Patent Publication Number: US-8530890-B2

Title: Aligned nanotube bearing composite material

Description:
This is a Divisional application of Ser. No. 12/364,435 filed Feb. 2, 2009, which is presently pending which is a Divisional application of Ser. No. 11/479,246, filed Jun. 29, 2006 which is now U.S. Pat. No. 7,534,648, Issued May 19, 2009. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the field of semiconductor manufacturing. In particular, the invention relates to composite materials for substrates and substrate cores. 
     BACKGROUND OF THE INVENTION 
     Modern high performance microelectronic devices (e.g. semiconductor chips) operate at substantially higher temperatures than their predecessors, which can lead to numerous performance and reliability problems. Some devices operate at temperatures high enough to ignite certain materials, presenting a thoroughly unacceptable fire danger. Some materials expand or contract in response to thermal variations at higher rates than other materials. When two or more materials with different coefficients of thermal expansion (CTE) are used in a microelectronic assembly, the extreme variance between operative and inoperative temperatures can cause materials to separate from one another, leading to device failure. High temperatures can also cause some materials to soften, particularly organic sheet materials, leading to structural and/or electrical failures in microelectronic assemblies. 
     As a result, a microelectronic assembly must be able to efficiently dissipate heat away from a high temperature microelectronic device. When designing and manufacturing electronic assemblies, the materials used to form substrates, packages, and other components closely associated with high temperature microelectronic devices must not only be able to withstand high temperatures without being damaged, but must also be highly thermally conductive. 
     Some methods used to increase the stiffness and lower the CTE of substrates or substrate core materials, include adding or increasing the amount of ceramic or glass filler (fiber) in the substrate materials. While this provides some benefits, it also reduces the manufacturability of substrates. In particular, it interferes with formation of holes through the substrate, such as plated through holes, by increasing the wear rate of drill bits, increasing the time required to drill holes, and reducing the number of substrates that may be drilled in a single drilling operation. Further, the reliability of the core material can be detrimentally affected by the increased amount of glass or ceramic filler. 
     Another approach is to use coreless substrates, but these can have problems such as increased warpage, low machinability, and blistering. Current materials and approaches simply do not provide a solution which combines reliability with highly efficient thermal dissipation in high temperature conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an embodiment of a method for forming a composite substrate core. 
         FIGS. 2   a - b  depict embodiments of aligned nanofilaments disposed at a surface of a substrate. 
         FIGS. 2   c  and  3  depict embodiments of an array of nanofilaments disposed at a surface of a substrate. 
         FIGS. 4 and 5  depict embodiments of an array of nanofilaments with spaces formed in the array. 
         FIG. 6  depicts an embodiment of a nanoparticle-filled epoxy resin disposed among nanofilaments of an array. 
         FIG. 7  depicts an embodiment of openings formed corresponding to spaces formed among nanofilaments of an array. 
         FIG. 8  depicts a cross-sectional view of an embodiment of a microelectronic substrate including a composite substrate core material. 
         FIG. 9  depicts an embodiment of a microelectronic package. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As depicted at  110  in  FIG. 1 , an embodiment of a method  100  includes disposing nanofilaments in an array at a surface of a substrate. Nanofilaments may include single-walled or multi-walled nanotubes (SWNT and MWNT, respectively) formed of carbon or boron nitride, or carbon nano-fibers. For some embodiments of electrically non-conductive substrates or substrate cores, electrically insulating nanotubes such as boron nitride are used. Conversely, for electrically conductive substrates or substrate cores, carbon nanotubes or carbon nanofibers can be used. Carbon nanofibers typically cost less than either carbon or boron nitride nanotubes, but have a lower thermal conductivity than either type of nanotube. Therefore, the electrical and/or thermal requirements of the microelectronic device in which an embodiment of the invention is implemented will influence the choice of which nanofilaments to use. 
