Patent Application: US-40301309-A

Abstract:
a method of manufacturing a thermal interface material , comprising providing a sheet comprising nano - scale fibers , the sheet having at least one exposed surface ; and stabilizing the fibers with a stabilizing material disposed in at least a portion of a void space between the fibers in the sheet . the fibers may be cnt &# 39 ; s or metallic nano - wires . stabilizing may include infiltrating the fibers with a polymerizable material . the polymerizable material may be mixed with nano - or micro - particles . the composite system may include two films , with the fibers in between , to create a sandwich . each capping film may include two sub films : a palladium film closer to the stabilizing material to improve adhesion ; and a nano - particle film for contact with a device to be cooled or a heat sink .

Description:
for the purposes of this application the terms “ nanometer realm ” or “ nano - scale ” are understood to mean approximately 1 - 100 nm , and preferably 1 - 10 nm . “ nano - particles ” will be in this same size range . micro range will be understood to mean 0 . 1 to about 10 microns . fig1 a is a schematic diagram of a tim system in accordance with the invention . fig1 a illustrates nano - scale fibers 101 , such as cnt &# 39 ; s stabilized in a matrix 102 and sandwiched between two capping layers 103 , such as silver nanoparticle paste . the fibers 101 may be vertically aligned or randomly oriented . there will be some expense associated with getting the fibers to be aligned , but aligned fibers are expected to demonstrate superior performance , and are therefore preferred . on the other hand , even in an unaligned state , a portion of the fibers 101 will be oriented normal to the interface plane . the matrix 102 may include organic or organic / inorganic hybrid material stabilizing the arrays or networks of nano - scale fibers 101 . the fibers 101 function as heat - passages for heat flux . other materials that might be used as fibers 101 , analogously to cnts , include silver or copper nanowires , carbon columns , and any other highly thermally conductive fiber , for example formed of metal alloys or organic or mineral materials . to achieve high bulk thermal conductivity , the fibers 101 preferably physically connect the two opposing solid surfaces 401 ( shown in fig4 ). any low conducting interfaces could be detrimental to overall performance . this preference for physical connection holds true no matter whether the fillers in use contain nanoparticles ( nps ) or microparticles , or whether the fibers are nanowires ( nws ), or nanotubes . examples of thermal properties of nanowires are discussed in several papers 13 , 14 . however , at the interfaces between the tim and solids ( such as a die or a heat sink ), direct contacts between the nano - scale fibers and solids direct connection is desirable but generally impossible to realize , except perhaps if the fibers are grown between the surfaces , in which case the density of fibers may be too low . therefore , an approximation of physical contact , which achieves thermal performance approaching that of true physical contact , is preferred . a deformable buffer layer 103 with good interfacial adhesion with both fibers 101 and the solid surface 401 ( shown in fig4 ) is therefore provided . fibers 101 are embedded between capping layers 103 . the capping layers 103 wet the solid surfaces , potentially eliminating voids due to surface roughness . the capping layers 103 also may serve to improve mechanical strength of the tim , making it more suitable for automatic processing , including being punched out and / or being picked up , carried , or placed , during device assembly . the capping layers 103 may also further serve a lateral heat dissipation function , perpendicular to the direction of heat transport provided by the fibers 101 . the matrix 102 may be , for example , a polymer with included nano - or micro - particles . the assembly shown in fig1 a will generally have a thickness in the range of 2 to 1000 microns , preferably in the 100 to 150 micron range . fig1 b and 1e are schematics of a side view . in the embodiment of fig1 b , the capping layers 104 are patterned . in the embodiment of fig1 e , both the matrix 102 and the capping layers 103 are patterned . by judiciously placing voids 104 in the capping layers 103 , or a different matrix material occupying these zones , both the mechanical characteristics and the processability could be improved . in general , the nanoparticle paste which preferably forms the capping layers 103 is expected to improve thermal conductivity . therefore paste should be in contact with anticipated hot spots in the circuitry , while voids 104 should be placed where the circuitry needs less cooling . fig1 c shows a top view of the tim of fig1 a and 1 b , also showing capping layer 103 patterning and voids 104 . fig1 a , 1 b and 1 c illustrate a sandwich - like tim with nano - scale fibers 101 between two capping layers 103 formed of paste . more layers of paste might be used . for instance , the first thin layer 105 would be applied to the fibers to promote the adhesion of the second layer 106 to the opposing surfaces as illustrated in fig1 d . this figure shows the fibers 101 , the polymer matrix 102 , a first capping layer 105 and a second capping layer 106 . preferably the first , thinner capping layer 105 is formed of palladium deposited from spattering or wax processing of nanoparticles or microparticles or other items in a paste that will go away during processing . there may be a modifier to make the palladium soluble in an