Abstract:
Described is an adhesive composition suitable for bonding a semiconductor device to a substrate comprising about 60-90 wt. % particulate silver; about 10-40 wt. % of an adhesive material comprising at least one nonelectrically conductive organic compound; and a filler comprising at least one electrically conductive organic compound capable of forming a conductivity bridge between silver particles. The adhesive composition may additionally include at least one curing catalyst for the adhesive material, a flexibilizer to render the composition more flexible, a surfactant to facilitate contact with a substrate and/or a material to aid in adjusting the rheology of the composition.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to adhesive compositions suitable for bonding a semiconductor device to a substrate. More particularly, the invention involves polymer-based die attach formulations useful for bonding a semiconductor device to flexible and laminate substrates. 
     Polymer-based die attach adhesives have been used with advantage to bond semiconductor devices to substrates in a variety of applications. The adhesive composition component of such formulations are nonelectrically conductive but are combined with electrically conductive fillers, such as silver, when an electrically conductive die attach adhesive is desired. 
     Electrically conductive polymers may be used in place of silver, however, although available, such conductive polymers by themselves do not present a conductivity advantage compared to silver and pure conductive polymers are normally unstable at process temperatures used for curing and solder reflow. 
     Electrically conductive polymeric compositions, although available, do not present a conductivity advantage compared to silver. Moreover, thermal instability inherent in conductive polymers suggest that they are not suitable for die attach adhesive formulations which generally require heating to elevated temperatures to cure the adhesive component. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an adhesive composition suitable for bonding a semiconductor device to a substrate comprising about 60-90 wt. % particulate silver; about 10-40 wt. % of an adhesive material comprising at least one nonelectrically conductive organic compound; and a filler comprising at least one electrically conductive organic compound capable of forming a conductivity bridge between silver particles, or functionally equivalent materials, unless otherwise indicated herein. The term particulate silver refers to powder or flake. The terms &#34;electrically conductive organic compound&#34; or &#34;electrically conductive filler&#34; are used throughout the present specification to refer to any electrically conductive polymer or any electrically conductive polymer composition. The adhesive composition may additionally include at least one curing catalyst for the adhesive material, a flexibilizer to render the composition more flexible, a surfactant to facilitate contact with a substrate and/or a material to aid in adjusting the rheology of the composition. 
     The electrically conductive filler advantageously comprises a conductive polymer compound which is present in an amount up to about 5 wt. % of the total adhesive formulation. Suitable electrically conductive fillers include the conductive polymers polyaniline, polypyrrole and composites of the conductive polymers on carbon, silicon, aluminum nitride, boron nitride, silicon carbide, etc, supports. The composite form of the electrically conductive filler is desirable because of its enhanced thermal stability over the pure form of the conductive polymer. Therefore, thermal treatments of the adhesive (such as curing, annealing, solder reflow, etc.) will not affect the electrical performance of the formulation. 
    
    
     DETAILED DESCRIPTION 
     As contemplated herein, the adhesive composition with an electrically conductive filler is especially useful for bonding semiconductor devices to plastic and flexible substrates where improved electrical conductivity is desirable. According to the invention electrically conductive polymers are included in the adhesive composition because, though they do not present a conductivity advantage compared to silver, they provide an additional organic filler with conductivity greater than the typical fillers used in die attached adhesives, such as silica, clays, alumina, orceramic materials normally used to control rheology. The electrically conductive fillers also have the advantage of allowing a greater opportunity for a conductivity bridge to form between the silver particlesin the adhesive formulation as well as between silver and electrode interfaces. Furthermore, compositions in accordance with the invention canbe formulated to enable the adhesive to be cured at temperatures as high as200° C. with exhibited temperature stability as high as 300° C. 
     In addition to the forgoing, these electrically conductive fillers are compatible with the resin component of the adhesive. Such polymers have better wetting properties with the monomeric binders than the common inorganic fillers such as silica, clays, alumina, silver, and other ceramic-based fillers. Conductive polymers generally do not have as high conductivity as pure silver (0.03125 vs. 0.00000159 ohms-cm resistivity) and as such, do not serve as a good alternative to silver. However the conductive polymers are much more conductive than insulating materials such as the polymer binding materials (including cyanate esters, epoxies, acrylates, polyolefins, polycyloxanes, urethanes, etc.) and inorganic fillers such as silicon oxide and aluminum oxide which have resistivities of 1×10 21  and 1×10 22  ohm-cm, respectively. Another feature of these material is that they are compatible with the organic-based vehicle or binder in the adhesive formulation and as such appear to maintain suspension in the resin matrix without settling like other fillers such as silver. This suspended nature enhances the probability of formation of conductive bridges mentioned previously and also helps to also maintain suspension of the silver. It has been found that by replacing nonelectrically conductive fillers with a conductive polymer, such as polyaniline and polypyrrole, the conductivity increases at least 10 to 100 times over the conventional silver-filled formulation. In addition, since the surface area of the conductive polymer is much higher than that of the typical particulate silver, powder or flake, filler (about 300 m 2  /g for the conductive polymer vs. about 0.4 m 2  /g for silver), even when used in small quantities the conductive polymer acts to modulate the rheology of the paste as well as increase theconductivity of a paste formed of the adhesive composition. Graphite particles or fibers may also be used as a conductivity enhancement filler,but graphite&#39;s extremely high porous nature and high surface area (greater than 1000 m 2  /g) creates viscosity problems and problems of compatibility with the polymer binders, even in low quantities, and therefore make it less desirable than the conductive polymer fillers used in the composition of the invention. In addition, they may not offer the same suspension stabilizing affect as the conductive polymer fillers. Another advantage of the adhesive composition described is that the conductive polymer appears to help decrease the resin bleed when added to replace up to about 1% silver in the composition. 
     To illustrate the adhesive composition, several formulations were prepared as described in Table 1. These formulations include particulate silver, e.g. flake, cyanate ester as the adhesive material component, a quaternaryammonium surfactant, &#34;Armak&#34;, two monomeric adhesive materials, trimethylopropanetriacrylate, and tetraethyleneglycoldiacrylate, 2,2&#39;- azobisisobutyronitrile (AIBN) catalyst, an accelerant for curing cyanate ester, nonylphenol, a rheology adjusting agent silica, and a commercially available conductive polymer Eeonomer P20-6 F. The surfactant Armak 1363 is available from Akzochemie America, Deer Park, Tex.; Ancamine 2014 as isavailable from Pacific Anchor, Allentown, Pa. and the conductive filler Eeonomer is available from Eeonyx, Pinole, Calif. and is based upon eithera polyaniline or polypyrrole coating on a carbon core. The cyanate ester isArocy L10 from Ciba/Geigy. 
     
