Carbon Nanotube Films And Microjet Cooling Devices For Thermal Management

The downsizing of electronic devices and the consequent increasing power densities pose thermal management challenges for the semiconductor industry. Since the present thermal solutions limit their cooling capacity, developing new cooling methods for electronic devices has become important. This dissertation presents two types of novel methods for heat dissipation from integrated circuits: One is the use of advanced thermal interface materials, such as carbon nanotubes (CNTs), to increase heat dissipation between the solid and solid surface, such as a chip and heat sink. The second method is the use of a microjet impingement device to improve heat transfer between a liquid and solid in a heat sink. As advanced interface materials, vertically aligned carbon nanotube films are promising because of their unique mechanical and thermal properties. The first part of the dissertation describes the design, fabrication, and testing of CNTs using resonators to characterize their mechanical properties. Discussed in detail is the preparation of carbon nanotubes using different recipes, resulting in varied thicknesses of single-walled carbon nanotube films and multi-walled carbon nanotube films. The measurements of the resonant frequency shifts due to the presence of the CNT films using a laser Doppler vibrometer system result in extracted moduli of 0.5-220 [Mu]m-thick nanotube films varying from 1 to 370 MPa. To show how the physics between the effective modulus and thickness are connected, an analysis for the height dependence of the modulus is provided. After an image analysis is presented, a nanotube dynamics simulation based on tube properties and film morphology is introduced to predict mechanical properties. In addition to discussing the proposed interface materials, the second part of the dissertation describes the design, fabrication and testing of microjet impingement cooling, which display high heat capacities, as an advanced thermal management solution. The design of single-jet and multi-jet arrays with different numbers of diameters, locations, and spacing is discussed. Specifically, this part demonstrates how the microjet hydrodynamics are quantified using two-dimensional images by [Mu]PIV techniques, enabling the reconstruction of the three-dimensional flow field. The results indicate that CNT films offer a mechanical compliance that is suitable for TIM applications and that the microscale liquid jet devices provide quantified flow physics for heat sink applications.

To celebrate Professor Avi Bar-Cohen's 65th birthday, this unique volume is a collection of recent advances and emerging research from various luminaries and experts in the field. Cutting-edge technologies and research related to thermal management and thermal packaging of micro- and nanoelectronics are covered, including enhanced heat transfer, heat sinks, liquid cooling, phase change materials, synthetic jets, computational heat transfer, electronics reliability, 3D packaging, thermoelectrics, data centers, and solid state lighting.This book can be used by researchers and practitioners of thermal engineering to gain insight into next generation thermal packaging solutions. It is an excellent reference text for graduate-level courses in heat transfer and electronics packaging.

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.

Efficient heat conduction is critical to the performance and reliability of a variety of devices. To ensure efficient heat removal from electronic devices, thermal interface materials (TIMs) are used to enhance heat conduction between two contacting surfaces. Ideal TIMs have high thermal conductivity, conform to the microscale roughness of the surfaces in contact, and accommodate stresses due to mismatch in thermal expansion coefficients. Commercial TIMs exhibit a tradeoff between these desired thermal and mechanical properties. Vertically aligned carbon nanotube (VACNT) films have great promise as TIMs because they have high through-plane thermal conductance and mechanical compliance. VACNT films are typically composed of a complex, entangled network of CNTs and the density and alignment of CNTs within a film affect its mechanical and thermal properties. Given the variety of growth techniques and variations in film morphology, it is important to understand how these factors influence film behavior. The present work combines three major aspects of VACNT film characterization: mechanical and thermal characterization, analysis of the film microstructure, and modeling of the film properties. The inhomogeneous VACNT film morphology is investigated by using image analysis to quantify CNT alignment and density and by measuring the elastic modulus at the top and bottom film surfaces using nanoindentation. The alignment and density information is input into a model of the mechanical response of the VACNT film, which is compared to measured data. The through-plane thermal properties of VACNT films are measured using thermoreflectance techniques. Multiwall VACNT films are grown directly on a thermoelectric material and the thermal properties of the films are characterized using nanosecond thermoreflectance. A set of eight single-wall VACNT films are measured using frequency domain thermoreflectance to extract four relevant thermal properties: thermal conductivity, heat capacity, and the upper and lower boundary resistances.

The main objective of this thesis is to study the characteristics of carbon nanotube thin lms and apply them as detectors of microwave power. Using measured reflection data, S11, the relative permittivity and loss tangent of carbon nanotubes is extracted. These parameters are used to define a custom, frequency dependent material in HFSS, an electromagnetic finite element analysis solver. This new material definition allows one to simulate structures more complicated than the Corbino disc. The basic idea is to use a simple device to characterize the carbon nanotubes and use the data to simulate more complicated devices. A new 2-port twinax structure is analyzed using HFSS in order to extract the scattering parameters. MAXWELL, an electromagnetic field solver, is used to determine the DC potential distribution in the structure and is compared with a simple analytical model. Experiments for power detection are done using carbon nanotube thin lms deposited on sapphire substrates with Corbino disc test structures. These structures have been used to realize a microwave power sensor that operates at and above room temperature. Current vs voltage curve traces have been collected at various temperatures to evaluate the temperature dependent resistance of the carbon nanotube thin lm Corbino discs. The thermal time constant of the carbon nanotubes was found in order to determine how fast the device may respond when used as a microwave bolometer. Corbino effect measurements were then taken to observe the effect of an applied magnetic field on the resistance of the device. Such tuning of resistance can be of potential use in performance optimization of the power detection scheme. Device degradation was observed due to exposure to extreme temperatures (200C). New devices are now needed to continue work on carbon nanotube thin films. Photolithography masks have been designed and a photolithography process developed in order to fabricate Corbino disc structures and the new twinax structures.

The thermal interface layer can be a limiting element in the cooling of microelectronic devices. Conventional solders, pastes and pads are no longer sufficient to handle the high heat fluxes associated with connecting the device to the sink. Carbon nanotubes(CNTs) have been proposed as a possible thermal interface material(TI M), due to their thermal and mechanical properties, and prior research has established the effectiveness of vertically arranged CNT arrays to match the capabilities of the best conventional TIMs. However, to reach commercial applicability, many improvements need to be made in terms of improving thermal and mechanical properties as well as cost and manufacturing ease of the layer. Prior work demonstrated a simple method to transfer and bond CNT arrays through the use of a nanometer thin layer of gold as a bonding layer. This study sought to improve on that technique. By controlling the rate of deposition, the bonding temperature was reduced. By using different metals and thinner layers, the potential cost of the technique was reduced. Through the creation of a patterned array, a phase change element was able to be incorporated into the technique. The various interfaces created are characterized mechanically and thermally.

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.

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.