Abstract:
A microcircuit package having a ductile layer between a copper flange and die attach. The ductile layer absorbs the stress between the flange and semiconductor device mounted on the flange, and can substantially reduce the stress applied to the semiconductor device. In addition, the package provides the combination of copper flange and polymeric dielectric with a TCE close to copper, which results in a low stress structure of improved reliability and conductivity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 60/858,020, filed on Nov. 9, 2006, the disclosure of which is incorporated by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     Microcircuit packages are known for containing a semiconductor device or circuit but are relatively costly to achieve acceptable levels of reliability and performance. The manufacturing goal is to produce a microcircuit package having a high performance at a low cost. However, microcircuit packages of presently known construction cannot achieve intended performance levels at a low cost. In general, known microcircuit packages employ a ceramic material to provide high thermal performance and high reliability. 
     To achieve low cost, high thermal performance and high reliability for a microcircuit package which will contain a semiconductor device or circuit, the following criteria should be met:
         1. High thermal conductivity in a low cost base material;   2. Low cost insulator material with a thermal coefficient of expansion (TCE) match to the base material; and   3. High thermal performance die attach.       

     Many high performance microcircuit packages are fabricated using ceramic dielectric materials with thermal dissipation structures using “flanges” of materials which have matched TCEs. Typical materials incorporated into ceramic packages as flanges include copper-tungsten, copper/molybdenum clad structures, and aluminum-silicon carbide (AlSiC). These materials have an advantage of TCEs fairly close to that of the semiconductor devices. Semiconductor devices typically have thermal coefficients of expansion in the range of 2.8-4.0 ppm/° C. The aforementioned flange materials have TCE values in the range of 6.0-10.0 ppm/° C. TCE values below 10.0 ppm/° C. are desirable so that expansion and contraction during temperature extremes do not cause high levels of stress to the semiconductor device which can cause the device to crack. The deficiencies in these materials are that the thermal conductivities are fairly low, i.e., in the range of 150-240 W/mK (Watt per meter Kelvin), and the cost of these materials is high. 
     A better flange material would be copper or a copper alloy for at least the following reasons. Copper is a material which is commonly available, has a low cost, and can be fabricated using high volume manufacturing techniques such as stamping. Also, copper and copper alloys have a thermal conductivity in the range of 350-400 W/mK. A technical barrier to using copper for flanges in these applications has been the fact that copper and copper alloys have a high TCE (about 17-20 ppm/° C.). This large difference between the TCEs of copper and that of semiconductor devices has resulted in large stresses applied to the semiconductor devices which can cause a failure during operation. In addition, conventional dielectric materials used for this application are ceramic based. The ceramic material has a TCE in the range of about 6-8 ppm/° C., and the combination of the traditional ceramic dielectric and copper flange result in a large mismatch of TCE and results in excessive warpage or cracking of the dielectric. 
     To minimize the effects of large stresses being applied to the semiconductor devices in conventional packages having copper flanges, one prior art approach employs an adhesive for the die attach. This allows use of a more ductile die attach but has a substantial drawback in that the adhesive has a very low thermal conductivity which limits the performance of the die attach. Another prior art approach uses high lead solder for the die attach, which allows use of a more ductile solder, but the high lead solder is a problem due to environmental issues. A further prior art approach uses a thick layer of gold, typically 300 micro-inches, applied to the backside of the semiconductor die, which allows for a buffer layer of gold on the die, but the thick layer of gold adds considerable cost to the product. A gold layer has been used on the backside of a gallium arsenide die which is soldered with AuSn eutectic solder to a copper substrate, but this approach has traditionally been limited to small devices &lt;3 mm on a side, and has been limited to devices which have a substantially square shape. 
     As previously noted, the TCE mismatch between the semiconductor device and the flange material results in failure of the semiconductor device or the die attach by reason of the stress induced by the TCE mismatch. In addition, when a ceramic dielectric material is used with a copper flange, the mismatch in TCE between ceramic and copper can cause large stresses to be developed in the structure, which results in excessive warpage or cracking of the dielectric. When the semiconductor device is soldered to the flange, the temperature of the solder at a liquidus point is 280° C. for gold-tin alloys, or 368° C. for a gold-silicon eutectic composition. For these eutectic compositions, the solder turns into a solidus at the aforementioned temperatures. At this solidus point a top layer of the flange material is frozen, and cooling to room temperature causes a bottom portion of the flange to contract more than the top portion, causing the flange to bend into a concave shape. This concave shape subjects the semiconductor device to a bending stress, and such a tensile stress in the semiconductor device can cause a failure of the device. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a reliable microcircuit package having the above-noted preferred criteria by use of a ductile layer between the copper flange and the die attach. The ductile layer absorbs the stress between the flange and semiconductor device, and can substantially reduce the stress applied to the semiconductor device. In addition, this invention provides the combination of copper flange and polymeric dielectric with a TCE close to copper. The polymeric material has a TCE about 17 ppm/° C. which is a closer match with copper. This combination results in a low stress structure that is robust when temperature cycled and which also demonstrates low cost and high thermal performance. In one example, the stress can be reduced by up to 40%. This invention, therefore, provides a microcircuit package having improved reliability and a significant improvement in thermal conductivity. In one example, the thermal conductivity can be improved by a factor of about 2. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will be more fully described in the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a perspective view of a circuit package, without a lid, according to one embodiment of the present invention; and 
         FIG. 2  is a perspective view of the circuit package of  FIG. 1  with a lid attached thereto; and 
         FIG. 3  is a diagrammatic elevation view of a circuit package in accordance with the invention illustrating the several layers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     One embodiment of a microcircuit package in accordance with the invention is shown in  FIG. 1 . The circuit package  100  includes a flange  102 , a frame  104  and two leads  106  and  108  extending from respective sides of the package. The frame  104  electrically insulates the leads  106  and  108  from the flange  102  and from each other. A semiconductor die  110  is attached to a die attach area  112  within the area defined by the frame  104 . The die  110  is attached to the die attach area  112  by a eutectic or other appropriate solder  114 . In the illustrated embodiment only one die is shown, although two or more dies can typically be attached to the die attach area  112  in accordance with application and user requirements. 