                       TABLE 1______________________________________             Sample No.                                 Con-                                 trol               061196.3 061196.4 Wt.Paste ID #          Wt. %    Wt. %    %______________________________________A. Silver Flake     74.34    74.47    75B. Cyanate Ester (Ciba/Geigy L-10)               12.92    12.94    12.9C. Armak (quaternary ammonium surfact-               0.2      0.2      0.2  ant)D. Ancamine (polyamine)               3.37     3.41     3.4E. Trimethylolpropane Triacrylate               3.51     3.51     3.5F. Tetraethyleneglycol Diacrylate               3.5      3.51     3.5G. AIBN,2,2&#39;-azobisisobutyronitrile               0.2      0.2      0.2H. Nonylphenol      1        1        1I. Silica           0.2      0        0.2J. Eeonomer (Conductive Polymer Filler)               0.75     0.75     0______________________________________ 
    
     It has been determined that the conductive filler has the most profound effect as a replacement for conventional silica fillers used for rheological control. It has also been found that rheology, bleed and conductivity are all controllable to some extent by using the conductive polymer filler. 
     Volume resistivity for these samples are shown in Table 2. Three replicate specimens were prepared on glass glides by masking off a strip measuring 0.1&#34; wide by approximately 3&#34;long and 1.2 mil thick. All test strips were made by doctor-blading, then cured at 175° C. for 2 hours. The testslides were initially measured for resistance on a Keithley 4 point probe station, followed by a comparison of multi-positioned (at 2.54 cm) kelvin probes, using a Valhalla resistance meter. 
     
                       TABLE 2______________________________________    Conductive    Polymer   Volume Resistivity                           Volume ResistivitySample ID #    Type      ohms-cm (Keithley)                           ohms-cm (Valhalla)______________________________________061196.3 Polyaniline              0.005099     0.000961061196.4 Polyaniline              0.00131      0.0000998Control  none      0.097578     0.0154______________________________________ 
    
     Since the density of the conductive polymer filler is very low (about 0.062g/cm 3 ), it is believed that keeping the filler dispersed within the formulation in this composition is easier than a similar sized metal powder which would have a tendency to settle. It is hypothesized thereforethat the dispersed conductive polymer filler acts as a conductive bridge between silver. In addition, we have observed the dispersion enhancement when formulating with the conductive polymer filler with silver filled compositions. Because of this dispersion enhancement quality surface contact between the paste and test electrodes is further enhanced. 
     This is especially important when considering the effect of the binder as the higher the insulating effect of the binder on the silver, the greater the need for a co-compatible particle that can break through the insulative layer as well as act as the conductive bridge. Conductive polymers, due to their organic nature can serve that function. Attempts atdissolving the conductive polymer filler resulted in no improvement in conductivity in these formulations. 
     Other examples of the invention are shown below. Three additional series offormulations were made as described in Tables 3, 4 and 5. Compositions in Table 3 were made from production samples of a die attach formulation &#34;JM 2000&#34; available from Johnson Matthey, Inc., San Diego, Calif.; Table 4 describes formulations of cyanate ester and epoxy and Table 4 describes formulations of cyanate ester and a second epoxy. Components A through J where used in Tables 3, 4 and 5 correspond to materials labels A through Jin Table 1. 
     
