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School of Materials Science and Engineering

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College of Engineering

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Now showing 1 - 10 of 38

High Thermal Conductivity Epoxy Composites In The Application Of 3D Semiconductor Packaging

(Georgia Institute of Technology, 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.
Transition Metal Complexes As Latent Catalyst And Adhesion Promoter In Epoxy Resin

(Georgia Institute of Technology, 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.
Epoxy/triazine based high performance molding compound for next generation power electronics packaging

(Georgia Institute of Technology, 2019-07-26) Li, Jiaxiong

The power electronics industry has been actively seeking encapsulant materials that can serve in harsher environments. For example, with the power semiconductors leading into SiC era, the higher operation temperature (250 ºC) have proposed great challenges on the packaging materials especially on epoxy molding compound (EMC) technologies, since the temperature exceeds the stability limit of typical epoxy (EP) chemistry. In this thesis, EP/triazine system was selected to develop high temperature stable resin system that can meet the temperature requirement of next generation power electronics packaging. In the first part of the thesis, different approaches were discussed to enhance the high temperature performance of a previously studied cyanate ester (CE)/ biphenyl EP blend which is impaired by the hydrolysis degradation of remaining cyanate groups. Firstly, the effects of different metal catalyst on the CE properties were discussed. Secondly, a triazine containing molecule triglycidyl isocyanurate (TGIC) was employed to increase the triazine content without increasing CE feed ratio to circumstance problem of unreacted cyanate groups. Finally, the high heat resistant novolac type CE was employed to form the NCE/EP blend, and their blends with different feed ratio were systematically evaluated. In the second part, a detailed characterization of a high heat resistant CE/novolac type EP blends and the investigation on their degradation under long-term high temperature storage were summarized. The effects of the CE concentration on the thermomechanical properties of the copolymer were explored, where a tradeoff behavior between the triazine content and crosslink density was accounted for the property change. In addition, the distinguished thermal degradation mechanisms in copolymer with different compositions were identified and illustrated. The knowledge obtained in this work can serve as a reference in the future to formulate EP/triazine based resin system for high temperature applications.
Encapsulation and design of scalable packaging materials for thin film perovskite solar cell applications

(Georgia Institute of Technology, 2019-04-25) Hah, Jinho

There have been many attempts to improve the stability of the environmentally-sensitive perovskite solar cells (PSCs) from adverse environments. The next generation encapsulation method should be compatible with roll-to-roll (R2R) processing, which can manufacture thin-film PSC modules at large scale and make solar electricity economically competitive with conventional electricity generation. This work investigates the interface chemistry between the polymer backsheet and the polymer encapsulants to understand the moisture, thermal, and UV stability of the packaging materials for PSCs. First, surface modification on the commercially available PET backsheets was done using various types of silane-based coupling agents, and their adhesion profiles were studied upon damp-heat exposure on these samples. Second, thorough XPS analysis was conducted on the delaminated PET surface from the PET/EVA/PET encapsulation architecture upon the UV, thermal, and moisture aging to understand the degradation mechanism at the interface. Moreover, this work also includes encapsulant design by combining the polymer blends to improve the mechanical and chemical bulk properties of a PV encapsulant. In short, this work serves to investigate on the encapsulation methods to improve the reliability and lifetime of PSCs.
Study on epoxy based composites for high temperature molding compounds

(Georgia Institute of Technology, 2018-07-19) Wu, Fan

Epoxy molding compound (EMC) is one of the most widely used encapsulation materials for electronic packaging. To provide substantial protection for the electronic packages, EMCs are frequently required to work at elevated temperature, especially when high power and high density devices are developing rapidly and more heat is generated during operation. This thesis discussed the study on epoxy based composites for high temperature molding compounds by investigating two components most important in EMC system, namely the polymer resin and the filler system. In order to increase the glass transition temperature (Tg) and thermal stability of epoxy resin, cyanate ester was incorporated into the polymer matrix. The copolymer network formed by epoxy and cyanate ester (CE/EP) exhibited excellent thermal stability and high Tg above 270 ℃ because of the thermally stable s-triazine structures formed by cyanate ester trimerization. However, cyanate ester was affected by the hydrolysis reaction and too much cyanate ester in the system led to blistering and Tg drops in high temperature and high humidity tests. The cyanate ester amount in this copolymer was optimized to be 33-50 %. Polyimide was incorporated into CE/EP system as an additive (CE/EP-PI) to further improve the thermal stability of this epoxy-based resin. Aromatic polyimide exhibited good compatibility with CE/EP for their structural similarity. Improvements in Tg, storage modulus, fracture toughness and long term high temperature performance were observed at 5-10 % polyimide loading. At high polyimide loading level (> 10 %), a secondary phase emerged which deteriorated the resin properties such as storage modulus. The second part of this thesis investigated a modified filler system with surface coated silicon carbide (SiC) for thermal conductivity enhancement. In this part, SiC with high thermal conductivity was adopted as a replacement for conventional silica fillers. After surface treatment by silane coupling agent (SiC-GPTMS) and reactive silicon rubber (SiC-A15), modified SiC increased the thermal conductivity of the composites from 0.11 W/mK to 0.28 W/mK at 30 % filler loading.
Functional polymer composite encapsulants for electronic packaging

(Georgia Institute of Technology, 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.
Uniform high-aspect-ratio 3D micro-and nanomanufacturing on silicon by (electro)-metal-assisted chemical etching: fundamentals and applications

(Georgia Institute of Technology, 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.
Metal-reduced graphene oxide for supercapacitors and alternating current line-filters

(Georgia Institute of Technology, 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.
Rational design of electrically conductive polymer composites for electronic packaging

(Georgia Institute of Technology, 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.
A thin film triode type carbon nanotube field electron emission cathode

(Georgia Institute of Technology, 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.