Increase The Packing Density Of Vertically Aligned Carbon Nanotube Array For The Application Of Thermal Interface Materials

To fulfill the potential of carbon nanotube (CNT) as thermal interface material (TIM), the packing density of CNT array needs improvement. In this work, two potential ways to increase the packing density of CNT array are tested. They are liquid precursor(LP)CVD and cycled catalyst deposition method. Although LP-CVD turned out to be no help for packing density increase, it is proved to enhance the CNT growth rate. The packing density of CNT array indeed increases with the cycle number. The thermal conductivity of the CNT array increases with the packing density. This work is believed to be a step closer to the real life application of CNT in electronic packaging industry.

As the integration scale of transistors/devices in a chip/system keeps increasing, effective cooling has become more and more important in microelectronics. To address the thermal dissipation issue, one important solution is to develop thermal interface materials with higher performance. Carbon nanotubes, given their high intrinsic thermal and mechanical properties, and their high thermal and chemical stabilities, have received extensive attention from both academia and industry as a candidate for high-performance thermal interface materials.\r : The thesis is devoted to addressing some challenges related to the potential application of carbon nanotubes as thermal interface materials in microelectronics. These challenges include: 1) controlled synthesis of vertically aligned carbon nanotubes on various bulk substrates via chemical vapor deposition and the fundamental understanding involved; 2) development of a scalable annealing process to improve the intrinsic properties of synthesized carbon nanotubes; 3) development of a state-of-art assembling process to effectively implement high-quality vertically aligned carbon nanotubes into a flip-chip assembly; 4) a reliable thermal measurement of intrinsic thermal transport property of vertically aligned carbon nanotube films; 5) improvement of interfacial thermal transport between carbon nanotubes and other materials.\r : The major achievements are summarized.\r : 1. Based on the fundamental understanding of catalytic chemical vapor deposition processes and the growth mechanism of carbon nanotube, fast synthesis of high-quality vertically aligned carbon nanotubes on various bulk substrates (e.g., copper, quartz, silicon, aluminum oxide, etc.) has been successfully achieved. The synthesis of vertically aligned carbon nanotubes on the bulk copper substrate by the thermal chemical vapor deposition process has set a world record. In order to functionalize the synthesized carbon nanotubes while maintaining their good vertical alignment, an in situ functionalization process has for the first time been demonstrated. The in situ functionalization renders the vertically aligned carbon nanotubes a proper chemical reactivity for forming chemical bonding with other substrate materials such as gold and silicon.\r : 2. An ultrafast microwave annealing process has been developed to reduce the defect density in vertically aligned carbon nanotubes. Raman and thermogravimetric analyses have shown a distinct defect reduction in the CNTs annealed in microwave for 3 min. Fibers spun from the as-annealed CNTs, in comparison with those from the pristine CNTs, show increases of ~35% and ~65%, respectively, in tensile strength (~0.8 GPa) and modulus (~90 GPa) during tensile testing; an ~20% improvement in electrical conductivity (~80000 S m−1) was also reported. The mechanism of the microwave response of CNTs was discussed. Such an microwave annealing process has been extended to the preparation of reduced graphene oxide.\r : 3. Based on the fundamental understanding of interfacial thermal transport and surface chemistry of metals and carbon nanotubes, two major transfer/assembling processes have been developed: molecular bonding and metal bonding. Effective improvement of the interfacial thermal transport has been achieved by the interfacial bonding.\r : 4. The thermal diffusivity of vertically aligned carbon nanotube (VACNT, multi-walled) films was measured by a laser flash technique, and shown to be ~30 mm2 s−1 along the tube-alignment direction. The calculated thermal conductivities of the VACNT film and the individual CNTs are ~27 and ~540 W m−1 K−1, respectively. The technique was verified to be reliable although a proper sampling procedure is critical. A systematic parametric study of the effects of defects, buckling, tip-to-tip contacts, packing density, and tube-tube interaction on the thermal diffusivity was carried out. Defects and buckling decreased the thermal diffusivity dramatically. An increased packing density was beneficial in increasing the collective thermal conductivity of the VACNT film; however, the increased tube-tube interaction in dense VACNT films decreased the thermal conductivity of the individual CNTs. The tip-to-tip contact resistance was shown to be ~1×10−7 m2 K W−1. The study will shed light on the potential application of VACNTs as thermal interface materials in microelectronic packaging.\r : 5. A combined process of in situ functionalization and microwave curing has been developed to effective enhance the interface between carbon nanotubes and the epoxy matrix. Effective medium theory has been used to analyze the interfacial thermal resistance between carbon nanotubes and polymer matrix, and that between graphite nanoplatlets and polymer matrix.

