 , filling the holes  130 . Semiconductor device  125  is placed with its semiconductor mating surface  139  opposing and aligned with carrier mating surface  135  so that device pads  141  engage carrier pads  137 . Except for the differences noted above, the resulting module or assembly  121  is further processed substantially as described with reference to the preceding embodiment shown in FIGS.  1 , and  2 .  
         [0041]    As a further alternative embodiment, the metal conductive pads  33 ,  133  may be formed to protrude from the plane of the surfaces on which the pads are defined. Such protrusions may be in the form of prefabricated dendrites or any other extending structure, with the result that field lines  47  are further concentrated. Such further concentration favors dendrite growth between mating pads and shortens the processing time.  
         [0042]    In addition to the advantages apparent from the foregoing description, the interconnection method and resulting structure are relatively simple and cost-effective to implement. As a further advantage, the dendrites  31  of the present invention form robust, metallurgical connections as opposed to mechanical bonds or connections. In addition, such interconnections are not only tolerant of surface irregularities on the conductive pads,  33 ,  133 , but such irregularities may in fact enhance dendrite growth according to the present invention. As still another advantage, the method and resulting module or assembly  21 ,  121 , are practiced and built, respectively, with reduced use of lead-based solder during assembly of device  25 ,  125  to carrier  23 ,  123 .  
         [0043]    Although illustrated and described above with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.