                       TABLE 3______________________________________Paste ID #     A      B      C   D   E   F   G   H   I   J______________________________________1473-60  (%)    75     12.9 0.2 3.4 3.5 3.5 0.2 1.0 0.2 01473-60A  (%)    74.25  12.9 0.2 3.4 3.5 3.5 0.2 1.0 0.2 0.75______________________________________ 
    
     
                       TABLE 4______________________________________                           CopperPaste ID #    A      B       BisF Epoxy                           Acetylacetonate                                     J______________________________________XO2244 %     75     13.7  11.24   0.06      0XO2246 %     74.65  13.489                     11.059  0.062     0.74______________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________Paste ID #  B  BisA Epoxy           BisF Epoxy                 HT 939                     C  A187                           A  J__________________________________________________________________________XO2243% 6.075     7.6   7.6   3.425                     0.125                        0.15                           75.0                              0XO2245% 6.081     7.608 7.608 3.428                     0.125                        0.15                           74.25                              0.751473-78A% 6.081     7.608 7.608 3.428                     0.125                        0.15                           74.25                              0.75__________________________________________________________________________HT939 = a polyamide from CIBA/GEIGYA187 = an epoxy silane coupling agent from Witco, Osi Specialities Group (Eudicott, NY)Armak = quaternary ammonium surfactant from Akzo Chemicals (Chicago, Ill)Note: All conductive polymers obtained from Eeonyx (Pinole, CA) and have a carbon core structure 
    
     Volume resistivity was measured and shown in Table 6. An even greater enhancement of conductivity was observed in the production sample, which may reflect differences in manufacturing processes from those samples in Tables 1 to 2. 
     
                                           TABLE 6__________________________________________________________________________                  Volume ResistivitySample #Resin Type      Conductive Polymer Type                  (Keithley)                           Volume Resistivity (Valhalla)__________________________________________________________________________1473-60JM2000      None        35900 ohm-cm                           52700 ohm-cm1473-60AJM2000      Polyaniline  0.019    0.0186XO2243CE/Epoxy B      None        off-scale                           off-scaleXO2245CE/Epoxy B      Polyaniline 69       741473-78ACE/Epoxy B      Polypyrrole 20.7     23.7__________________________________________________________________________ 
    
     Back-side resistance was measured using a gold lid (500 mil×500 mil) attached to a gold coated ceramic substrate or gold like substrate using abondline of the adhesive or approximately 0.5 to 1.0 mi. The adhesive is cured and conductivity tested using the two gold surfaces as the electrodes. The results are reported in Table 7 and Table 8. In Table 7 a 30 minute cure was used and in Table 8 a 120 minute cure was used. The Simpson is another resistance meter which is a more sensitive instrument than either the Keithley or Valhalla. 
     
                                           TABLE 7__________________________________________________________________________                               Resistance (Valhalla)Sample #Resin Type      Conductive Polymer Type                  Resistance (Simpson) OHMS                               OHMS__________________________________________________________________________1473-60JM2000      None        0.047-1.15   --1473-60AJM2000      Polyaniline 0.0119-0.0697                               --XO2244CE/Epoxy A      None        off-scale     40 × 10.sup.6XO2246CE/Epoxy A      Polyaniline off-scale    0.1 × 10.sup.6XO2243CE/Epoxy B      None        10.9-12.0    11.7-13.4XO2245CE/Epoxy B      Polyaniline 1.19-2.00    1.15-1.951473-78ACE/Epoxy B      Polypyrrole 3.71-5.14    --__________________________________________________________________________ 
    
     
                                           TABLE 8__________________________________________________________________________                               Resistance (Valhalla)Sample #Resin Type      Conductive Polymer Type                  Resistance (Simpson) OHMS                               OHMS Range__________________________________________________________________________1473-60JM2000      None        0.075-2.3    --1473-60AJM2000      Polyaniline 0.014-0.062  --XO2244CE/Epoxy A      None        off-scale    off-scaleXO2246CE/Epoxy A      Polyaniline off-scale    0.72-1.9 × 10.sup.5XO2243CE/Epoxy B      None         7.7-10.3     8.9-10.9XO2245CE/Epoxy B      Polyaniline 3.2-5.3       3.1-5.31473-78ACE/Epoxy B      Poiypyrrole 2.0-6.1      --__________________________________________________________________________ 
    
     It is apparent from the forgoing that various changes in modifications may be made without departing from the invention. Accordingly, the scope of the invention should be limited only by the appended claims, wherein what is claimed is: