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search.filters.applied.f.advisor: Wong, C. P. × search.filters.applied.f.resourcesubtype: Dissertation ×

Publication Search Results

Now showing 1 - 10 of 22
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High Thermal Conductivity Epoxy Composites In The Application Of 3D Semiconductor Packaging

2022-11-11 , Sun, Zhijian

With the ultra-fast development of high-performance semiconductor devices through the increase of power and on-chip integration density, heterogeneous integration heat dissipation is becoming more crucial to maintain desired operating temperatures for chips. Excellent thermal management in 3D electronic encapsulation is very important because it can ensure the performance and reliability of the electronic device. Epoxy-based composites are one of the most common thermal management materials in electronic packaging due to their excellent adhesion strength, low cost, light weight, good processibility, etc. However, epoxy itself only has a thermal conductivity of around 0.2 W/mK, so it needs to combine with thermally conductive fillers, such as aluminum oxide, aluminum nitride, and metal particles to improve its thermal conductivity. However, traditional thermal management materials struggle to dissipate large amounts of heat efficiently to meet the requirements of next generation microelectronic devices. Therefore, new epoxy composites, especially those with novel nanofillers, need to be explored to maximize heat transfer efficiency. In this dissertation, graphene nanosheets are chosen as one of fillers to combine with epoxy for achieving a high thermal conductivity because of its ultrahigh thermal conductivity of 3500–5300 W/mK and large surface area of 2630 m2/g. However, graphene nanosheets easily aggerate, similar to particulate graphite platelets with low surface area, due to strong van der Waals attraction. Additionally, their surface is too smooth, resulting in poor interfacial connections with the polymer matrix. This ultimately causes phonon scattering that lowers the thermal conductivity of composites. Thus, modifying graphene nanosheets, including surface modification and morphology change, are discussed to solve these issues. In addition to the thermal conductivity of graphene-based epoxy composites, other properties like viscosity, CTE, storage modulus, and so on are also discussed for meeting the requirements of electronic packaging materials. Another filler is boron nitride nanosheets (BNNS), also known as white graphene, and it has attracted much attention due to its high thermal conductivity (200–600 W/mK), low density, and a large band gap (nearly 5.9 eV), excellent thermal stability, and superior anti-oxidation ability. These properties make it suitable for applications, requiring electrical insulation, in thermal management materials in semiconductor packaging. The modification of BNNS and pre-formed network of BNNS will also be explored. These two nanofillers can be used to create epoxy composites whose resulting properties could support the idea that these composites have potential to be applied in the next generation of semiconductor packaging materials for high-power and high-density ICs.

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Uniform high-aspect-ratio 3D micro-and nanomanufacturing on silicon by (electro)-metal-assisted chemical etching: fundamentals and applications

2016-06-10 , Li, Liyi

This dissertation is focused on a novel wet etching method, named metal-assisted chemical etching (MaCE), for fabrication of uniform high-aspect-ratio (HAR) structures on silicon (Si) in micro- and nanometer scale. In MaCE, a layer of noble metal thin film is deposited on the surface of Si and serves as the catalyst. The metal-loaded Si is immersed in hydrofluoric acid (HF)-hydrogen peroxide (H2O2) solution. A redox reaction occurs at the metal-Si interface where the Si under the metal film is preferentially etched. The metal catalyst can continue etching into Si to form HAR structures. In this dissertation, the challenge of obtaining uniform HAR structures by MaCE is firstly addressed where random movements of the metal catalyst during MaCE are observed. Then suitable experimental conditions are presented, under which uniform HAR holes and trenches on Si are successfully fabricated. The uniform MaCE phenomena are explained by the microscopic transport processes of HF and electronic holes (h+). Further, the influence of h+ transport on the 3D etching profiles is discussed. By applying external electric bias, the 3D etching profiles is effectively controlled. Further, the transport of h+ is also found to be influenced by the dopants type and the doping level of the Si substrates. Based the above findings, HAR trenches and holes with vertical sidewalls are successfully fabricated and devices built on these structures are demonstrated to work properly. The established method further shows compatibility with a novel low-cost lithography method, constituting an economic overall approach for HAR structures fabrication. Finally, uniformity of MaCE is achieved across multiple wafers that are etched simultaneously, paving the way for its application in high-volume manufacturing.

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A thin film triode type carbon nanotube field electron emission cathode

2013-10-17 , Sanborn, Graham Patrick

The current technological age is embodied by a constant push for increased performance and efficiency of electronic devices. This push is particularly observable for technologies that comprise free electron sources, which are used in various technologies including electronic displays, x-ray sources, telecommunication equipment, and spacecraft propulsion. Performance of these systems can be increased by reducing weight and power consumption, but is often limited by a bulky electron source with a high energy demand. Carbon nanotubes (CNTs) show favorable properties for field electron emission (FE) and performance as electron sources. This dissertation details the developments of a uniquely designed Spindt type CNT field emission array (CFEA), from initial concept to working prototype, to specifically prevent electrical shorting of the gate. The CFEA is patent pending in the United States. Process development enabled fabrication of a CFEA with a yield of up to 82%. Furthermore, a novel oxygen plasma etch process was developed to reverse shorting after CNT synthesis. CFEA testing demonstrates FE with a current density of up to 293 μA/cm² at the anode and 1.68 mA/cm² at the gate, with lifetimes in excess of 100 hours. A detailed analysis of eighty tested CFEAs revealed three distinct types of damage. Surprisingly, about half of the damaged chips are not electrically shorted, indicating that the CFEAs are very robust. Potential applications of this technology as cathodes for spacecraft electric propulsion were explored. Exposure to an operating electric propulsion thruster showed no significant effect or damage to the CFEAs, marking the first experimental study of CNT field emitters in an electric propulsion environment. A second effort in spacecraft propulsion is a collaboration with the Air Force Institute of Technology (AFIT). CFEAs are the payload on an AFIT developed Cube Satellite, called ALICE, to test electron emission in the space environment. ALICE has passed flight tests and is awaiting launch scheduled for 5 December 2013.

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Carbon nanotubes for thermal interface materials in microelectronic packaging

2011-11-14 , Lin, Wei

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. 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. The major achievements are summarized. 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. 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. 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. 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. 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.

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Transition Metal Complexes As Latent Catalyst And Adhesion Promoter In Epoxy Resin

2022-11-09 , Li, Jiaxiong

Epoxy based materials are widely used in electronic packaging, serving as key enablers for many structures and in various aspects determining the process efficiency and package reliability. As a thermosetting polymer, epoxy curing control towards designed temperature response and thermal profile are desirable to fulfill the needs of specific applications such as no-flow underfill in advanced flip-chip packages. As such, controllable latent catalysts have been pursued for decades. Epoxy-copper interfaces are commonly found at encapsulant, substrate and printed circuit board applications where the contacts of epoxy composites are made with lead frame, mentalizations and bond wires. The delamination and crack of epoxy-copper interfaces is one of the major failure mechanisms of a package. Traditional approaches for improving the epoxy/copper adhesion include pre-treatment of substrate with physical/chemical etching and applying coupling agents. Adhesion promoter additives in the epoxy resin would be further appreciated for saving cost and process time, as well as for accessing numerous novel structures. In addition, the covalent bond or hydrogen bond formation assisted by coupling agents are susceptible to hydrolysis degradation under moisture aging. Coordination bonds between transition metal species and organic ligands with O or N doners on the other hand are more stable against moisture, in the meantime benefitting the crosslinks at interface without being consumed by the bulk. Targeting at these issues, this dissertation explores in-formulation metal complex based chemistry for latent catalyst and adhesion promoter in epoxy resins. This dissertation systematically studies the effects of introducing a series of transition metal chelates on the curing kinetics and copper-adhesion performances of epoxy/anhydride resin systems. First row transition metal (Co(II), Ni(II), Cu(II), Zn(II)) chelate-based modifiers bearing different β-diketone ligands were used as model compounds to differentiate metal and ligand effects. The first part of the dissertation introduces the controllable curing kinetics of epoxy resin using metal chelate additives. The interaction between metal β-diketonate with Lewis base phosphine catalyst manifested distinguished and useful thermal latent cure characteristics. It was found that other than the species of metal cation, inductive effects of the diketone ligands played a crucial role in determining the metal-phosphine interaction and thus the catalytic response of the resin. In-depth feed ratio studies on the Co(II) based metal complexes in curing control helped reveal a chemical equilibrium nature of these coordination reactions. The temperature induced coordination paradigm shift in especially the hexafluoroacetylacetonate (6Facac2) chelates were examined in the second part, and the underlying ligand mediated metal-base interaction strength upon heat treatment was analyzed in detail through structural characterizations and calculations. The third part of the dissertation presents the effects of the metal complexes on the adhesion strength of epoxy-copper joints and the resistance to moisture aging. The parametric studies on transition metal complexes with different metal and ligand types provided trend plots revealing metal and ligand dependence in the resin adhesion enhancement. The mechanisms of such adhesion improvements were investigated through extensive chemical characterizations of the fractured surfaces, along with associated understandings of both copper and epoxy behaviors when incorporating transition metal chelate species. Both the favorable interfacial chemical reactions, which is related to the cure kinetics regulation discussed in the first section, and the metal-polymer coordination effects were determined to be responsible for adhesion enhancement in the metal complex doped resin systems.

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Metal-reduced graphene oxide for supercapacitors and alternating current line-filters

2015-07-22 , Wu, Zhenkun

We design a facile approach to investigate the role benzene derivatives play in the capacitance enhancement of graphene-based supercapacitors. The main reason is attributed to the pseudocapacitance of the aromatic molecules rather than the former one. Meanwhile, we find that the para and ortho substituted benzene derivatives contribute much more than the meta substituted ones. In addition, we fabricate an all-solid-state flexible MSC based on metal-reduced GO. The as-fabricated MSC shows high areal capacitance and excellent reliability, which makes it a promising energy storage candidate for wearable electronics. Based on the work of MSC, we achieve a flexible ac line-filter that is not only competitive against commercial product but also suitable for mass production. Meanwhile, we produce a three-dimensional graphene/polydimethylsiloxane composite that gives a thermal resistance as small as 14 mm2K/W, which is comparable to commercial products. What’s more, a convenient transient program that saves much time is developed to measure the thermal resistance.

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Superhydrophobic surfaces for electronic packaging and energy applications

2013-05-17 , Liu, Yan

Superhydrophobic surfaces, which display water contact angles of larger than 150°, have attracted more and more attention due to their importance in both fundamental research and practical applications. This dissertation is mainly focused on the fundamental understanding and exploring applications of superhydrophobic surfaces. First, some specific examples of superhydrophobic surface fabrication were given, which include superoleophobic Si surface, robust superhydrophobic SiC surface, and reversible wettability nanocomposite films. Based on the study of superhydrophobic surfaces, the application of superhydrophobic surfaces in electronic packaging were explored. Superhydrophobic silica/epoxy nanocomposite coating serves as an encapsulant to improve the electronic device reliability. Such superhydrophobic coating showed good stability under humidity at elevated temperatures and was applied on the triple track resistors test coupons. In addition, the applications of superhydrophobic surfaces in solar cells were studied. Two multi-functional hierarchical structure solar cells with self-cleaning, low reflection and high efficiency properties were built up by coating or etching methods.

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Functional polymer composite encapsulants for electronic packaging

2017-07-28 , Tuan, Chia-Chi

Polymer-based materials have attracted more and more interests in recent years for fundamental studies and for practical applications, for they combine material benefits of both the polymer matrix and the inorganic filler. In electronic packages, polymer composites are commonly used for the applications of encapsulants, underfills, and molding compounds using their mechanical, thermomechanical, and optical properties. This thesis is mainly focused on the understanding and applications of nanocomposite materials in electronic packaging. First, high refractive index, silicone-based LED encapsulants were fabricated by incorporating TiO2 nanoparticles. The surfaces of nanoparticles were modified with silane surfactants during and after nanoparticle syntheses, and the method of surface modification significantly affected the particle dispersion and size control, both of which were shown to be correlated to the optical performance of nanocomposite encapsulants. Encapsulant with refractive index > 1.7 and relative transmittance > 90% was demonstrated, and the nanocomposite also showed resistance to thermal cycling degradation under high humidity conditions. Expanding from the study of filler dispersion, the interface between filler and polymeric matrix was further investigated in silica-epoxy nanocomposites for underfill application. A two- layer silica surface modification method was employed, where the inner layer served as coupling agent and the outer polysiloxane layer served to absorb stress and toughen the nanocomposite. Compared to unmodified or silane-modified silica, the two-layer modified silica fillers also showed improved interphase properties as shown in thermomechanical and mechanical properties, including higher glass transition temperatures, lower thermal expansion in the underfills, and stronger silica-epoxy adhesion. With the understanding of underfill composition and properties, we further explored methods to control the flow of nanocomposite underfill and to reduce filler entrapment in solders for 3D IC packaging. Fluid control on hydrophobic/hydrophilic patterned surfaces were simulated to determine the critical contact angles the surface. Superhydrophobic Cu bond pads and hydrophilic Si3N4 were fabricated according to the computational analyses. Self-patterning of underfill was demonstrated, as well as the interconnection bonding using the superhydrophobic Cu. Filler entrapment is shown to be reduced using this technology for enhanced interconnect reliability.

