Carbon Nanotubes for Electronic Packaging: Growth, Novel Devices and 3D Networks
Abstract: Carbon nanotubes (CNTs) have shown great potential of application in electronics because of their attractive physical properties, such as high surface-to-volume ratio, high electron mobility, high Young’s modulus, high thermal conductivity, low thermal expansion coefficient, etc. However, many obstacles are yet to be removed to use CNTs as building blocks in electronic systems. This thesis is attempting to overcome some of the challenges that hinder the implementation of CNTs in electronic packaging. The first section concerns about the most essential issue in CNT application, the growth of CNTs. A new method is presented and it is capable of highly selective growth of CNTs, which is one of the main challenges in the CNT technology field. By growing CNTs on gold films using thermal chemical vapor deposition (TCVD) method, the number of CNT walls can be well controlled and statistical study shows that a selectivity of 79.4% can be achieved for double-walled CNTs. The second section presents the design and fabrication of a CNT based on-chip cooler, attempting to utilize CNTs for heat dissipation from electronic systems. A test platform with integrated heating elements and temperature sensors is designed and fabricated for this purpose. A new CNT transfer technology is developed to transplant the CNT microfins with pre-defined structures to desired positions on the test platform. Electrical and mechanical characterizations demonstrate that the CNT-substrate interface has been dramatically improved after the transfer process therefore addressed two of the most challenging tasks on integrating CNTs into electronics, i.e. decreasing the huge interfacical contact resistance and improving the weak adhesion between CNTs and substrates. A double-coated single photoresist structure for thick materials deposition is also developed to facilitate and simplify the CNT transfer process. Multi-scale modeling is performed to help design the CNT microfin structure. A molecular dynamics simulation (MDS) is carred out to investigate the interaction between water and CNTs at the nanoscale and a finite element method (FEM) modeling is executed to analyze the fluid field and temperature distribution at the macroscale. A novel packaging process using polydimethylsiloxane (PDMS) is also developed to assemble the CNT microfins, the test platform, the fluid channel and the supporting substrate into an integrated system. Cooling experiments have demonstrated the high efficiency of the CNT-based on-chip cooler. The third section describes a novel concept to grow covalently bonded three dimensional CNT networks, which is potentially applicable to facilitate thermal transportation in micro systems. A nickel nitrate dissolved polymer is electrospun into inter-connected porous nano fiber networks acting as precursor. The polymer fibers are then burned out and nickel nitrate is decomposed at high temperature into nickel oxide fibers. These fibers are subsequently reduced into pure nickel in hydrogen environment. The as-reduced nickel nano fiber network serves as catalyst to grow graphite layers on their surfaces. Ultimately, the nickel core is etched by iron chloride so that graphite tubes, i.e. CNT structures are left. Since the nickel fibers are inter-connected with each other and graphite layers can only grow on the surface, the as-grown CNT network is covalently bonded. Atomic force microscopy (AFM) based bending tests show that the Young’s modulus of the as-grown CNTs is about 〖391〗_(-172)^(+270) GPa, and the covalently bonded CNT structure can effectively distribute external loading throughout the network to improve the mechanical strength of the material.
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