appropriate solution . several different methods of synthesizing nano - scale fibers exist . in particular , with reference to cnts , there are bulk randomly oriented cnts , random cnts in a thin mat , and vertically aligned cnts ( vacnt ) on substrates . those cover a broad tim performance range while retaining high performance / cost ratios . because processes such as chemical purification and mechanical mixing break cnts and introduce defects , preferably the skilled artisan will chose as high a quality cnts as are currently available and practicable in terms of cost and meet the functional requirements . a number of papers describe methods for synthesizing aligned cnt 15 , 16 . randomly oriented clean and long cnts may be synthesized in large quantities using chemical vapor deposition ( cvd ). the density of as - synthesized cnt powders can be as low as 30 mg / cm 3 , which can be tuned to optimize the eventual density in composites and the convenient incorporation of matrix materials . random cnt mats may be obtained through known methods 17 . in addition to the quality of cnts , density and thickness are characteristics of cnt mats . synthesis conditions generally control the density cnt mat and thickness , which is generally achievable in the range of tens of microns . high density vacnts of controlled thickness at the vicinity of 10 microns may also be synthesized . fibers , such as cnt networks or arrays , can be stabilized by infiltrating the fibers with a filler , such as monomers or mixture of monomers and nanoparticles ( nps ) or microparticles followed by polymerization . preferably , the fibers are placed in an evacuated chamber to allow entry of the monomers , which otherwise do not easily wet the fibers . the chamber is then ventilated to push the monomers further into the fibers . voids around fibers are then filled with polymer . this polymer then leaves fiber configuration intact , whether it is an entangled network or aligned tubes . alternatively , monomers may be pushed in by filtration . in the latter case , nps loaded in monomers are retained in fibers and accumulated to high concentrations . high concentration means 20 - 60 % by volume . high volume fraction of metallic nano - or micro - particles in the matrix allows for formation of interconnected thermal passages upon np fusing . the monomers are then polymerized . hence , the thermal conductivity of the matrix is enhanced greatly . the better thermal conductivity is due to the network passages which form upon the fusion of high concentration nano - or micro - particles . polymerization of monomers provides mechanical integrity of the structures . fracture surface morphology of a vacnt composite in accordance with the present process is shown in fig3 , showing that the process does not destroy the ordering . embedded cnts remain well - aligned . thus stabilizing the fibers , separate from the device requiring a thermal interface , rather than growing the fibers on the device , allows for more flexibility and lower cost of manufacturing . for example , the fiber mats may be grown under uniform , optimized and tightly controlled conditions , which may be unavailable when seeking to grow the fibers on the device itself . orientation of cnts in polymer composites may also be introduced to initially randomly - oriented cnt / polymer composite 9 , 10 , 18 , 19 . in this approach , composites using bulk cnt powders are compressed biaxially followed by curing and polymerization . biaxial confinement deforms cnt networks , orienting cnts along the third , or the neutral axis . with deformed cnts fixed by np fusion and monomer polymerization , composite films obtained by cutting perpendicular to the neutral axis contain aligned cnts , resembling the morphology of a composite film prepared using vacnts . fig5 a shows cnts aligned along a neutral axis responsive to shear from compression . fig5 b shows cutting cnt &# 39 ; s perpendicular to the neutral axis . preferably such compression and cutting will be performed prior to application of the capping layer or layers . the cnt may be cut or patterned , for example with a die , laser , water jet , chemical process , optical process , ablative electrical current , or other known cutting tool or mechanism . indeed , the matrix 102 may be selectively processed after polymerization , to weaken it , and thus permitting a separation . if cnt mats are used , it is possible to fabricate tims with the patterned matrix and leave regularly arranged voids in composite films . inkjet printing or aerosol deposition may be used to deliver monomers ( with or without nps or microparticles ) to targeted locations . a known machine for aerosol deposition is the optomec m3d printer . patterning per fig1 b is particularly desirable to address the issue of local hot spots , where rapid dissipation in directions both perpendicular and parallel to die surface is required . patterning is a matter of design optimization , a balance of performance vs . cost . patterning could reduce the cost while still get work done , i . e . dissipating heat from a hot spot . for example , use of precious metals used in the capping layers are may be minimized . likewise , by reducing the contact area and providing intentional voids , more even contact between the opposed surfaces at the tim locations may be assured after compressing the tim between them . after stabilization with the filler , composite films may be etched using plasma or reactive ions in order to expose the ends of cnts or other nano - scale fibers at both surfaces of the film . therefore , it is preferred that the exposing step does not substantially degrade the fibers . np ( such as ag ) films of a few microns thick are coated to both surfaces of the fiber composite film . individual nps are coated with wax - like organic shells , hence np films are readily deformable under pressure . seamless joints form between the tim buffer layers and the solid surfaces after they are pressed between two solid surfaces at 100 ° c . a heat treatment at about 100 ° c . drives away the waxy organic molecules in the np shell , triggering the fusion of nps to form a contiguous metallic layer . this np layer may therefore be sintered . this layer may also conform to the roughness features of the solid surfaces , connecting cnts or other nano - scale fibers from one solid surface to the other with high thermal conductivity passages , especially when formed in situ between the opposed surfaces to be connected by the tim . a substantial fraction of the fibers are preferably oriented so that their ends are at the surfaces to achieve thermal conductivity . preferably substantially all of the fibers are so oriented , though as discussed above , the alignment requirement varies in dependence on the application and design criteria . alternatively , microparticles may be used . while using both filler and capping layers is expected to enhance mechanical strength and thermal properties — using one or more capping layers alone , without filler between the fibers , may provide an adequate tim . also , the fibers stabilized with filler , and without capping layers , may also provide an adequate tim . it is further noted that the process may be asymmetric , with the process according to embodiments of the invention provided for only one face of the tim , with other processes used to interface the other face of the tim with a respective surface . using printing ( electrostatic , electrophoretic , ink jet , impact ) or lithography techniques , capping layers with carefully designed patterns ( such as lines , grids , or pads ) may be deposited . a design criterion is to deliver just the right amount of materials to fill the gap between tim and solid surfaces and create seamless contact . fig2 shows a portion of the assembly process of a tim in accordance with an aspect of the invention . in this figure , a conveyor 201 carries fiber composites 202 past an ink jet printer 203 , which deposits the nano - particle paste on the fiber composites to form patterns such as shown in fig1 b and 1c . there are several advantages of fabricating patterned tim films . the voids in the matrix layer serve as breathing channels that release the trapped air during assembly , and organic molecules during curing nps capping layers ; the voids accommodate thermal expansion cycles and therefore improve mechanical performance and longevity ; and the voids also make films more compressible , facilitate the flow of the np paste to form better contacts , and reduce the packaging pressure . once the tim is completed , it will preferably be added to a device . adding the tim to a device will involve temperature and pressure sufficient to remove excessive ( and unintended ) voids in the material . fig4 shows how sintered metal paste 402 helps the tim bond with device surfaces 401 . sandwiched between the capping layers 405 are the nano - scale fibers 403 . each fiber has a first and second end . substantially all of the fibers 403 are preferably oriented so that their first end is at a first device surface and their second end is at a second device surface . the fibers 403 are stabilized with a filler 404 , while the sintered metal paste 402 serves as the capping layers . the paste has voids 406 which give flexibility and improved stress reduction . the paste 402 is preferably located at anticipated hot spots on at least one of the device surfaces 401 . fig6 shows a device incorporating a tim in accordance with the invention . at the top is an air - cooled heat sink 601 . below that is a layer 609 of tim ( tim 2 ) in accordance with the invention . below that is a lid 602 , which functions as a heat spreader . below that is another layer 610 of tim in accordance with the invention ( tim 1 ). below that is the chip 608 . below that is a layer of first level interconnect c 4 bumps 603 , also known as flip chip solder bumps , where c 4 is the acronym for controlled collapse chip connection . the bumps 603 are interspersed with underfill 607 . the bumps 603 and the underfill 607 rest on a substrate 604 . the substrate is connected to the printed circuit board ( pcb ) 606 via a second level interconnect 605 , similar to elements 603 and 607 . a tim in accordance with the invention is a hybrid materials system . preferably this system will include various forms of nanomaterials . this tim will preferably realize one or more of the following goals : low temperature application of the tim , i . e . below 200 ° c ., comparable to the temperature of operation of the device and / or it electrical assembly , to reduce thermal stress during operation ; minimizing thermal resistance between two solid surfaces , usefulness for any applications requiring very high thermal dissipation by joining two solid surfaces , high bulk thermal conductivity close to theoretical limit of a composite containing cnts , readily deformable surfaces to form intimate contacts with solid surfaces of varying topological and roughness features , variable thickness that can be minimized to a few microns , mechanical robustness for easy processing and application of tim as well as long term stability for thermal cycling , and chemical stability therefore environmental and manufacturing friendliness . individual features could be