Patent Publication Number: US-9837328-B2

Title: Semiconductor device packages

Description:
RELATED APPLICATION 
     This application is a continuation of co-pending, U.S. patent application Ser. No. 14/068,496, filed on Oct. 31, 2013. 
    
    
     BACKGROUND 
     This disclosure relates generally to semiconductor device packaging, and more specifically, to a low-temperature process for fabricating high temperature and high performance semiconductor device packages. 
     A variety of semiconductor device packages include a combination of ceramic, organic, and metallic materials. In order to form a usable structure for the semiconductor device package, these differing materials are in contact with one another. These differing materials often have significantly different material properties that can cause failures of a semiconductor device package incorporating the materials. It is therefore desirable to have semiconductor device packages that incorporate materials having differing material properties but that are not subject to failures due to the differing material properties (e.g., coefficient of thermal expansion). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
         FIG. 1  is a simplified block diagram illustrating a cross-sectional view of an air cavity package. 
         FIG. 2  is a simplified block diagram illustrating a plan view of the air cavity package. 
         FIG. 3  is a simplified block diagram of a cross sectional view of a system at a stage in processing in which device die are coupled to metal coins or slugs embedded in a PCB or other package substrate. 
         FIG. 4  is a simplified block diagram of a cross sectional view of a system at a stage in processing in which device die are coupled to sintered silver vias in a PCB or other package substrate. 
         FIG. 5  is a simplified block diagram of a cross sectional view of a system at a stage in processing in which a copper slug is incorporated in a sintered silver large via. 
         FIG. 6  is a simplified block diagram of a cross sectional view of a system at a stage in processing in which a pair of metal slugs are incorporated in a sintered silver large via. 
         FIG. 7  is a simplified block diagram of a cross sectional view of a system at a stage in processing in which a passive component is incorporated in a sintered silver large via. 
     
    
    
     The use of the same reference symbols in different drawings indicates identical items unless otherwise noted. The figures are not necessarily drawn to scale. 
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide for a semiconductor device package that incorporates a combination of ceramic, organic, and metallic materials that are coupled using silver. The silver is applied in the form of fine particles (e.g., nano particle silver) under pressure and a low temperature (e.g., sintering at 250° C.). After application, the silver forms a solid that has a typical melting point of silver (i.e., approximately 962° C.), and therefore the finished package can withstand temperatures significantly higher than the manufacturing temperature. Further, since the silver is an interfacial material between the various combined materials, the effect of differing material properties between ceramic, organic, and metallic components, such as coefficient of thermal expansion, is reduced due to low temperature of bonding and the ductility of the silver. In other embodiments, the silver can be used in place of or in conjunction with copper slugs in printed circuit boards for attachment of heat sinks or large through vias. Such embodiments provide a thinner PCB than is typical for included copper slugs. 
     Different types of high performance semiconductor device packages incorporate a combination of ceramic, organic, and metallic materials. But differing property characteristics of these materials can cause failures in the resultant package. For example, widely differing coefficients of thermal expansion between ceramic and copper can cause high stress in the package that can cause warping and cracking of the package near the connection between these materials. 
     One example of such packages is an air cavity package that typically includes one or more semiconductor device die attached to a base plate and an insulative window frame surrounding the die. A cap is placed over the window frame, sealing the die in a cavity of air. Air cavity packages can be used to house high frequency devices such as radio-frequency (RF) die. Packaging a high frequency semiconductor device in encapsulated air can improve high frequency properties of the die and electrical leads, as compared to encapsulation in a molding compound that has a higher dielectric constant than air. 
       FIG. 1  is a simplified block diagram illustrating a cross-sectional view of an air cavity package  100 .  FIG. 2  is a simplified block diagram illustrating a plan view of air cavity package  100 . An air cavity package uses a conductive metal base plate  110  that can incorporate a die attach region  120 . A window frame  130  made of a ceramic material is attached to the conductive metal base plate. Window frame  130  is generally attached to the conductive metal base plate prior to a die attach process. Conductive leads  140  are disposed on a top surface of the window frame and are used to make electrical contact  142  and  144  to a die  150  included in the package. Conductive leads  140  can be inserted into a recess along the top of window frame  130 . A cap  160  is attached to the top of the leads and the window frame, which seals a cavity  170 . 
     Conventional processes for forming an air cavity package, such as that illustrated in  FIG. 1 , involve attaching window frame  130  to conductive metal base plate  110  using a high temperature brazing process (e.g., 850° C.). Such high processing temperatures preclude use of copper base plates with ceramic window frames because a coefficient of thermal expansion (CTE) mismatch between copper and ceramic materials induces cracks in the ceramic window frame at brazing temperatures. Thus, conductive metal base plate  110  is typically made of a CuMOCu or Cu(CuMo)Cu laminate or CuW when a ceramic window frame is used. But both CuMOCu and Cu(CuMo)Cu laminates, as well as CuW, have significantly lower thermal conductivity than pure copper, which reduces the overall thermal performance of the package. 
     While epoxies can be used to attach a ceramic window frame to a metal base plate prior to die attach, epoxies can be damaged during subsequent high temperature die attach processes and thereby have a lower reliability. While some conventional air cavity techniques can involve attaching the window frame after die attach, those window frames are typically constructed of plastic which has a much lower thermal conductivity and capacitance than a ceramic window frame. This can limit the use of air cavity packages with plastic window frames to lower power applications. 
     Embodiments of the present invention provide for using silver  190 ,  192  as an interface material between ceramic window frame  130  and both conductive metal base plate  110  and conductive leads  140  (e.g., at interface regions  175  and  180 ). The silver  190 ,  192  is applied using a low-temperature sintering technique in which a fine particle silver paste, powder, or film is applied to the region of interest under heat and pressure. The silver particles are of nano-scale, and therefore surface energies of the molecules forming the particles can dominate interactions between the particles, including surface tension, thereby allowing for formation of solid silver at a temperature significantly lower than the melting point of silver. Once the solid silver region is formed, a typical melting point of the silver applies (i.e., approximately 960° C.). 
     In the air cavity package illustrated in  FIGS. 1 and 2 , a silver sintering process can be performed by applying fine particle silver  190  to conductive metal base plate  110  at interface region  175 . Ceramic window frame  130  is then applied to the fine particle silver at a temperature and pressure sufficient to cause the silver particles to bond to one another and to the materials of the ceramic window frame and the conductive metal base plate. In order to enhance the bonding between the silver and the ceramic window frame, the ceramic window frame can be metalized using a number of metalizing techniques known in the art (metallization layer  135 ). For example, the ceramic window frame can be metalized using a direct plated copper process, a directed bonded copper process, a refractory metal fire plus nickel/gold plating, or a thin film process using TiNiAu, TiW, gold, and the like. 
     In one embodiment, the temperature used during the sintering process is between approximately 200-300° C., and typically around 250° C., well below brazing temperatures used for prior art air cavity packages. Similarly, fine particle silver  192  can be applied at interface region  180  on ceramic window frame  130  and conductive leads  140  can be applied under a temperature and pressure to cause the silver particles to bond. The sintered silver  190 ,  192  at the interfaces provides a dual effect, an adhesive coupling of the differing materials together and an electrical/thermal coupling. 
     Fine particle silver can be applied using a variety of techniques. A paste containing the fine particle silver can be used. Such a paste can be sprayed on, printed on, or otherwise applied. A powder form of the fine particle silver can also be used, and applied using similar methods. Alternatively, a pre-formed film incorporating fine particle silver and organic materials can be used by placing the film between parts desired to be bonded under a low temperature (e.g., 250° C.) and added pressure. The organic material of the film is removed under such process conditions and the solid silver is formed. 
     Since the bonding process is performed at significantly lower temperatures than that of prior art brazing techniques, effects due to the differing CTE of ceramic and metal are avoided (e.g., ceramic cracking and warping). Further, less expensive and more efficient materials can be used for conductive metal base plate  110 . For example, a solid copper flange can be used instead of a laminate flange as the conductive metal base plate. In addition, conductive leads  140  can be formed of a solid copper rather than an alloy. In both cases, use of solid copper, rather than a laminate or an alloy, improves electrical conductivity and, in the case of the conductive metal base plate, thermal conductivity. Solid copper also provides significant cost benefits over laminates and alloys. A die attach region for the flange can include, for example, AuSi, AuSn, or sintered silver. In addition, according to the needs of the application, the package system can be plated to cover the sintered silver, using, for example, NiAu or NiPdAu at various stages of the package buildup (e.g., before die attach and after hermetic lidding). 
     Another semiconductor device package structure that can derive benefits from use of sintered silver is for high power devices employing metal coins in a package substrate or PCB. Traditionally, power components are provided to a system using individually packaged components that are coupled to the system PCB. A heat sink is coupled to the power component package. Therefore, there can be multiple connections between a power device die in the component package and the heat sink. This can result in an inefficient transfer of heat from the power device die to the heat sink. In addition, the power component package, along with all the various connections, can take up significant space in the system package. Further, since there are limited provided geometries of the packaged components, the use of those packaged components limits the flexibility of system geometries. 
       FIG. 3  is a simplified block diagram of a cross sectional view of a system  300  at a stage in processing in which power device die are coupled to metal coins or slugs embedded in a PCB or other package substrate (alternatively, the power device die or passive device die could be coupled to the metal coins or slugs to enhance grounding). System  300  includes a package substrate  310 , such as a PCB. Package substrate  310  has embedded metal coins  320  and  330 . The metal coins have a high thermal or electrical conductivity, depending upon the application. For many thermal applications, a copper coin is used because of copper&#39;s high electrical and thermal properties. Further, copper can be readily incorporated into circuit board designs. In the examples below, the metal coins are specifically discussed to be copper, but other metals (e.g., aluminum) and composites (e.g., AlSiC, Ag diamond, and Cu graphite) with high thermal or electrical conductivity can be used, as the application warrants. 
     As will be discussed more fully below, embedded copper coins  320  and  330  can be embedded using methods known in the art, as appropriate to the application. Power device die  340  and  350  are coupled to embedded copper coins  320  and  330  respectively. As will be discussed more fully below, the methods for coupling the power device die to the embedded copper coins depend upon the application. Power device die  340  and  350  are coupled to the embedded copper coins at coupling regions  342  and  352 , respectively. The coupling regions can be thermal or electrical or both, depending upon the application. On the major surface of the power device die opposite the major surface including the coupling regions, power device die  340  includes terminal pads  344  and  346 , while power device die  350  includes signal pads  354  and  356 . A device die  360  is shown as adhesively coupled to package substrate  310 . Device die  360  can be any component not needing the advantages of being coupled to an embedded copper coin, such as a low power device die or a passive component. Device die  360  includes terminal pads, such as terminal pad  362 , on the major surface opposite that coupled to the package substrate. 
     The power device die can be coupled to the embedded copper coins in a variety of ways, depending upon whether the copper coin is embedded in a preassembled PCB or is initially separate from a PCB and the PCB then assembled around the copper coin after attachment to the power device die. In those instances where the copper coin is embedded in a preassembled PCB, methods for attaching the power device die to the copper coin should take into account temperature limitations associated with processing of a premade PCB. That is, if too a high temperature is used in coupling the power device die to the embedded copper coin, then damage may occur to other areas of the premade PCB. Such a coupling can be performed using a low-temperature silver die attach. Low-temperature silver die attach methods known in the art can form an acceptable bond between the silicon die and the copper coin using temperatures of approximately 250° C. Such low-temperature die attach techniques include the use of nanoscale silver pastes or sintered silver, as discussed above, and typically provide better electrical, thermal and thermomechanical properties than traditional solder techniques. As stated above, another advantage of using low-temperature die attach techniques to couple the power device die to the embedded copper coins is avoiding damage to the remaining portions of the PCB. 
     Wire bonds  370 ,  375 ,  380 , and  385  are used to couple the contact pads on the active surface of power device die  340  and  350  and device die  360  with one another and with contact pads provided on substrate  310  (not shown). An interconnect and other circuitry provided on the substrate (not shown) can provide additional connections between the various device die. Subsequent to formation of the communication net provided by the wire bonds, a molding material is applied over and around the power device dies, the device die, wire bonds, and over the substrate, forming an encapsulant  390  that encapsulates the structures within the molding material and forms a panel. 
     The molding material can be any appropriate encapsulant including, for example, silica-filled epoxy molding compounds, plastic encapsulation resins, and other polymeric materials such as silicones, polyimides, phenolics, and polyurethanes. The molding material can be applied by a variety of standard processing techniques used in encapsulation including, for example, printing, pressure molding, and spin application. Once the molding material is applied, the panel can be cured by exposing the materials to certain temperatures for a period of time, or by applying curing agents, or both. In a typical encapsulation process, a depth of encapsulant  390  can exceed a maximum height of structures embedded in the molding material. 