     The eutectic solder  114  electrically bonds the die  110  to the confronting surface of the flange  102 . The leads  106  and  108  are connected to contact areas of the die  110  by wire bonded leads  120  and  122 . A lid  200  is attached to the confronting periphery of the frame  104  to enclose the die, as illustrated in  FIG. 2 . 
     The flange  102  forms a base to which other parts of the circuit package are attached, and also serves as a heat sink to conduct heat from the one or more semiconductor dies mounted in the package. The flange is preferably made of copper or a high copper alloy to provide high electrical and thermal conductivity. The frame  104  is made of an injection molded thermoplastic and is molded to the flange  102  and to the leads  106  and  108 . 
     The frame material is preferably a liquid crystal polymer (LCP) that can withstand die attach temperatures which typically are 280-330° C. for AuSn soldering, or 390-420° C. for AuSi soldering. Preferable high temperature LCP frame materials are further described, for example, in Applicant&#39;s prior U.S. Pat. No. 7,053,299. The high temperature polymeric material can have a composition which includes one of the following chemical groups: hydroquinone (HQ), 4,4 bisphenol (BP), bis (4-hydroxylphenyl ether) (POP), terephthalic acid (TPA), 2,6 naphthalene dicarboxylic acid (NPA), 4,4 benzoic acid (BB), 4-hydroxybenzoic acid (HBA), 6-hydroxy-2-napthoic acid (HNA). 
     The leads  106  and  108  are preferably made of an alloy of copper which may be of many alternative compositions such as those described in the aforesaid U.S. Pat. No. 7,053,299. The copper alloys include those known under the UNS designations C19400, C15100, C19500, C19700, C50710, C19210, C19520, C18070, C19010, C70250, EFTEC-64T, KLF-25 and MF224. 
     In accordance with the present invention, a ductile layer is provided between the flange and the die attach. The multi-layer structure of a preferred embodiment is illustrated in  FIG. 3 . Referring to  FIG. 3 , the microcircuit package comprises a flange or substrate  200  of copper or copper alloy, having a ductile layer  210 , typically of copper or silver, applied on a surface of the flange. A barrier layer  212  of nickel or nickel cobalt is applied over the ductile layer, and a gold layer  214  is applied over the nickel layer. A eutectic solder  216 , typically of gold-tin (AuSn), gold-silicon (AuSi), or gold-germanium (AuGe)is applied over the gold layer, and one or more semiconductor dies  218  are attached to the eutectic solder. The semiconductor dies can be fabricated from materials such as silicon, gallium arsenide, gallium nitride or any other suitable semiconductor material. 
     The flange  200  typically has a thickness in the range of about 0.040-0.060 inches. The ductile layer  210  has a thickness in the range of about 100-500 micro-inches. The barrier layer  212  has a thickness in the range of about 100-200 micro-inches. The semiconductor dies  218  typically have a thickness in the range of about 0.002-0.010 inches. The thickness of the gold layer  214  will depend upon the type of eutectic solder employed. For gold-tin (AuSn) solder, the gold layer on the flange has a thickness in the range of about 30-50 micro-inches. 
     In another embodiment, for a package having gold-tin solder, a gold layer of about 25 micro-inches is applied over a palladium layer of about 5 micro-inches. 
     When a eutectic solder of gold-silicon (Ausi) or gold germanium (AuGe) is used, the gold layer has a thickness in the range of about 100-200 micro-inches. 
     The eutectic solder can be a lead-free solder such as tin-silver-copper (SnAgCu), tin-silver (SnAg), antimony-tin (SbSn), tin-zinc (SnZn), bismuth (Bi) and tin-indium (SnIn). For use with these lead-free solders, a layer of nickel is applied at a thickness of about 150 micro-inches over the ductile layer. Optionally, a “flash” coating of about 5 micro-inches of gold can be applied over the nickel layer to prevent oxidation of the nickel. 
     The ductile layer  210  can be copper, silver, or an alloy of copper and silver, and the ductile layer can be provided in several different ways such as by plating, cladding, evaporation, and sputtering. The ductile layer has hardness less than about 80 Knoop and a thickness in the range of about 100 to 1000 micro-inches, and preferably in the range of about 100 to 500 micro-inches. 
     For efficient manufacturing, the flange  200  can be made of a harder form of copper or other suitable material, which is more resistant to damage during manufacturing such as scratches, nicks, and the like. A preferred hardness is greater than 80 Knoop and preferably in the range of about 85-100 Rockwell F. 
     In a preferred embodiment the flange  10  can be made of a copper zirconium alloy (CDA 151) which has hardness in the intended range of 85-100 Rockwell F. 
     As a comparison, the widely used oxygen-free copper (CDA101/102) is very soft and is prone to nicks, scratches and the like and has an annealing temperature of about 350° C. In contrast, CDA 151 copper has an annealing temperature greater than 500° C. and a significantly greater hardness. Thus, the harder flange material is more stable and suitable for efficient manufacturing processes. 
     The invention is not to be limited by what has been particularly shown and described but is to encompass the full spirit and scope of the claims.