adjusted to optimize the overall performance of the tim system . in addition to heat transport , a tim should reduce or minimize stresses that arise between the devices coupled by the tim . these stresses may be thermo - mechanical in the sense that they are caused by differences in coefficients of thermal expansion between device areas . stresses may also be caused by differences in temperatures in different regions of the devices to be coupled . polymers used in this tim are preferably chosen to have a low modulus of elasticity . also , when the tim is applied , some areas may be unattached to the devices to be coupled to reduce stresses . similarly there may be voids in the fibers , filler , or capping materials to reduce stresses . during application to a device , a tim in accordance with the invention may be further processed to expose the ends of the nano - particles immediately prior to application to a device requiring a thermal interface . such further processing might include mechanical means , chemical means , or laser ablation . from reading the present disclosure , other modifications will be apparent to persons skilled in the art . such modifications may involve other features which are already known in the design , manufacture and use of thermal insulating materials and nano - scale fibers and which may be used instead of or in addition to features already described herein . although claims have been formulated in this application to particular combinations of features , it should be understood that the scope of the disclosure of the present application also includes any novel feature or novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof , whether or not it mitigates any or all of the same technical problems as does the present invention . the applicants hereby give notice that new claims may be formulated to such features during the prosecution of the present application or any further application derived therefrom . k i thermal conductivity of the material i = 1 , 2 , 3 , w / mk q 1 heat flux applied per unit area , w / m 2 q = q 0 l 1 + q 1 , effective heat flux , w / m 2 r eff effective thermal resistance of the thermal interface layer , k / w k eff effective thermal conductivity of the thermal interface layer , w / mk an analysis was conducted of the tim system for configurations in which the thickness of the heat source is also taken into account . both specified heat generation and specified uniform heat flux can be applied to the system . the details of the analytical solution are given in desai et al . 21 , expressly incorporated herein by reference . in desai et al . 22 , expressly incorporated herein by references , numerical and analytical models are built for a periodic element ( or a unit cell element ) of the system of vertically aligned nanotubes between silicon and aluminum surfaces . the size of the periodic element is determined by the size of the nanotubes , and the percentage of area they occupy on the silicon surface ( assuming they are uniformly distributed on the silicon surface ). the size of the silicon surface is 1cm × 1 cm . the periodic element is assumed to be cylindrical . fig7 represents one such periodic element . as can be seen from the micrographs shown in fig8 and 9 , the vertically aligned nanotubes grown on a silicon substrate do not have the same height . to take into account the size variation and to analyze the effect of this variation on the effective thermal conductivity of the system , a statistical approach is applied . an analytical solution presented in desai et al . 21 is used along with a random number generator to represent variations in heights of nanotubes over the chip area . a statistical analysis may then be carried out on the different heights of the tubes and a corresponding temperature drop calculated for that system ( combination of many unit cells ). the results obtained indicate that considering a small system is sufficient to accurately model the effect of variation of height over the chip area . in practice , the nanotubes are grown off a surface ( silicon ) and the height to which the nanotubes grow cannot be controlled with great precision . hence , there will be a small gap between some of the nanotubes and the aluminum interface . the analytical solution from desai et al . 21 may be used for modeling a unit cell as shown in fig7 . the variation in height is accounted for by taking the resultant temperature drop in the gap between the end of the nanotube and the aluminum surface in short tubes and applying the same as an interface temperature drop , as given by relation ( 1 ) below . where k gap is the thermal conductivity of the gap , a gap is the area occupied by the gap , and l gap is the length of the gap . q is the heat flowing through the nanotube . this model is then coupled with a random number generator , which assigns a height to the tubes randomly , and results obtained for a series of interations . the thermal resistance of each of the nanotubes is stored . the effective resistance of the thermal interface layer is calculated by combining the individual resistances in parallel . the effective resistance is then used to evaluate the one - dimensional effective thermal conductivity of the tim layer using the relation , k eff = l r eff ⁢ a . ( 3 ) the result is a model of many vertical nanotubes to form a miniature version of the tim system . two different random distributions are considered . first is a normal random distribution with mean as the mean height of the nanotubes and standard deviation σ = 1 micron . the second distribution is a uniform random distribution , which generates random numbers whose elements are uniformly distributed in the range of the mean , +/− 3 micron . the results are compared for these two random distributions . two different analyses are considered for modeling the effects of height variation across the thermal interface material . in the first analysis it is assumed that the nanotubes which are smaller than the mean height do not contribute to the effective thermal conductivity ( i . e ., the resistance of the matrix is very high so there is essentially no heat flowing through these tubes , k gap = 0 . 001 w / mk ). the second analysis uses equation ( 1 ) to determine the resistance of the short tube ( k gap = 4 w / mk ), and then uses the resistance of the matrix material and the spreading resistance of the tube with the matrix material added . in the second case the short tubes also contribute to the effective conductivity calculation . this results in four different cases : 1 ) normal finite — normal random distribution with finite resistance for the short tube . in this case , short tubes contribute to the effective thermal conductivity of the tim . 2 ) normal infinite — normal random distribution with infinite resistance for the short tube . so that short tubes do not contribute to the effective thermal conductivity of the tim . 3 ) uniform finite — uniform random distribution with finite resistance for the short tube . in this case , short tubes also contribute to the effective thermal conductivity of the tim . 4 ) uniform infinite — uniform random distribution with infinite resistance for the short tube , so that short tubes do not contribute to the effective thermal conductivity of the tim . fig1 , shows a plot of number of runs ( same as the number of unit cells used in the model ) versus the effective thermal conductivity of the matrix for a normal distribution with infinite resistance of the shorter nanotubes . 300 iterations were required to obtain the required convergence for case 2 . in the other cases similar convergence analyses were performed . for case 1 , 300 iterations gave a converged solution . for case 3 , 200 iterations and , for case 4 , 100 iterations gave a converged solution . table 1 and fig1 are the results obtained for the normal distribution with finite effective thermal conductivity analyses . the effective thermal conductivity is scaled with the bulk thermal conductivity of the nanotubes and is plotted against the percentage of area occupied by the nanotubes . the results indicate that taking the average of more than six runs ( three lines shown in the plot ) would result in the three lines shown here collapsing into a single line . the results for normal distribution with infinite resistance are presented in table 2 and fig1 . plotting the dimensionless thermal conductivity against the percentage of area occupied by the nanotubes results in three lines that lie nearly on top of each other , converging all the data into a single line . table 3 and fig1 show the results obtained for the uniform distribution with finite effective thermal conductivity analyses . the effective thermal conductivity is scaled with the bulk thermal conductivity of the nanotubes and plotted against the percentage of area occupied by the nanotubes . the results indicate that taking average of more than six runs ( three lines shown in the plot ) would result in the three lines shown here collapsing into a single line . the results for uniform distribution with infinite resistance are presented in table 4 and fig1 . in fig1 , the collapsed single line ( linear fit line through all the lines in case of infinite resistance case and the centre line in case of the finite resistance case ) in each case is plotted against the percentage of area occupied by nanotubes . the two lower lines lying on top of each other in fig1 are the lines for infinite resistance case with normal and uniform distributions . they lie nearly on top of each other , as in both cases there are 50 % of the tubes that are longer or of equal height as the gap and they contribute to the effective thermal conductivity and the other 50 % do not contribute at all . in the uniform distribution with finite resistance the tubes shorter than the gap height contribute to the effective conductivity and hence this gives a higher effective thermal conductivity then the infinite resistance case . in the normal distribution with finite resistance case the tubes shorter than the gap height contribute to the effective conductivity with a tighter distribution of the height of the nanotubes towards the mean ( normal distribution with σ = 1 ) and hence this gives a higher effective thermal conductivity then the uniform distribution with finite resistance case . the results indicate that the normal distribution with finite resistance of short nanotubes case give the highest thermal conductivity of all the four cases . also , a parametric analysis is carried out by varying the thermal conductivity of the nanotubes and the percentage of area they occupy on the silicon surface . by scaling the thermal conductivity with the bulk conductivity and plotting this against the percentage of area occupied , all the lines converge into a single line . the results indicate that , despite the effects of height variation , a thermal interface material with vertically aligned carbon nanotubes has the potential to be a high thermal conductivity thermal interface material . the word “ comprising ”, “ comprise ”, or “ comprises ” as used herein should not be viewed as excluding additional elements . the singular article “ a ” or “ an ” as used herein 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