Thermal management in electronic devices is one of the biggest challenges faced by the semiconductor industry. Thermal Interface Materials (TIMs) are used in electronics to fill air gaps between the surfaces of integrated circuit (IC) chips to dissipate heat. Polymer-graphene composites, a very promising choice as TIMs also have a drawback of high interfacial thermal resistance and a low thermal conductivity of polymer. It is known from the theoretical models that application of pressure may affect the thermal conductivity in a desirable manner, but quantitative simulations were not available. In this paper, the pressure dependence of thermal conductivity of epoxy and interfacial resistance at epoxy-graphene interface is studied using non-equilibrium molecular dynamics (NEMD) simulations. The results show that the thermal conductivity of epoxy increases with increase in pressure, and they compare well with the predictions using a theoretical model. The interfacial thermal resistance at epoxy-graphene interface reduces with increase in pressure. The reduction is sharp in the beginning and slowly reaches saturation as pressure increases. At 10 GPa compressive pressure, a 90-95% decrease in interfacial thermal resistance is observed.

Miniaturization of electronic devices for enhancing their performance is associated with higher heat fluxes and cooling requirements. Surface modifi cation by texturing or coating is the most cost-effective approach to enhance the cooling of electronic devices. Experiments on carbon nanotube coated heater surfaces have shown heat transfer enhancement of 60 percent. In addition, silicon nanotubes etched on the silicon substrates have shown heat flux enhancement by as much as 120 percent. The heat flux augmentation is attributed to the combined effects of increase in the surface area due to the protruding nanotubes (nano- n eff ect), disruption of vapor lms and modi fication of the thermal/mass di ffusion boundary layers. Since the e ffects of disruption of vapor lms and modifi cation of the thermal/mass di ffusion boundary layers are similar in the above experiments, the difference in enhancement in heat transfer is the consequence of dissimilar nano- n eff ect. The thermal conductivity of carbon nanotubes is of the order of 6000 W/mK while that of silicon is 150 W/mK. However, in the experiments, carbon nanotubes have shown poor performance compared to silicon. This is the consequence of interfacial thermal resistance between the carbon nanotubes and the surrounding fluid since earlier studies have shown that there is comparatively smaller interface resistance to the heat flow from the silicon surface to the surrounding liquids. At the molecular level, atomic interactions of the coolant molecules with the solid substrate as well as their thermal-physical-chemical properties can play a vital role in the heat transfer from the nanotubes. Characterization of the e ffect of the molecular scale chemistry and structure can help to simulate the performance of a nano fin in diff erent kinds of coolants. So in this work to elucidate the eff ect of the molecular composition and structures on the interfacial thermal resistance, water, ethyl alcohol, 1-hexene, n-heptane and its isomers and chains are considered. Non equilibrium molecular dynamic simulations have been performed to compute the interfacial thermal resistance between the carbon nanotube and different coolants as well as to study the diff erent modes of heat transfer. The approach used in these simulations is based on the lumped capacitance method. This method is applicable due to the very high thermal conductivity of the carbon nanotubes, leading to orders of magnitude smaller temperature gradients within the nanotube than between the nanotube and the coolants. To perform the simulations, a single wall carbon nanotube (nano-fin) is placed at the center of the simulation domain surrounded by fluid molecules. The system is minimized and equilibrated to a certain reference temperature. Subsequently, the temperature of the nanotube is raised and the system is allowed to relax under constant energy. The heat transfer from the nano- fin to the surrounding fluid molecules is calculated as a function of time. The temperature decay rate of the nanotube is used to estimate the relaxation time constant and hence the e ffective thermal interfacial resistance between the nano-fi n and the fluid molecules. From the results it can be concluded that the interfacial thermal resistance depends upon the chemical composition, molecular structure, size of the polymer chains and the composition of their mixtures. By calculating the vibration spectra of the molecules of the fluids, it was observed that the heat transfer from the nanotube to the surrounding fluid occurs mutually via the coupling of the low frequency vibration modes.