Electronic Chip cooling has become an important issue with the ever increasing transistor densities and computing power demands. One of the crucial components of the thermal management system is high-performance thermal interface materials (TIMs), the materials connecting various solid-solid interfaces in packaged electronic devices. Ideal TIMs have the characteristics of high mechanical compliance and high intrinsic thermal conductivity. Vertically Aligned Carbon Nanotube (CNT) arrays are promising for advanced TIMs since they possesses both characters yet poor contact to the target surface can limit the overall performance. Recently, indium-assisted bonding has been found to enhance the contact conductance by a factor of 10, which inspires a comprehensive study of the CNT-array thermal transport properties. This thesis presents a systematic study on the thermal transport and mechanical properties of CNT arrays. The CNT array density and length are controlled via the thermal annealing duration and ethylene exposure duration in water-assisted chemical vapor deposition synthesis. The thermal transport properties are measured accurately by phase-sensitive photo thermal reflectance thermometry. The thermal contact conductance between CNT array and Glass increased close to linearly by increasing the volume fraction of the CNT array. The increase of volume fraction can potentially increase the number of contacting tubes which further enhance the contact area. In addition, the effective thermal conductivity increases monotonically with the increase of volume fraction of the CNT array. Quantitatively, it has been found that the increasing trend of thermal conductivity is larger than the increasing trend of volume fraction. The strain and buckling behavior of CNT arrays under compressive stress were systematically studied. It has been verified both experimentally and analytically that buckling in lower density CNT array results in a further decrease of thermal conductivity. The thermal conductivity of CNT array decreases as the structure changes from vertically aligned to buckled, while the thermal conductivity rises back as the buckling structure becomes more compact. The rise of thermal conductivity with the buckling structure is attributed to the rise of the thermal contact conductance between tubes. The thermal contact conductance between CNT array and glass increases as the compressive stress increases to certain degree, while further increase of stress causes fatigue at the contacts, which decreases the contact conductance. These results demonstrate how thermal transport properties vary as a function of CNT array density and as a function of the strain of CNT array. With such trends, the thermal properties can be further increased by understanding the underlying mechanisms for such trends.

It has been known that a significant part of the thermal budget of an electronic package is occupied by the thermal interface material which is used to join different materials. Research in reducing this resistance through the use of vertically aligned multiwall carbon nanotube based thermal interface materials is presented. Transferred arrays anchored to substrates using thermal conductive adhesive and solder was analyzed through a steady-state infrared measurement technique. The thermal performance of the arrays as characterized through the measurement system is shown to be comparable and better than currently available interface material alternatives. Furthermore, a developed parametric model of the thermal conductive adhesive anchoring scheme demonstrates even greater potential for improved thermal resistances. Additionally, a developed transient infrared measurement system based on single point high speed temperature measurements and full temperature mappings is shown to give increased information into the thermophysical properties of a multilayer sample than other steady-state techniques.

With technological advancements and ever-growing competition, the need for Carbon Nanotubes (CNTs) is now greater than ever. Some engineering applications for CNTs are sensors, field emission devices, energy storage, composite materials, and heat dissipation sinks. Heat transfer applications like the heat dissipation in computers remain a challenge. It has been theoretically proven that the thermal conductivity of Multiwalled Carbon Nanotubes (MWCNTs) can reach 3000 W/mK. Experimental measurements, however, have shown much lower values, although higher than those of carbon micro fibers and polymeric matrices. For polymeric composite materials, in-plane thermal conductivity is governed by the carbon fibers but the out of plane conductivity is dominated by the polymeric matrix. Using aligned CNT arrays in the transverse direction is expected to substantially increase the thermal conductivity. In this thesis a study was conducted to better understand heat conduction in CNT arrays and quantify their thermal conductivity. A method was devised to measure the thermal conductivity of carbon nanotube arrays based on Fourier's law. The method relied on using a strain gage as a heater and maintaining a steady state one dimensional flow. Heat was provided with a power source and thermocouples were placed at various points on the sample and connected to a thermocouple reader. Various parameters that affect the thermal conductivity of CNTs are the alignment, density, chirality, functionalization, and interface resistance. Interface resistance is one of the major parameters that affects the thermal conductivity. This thesis presents the results of a study on the effect of interface materials, array density and height on the thermal conductivity of CNT arrays.

