Abstract:
An apparatus and system, as well as fabrication methods therefor, may include a thermal interface material comprised of an array of carbon nanotubes and a buffer layer disposed between the thermal interface material and one of a die or a heat spreader.

Description:
TECHNICAL FIELD  
         [0001]    The subject matter relates generally to apparatus, systems, and methods used to assist in transferring heat from one element or body, such as a circuit, to another, such as a heat sink.  
         BACKGROUND INFORMATION  
         [0002]    Electronic components, such as integrated circuits, may be assembled into component packages by physically and electrically coupling them to a substrate. During operation, the package may generate heat which can be dissipated to help maintain the circuitry at a desired temperature. Heat sinks, including heat spreaders, may be coupled to the package using a suitable thermal interface to assist in transferring heat from the package to the heat sink. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]    [0003]FIG. 1 is an cross-section view of an apparatus according to various embodiments;  
         [0004]    [0004]FIG. 2 is a detail of the view shown in FIG. 1;  
         [0005]    [0005]FIGS. 3-5 are flow charts illustrating several methods according to various embodiments; and  
         [0006]    [0006]FIG. 6 is a depiction of a computing system according to an embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0007]    In the following detailed description of various embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration, and not of limitation, specific embodiments in which the subject matter may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be utilized and derived therefrom, such that compositional, structural, and logical substitutions and changes may be made without departing from the scope of this disclosure. The following detailed description, therefore, is not to be taken in a limiting sense.  
         [0008]    Examples and embodiments merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in or substituted for those of others. The following description is, therefore, not to be taken in a limiting sense.  
         [0009]    [0009]FIG. 1 is a cross section view of an apparatus  10  according to various embodiments. Apparatus  10  includes a package substrate  12 , a die  14  and a thermal management aid such as a heat sink or an integral heat spreader  16  which is mounted adjacent the die  14  and separated from it by a gap.  
         [0010]    The substrate of die  14  is typically made of silicon and has frontside and backside surfaces. The die  14  also has an integrated circuit  20  and solder bump contacts  22  on the frontside surface. The contacts  22  connect with contact pads (not shown) on the upper surface of package substrate  12 . In some embodiments, the contacts  22  are manufactured according to a commonly used controlled collapse chip connect (C 4 ) process.  
         [0011]    In use, electric signals and power are provided to the integrated circuit  20 . Operation of the integrated circuit  20  causes heating of die  14 . Heat is transferred from the integrated circuit  20  through the substrate of die  14  to the heat spreader  16  through a thermal interface material  24  interposed in the gap between them.  
         [0012]    In FIG. 1, the heat spreader  16  includes a top plate  17  and supporting side walls  19 . In some embodiments the side walls  19  completely surround the die  14 . In one embodiment the heat spreader  16  is coupled to a further heat sink (not shown) which may or may not be actively cooled.  
         [0013]    In some embodiments, the thermal interface material  24  comprises an array  25  of either densely packed multi-walled or single walled carbon nanotubes or a combination of both single and double walled nanotubes. In some embodiments, either the heat spreader  16  or a lower surface thereof, is formed of, or coated with, a high thermal conductivity metal (and its alloys) such as molybdenum or copper, semiconductor material such as silicon or compounds as SiC (silicon carbide).  
         [0014]    In some embodiments, a first layer  26  is deposited on the surface of the heat spreader  16  prior to growing the array of carbon nanotubes  25  in order to prevent a chemical reaction at the surface of the substrate of heat spreader  16  between the heat spreader surface material and reactive gases which may be used to support the growth of the nanotubes in either a thermal or plasma assisted chemical vapor deposition (CVD) method. When grown by the CVD process, some of the carbon nanotubes of the array of nanotubes  25  are aligned with a substantial number of the carbon nanotubes oriented substantially perpendicular to the surface of heat spreader  16 .  
         [0015]    Carbon nanotubes have a coefficient of thermal conductivity along their longitudinal axis which is relatively high relative to the conductivity orthogonal to the longitudinal axis. The thermal conductivity of carbon nanotubes along their longitudinal axes is substantially higher than that of other materials used for thermal intermediates. The thermal conductivity of multi-walled nanotubes is about 3000 to 4000 W/m-K and theoretically about 6000 W/m-K for single walled nanotubes.  
         [0016]    In some embodiments, a catalyst is applied to either the surface of the heat spreader  16  or as a separate layer of layer  26 . In some embodiments such catalysts include metals such as nickel, iron and cobalt which are selected to improve the efficiency of the deposition process.  
         [0017]    After the prepared thermal interface material  24  has been formed on either the heat spreader  16  or, alternatively on layer  26  which was previously applied to the heat spreader substrate, the structure  24  is then thermally coupled to the backside surface of the die  14 . The thermal coupling is improved by orienting the free ends of the nanotubes to engage the backside of silicon die  18 . In some embodiments the coupling is directly to the backside of die  14  and in other embodiments the coupling is to a layer  30  on die  14 .  
         [0018]    In some embodiments, spacers  28  are inserted between die  14  and heat spreader  16  to define a minimum gap width. On such spacer is shown in the detail view of FIG. 2. In some embodiment multiple spacers  28  may be distributed about the perimeter of thermal interface material  24 .  
         [0019]    If the height h of the spacers  28  is less than the thickness of thermal interface material  24  by a predetermined amount when the die and the heat spreader are forced together during assembly until they both contact spacers  28 , a controlled bias force is applied between the free ends of some carbon nanotubes of the array of carbon nanotubes  25  and the backside surface of die  14  to provide a good thermal contact. By increasing the difference between the thickness of the thermal interface material  24  and the height of the gap defined by spacer  28 , the bias force for the junction between thermal intermediate  24  and die  14  can be controllably increased when a sufficient loading force is applied to the heat spreader  16  to have the spacer  28  engage both the surface of both heat spreader  16  and die  14 . By controlling the loading force and limiting it to a predetermined maximum force, the array of nanotubes  25  of the thermal interface material  24  will be deformed elastically by the bias force so that the highly conductive longitudinally oriented carbon nanotubes remain intact while they establish a highly conductive path between the die and heat spreader.  
         [0020]    In some embodiments. the free ends of some of the individual nanotubes of the array of carbon nanotubes in the array of nanotubes  25  in thermal interface material  24  make generally perpendicular contact with the entire surface of die substrate  18 . This also allows the thermal interface material  24  to efficiently engage the entire surface of die  14  even if it is not perfectly smooth.  
         [0021]    Substantial alignment of the longitudinal axes of many nanotubes of the of the array of carbon nanotubes  25  so that they are substantially perpendicular to the surface of die  14  provides a thermal path with direct, low thermal resistance between the surface of die  14  and heat spreader  16 .  
         [0022]    In some embodiments, free ends of the carbon nanotubes of the carbon nanotube array of the thermal interface material may contact a layer  30 . In some embodiments, layer  30  is a metal film of gold or silver or other suitable metal or alloy having high thermal conductivity and which may be readily deposited on the surface of die substrate  18 . Layer  30  provides a reduced thermal resistance thermal coupling between the surface of die  14  and the carbon nanotubes of thermal interface material  24 .  
         [0023]    In some embodiments, at least some of the carbon nanotubes of the thermal interface material  24  are coated or partially coated with gold, silver, platinum or other suitable metals or alloys by physical deposition or sputtering methods which are known. Such metal coated nanotubes may further reduce the contact thermal resistance between nanotubes of the thermal interface material  24  and either the surface of die  14  or the buffer  30  on the surface of die  14 .  
         [0024]    Some embodiments include a number of methods. For example, FIG. 3 is a flow chart illustrating several methods according to various embodiments. Thus, a method  311  may (optionally) begin at block  321  with coupling a heat source to a first surface of an array of substantially aligned carbon nanotubes and interposing a layer between at least one of either a heat source or a heat sink and at least one of either the first or a second surface of the array of carbon nanotubes in a block  325 . The method may include bonding another surface of the heat source to a substrate in block  327 .  
         [0025]    In some embodiments block  321  of method  311  may also include forming a layer on the heat source and growing the array of substantially aligned carbon nanotubes on the layer.  
         [0026]    In some embodiments block  325  of method  311  may also include applying an adhesion promoting layer between the heat sink and the array of carbon nanotubes.  
         [0027]    [0027]FIG. 4 is a flow chart of a method  411  illustrating several methods according to various embodiments. Thus, a method  411  may (optionally) begin at block  421  with growing an array of substantially aligned carbon nanotubes from a surface of a die and contacting the surface of a heat sink with free ends of some of the carbon nanotubes of the array of carbon nanotubes in block  423 .  
         [0028]    In some embodiments method  411  also comprises forming an adhesion layer on the surface of the die according to block  423 .  
         [0029]    In some embodiments method  411  also comprises forming an adhesion layer on some of the carbon nanotubes of the array of carbon nanotubes according to block  425 .  