     In an alternative embodiment, the wire bonded structures of  FIG. 3  can be packaged as an air cavity system. In such a case, encapsulant would not be used to cover the various components of the system. Instead, a pre-molded cavity package can be used to surround the various components and a cap or a lid can replace the encapsulant to protect the components within the cavity package. In some instances, a silicone gel can be used to further protect the components by being applied over and around the components and the wire bonds. In a further alternative embodiment, the wires can be replaced with a redistributed chip scale package system to provide the package interconnects. 
     As discussed above, embedded copper coins  320  and  330  are typically embedded in substrate  310  using a variety of techniques known in the art. One technique involves building up the substrate (e.g., PCB) around the copper coins. The coins are at least mechanically attached to the built up substrate, and can also be electrically coupled to the substrate as the substrate is built up, according to the needs of the application. Another technique involves press fitting the coins in a pre-built up substrate. In such a situation, the embedded copper coins are mechanically attached to the substrate. A disadvantage of both of these prior art methods for incorporating embedded copper coins is the necessary thickness of the substrate. In typical applications, the substrate would be between 32 (e.g., built up) and  40  (e.g., press fit) mils. 
       FIG. 4  is a simplified block diagram of a cross sectional view of a system  400  at a stage in processing in which power device die are coupled to sintered silver vias in a PCB or other package substrate.  FIG. 4  is an alternative to the structure of  FIG. 3  in that the embedded copper coins of  FIG. 3  are replaced by large vias  430  and  440 . Large vias  420  and  430  are formed from sintered silver and bonded to the organic material of substrate  410 . The sintered silver material can be directly bonded to the organic material of substrate  410  or, as shown, through an intermediate bonded plating metal  425  and  435  along edges of a drilled hole through the substrate for vias  420  and  430 , respectively. As discussed above, sintered silver vias  420  and  430  can be formed using a fine particle silver paste applied within the via holes under a low temperature and added pressure, or using a pre-formed film under low temperature and added pressure. The silver bonds to the bonded plating metal which, in turn, is bonded to the multi-layer printed circuit board. Bonded plating metal  425  and  435  be a variety of plating metals, as appropriate to the application, and can typically be silver, copper, palladium, or gold. 
     As illustrated in  FIG. 4 , one advantage of using large vias formed of sintered silver is a significantly reduced thickness of the printed circuit board over PCBs incorporating embedded copper coins. Power device die  340  and  350  are thermally and/or electrically coupled to the large vias using techniques known in the art for coupling to silver (alternatively, passive semiconductor devices can be coupled to the large vias). The other components illustrated for system  400  correspond to similarly numbered elements of system  300  in  FIG. 3 . 
     Large via formation using sintered silver also provides opportunities for incorporation of other materials and devices within the large via region. This is due to the low temperature formation of the silver, and the electrical and thermal coupling properties of the formed silver. 
       FIG. 5  is a simplified block diagram of a cross sectional view of a system at a stage in processing in which a copper slug is incorporated in a sintered silver large via. Substrate  510  includes a large via  515 . Large via  515  can be plated along the edges using a plating metal such as gold, silver, or copper using techniques known in the art. Large via  515  further includes sintered silver  530  and a copper slug  540 . Copper slug  540  can be placed into position before or after the application of a fine particle silver used for formation of sintered silver  530  (e.g., a nano-silver paste). The sintering process can then be applied in the large via region to form the large via having both sintered silver and the copper slug. Such a large via can incorporate both the advantages of the sintered silver (e.g., thinness, electrical, and cost) and the thermal advantages of copper. Alternative metallic slugs can be used in place of a copper slug, according to the needs of the application. Further, alternatively diamond or ceramics, such as low-temperature cofired ceramics (LTCC) having dielectric and conductive elements, can be included in the large via in place of copper slug  540 . 
       FIG. 6  is a simplified block diagram of a cross sectional view of a system at a stage in processing in which a pair of metal slugs are incorporated in a sintered silver large via. Substrate  610  includes a large via  615 . The large via can be formed by, for example, a drilling process or during build up of the substrate. The area of the through via along the major surfaces can be larger than that within the PCB. The sides of the large via are metalized using a plating metal such as gold, silver, or copper (e.g.,  620 ). Sintered silver  630  and metal slugs  640  and  645  are incorporated in the via hole, wherein the silver is bonded to the metalizing  620 . As with the embodiment in  FIG. 5 , the silver is introduced to large via  615  in the form of fine particle silver (e.g., a nano-silver paste) under a low temperature (e.g., 250° C.) and, in certain applications, an added pressure in order to form the sintered silver. Metal slugs  640  and  645  can be copper slugs or another metal, according to the application. Plating metal  620  can provide both a physical connection to the organic portion of the PCB along with an electrical connection to metalized interconnects within a laminate of the PCB or built up along the surfaces. 