     At least one embodiment for disposing nanofilaments thusly employs the Langmuir-Blodgett technique, wherein a monolayer of nanofilaments and surfactant are uniaxially compressed on an aqueous sub-phase, and the resulting axially aligned (along the long axis) nanofilaments are then transferred to a planar surface of a substrate, (e.g., a silicon substrate). The separation distance between centers of adjacent nanofilaments is controlled by the compression process, producing an arrangement  200  of approximately parallel (aligned) nanofilaments  201  at the substrate surface  250 , as depicted in  FIG. 2   a.    
     A plurality of arrangements may be formed as described above. As in the embodiments depicted in  FIGS. 2   b  and  2   c , at least a second arrangement  202  of approximately aligned nanofilaments  203  is produced and disposed at a surface of a substrate  251 , with the long axis of the nanofilaments  203  oriented differently from those of the first arrangement. For example, the nanofilaments  203  in the arrangement  202  depicted in  FIG. 2   b  are disposed and orientated approximately perpendicularly to those depicted in  FIG. 2   a . Embodiments of the invention include disposing an arrangement of aligned nanofilaments at a substrate surface  252  so as to superimpose at least one other arrangement of nanofilaments already disposed at the substrate surface  252 , wherein the nanofilaments  205  of one arrangement are aligned and oriented approximately perpendicular to the nanofilaments  206  of the other arrangement to form an array  204  of nanofilaments in a criss-cross pattern as in  FIG. 2   c . Accordingly, at least one nanofilament  205  in one arrangement will cross over at least one nanofilament  206  of the other arrangement, as shown at  207 . Although nanofilaments  205 ,  206  of an array  204  so formed have some nominal thickness, the thickness of the overall array  204  remains sufficiently small, such that nanofilaments  205 ,  206  of the arrangements within the array  204  are approximately coplanar with each other, with respect to the substrate. 
     While  FIG. 2   c  depicts a relatively small array, an exemplary embodiment includes an arrangement of a plurality of such relatively small arrays  520 , as depicted in  FIG. 5 . Other exemplary embodiments include a relatively larger array  315  formed at a surface of a substrate  300 , as depicted in  FIG. 3 , or a combination of relatively larger and relatively smaller arrays. A relatively larger array  315  may also be formed by combining a plurality of relatively smaller arrays  204  placed closely together at the surface of a substrate  300 . In such embodiments, a portion of the periphery of each adjacent array can overlap a portion of the periphery of at least one other adjacent array to eliminate gaps between the arrays, or be placed closely proximate to each other without overlapping. In this way, nearly any size of array is formed by combining a plurality of arrays at the surface of a substrate. 
     An embodiment of a relatively larger array  415 , depicted in  FIG. 4 , also includes spaces  420  formed in the array  415  among the plurality of nanofilaments. A space  420  provided in an array  415  is an area at a surface of a substrate  400 , within the boundaries of which few or no nanofilaments are disposed. The size and position of the spaces  420  within the array  415  generally correspond to the size and position of holes formed in embodiments of a composite material, as will be described. For example, if holes with a diameter of approximately 300 micrometers (‘microns’, or ‘μm’) are to be formed, with an approximately 400-600 μm pitch between centers of adjacent holes, spaces  420  provided in the array  415  will generally also have their centers positioned at approximately a 400-600 μm pitch, and a minimum clear area (without nanofilaments) of approximately 300 μm as measured in any direction within the boundaries of each space  420 . Embodiments of the invention will typically vary substantially according to the requirements of different product designs in which they are to be used. Therefore, the design requirements will impact the size and position of spaces  420  to be provided within an array  415 . Likewise, spaces  525  can be provided between arrays  520  in embodiments including a plurality of relatively smaller arrays  520 , as in  FIG. 5 , and the design requirements will drive the layout and spacing of the arrays  520  relative to each other and relative to locations where holes are to be provided. 
     In one embodiment, nanofilaments are initially disposed only in those areas at the surface of a substrate corresponding to the array, but not disposed in those areas corresponding to a space. In another embodiment, spaces are formed by selectively removing nanofilaments in an area corresponding to a space after an array has been formed at a surface of a substrate. For example, areas of a substrate surface corresponding to a space are treated with a sacrificial material. After disposing nanofilaments at the surface of the substrate, including such treated areas, the sacrificial material is removed, also removing the nanofilaments disposed in the treated areas. In another example, after an array is formed at a surface of a substrate, a masking layer is disposed over the array with openings formed in the masking layer corresponding with areas where spaces are to be formed. Nanofilaments exposed by the openings are treated with a surfactant, a solvent, or some other agent capable of removing the nanofilaments from the surface of the substrate. Once spaces are formed, the masking layer is removed leaving an array having spaces formed in areas corresponding to holes. 
     As shown at  120  and  130  in  FIG. 1 , nanoparticles are dispersed throughout an uncured epoxy resin, and the resin is disposed among the nanofilaments of an array at the surface of a substrate. Using nanoparticles as fillers in an epoxy resin provides numerous benefits, including improved thermal stability, due to the low coefficient of thermal expansion (CTE) of nanoparticles, greater flame retardancy, reduced epoxy resin viscosity, and improved adhesion to external surfaces of a composite material formed with the epoxy resin. Depending on the epoxy resin used and what performance characteristics are desired in a particular application, the level of nanoparticle loading can be increased or decreased to alter those characteristics. 
     According to alternate embodiments, nanoparticles are alumina or silica nanoparticles with a diameter of approximately 30 nanometers (‘nm’). In an exemplary embodiment, a loading of approximately 0.5-1.0 weight percent (%) of nanoparticles are dispersed in an uncured epoxy resin by conventional sonication or a solution mixing process. In other embodiments, nanoparticle loadings levels of up to approximately 3.0 weight % are used without significant difficulties in nanoparticle dispersal or epoxy resin viscosity. As the increase in nanoparticle loading levels increases, uniform dispersal of nanoparticles throughout an epoxy resin can become more difficult, and/or the viscosity of a resin can increase, potentially hindering complete dispersal of epoxy resin among a nanofilaments array. If relatively higher levels of nanoparticle loading are beneficial to a substrate or substrate core, for example, up to approximately 10-15 weight % of nanoparticles, an epoxy with a lower initial viscosity can accommodate relatively higher loading while still providing sufficient infiltration into a nanofilaments array. In embodiments with relatively low nanoparticle loading, generally untreated particles will be used. However, in embodiments with relatively higher levels of nanoparticle loading, or when dispersion of nanoparticles throughout an epoxy resin is poor, silane-treated nanoparticles will also generally provide enhanced dispersion characteristics. 
     As depicted in an exemplary embodiment in  FIG. 6 , nanoparticle loaded epoxy resin  630  is disposed at the surface of the substrate  600 , and the resin  630  infiltrates among the arrangements of nanofilaments constituting the array(s)  620 . Epoxy resins, according to embodiments, are any of a class of organic, generally viscous liquid resins, including thermosetting or thermoplastic modified resins typically used in semiconductor or electronic packaging applications. When so used, epoxy resins can be functionalized to improve thermomechanical and/or electrical properties, adhesion, and other beneficial properties. In embodiments, epoxy resins include ether linkages and epoxy groups. When used in conjunction with hardeners, curing agents, or catalysts, epoxy resins produce resin systems/networks with excellent electrical and/or thermomechanical properties and good chemical resistance. Although an exemplary embodiment of epoxy resin is referred to throughout this description, the embodiments are not so limited. Non-epoxy based resin systems including but not limited to novalac-based, cyanate ester-based, biamaleimde-based, and/or polymide-based systems are used according to other embodiments. 
     As most organic liquids are known to wet the surface of nanotubes, a good interface will generally form between the nanofilaments and epoxy resin  630 . As described, an epoxy resin having a relatively low viscosity will tend to infiltrate more easily among the nanofilaments of an array  620  than an epoxy resin with a relatively higher viscosity, but embodiments of the invention are not so limited. A number of factors can contribute to effective infiltration of even a relatively more viscous epoxy resin  630 , including the separation distance between nanofilaments, type and diameter of nanofilaments in the array, the number of overlying arrangements of nanofilaments in the array and the alignment angles of nanofilaments in the respective arrangements. 
     As depicted at  140  in  FIG. 1 , the disposed nanoparticle-epoxy resin mixture is then cured in situ. Curing is preceded in embodiments by degassing, to remove dissolved gasses from the epoxy resin. Curing of thermoset epoxy resins is achieved by exposing the resin to elevated thermal conditions, according to some embodiments. Curing of epoxy resins in other embodiments is achieved by mixing the resin with an amine or anhydride curing agent, producing a catalytic polymerization reaction. However curing is carried out in alternate embodiments, a cured epoxy resin forms a relatively hard, resistant plastic composite material including the array(s) of nanofilaments and dispersed nanoparticles. 
     Following curing of the epoxy, the resulting composite material is separated from the substrate according to numerous embodiments. In one embodiment, the composite material is delaminated from the substrate by applying a mechanical force sufficient to overcome the adhesive force bonding the composite material with the substrate. For example, a mechanical force generated by forcing a wedge into the interface between the composite material and the substrate will exert a separating force, as will a peeling force caused by pulling a substrate and a composite material in opposite directions. Delamination can be aided by applying a release material to the surface of the substrate prior to disposing the nanofilaments and epoxy at the substrate surface, and/or by using a surfactant during separation, thus reducing the adhesive forces between the composite material and the substrate. In another embodiment, a sacrificial material is disposed at the surface of the substrate prior to disposing the nanofilaments and forming the composite material. After forming the composite material, the sacrificial material is dissolved by exposure to a solvent or by heating and melting the sacrificial material, thus releasing the composite material from the substrate. In still another embodiment, the substrate itself is a sacrificial material that may be dissolved, abrasively ground away, or otherwise destructively removed from the composite material. In other embodiments, however, the substrate remains coupled with the composite material, and forms a portion of a microelectronic substrate or substrate core. 
     As a portion of a substrate or substrate core, a composite material may be in contact with other electrically conductive materials or elements, such as conductive pathways or planes. In such situations, an electrically conductive composite material could cause electrical shorting between the conductive elements, or between one conductive layer and another conductive layer or elements comprising portions of a microelectronic substrate. To prevent shorting in embodiments, a non-conductive masking material is disposed at the surface of at least a portion of the conductive composite material, interposed between the conductive composite material and the conductive elements or layers. A non-conductive masking material includes a polymer, a spin-on glass (SOG) material, a nitride or oxide material (e.g., silicon nitride), or another non-conductive material, according to various embodiments. In other embodiments, the non-conductive material comprises the substrate upon which the composite material was formed, or a layer formed between the substrate and the composite material, and used to separate the composite material from the substrate. A non-conductive masking material may be disposed across an entire surface of a composite material or only selected portions of it, as needed to prevent electrical shorting. 
     As depicted in  FIG. 1  at  150 , openings  725  (e.g., holes) are formed through the composite material  730  in embodiments. In some embodiments, openings  725  in a composite material  730  align with corresponding openings in other materials of a microelectronic substrate to provide an opening formed from one exterior surface of a substrate through to another exterior surface of the microelectronic substrate, (e.g., through holes). In other embodiments, openings  725  formed in a composite material  730  are presented to an exterior surface of a microelectronic substrate, but are not formed through to and presented at another exterior surface of the microelectronic substrate, (e.g., blind vias). Openings  725 , in an exemplary embodiment, are formed as plated through holes, which may include vias, microvias, or other conductive through structures of a microelectronic substrate. 