The development of wide band gap semiconductors for power and RF electronics as well as high power silicon microelectronics has pushed the need for advanced thermal management techniques to ensure device reliability. While many techniques to remove large heat fluxes from devices have been developed, fewer advancements have been made in the development of new materials which can be integrated into the packaging architecture. This is especially true in the development of thermal interface materials. Conventional solders are currently being used for interface materials in the most demanding applications, but have issues of high cost, long term reliability and inducing negative thermomechanical effects in active die. Carbon nanotubes have been suggested as a possible thermal interface material which can challenge solders because of their good thermal properties and 1-D structure which can enhance mechanical compliance between surfaces. In this work, we have developed a novel growth and transfer printing method to manufacture vertically aligned CNTs for thermal interface applications. This method follows the nanomaterial transfer printing methods pioneered at Georgia Tech over the past several years. This process is attractive as it separates the high growth synthesis temperatures from the lower temperatures needed during device integration. For this thesis, CNTs were grown on oxidized Si substrates which allowed us to produce high quality vertically aligned CNTs with specific lengths. Through the development of a water vapor assisted etch process, which takes place immediately after CNT synthesis, control over the adhesion of the nanotubes to the growth surface was achieved. By controlling the adhesion we demonstrated the capability to transfer arrays of vertically aligned CNTs to polyimide tape. The CNTs were then printed onto substrates like Si and Cu using a unique gold bonding process. The thermal resistances of the CNTs and the bonded interfaces were measured using the photoacoustic method, and the strength of the CNT interface was measured through tensile tests. Finally, the heat dissipation capabilities of the vertically aligned CNTs were demonstrated through incorporation with high brightness LEDs. A comparison of LED junction temperatures for devices using a CNT and lead free solder thermal interface was made.

The total thermal resistance of a thermal interface material (TIM) depends on its thermal conductivity, bond line thickness (BLT) and the contact resistances of the TIM with the two bounding surfaces. This work reports development and thermal characterization of tin-capped vertically aligned multi-walled carbon nanotube (VA-MWCNT) array-epoxy composites for thermal energy management in load-bearing structural applications. The epoxy matrix is expected to impart mechanical strength to these systems while the VA-MWCNTs provide avenues for high thru-thickness thermal conductivity across the material interface. A transition zone (capping layer) comprising of a Sn thin film is introduced at the interface between the MWCNTs and the bounding surfaces to minimize the total interface thermal resistance of the TIM. Three-omega measurement method is utilized to characterize thermal conductivity in the tin-capped VA-MWCNT-epoxy composites as well as in its individual constituents, i.e. bulk EPON-862 (matrix) from 40K-320K, tin thin films in the temperature range 240K-300K and in individual MWCNTs at room temperature, taken from the same VA-MWCNT batch as the one used to fabricate the CNT-epoxy composites. Multilayer thermal model that includes effects of thermal interface resistance is developed to interpret the experimental results. The thermal conductivity of the carbon nanotube-epoxy composite is estimated to be ~ 5.8 W/m-K, and exhibits a slight increase in the temperature range of 240 K to 300 K. The results of the study suggests that the morphological structure/quality of the individual MWCNTs as well as the tin thin layer on the VA-MWCNT array are dominating factors that control the overall thermal conductivity of the TIM. These results are encouraging in light of the fact that the thermal conductivity of a VA-MWCNT array can be increased by an order of magnitude by using a standard high temperature post-annealing step. In this way, multifunctional (load bearing) TIMs with effective through thickness thermal conductivities as high as 25 W/m-K, can potentially be fabricated. Recently, tin has been identified as an attractive electrode material for energy storage/conversion technologies. Tin thin films have also been utilized as an important constituent of thermal interface materials in thermal management applications in the first part of this thesis. In this regards, in the present work, we also investigate thermal conductivity of two nanoscale tin films, (i) with thickness 500 ± 50nm and 0.45% porosity, and (ii) with thickness 100 ± 20nm and 12.21% porosity. Thermal transport in these films is characterized over the temperature range from 40K-310K, using a three-omega method for multilayer configurations. The experimental results are compared with analytical-numerical predictions obtained by considering both phonon and electron contributions to heat conduction as described by frequency-dependent phenomenological models and Born-von-Karman (BvK) dispersion for phonons. The thermal conductivity of the thicker tin film (500nm) is measured to be 46.2W/m-K at 300K and is observed to increase with reduced temperatures; the mechanisms for thermal transport are understood to be governed by strong phonon-electron interactions in addition to the normal phonon-phonon interactions within the temperature range 160K-300K. In the case of the tin thin film with 100nm thickness, porosity and electron-boundary scattering supersede carrier interactions, and a reversal in the thermal conductivity trend with reduced temperatures is observed; the thermal conductivity falls to 1.83 W/m-K at 40K from its room temperature value of 36.1 W/m-K, which is still more than an order of magnitude higher than predicted by the minimum thermal conductivity model. In order to interpret the experimental results, we utilize analytical models that account for contributions of electron-boundary scattering using the Mayadas-Shatzkes (MS) and Fuchs-Sondheimer (FS) models for the thin and thick films, respectively. Moreover, the effects of porosity on carrier transport are included using a treatment based on phonon radiative transport involving frequency-dependent mean free paths and the morphology of the nanoporous channels. The systematic modeling approach presented in here can, in general, also be utilized to understand thermal transport in semi-metals and semiconductor nano-porous thin films and/or phononic nanocrystals.

