Processing Of Vertically Aligned Carbon Nanotubes For Heat Transfer Applications

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.

This book gives a survey of the physics and fabrication of carbon nanotubes and their applications in optics, electronics, chemistry and biotechnology. It focuses on the structural characterization of various carbon nanotubes, fabrication of vertically or parallel aligned carbon nanotubes on substrates or in composites, physical properties for their alignment, and applications of aligned carbon nanotubes in field emission, optical antennas, light transmission, solar cells, chemical devices, bio-devices, and many others. Major fabrication methods are illustrated in detail, particularly the most widely used PECVD growth technique on which various device integration schemes are based, followed by applications such as electrical interconnects, nanodiodes, optical antennas, and nanocoax solar cells, whereas current limitations and challenges are also be discussed to lay the foundation for future developments.

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.

Contains 32 papers from the following seven 2013 Materials Science and Technology (MS&T'13) symposia: Innovative Processing and Synthesis of Ceramics, Glasses and Composites Advances in Ceramic Matrix Composites Advanced Materials for Harsh Environments Advances in Dielectric Materials and Electronic Devices Controlled Synthesis, Processing, and Applications of Structure and Functional Nanomaterials Rustum Roy Memorial Symposium: Processing and Performance of Materials Using Microwaves, Electric and Magnetic Fields, Ultrasound, Lasers, and Mechanical Work Solution Based Processing for Ceramic Materials

Transfer printing (TP) is a class of techniques for the deterministic assembly of disparate micro/nanomaterials into functional devices, and has become an emerging suite of technologies for micro/nanofabrication. Systems enabled by transfer printing range from complex molecular-scale materials, to high-performance hard materials, to fully integrated devices. A variety of sub-techniques for different purposes have grown significantly in the past decade, leading to non-conventional electronics, optoelectronics, photovoltaics, and photonics, and enabling the development of non-planar and flexible electronics.Highlights breakthrough results and systems enabled by novel TP techniques.Highlights breakthrough results and systems enabled by novel TP techniques.Transfer Printing Technologies and Applications is a complete guide to transfer printing techniques and their cutting-edge applications. The first section of the book provides a solid grounding in transfer printing methods and the fundamentals behind these technologies. The second part of the book focuses on state-of-the-art applications enabled by transfer printing techniques, including areas such as flexible sensors, flexible transistors, wearable devices, thin film-based energy systems, flexible displays, microLED-based displays, metal films, and more. A concluding chapter addresses current challenges and future opportunities in this innovative field.Highlights breakthrough results and systems enabled by novel TP techniques.Highlights breakthrough results and systems enabled by novel TP techniques.This book is of interest to researchers and advanced students across nanotechnology, materials science, electrical engineering, mechanical engineering, chemistry, and biomedicine, as well as scientists, engineers, and R&D professionals involved with nanomaterials, micro- or nano-fabrication, microelectromechanical systems (MEMS), display technology, biotechnology, and devices. Highlights breakthrough results and systems enabled by novel TP techniques. Highlights breakthrough results and systems enabled by novel TP techniques.Highlights breakthrough results and systems enabled by novel TP techniques.Highlights breakthrough results and systems enabled by novel TP techniques. Highlights breakthrough results and systems enabled by novel TP techniques. Examines a range of transfer printing technologies and their specific features for different applications Highlights breakthrough results and systems enabled by novel TP techniques Offers an insightful outlook into trends and future directions in each sub-area of transfer printing

This book provides a single-source reference on the use of carbon nanotubes (CNTs) as interconnect material for horizontal, on-chip and 3D interconnects. The authors demonstrate the uses of bundles of CNTs, as innovative conducting material to fabricate interconnect through-silicon vias (TSVs), in order to improve the performance, reliability and integration of 3D integrated circuits (ICs). This book will be first to provide a coherent overview of exploiting carbon nanotubes for 3D interconnects covering aspects from processing, modeling, simulation, characterization and applications. Coverage also includes a thorough presentation of the application of CNTs as horizontal on-chip interconnects which can potentially revolutionize the nanoelectronics industry. This book is a must-read for anyone interested in the state-of-the-art on exploiting carbon nanotubes for interconnects for both 2D and 3D integrated circuits.

One of the driving forces behind nanotechnology research is the miniaturization of electronic devices. Electrical and thermal transport properties of device materials at micrometer and nanometer scales become very important in such applications. Carbon materials, especially carbon nanotubes (CNTs), have exceptionally low density and superior electrical, thermal, and mechanical properties. Vertically aligned CNTs attached to lightweight carbon substrates may hold the key to fully use these outstanding properties. However, the majority of studies reported to date involve either loosely unattached CNTs or CNTs attached to traditional electronic grade silicon, which have limited use in lightweight electronic components. Studies of CNT arrays attached to carbon substrates are extremely scarce, but if successful, such a composition could lead to unprecedented lightweight electronic devices with superior electrical and thermal transport properties. This dissertation is aimed at performing detailed investigation of such structures. This work investigates the synthesis-structure-property relationships of CNT arrays attached to carbon surfaces relevant to power electronic applications. Several detailed investigations were performed to achieve the goal of creating multiscale combination materials and to test their feasibility as high power electronic devices. Background studies were piloted to determine the most practical growth technique and growth parameters in order to achieve dense CNT growth. Floating catalyst chemical vapor deposition was determined to be the most effective, scalable, and reliable growth method. In addition, an oxide buffer layer was deemed necessary for dense CNTs growth on carbon substrate. Several oxides were compared in order to determine the most suitable for CNTs growth while providing superior thermal properties. Among the buffer oxides investigated in this study, the ALD Al2O3 buffer layer provided the fastest CNT nucleation and most uniform size distribution. However, Al2O3 buffer layer was plagued by adhesion issues, which may limit future applications. Plasma SiO2 offers a slower initial nucleation rate, but yields the tallest carpet height in identical growth conditions, and also appears to be the most stable and repeatable. Thermal properties investigations were conducted on the final products, which consisted of aligned CNTs arrays of different carpet heights on carbon substrates. Observations show that the thermal resistance of the CNT array varies linearly with CNT carpet height, as expected. This variation was used to estimate the thermal conductivity of multi-walled nanotube in the carpet, and found to be approximately 35W/m-K. This value shows promise that such lightweight structure can replace current commercially available products. This dissertation will reveal key results and discuss the investigations from the following areas: comparing chemical vapor deposition growth techniques, the importance of oxide buffer layer on carbon substrates, the effect of buffer layer composition, structure and thickness on CNT growth, and the feasibility of such lightweight structures for power electronics through thermal analysis investigations.

This book shows the recent advances of the applications of carbon nanotubes (CNTs), in particular, the polymer functionalized carbon nanotubes. It also includes a comprehensive description of carbon nanotubes' preparation, properties, and characterization. Therefore, we have attempted to provide detailed information about the polymer-carbon nanotube composites. With regard to the unique structure and properties of carbon nanotubes, a series of important findings have been reported. The unique properties of carbon nanotubes, including thermal, mechanical, and electrical properties, after polymer functionalization have been documented in detail. This book comprises 18 chapters. The chapters include different applications of polymer functionalization CNTs, e.g. photovoltaic, biomedical, drug delivery, gene delivery, stem cell therapy, thermal therapy, biological detection and imaging, electroanalytical, energy, supercapacitor, and gas sensor applications.

Vertically-aligned carbon nanotube (VACNT) arrays are both an important technological system, and a fascinating system for studying basic principles of nanomaterial synthesis. However, despite continuing efforts for the past decade, important questions about this process remain largely unexplained. Recently, nanotube research investigations have been conducted, aiming at revealing the underlying growth mechanisms, rather than merely studying the feasibility on new growth methods. Nonetheless, growth deactivation and the accompanying termination mechanisms still remain a topic of nanotube synthesis science. Due to the extremely small size, however, direct characterization of various transport and conversion events occurring at the catalyst surface is not an easy task. Thus investigations on growth kinetics are the first step to resolve questions about growth mechanism. Before exploring kinetic aspects of the growth process, one must achieve reliable growth conditions since growth non-reproducibility retards obtaining reliable growth data and undermines the scientific value of the data. In order to improve growth reliability, several factors that may contribute to growth non-reproducibility were identified and thereafter mitigated. Firstly, a simulation study was conducted to achieve insight into temperature and velocity profile of gases inside the reactor since gas flow dynamics can render growth environment near the substrate non-uniform. Interestingly, when argon gas was used as the main carrier gas, natural convective flow emerged, generating flow circulation before the gas reached the substrate placed at the center of the tube reactor. This flow circulation was not favorable for controlled gas introduction. This problem could be resolved by using a more heat- and momentum- conductive gas such as helium. Secondly, atomic force microscopy of annealed catalyst revealed that the aluminum sub-layer was not thermally stable at the growth temperature although this material has been widely used as a barrier layer to avoid silicide formation of catalyst on silicon substrates. In this respect, aluminum oxide should be a better choice, but under-stoichiometry of the aluminum oxide layer, which originated from sputter target degradation, affected thermal stability of the layer. Reactive sputtering by oxygen addition greatly enhanced thermal stability, and finally defect-free catalyst nanoparticles were formed by thermal annealing. Thirdly, the effect of the small part-per-million levels of oxygen-containing species on VACNT growth revealed that oxygen-containing gas impurities in nominally pure gas sources have a great influence on growth kinetics in a positive way; their presence increases catalyst lifetime and growth yield. However, the kinetic behavior that is highly sensitive to gas purity is prone to showing an interfering kinetic trend where the real mechanism is masked by the significant gas impurity effect. The stark difference in catalytic lifetime after the introduction of high-performance gas purifiers shows that extremely tight control of the reaction gas composition purity is necessary to obtain controlled growth of CNTs under atmospheric chemical vapor deposition (CVD) conditions. Finally, more reliable growth of VACNTs was achieved, and thereafter the next step for fundamental growth kinetics measurement was followed. Finally, the CVD system was equipped with an optical micrometer that enables in-situ measurement of the height of growing VACNTs, which have advantageous structure facilitating measurement of growth kinetics since the array height has a robust correlation with growth yield and thereby growth rate. Various ethylene and hydrogen combinations were examined to capture growth kinetics related to different gas environment. The measured initial growth rates were linearly proportional to ethylene concentration, whereas a reciprocal relation was observed with respect to hydrogen concentration. The apparent activation energy was higher than reported in references. Flow rate variation experiments revealed that gas phase reaction is involved as the crucial growth step, which supports the observed high activation energy. Consequently, a growth model was proposed so that it could reasonably fit the initial growth rate data. Kinetic aspects related to growth deactivation were explored by measuring the final growth height and catalyst lifetime. Unlike growth with unpurified gases, growth became much less sensitive to gas composition after purification. Importantly, it was observed that growth deactivates by deficit of carbon source when relatively low ethylene was introduced. This result is surprising since ethylene pressure should be high enough at the catalyst, considering the calculated sticking coefficient of ethylene is very low, approximately 10^-5. Thus it substantiates the idea that catalyst-mediated gas pretreatment process is critical to sustain nanotube growth. Importantly, this idea challenges the widely accepted growth termination concept whereby nanotube stops growing due to catalyst encapsulation by excessive carbon. Indeed, reduced flow rate of gas mixture increased growth yield remarkably by promoting the gas pretreatment over the catalyst. Catalyst ripening, or steric hindrance by interaction of nanotubes can be an alternative reason for growth termination, but analysis of morphologies of the annealed catalyst and as-grown nanotubes revealed that their effects were not significant for the corresponding growth conditions.