         [0030]    [0030]FIG. 5 is a flow chart of a method  511  illustrating several methods according to various embodiments. Thus, a method  511  may (optionally) begin at block  521  with coupling a heat sink to a first surface of an array of carbon nanotubes and, in block  523 , applying an adhesion promoting coating to at least one of either the surface of a heat source or some of the carbon nanotubes of the array of carbon nanotubes. In some embodiments method  511  also comprises coupling the heat source to a second surface of the array of carbon nanotubes in block  525 ,  
         [0031]    In some embodiments applying an adhesion promoting coating to the surface of the heat source of block  523  comprises applying a metal.  
         [0032]    In some embodiments applying an adhesion promoting coating to some of the carbon nanotubes of block  523  comprises sputtering a metal coating on the carbon nanotubes.  
         [0033]    In some embodiments applying an adhesion layer to the heat sink of block  523  comprises applying a chemical adhesion promoting layer.  
         [0034]    [0034]FIG. 6 is a depiction of a computing system according to an embodiment. One or more of the embodiments of apparatus with one or more dies having a thermal intermediate with a thermal interface layer and a buffer layer interposed between the die and a heat spreader may be used in a computing system such as a computing system  600  of FIG. 6. The computing system  600  includes at least one processor (not pictured), which is enclosed in a microelectronic device package  610 , a data storage system  612 , at least one input device such as a keyboard  614 , and at least one output device such as a monitor  616 , for example. The computing system  600  includes a processor that processes data signals, and may include, for example, a microprocessor available from Intel Corporation. In addition to the keyboard  614 , an embodiment of the computing system includes a further user input device such as a mouse  618 , for example.  
         [0035]    For the purposes of this disclosure, a computing system  600  embodying components in accordance with the claimed subject matter may include any system that utilizes a microelectronic device package, which may include, for example, a data storage device such as dynamic random access memory, polymer memory, flash memory and phase change memory. The microelectronic device package can also include a die that contains a digital signal processor (DSP), a micro-controller, an application specific integrated circuit (ASIC), or a microprocessor.  
         [0036]    Embodiments set forth in this disclosure can be applied to devices and apparatus other than a traditional computer. For example, a die can be packaged with an embodiment of the thermal interface material and buffer layer, and placed in a portable device such as a wireless communicator or a hand held device such as a personal data assistant or the like. Another example is a die that can be coupled to a heat sink with an embodiment of the thermal interface material and buffer layer and placed in a dirigible craft such as an automobile, a watercraft, an aircraft or a spacecraft.  
         [0037]    The apparatus  10 , substrate  12 , die  14 , heat spreader  16 , integrated circuit  20 , solder bumps  22  thermal interface material  24  and layers  26  and  30 , spacer  28  and aligned nanotube array  24  may all be characterized as “modules” herein. Such modules may include hardware circuitry, and/or a processor and/or memory circuits, software program modules and objects, and/or firmware, and combinations thereof, as desired by the architect of the apparatus  10  and system  600 , and as appropriate for particular implementations of various embodiments. For example, such modules may be included in a system operations simulation package, such as a software electrical signal simulation package, a power usage and distribution simulation package, a thermo-mechanical stress simulation package, a power/heat dissipation simulation package, and/or a combination of software and hardware used to simulate the operation of various potential embodiments.  
         [0038]    It should also be understood that the apparatus and systems of various embodiments can be used in applications other than for coupling and heat transfer between dice and heat sinks, and thus, these embodiments are not to be so limited. The illustrations of apparatus  10  and system  600  are intended to provide a general understanding of the elements and structure of various embodiments, and they are not intended to serve as a complete description of all the features of compositions, apparatus, and systems that might make use of the elements and structures described herein.  
         [0039]    Applications that may include the novel apparatus and systems of various embodiments include electronic circuitry used in high-speed computers, communication and signal processing circuitry, data transceivers, modems, processor modules, embedded processors, and application-specific modules, including multilayer, multi-chip modules. Such apparatus and systems may further be included as sub-components within a variety of electronic systems, such as televisions, cellular telephones, personal computers, workstations, radios, video players, vehicles, and others.  
         [0040]    It should be noted that the methods described herein do not have to be executed in the order described, or in any particular order. Moreover, various activities described with respect to the methods identified herein can be executed in serial or parallel fashion.  
         [0041]    Although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. It is to be understood that the above description has been made in an illustrative fashion, and not a restrictive one. Combinations of the above embodiments, and other embodiments not specifically described herein will be apparent to those of skill in the art upon reviewing the above description. Thus, the scope of various embodiments includes any other applications in which the above compositions, structures, and methods are used.  
         [0042]    It is emphasized that the Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b), requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate preferred embodiment. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein,” respectively. Moreover, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.