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Rational design of electrically conductive polymer composites for electronic packaging

2014-04-07 , Li, Zhuo

Electrically conductive polymer composites, i.e. polymers filled with conductive fillers, may display a broad range of electrical properties. A rational design of fillers, filler surface chemistry and filler loading can tune the electrical properties of the composites to meet the requirements of specific applications. In this dissertation, two studies were discussed. In the first study, highly conductive composites with electrical conductivity close to that of pure metals were developed as environmentally-friendly alternatives to tin/lead solder in electronic packaging. Conventional conductive composites with silver fillers have an electrical conductivity 1~2 orders of magnitude lower than that of pure, even at filler loadings as high as 80-90 wt.%. It is found that the low conductivity of the polymer composites mainly results from the thin layer of insulating lubricant on commercial silver flakes. In this work, by modifying the functional groups in polymer backbones, the lubricant layer on silver could be chemically reduced in-situ to generate silver nanoparticles. Furthermore, these nanoparticles could sinter to form metallurgical bonds during the curing of the polymer matrix. This resulted in a significant electrical conductivity enhancement up to 10 times, without sacrificing the processability of the composite or adding extraneous steps. This method was also applied to develop highly flexible/stretchable conductors as building block for flexible/stretchable electronics. In the second study, a moderately conductive carbon/polymer composite was developed for use in sensors to monitor the thermal aging of insulation components in nuclear power plants. During thermal aging, the polymer matrix of this composite shrank while the carbon fillers remained intact, leading to a slight increase in filler loading and a substantial decrease in the resistivity of the sensors. The resistivity change was used to correlate with the aging time and to predict the need for maintenance of the insulation component according to Arrhenius’ equation. This aging sensor realized real-time, non-destructive monitoring capability for the aging of the target insulation component for the first time.

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Development of metal-assisted chemical etching as a 3D nanofabrication platform

2012-05-07 , Hildreth, Owen James

The considerable interest in nanomaterials and nanotechnology over the last decade is attributed to Industry's desire for lower cost, more sophisticated devices and the opportunity that nanotechnology presents for scientists to explore the fundamental properties of nature at near atomic levels. In pursuit of these goals, researchers around the world have worked to both perfect existing technologies and also develop new nano-fabrication methods; however, no technique exists that is capable of producing complex, 2D and 3D nano-sized features of arbitrary shape, with smooth walls, and at low cost. This in part is due to two important limitations of current nanofabrication methods. First, 3D geometry is difficult if not impossible to fabricate, often requiring multiple lithography steps that are both expensive and do not scale well to industrial level fabrication requirements. Second, as feature sizes shrink into the nano-domain, it becomes increasingly difficult to accurately maintain those features over large depths and heights. The ability to produce these structures affordably and with high precision is critically important to a number of existing and emerging technologies such as metamaterials, nano-fluidics, nano-imprint lithography, and more. Summary To overcome these limitations, this study developed a novel and efficient method to etch complex 2D and 3D geometry in silicon with controllable sub-micron to nano-sized features with aspect ratios in excess of 500:1. This study utilized Metal-assisted Chemical Etching (MaCE) of silicon in conjunction with shape-controlled catalysts to fabricate structures such as 3D cycloids, spirals, sloping channels, and out-of-plane rotational structures. This study focused on taking MaCE from a method to fabricate small pores and silicon nanowires using metal catalyst nanoparticles and discontinuous thin films, to a powerful etching technology that utilizes shaped catalysts to fabricate complex, 3D geometry using a single lithography/etch cycle. The effect of catalyst geometry, etchant composition, and external pinning structures was examined to establish how etching path can be controlled through catalyst shape. The ability to control the rotation angle for out-of-plane rotational structures was established to show a linear dependence on catalyst arm length and an inverse relationship with arm width. A plastic deformation model of these structures established a minimum pressure gradient across the catalyst of 0.4 - 0.6 MPa. To establish the cause of catalyst motion in MaCE, the pressure gradient data was combined with force-displacement curves and results from specialized EBL patterns to show that DVLO encompassed forces are the most likely cause of catalyst motion. Lastly, MaCE fabricated templates were combined with electroless deposition of Pd to demonstrate the bottom-up filling of MaCE with sub-20 nm feature resolution. These structures were also used to establish the relationship between rotation angle of spiraling star-shaped catalysts and their center core diameter. Summary In summary, a new method to fabricate 3D nanostructures by top-down etching and bottom-up filling was established along with control over etching path, rotation angle, and etch depth. Out-of-plane rotational catalysts were designed and a new model for catalyst motion proposed. This research is expected to further the advancement of MaCE as platform for 3D nanofabrication with potential applications in thru-silicon-vias, photonics, nano-imprint lithography, and more.

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