     Openings  725  are formed in embodiments by drilling though a composite material  730  in an area corresponding to a space  722  provided in the nanofilament array(s)  720 . Drilling may be accomplished using a drill bit, a laser, by selective etching (e.g., dry etch), or other methods. In an exemplary embodiment, projecting members, having a diameter corresponding to the diameter of an opening  725  to be formed, are provided extending approximately perpendicularly from the surface of the substrate at which a composite material  730  is to be formed. When the epoxy resin is disposed at the surface of the substrate, it flows around and includes at least a portion of the projecting member. When the composite material  730  is separated from the substrate, the projecting members are also withdrawn from the composite material  730 , leaving openings formed through the composite material  730 , similar to techniques sometimes used in injection molding and other molding processes to form openings in a single molding step. In another similar embodiment, the projecting members are not withdrawn from the composite material  730 , but may be a sacrificial material that is dissolved or otherwise destructively removed, leaving an opening in each portion of the composite material  730  previously occupied by a projecting member. In still another embodiment, such as when the substrate remains coupled with the composite material  730 , the projecting members are separated from the substrate and independently withdrawn from the composite material  730  to leave openings  725 . Another embodiment includes an assembly including projecting members attached to a main body portion of the assembly, and arranged to correspond to openings  725  to be formed in a composite material  730 . This assembly is placed so that the ends of each projecting member opposite from the main body portion are in simultaneous contact with or at least partially penetrate the surface of the substrate. The epoxy resin is then disposed and cured, and the assembly is used to simultaneously withdraw all projecting members, leaving openings in the composite material  730 . The embodiments described above are not limiting, and are not to be construed as excluding openings  725  formed by different approaches in other embodiments. 
     As described, embodiments of the invention comprise a composite material including an arrangement of approximately aligned nanofilaments overlying at least another arrangement of approximately aligned nanofilaments, the longitudinal axis of the nanotubes of the first arrangement being approximately perpendicular to the longitudinal axis of the nanotubes of the other arrangement, and the arrangements forming at least one array. An epoxy material having a nanoparticles dispersed throughout is disposed among the array(s) of nanofilaments, and cured, and openings may be formed into or through the composite material corresponding to spaces provided in the array of nanofilaments. A composite material according to embodiments forms a microelectronic substrate or some portion thereof, such as a substrate core. 
     A microelectronic substrate is a package substrate  800 , as in  FIG. 8 , to which a semiconductor device is physically and/or electrically coupled to form a semiconductor package in an embodiment of the invention. In embodiments where a composite material  815  forms a substrate core, the overall substrate  800  containing the composite substrate core  815  is likewise used as a microelectronic substrate for a semiconductor package. A composite material  815  will be an interior layer (e.g. substrate core) of a microelectronic substrate  800  located between adjacent materials  805 ,  820  in embodiments as depicted in  FIG. 8 , or will be an outer layer, or a plurality of layers of a microelectronic substrate  800 . In embodiments, a nonconductive mask layer  810  is disposed between a surface of a composite substrate core  815  and another layer  805 , material, or feature (i.e. conductive feature) of a microelectronic substrate  800 . Openings formed through a composite substrate core may transit completely through the microelectronic substrate, as at  825 , or they may penetrate only partially though the microelectronic substrate, as at  830 . 
     As in an embodiment depicted in  FIG. 9 , a composite substrate core material forms a portion of a microelectronic device package  900  comprising a microelectronic device  903  electrically and/or physically coupled with a microelectronic substrate  915 . A microelectronic device package  900 , according to alternate embodiments, is used in a computer system such as a normally stationary computer system (e.g., desktop or server) or a portable computer system (e.g., notebook computer, palmtop computer, tablet), a portable audio playing system (e.g., memory chip or disc drive based audio players), a video game system (e.g., configured for connection to a television display), or another electronic system. A microelectronic device package, according to alternative embodiments, include a flip chip device, a multichip module, a multiple-core single chip device, or others as are known and used in electronic systems. 
     The foregoing detailed description and accompanying drawings are only illustrative and not restrictive. They have been provided primarily for a clear and comprehensive understanding of the embodiments of the invention, and no unnecessary limitations are to be understood therefrom. Numerous additions, deletions, and modifications to the embodiments described herein, as well as alternative arrangements, may be devised by those skilled in the art without departing from the spirit of the embodiments and the scope of the appended claims.