Individual carbon nanotubes (CNTs) have been reported to have the highest thermal conductivities of any known material. However, significant variability exists both for the reported thermal conductivities of individual CNTs and the thermal conductivities measured for macroscopic CNT assemblies (e.g. CNT films, buckypapers, arrays, and fibers), which range from comparable to metals to aerogel-like. This chapter reviews the current status of the field, summarizing a wide selection of experimental results and drawing conclusions regarding present limitations of the thermal conductivity of CNT assemblies and opportunities for improvement of the performance of nanotube superfiber materials.

Currently out of plane thermal conductivity (Kz) in adhesive joints fails to meet the needed Kz at the overall system level. Carbon nanotubes theoretically have an extremely high thermal conductivity along the longitudinal axis and according to molecular dynamics simulations the value can be as high as 3500 W/mK at room temperature for multi-walled carbon nanotubes (MWCNT). The thermal conductivity along the radial axis for MWCNTs is between 10-15 W/sq mK. Studies to increase Kz for adhesive joints only had minimal enhancement in the thermal conductivity. In order to utilize the superior thermal conductivity of the MWCNTs along the axial direction; vertically aligned MWCNTs have been used in this study. Vertically aligned MWCNTs have been grown on silicon wafers. The aligned nanotube array has been partially infused with epoxy. Selective reactive ion etching (RIE) of the epoxy revealed the nanotube tips. In order to reduce the impedance mismatch and phonon scattering at the interface, gold is thermally evaporated on the nanotube tip. A MEMS based steady state thermal conductivity measurement technique has been designed to assess the thermal conductivity of the device with special attention to the interface/transition zone.

To improve the energy efficiency in many electronics and machinery applications, advanced Thermal Interface Materials (TIMs) with high heat dissipation ability and more pliability must be employed. Among a variety of promising choices to make the advanced TIMs, Vertically Aligned Carbon Nanotube (VACNT) turfs (arrays) outstand with their exceptional mechanical and thermal properties. Individual CNTs are quite flexible due to their quasi-one-dimensional structure and presence of strong sp2 bonds among the carbon atoms gives them great strength. Also, the dominance of ballistic phonon transport in the CNTs endows them superior thermal conductivity when compared to many metallic substrates. However, the defects in CNTs, misaligned axial contacts between CNTs in a CNT turf, and the CNTs/substrate resistance reduce the practical thermal conductivity of the material. It is hypothesized that the application of metal coatings on each CNT in a CNT turf would enhance the overall thermal conductivity of the material and improve the connectivity between the CNT turfs and the metallic substrate. As the diameter of the CNTs in a CNT turf is in the order of several nanometers, Molecular Dynamics (MD) atomistic simulations is selected as a tool which provide a deeper understanding in studying the thermal transport at the fundamental level. Thermal conduction in the metals is electron dominant whereas regular MD procedures are incapable of considering the energy exchange between these electrons and phonons. Therefore, a different mechanism called Two-temperature Model (TTM) coupled with Non-Equilibrium MD is used in this study and proved to be effective. MD code to procure the coefficient of thermal conductivity (kappa) was developed and the effects of the metal thickness, number of walls in the CNT and the role of diameter of CNT on kappa of the metal-coated CNTs was individually investigated. It was shown that the increase in the thickness of metal coating would impede the kappa of individual CNTs following an inverse power trend. Also, it was found that among the number of shells in the CNT and its diameter, the former parameter tends to contribute more towards the thermal transport than the latter. The results of this work are capable of predicting the optimal design structure for metal-coated VACNT composite for advanced thermal management applications.