The objective of this study is to synthesize nanomaterials by mixing molten salt (alkali nitrate salt eutectics) with inorganic nanoparticles. The thermo-physical properties of the synthesized nanomaterials were characterized experimentally. Experimental results allude to the existence of a distinct compressed phase even for the solid phase (i.e., in the nanocomposite samples). For example, the specific heat capacity of the nanocomposites was observed to be enhanced after melting and re-solidification - immediately after their synthesis; than those of the nanocomposites that were not subjected to melting and re-solidification. This shows that melting and re-solidification induced molecular reordering (i.e., formation of a compressed phase on the nanoparticle surface) even in the solid phase - leading to enhancement in the specific heat capacity. Numerical models (using analytical and computational approaches) were developed to simulate the fundamental transport mechanisms and the energy storage mechanisms responsible for the observed enhancements in the thermo-physical properties. In this study, a simple analytical model was proposed for predicting the specific heat capacity of nanoparticle suspensions in a solvent. The model explores the effect of the compressed phase – that is induced from the solvent molecules - at the interface with individual nanoparticles in the mixture. The results from the numerical simulations indicate that depending on the properties and morphology of the compressed phase – it can cause significant enhancement in the specific heat capacity of nanofluids and nanocomposites. The interfacial thermal resistance (also known as Kapitza resistance, or “Rk”) between a nanoparticle and the surrounding solvent molecules (for these molten salt based nanomaterials) is estimated using Molecular Dynamics (MD) simulations. This exercise is relevant for the design optimization of nanomaterials (nanoparticle size, shape, material, concentration, etc.). The design trade-off is between maximizing the thermal conductivity of the nanomaterial (which typically occurs for nanoparticle size varying between ~ 20-30nm) and maximizing the specific heat capacity (which typically occurs for nanoparticle size less than 5nm), while simultaneously minimizing the viscosity of the nanofluid. The specific heat capacity of nitrate salt-based nanomaterials was measured both for the nanocomposites (solid phase) and nanofluids (liquid phase). The neat salt sample was composed of a mixture of KNO3: NaNO3 (60:40 molar ratio). The enhancement of specific heat capacity of the nanomaterials obtained from the salt samples was found to be very sensitive to minor variations in the synthesis protocol. The measurements for the variation of the specific heat capacity with the mass concentration of nanoparticles were compared to the predictions from the analytical model. Materials characterization was performed using electron microscopy techniques (SEM and TEM). The rheological behavior of nanofluids can be non-Newtonian (e.g., shear thinning) even at very low mass concentrations of nanoparticles, while (in contrast) the pure undoped (neat) molten salt may be a Newtonian fluid. Such viscosity enhancements and change in rheological properties of nanofluids can be detrimental to the operational efficiencies for thermal management as well as energy storage applications (which can effectively lead to higher costs for energy conversion). Hence, the rheological behavior of the nanofluid samples was measured experimentally and compared to that of the neat solvent (pure molten salt eutectic). The viscosity measurements were performed for the nitrate based molten salt samples as a function of temperature, shear rate and the mass concentration of the nanoparticles. The experimental measurements for the rheological behavior were compared with analytical models proposed in the literature. The results from the analytical and computational investigations as well as the experimental measurements performed in this proposed study – were used to formulate the design rules for maximizing the enhancement in the thermo-physical properties (particularly the specific heat capacity) of various molten salt based inorganic nanomaterials. The results from these studies are summarized and the future directions are identified as a conclusion from this study.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21369号 / 工博第4528号 / 新制||工||1705(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 中村 裕之, 教授 安田 秀幸 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

This research is focused on the processing of an innovative three-dimensional packaging architecture for power electronics building blocks with soldered device interconnections and subsequent characterization of the module's critical interfaces. A low-cost approach termed metal posts interconnected parallel plate structure (MPIPPS) was developed for packaging high-performance modules of power electronics building blocks (PEBB). The new concept implemented direct bonding of copper posts, not wire bonding of fine aluminum wires, to interconnect power devices as well as joining the different circuit planes together. We have demonstrated the feasibility of this packaging approach by constructing PEBB modules (consisting of Insulated Gate Bipolar Transistors (IGBTs), diodes, and a few gate driver elements and passive components). In the 1st phase of module fabrication with IGBTs with Si₃N₄ passivation, we had successfully fabricated packaged devices and modules using the MPIPPS technique. These modules were tested electrically and thermally, and they operated at pulse-switch and high power stages up to 6kW. However, in the 2nd phase of module fabrication with polyimide passivated devices, we experienced significant yield problems due to metallization difficulties of these devices. The under-bump metallurgy scheme for the development of a solderable interface involved sputtering of Ti-Ni-Cu and Cr-Cu, and an electroless deposition of Zn-Ni-Au metallization. The metallization process produced excellent yield in the case of Si₃N₄ passivated devices. However, under the same metallization schemes, devices with a polyimide passivation exhibited inconsistent electrical contact resistance. We found that organic contaminants such as hydrocarbons remain in the form of thin monolayers on the surface, even in the case of as-received devices from the manufacturer. Moreover, in the case of polyimide passivated devices, plasma cleaning introduced a few carbon constituents on the surface, which was not observed in the case of Si₃N₄ passivated devices. X-Ray Photoelectron Spectroscopy (XPS) Spectra showed evidence of possible carbon contaminants, such as carbide (~282.9eV) and graphite (~284.3eV) on the surface at binding energies below the binding energy of the hydrocarbon peak (C 1s at 285eV). Whereas above the hydrocarbon peak energy level, carbon-nitrogen compounds, single bond carbon compounds (~285.9eV) and double bond carbon compounds (~288.5eV) were evident. The majority of the carbon composition on the pad surface was associated with hydrocarbons, which were hydrophobic in nature, thus making the device contact pad less wettable. XPS data showed that, after the plasma cleaning process, absorbed monolayers on the Si₃N₄ passivated and polyimide passivated surfaces consisted of different chemical compositions and accordingly, the attraction forces of these absorbed layers are also different, which affects the bonding properties of the subsequent metallization, resulting in different contact resistances. On the other hand, with an electroless Zn-Ni-Au deposition, it was found that the polyimide passivation on the devices degraded due to due alkaline exposure in the plating baths, thus lowering the device breakdown voltage significantly. Furthermore, interfacial thermal resistances of solder preform, solder paste and silver epoxy (between the power module and the heat spreader) were characterized for process optimization. Void content at the resulting interface was found to be dependent on the flux content and flux activity. Solder preform with no-clean flux, reflowed in nitrogen results in the least resistant and minimized void-content interface. It is most likely that the flux added to the preform had a higher fluxing action than the flux contained in the solder paste. On the other hand, the outgassing of the entrapped flux profoundly affects the void formation and a lower void content indicates a lesser amount of trapped flux. In the case of a solder paste, the flux is in direct contact with the surface oxide of the powders and the surface to be soldered. Consequently, during reflow, any residual oxide can be expected to have some flux adhered to it. In the case of solder preform with added flux, the higher activity flux eliminated the oxide more rapidly and more thoroughly, thus leaving fewer spots for the flux to adhere to. Void contents in all cases of nitrogen reflow are consistently lower than the air-reflowed samples. Silver epoxy with a higher thermal conductivity (60W/mK) than Pb-Sn eutectic solder did not produce low-resistance interfaces. We found that thermal conductivity of the interface material is not the most crucial factor in reducing thermal resistance, rather it is the contact thermal resistance of the interfaces, which constitutes the largest part of the total interfacial thermal resistance. Process optimization with applied pressure and nitrogen reflow resulted in a significant lowering of contact resistance (from 0.55°C/W to 0.25°C/W) for the solder preform interfaces. We concluded that contact resistance needs to be duly accounted for in thermal modeling for an accurate representation of an interface; at the same time, the module attachment process must be tailored to reduce contact resistance for improved thermal management. / Ph. D.

<p>The thesis covers the following topics:</p> <p>1. aggregation and size effect on metal-polymer nanocomposite thermal interface materials</p> <p>2. diffusion limited cluster aggregation lattice simulation on thermal conductivty</p> <p>3. thermal interfacial resistance reduction between metal and dielectric materials by inserting an intermediate metal layer</p> <p>4. absence of coupled thermal interfaces in al2o3/ni/al2o3 sandwich structure</p> <p>5. ultra-efficient low-cost radiative cooling paints</p>

De nos jours, l'industrie microélectronique utilise des fréquences de fonctionnement plus élevées dans les composants commercialisés. Ces fréquences entraînent des températures de fonctionnement plus élevées et limitent donc l'intégrité et la durée de vie des composants électroniques. Cependant, les besoins actuels nécessitent des dispositifs miniaturisés et de haute densité de puissance. De ce fait, la dissipation thermique dans les composants microélectroniques s’avère capitale. Ainsi, des drains thermiques sont utilisés pour évacuer la chaleur produite par le fonctionnement du composant. Les drains thermiques actuels sont composés de métaux, tels que le cuivre et l’aluminium, présentant des conductivités et des coefficients de dilatation thermiques élevés. Néanmoins, les coefficients de dilatation thermique des différents matériaux présents dans un circuit peuvent induire des contraintes thermo-mécaniques aux interfaces et engendrer une défaillance des composants après plusieurs cycles de fonctionnement. Dans ce contexte, nous proposons de remplacer ces drains métalliques par un système composite à matrice cuivre renforcée par du carbone, sur lequel est déposé un diffuseur thermique sous forme de diamant. Ces composites Cu/C présentent des propriétés thermo-mécaniques adaptatives pouvant palier aux contraintes induites durant l’utilisation des composants. Le transfert optimal des propriétés dans les MMC est souvent compromis par l'absence de liaison chimique interfaciale, en particulier dans les systèmes non réactifs telsque Cu/C. Cependant, pour un assemblage thermiquement efficace, l'interface devrait permettre un bon transfert de charges thermo-mécaniques entre les matériaux. L'objectif de cette étude est de combiner les propriétés exceptionnelles du diamant et les propriétés thermo-mécaniques adaptatives des MMC. Les composites à matrice de cuivre renforcés au carbone sont synthétisés à l'aide d'un processus dit semi-liquide pour obtenir des gradients de composition et des propriétés optimisées d'interface matrice - renfort. Par conséquent, des éléments d'alliage sont insérés dans le matériau pour former des interphases de carbure à l'interface Cu/C. Le film mince de diamant est obtenu par dépôt chimique en phase vapeur assisté par laser. Cette méthode de dépôt permet d’agir sur la qualité du film ainsi que sur l’adhésion avec le substrat composite. Finalement, une importance particulière est portée à l’influence des interfaces sur les propriétés thermiques tant au sein du matériau composite (interface matrice – renfort), qu’au sein de l’assemblage film diamant – MMC.Ces travaux ont été menés dans le cadre d’un accord franco-américain de cotutelle de thèse entre l’Institut de Chimie de la Matière Condensée de l’Université de Bordeaux, en France, et le département d’Ingénierie Electrique de l’Université du Nebraska-Lincoln, aux Etats-Unis. Ils ont été financés, en France, par la Direction Générale de l’Armement (DGA), et par l’équivalent Américain aux Etats-Unis. / Today, the microelectronics industry uses higher functioning frequencies in commercialized components. These frequencies result in higher functioning temperatures and, therefore, limit a component’s integrity and lifetime. Until now, heat-sink materials were composed of metals which exhibit high thermal conductivities (TC). However, these metals often induce large coefficient of thermal expansion (CTE) mismatches between the heat sink and the nonmetallic components of the device. Such differences in CTEs cause thermomechanical stresses at the interfaces and result in component failure after several on/off cycles.To overcome this issue, we suggest replacing the metallic heat sink materials with a heat-spreader (diamond film) deposited on metal matrix composites (MMCs), specifically, carbon-reinforced copper matrices (Cu/C) which exhibit optimized thermomechanical properties. However, proper transfer of properties in MMCs is often compromised by the absence of effective interfaces, especially in nonreactive systems such as Cu/C. Therefore, the creation of a chemical bond is ever more relevant. The goal of this research was to combine the exceptional properties of diamond by means of a thin film and the adaptive thermomechanical properties of MMCs. Carbon-reinforced copper matrix composites were synthesized using an innovative solid-liquid coexistent phase process to achieve designed composition gradients and optimized matrix/reinforcement interface properties. In addition, the lack of chemical affinitybetween Cu and C results in poor thermal efficiency of the composites. Therefore, alloying elements were inserted into the material to form carbide interphases at the Cu/C interface. Their addition enabled the composite’s integrity to be optimized in order to obtain thermally efficient assemblies. The diamond, in the form of a thin layer, was obtained by laser-assisted chemical vapor deposition. This process allowed action on the film’s phase purity and adhesion to the substrate material. Of particular importance was the influence of the interfaces on thermal properties both within the composite material (matrix-reinforcement interface) and within the diamond film-MMC assembly. This work was carried out within the framework of a Franco-American agreement between the Institute of Condensed Matter Chemistry of the University of Bordeaux in France and the Department of Electrical Engineering at the University of Nebraska-Lincoln, in the United States. Funding, in France, was provided by the Direction Générale de l’Armement (DGA), and by the American equivalent in the United States.