       FIG. 7  is a simplified block diagram of a cross sectional view of a system at a stage in processing in which a passive component is incorporated in a sintered silver large via. Substrate  710  includes a large via  715 . The large via can be formed by, for example, a drilling process or during build up of the substrate. The area of the through via along the major surfaces can be larger than that within the PCB. The sides of the top and bottom regions of the large via can be metalized using a plating metal such as gold, silver, or copper (e.g.,  720 ). Sintered silver  730  is within the large via along with a low cost chip component  740 . Low cost chip component  740  can be, for example, a high capacitance (HiC) component in an electrical pathway defined by sintered silver  730  and plating metal  720 . As with the embodiment in  FIG. 6 , plating metal  720  can provide both a physical connection to the organic portion of the PCB along with an electrical connection to metalized interconnects within a laminate of the PCB or built up along the surfaces. 
     By now it should be appreciated that there has been provided a semiconductor device package that includes a first material portion that includes one of a ceramic or an organic material, a second material portion that includes a metallic material, and a sintered silver region disposed the adhesively couple the first and second material portions of the semiconductor device package. In one aspect of the above embodiment, the semiconductor device package is an air cavity package, the first material portion includes a metalized ceramic window frame, and the second material portion includes a conductive metal base plate. In a further aspect, the second material portion includes a solid copper metal base plate. 
     In another aspect of the above embodiment, the semiconductor device package is an air cavity package, the first material portion includes a metalized ceramic window frame, and the second material portion includes a conductive metal lead. A further aspect includes an air cavity cap. The air cavity cap is adhesively coupled to a surface of the metalized ceramic window frame and a surface of the conductive metal lead using a layer of sintered silver. 
     In another aspect of the above embodiment, the sintered silver region is formed by applying fine particles silver at a formation temperature lower than the melting point of silver. In a further aspect, the formation temperature is between approximately 200° C. and 300° C. 
     In another aspect of the above embodiment, the first material portion includes a printed circuit board, and the sintered silver region is disposed within a hole formed in the printed circuit board. In a further aspect, the second material portion is a metallic slug disposed within the sintered silver region. A still further aspect includes a semiconductor device die thermally coupled to the metallic slug where the metallic slug includes copper. Another further aspect includes a second metallic slug disposed within the sintered silver, where the sintered silver thermally and electrically couples the metallic slug and the second metallic slug. In another further aspect, the second material portion includes a passive electronic device disposed within the sintered silver region, where the sintered silver electrically couples the passive electronic device to an interconnect formed on the printed circuit board. In a still further aspect, the passive semiconductor device includes a high capacitance material. 
     Another embodiment of the present invention provides a method for forming a semiconductor device package. The method includes: forming, from a first material, a first material portion of the semiconductor device package where the first material includes one of a ceramic or an organic material; providing a second material portion of the semiconductor device package where the second material includes one of a ceramic or a metallic material; and, adhesively coupling the first and second material portions of the semiconductor device package using a sintered silver region. 
     In one aspect of the above embodiment, the semiconductor device package is an air cavity system, said forming the first material portion further includes forming a ceramic window frame, and the second material portion includes a conductive metal base plate. In another aspect, the semiconductor device package is an air cavity system, said forming the first material portion further includes forming a ceramic window frame, and the second material portion includes a conductive metal lead. 
     In another aspect of the above embodiment, adhesively coupling the first and second material portions further includes forming a sintered silver region by applying a fine particle silver at a formation temperature lower than the melting point of silver. In a further aspect, the formation temperature is between approximately 200° C. and 300° C. 
     In another aspect of the above embodiment, forming the first material portion further includes forming a printed circuit board and forming a hole in the PCB, and the adhesively coupling includes forming the sintered silver region within the hold in the PCB. A further aspect includes disposing the second material portion in the sintered silver region, where the second material portion is a metallic slug. 
     Because the apparatus implementing the present invention is, for the most part, composed of electronic components and circuits known to those skilled in the art, circuit details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. 
     Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. 
     Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, metallic materials other than copper can be used in the various packages. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims. 
     The term “coupled,” as used herein, is not intended to be limited to a direct coupling or a mechanical coupling. 
     Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or an limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. 
     Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements.