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School of Materials Science and Engineering

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College of Engineering

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Characterization of Commercial Dielectric Zaristo-700 as a Redistribution Layer Material for Advanced Packaging

(Georgia Institute of Technology, 2023-12-06) Madelone, Sophia Marie

This body of work, in detail, outlines the fundamental steps taken to characterize a material for novel use in RDL build-up layers for advanced packaging. The material (Zaristo-700) discussed in this thesis was only used in RF applications, and now we are exploring its use in the wiring layers. In the PRC, research into thin films, spin-on films, and many other dielectrics have been published before. It is essential to understand that this work is necessary to establish a “library” or catalog of information on all the materials we use to provide the correct material, depending on the goals of future projects. The material and electrical properties of Zaristo-700 are characterized through JEDEC adhesion testing (Peel test), leakage current measurements on ITO glass slides before and after Highly Accelerated Stress Testing (HAST) treatment, a series of dose tests to document the most optimized pitch-scalability at 8.0 m L/S, and lastly Shadow-Moire warpage studies of one layer and three-layer RDL samples. Leakage current measurements taken before and after HAST stayed at or below 2.0 nA. As we will explore, the CTE and adhesion of Zaristo-700 are excellent and contribute to making a great material for the RDL wiring layers. Taiyo Ink. has stated that this version of the dielectric film accounts for issues such as stability in how long it can sit, delamination during or after curing, delamination during fabrication processes, and so on. Whereas some of the dielectric films of other companies still have these problems. This research is working towards answering the unknowns about this dielectric and how well it will function as a future RDL build-up material through characterization and analysis of its properties. These results are a positive indication for use as an RDL dielectric in advanced packaging.
Atomic Layer Deposition As A Method Of Fine Tuning The Surface Chemistry Of Oxide Materials

(Georgia Institute of Technology, 2023-12-05) Yom, Typher

Atomic layer deposition (ALD) is a vapor-phase synthesis method in which a material is deposited onto a surface with precise atomic thickness. Through ALD, ultra-thin monolayers of oxide materials can be deposited onto powders, creating mixed oxide surfaces with tunable surface chemistries, enabling their usefulness towards catalytic processes in the petrochemical and fine chemical industries. ALD holds an advantage over typical solution-phase methods of creating mixed oxide materials due to the latter’s difficulty in controlling the surface composition, making analysis difficult. However, if we can better understand the interactions of the surface in solution, it can be used to design more effective catalysts. One way to observe this is by studying the zeta potential of the surface, which is directly correlated with surface charge and is a product of these acid-base interactions at the interface. Each material can be identified using the isoelectric point, which is the point at which the zeta potential/net surface charge is zero. For mixed metal oxides, their isoelectric points were calculated in the literature to be the summation of each individual component’s isoelectric point multiplied by its surface coverage. However, this calculation assumes that the components do not interact with each other when mixed. In order to investigate this discrepancy, we used ALD to deposit thin layers of titanium oxide onto silicon oxide powders. If we were to assume the equation used in the literature, we can assume that one single monolayer over the surface would be sufficient to convert the isoelectric point from that of silicon oxide to that of titanium oxide. However, our results have indicated that the isoelectric point did not reach that of titanium oxide until multiple monolayers were deposited, indicating that a different model/equation must be utilized to better elucidate the surface behavior. Additionally, during these studies of the isoelectric point, we have formulated an equation that can correlate the thickness of ALD-deposited films with the material’s relative atomic percent. This equation was created by assuming that the shape of the particle + film retains its shape, and therefore its volume formula, allowing it to work for ultra-thin films, but not for much thicker films. Finally, this thesis highlights the importance of being mindful of the precursor used for powder ALD: precursors like TiCl4 can create byproducts like HCl from the reactor walls and the powder itself. These byproducts can then adsorb onto the powder surface, which can block film growth or affect the pH of the resulting solution when the powder is dispersed in water. Extra measures, such as a double dose or a post-process washing step, were implemented, and should be used when performing powder ALD.
Stability Studies of ALD Films and Infiltrated Hybrid Materials

(Georgia Institute of Technology, 2022-12-19) Fairach, Selma Raquel

Aluminum oxide (alumina) thin films deposited through atomic layer deposition (ALD) are of great interest in chemical barrier and corrosion protection applications. However, the stability of ALD alumina in aqueous solutions is still not fully understood. Due to its metastable amorphous phase, the hydration and degradation behavior of ALD alumina films behaves differently from its crystalline Al2O3 counterpart. A full understanding of why these films hydrate and/or dissolve requires the exploration of different deposition conditions and ion content in solutions used. This thesis will discuss efforts to further elucidate the hydration and dissolution behavior of ALD alumina films. For this study, alumina thin films were ALD deposited onto silicon substrate at 150 °C using trimethylaluminum (TMA) and H2O. These films were then studied in Type 1 DI water and different concentrations of NaCl solutions at room temperature. Films were gently dried using a nitrogen gun and thickness was measured using a Cauchy ellipsometry model. After 15 days of immersion in Type 1 DI water, significant thickness growth is observed at twice (27 days) and 2.5 times (33 days) the normalized thickness. Similar hydration is not observed in salt-containing aqueous solutions nor upon exposure to air. This thesis will discuss the possible effects of CO2 dissolution and carbonate formation as well as ionic species on the hydration and dissolution processes of these alumina films. Similar stability applications are observed in the infiltration of hybrid organic-inorganic electronic devices through vapor phase infiltration (VPI). Spiro-OMeTAD is one of the most-studied hole transport layer (HTL) materials in perovskite solar cells, but it is known to degrade quickly due to thermal effects as well as gold diffusion into the layer. This thesis will discuss how infiltration on TiCl4 and H2O into Spiro-OMeTAD layers can change the thermal properties and prevent early degradation. Upon infiltration of TiOx, the formation of crystals observed on Spiro-OMeTAD layers decrease, and at 10 hours of infiltration, the glass transition temperature of the film is found to decrease almost 15 °C. Similar behavior is not found in samples that are thermally controlled, indicating that this behavior is not an annealing effect. This thesis will discuss all the different thermal stability results with varying infiltration times and precursors used.
Phase Transformations in Atomic Layer Deposited (ALD) Titanium Dioxide (TiO2) Thin Films

(Georgia Institute of Technology, 2022-08-24) Wooding, Jamie Pearce

Atomic layer deposition (ALD) is a widely employed chemical vapor deposition (CVD) technique designed to grow thickness-controlled and highly conformal thin films. ALD can be used to functionalize surfaces, build multi-layer thin film stacks, or develop nano-structured composites. As such, ALD is a crucial thin film deposition process suitable for application in semiconductor processing, microelectromechanical systems (MEMS), stabilization of photovoltaics and energy storage devices, and development of catalysts and sensors. The defining feature of ALD is the self-limiting chemistry and surface half-reactions that, followed by sufficient inert gas purges, can ensure conformal, pin-hole free thin films. Once process is tailored to ensure ideal ALD behavior, deposited film properties are primarily governed by process temperature, substrate, film thickness, precursor selection/film impurities, and plasma enhancement, which cause changes in the as- deposited structure and result in shifts in electrical, optical, chemical, and mechanical properties. In vapor deposition literature, the impingement flux of the gaseous precursor on the growing film is a key parameter in ensuring depositing film quality and properties. But in ALD literature, the emphasis on self-limiting surface half reactions, precursor chemistry, and industrial requirements to reduce total process time have obscured the potential for structural rearrangement during/after the ALD cycle. As such, the purpose of this work is to improve understanding of as-deposited film structure and introduce cycle time and atmosphere selection as ALD process parameters. Selecting titanium dioxide (TiO2) as the model material system, first I study the effect of low-temperature post-deposition annealing on amorphous as-deposited TiO2-ALD thin films. Resultant TiO2-anatase grain size is found to be dependent on ALD deposition temperature and not on post-deposition annealing temperature. This implies a structure difference in as-deposited amorphous films as a function of ALD deposition temperature. Post-deposition annealing (PDA) is performed to study the amorphous to anatase phase transformation kinetics. Anatase is found to form via a two-dimensional growth mode. The phase transformation reaction rate is deconvoluted into nucleation rate and growth rate. Nucleation is found to be the rate-limiting step for the phase transformation. Further, the nucleation rate frequency factor is found to increase with increasing deposition temperature, implying amorphous films deposited at higher temperature have increased vibrational modes. I develop a model for understanding the resultant microstructure with changes in deposition temperature, nucleation rate, and grain growth rate. Second, I study what is limiting crystallization during ALD of TiO2. Thermal-ALD of TiO2 films from the alkyl amide precursor and water chemistry grow amorphous for deposition temperatures up to 220 °C while TiO2 films from ALD of the chlorinated precursor are crystalline as-deposited above 150 °C. I introduce an intermittent controlled atmosphere (ICA) annealing step during the ALD cycle to encourage growth of fully crystalline TiO2 thin films at 180 °C and less than 50 nm. As-deposited films without the air anneal are amorphous and sub-oxidized while those with the in situ air anneal are crystalline with fewer Ti3+ states. Additionally, I vary process conditions to highlight the effectiveness of atomic rearrangement during the ALD cycle compared against bulk diffusion during PDA. Finally, I present results regarding the importance of purge time during ALD for crystal formation and regarding TiO2-brookite formation with post- deposition annealing (PDA). Overall, a PDA method is developed to probe as-deposited amorphous film structure, an ALD process variation (ALD-ICA) is introduced to encourage phase transformation during deposition, and I propose that oxidation state is limiting TiO2 crystallization during growth from tetrakis(dimethylamino)titanium(IV) TDMAT/H2O thermal-ALD.
Quaternary Ammonium Cation bis-MPA Dendron PDMS Hybrid Copolymers: Synthesis, Characterization, Behavior And Functional Use As Macromolecular Additives In Sylgard 184

(Georgia Institute of Technology, 2022-05-23) Marks, Monica Abigail

This work aims to address the research question: Can quaternary ammonium cation dendron-PDMS hybrid copolymers be utilized as surface active macromolecular additives in Sylgard 184? In attempts to answer this question several steps were taken: • Synthesis of a library of quaternary ammonium cation (QAC) small molecule zwitterion building blocks of different hydrophobicity (alkyl chain lengths C4, C8, C12, C16) • Synthesis of a library of QAC bis-MPA dendrons of different branching (G1, G2 G3, G4) and different hydrophobicity (C4, C8, C12, C16) using esterification chemistry • Synthesis of a library of QAC bis-MPA dendron-PDMS hybrid copolymers of different branching (G1, G2, G3, G4) and different hydrophobicity (C4, C8, C12, C16) using CuAAC “click” chemistry • Development of processing and cure conditions in order to incorporate hybrid additives into Sylgard 184 QAC bis-MPA dendrons were tested in solution for antimicrobial activity against gram negative E. coli, gram positive S. epidermidis and yeast S. cerevisiae. Preliminary surface testing was performed in combination with cure profile. In addition, dendron-PDMS hybrid behavior was probed via several physical characterization techniques. Ultimately, this work illustrates the first reported QAC bis-MPA type dendrons and their biocidal activity, the first reported dendron-PDMS hybrid copolymers and preliminary studies displaying their potential use as surface active additives in Sylgard 184.
Quantifying Charge Transport in Chemically Doped Semiconducting Polymers

(Georgia Institute of Technology, 2022-04-27) Gregory, Shawn A.

Semiconducting polymers are a class of materials that engenders the solution processibility, mechanical compliancy, and biocompatibility of archetypal polymeric materials with the charge transport properties, optical properties, and device physics of archetypal inorganic semiconductors. Oftentimes, pristine semiconducting polymers are electrically insulative (σ<〖~10〗^(-4) S cm-1) with comparatively few mobile charge carriers with low mobilities. The charge carrier density and mobility can be increased via chemical doping, and chemical doping oftentimes involves adding or removing charge carriers from the pristine polymer via a redox chemical reaction. Ultimately, the resulting optical and electronic properties of chemically doped semiconducting polymers is a convoluted function of multiple parameters, including polymer chemistry, dopant chemistry, and processing techniques. While this convolution enables a nearly infinite number of permutations, each of which can be designed for a specific application, this convolution obfuscates the establishment of charge transport models, and fundamental process-structure-property relationships. In this thesis, I developed and compiled experimental methods, which are used to create and substantiate novel charge transport models, which are then used to contextualize the charge transport properties of chemically doped semiconducting polymers. This thesis begins by reviewing the electronic structure of materials, solid-state charge transport physics, and state of the art literature in organic electronics and thermoelectrics. Afterwards, a novel charge transport model (semi-localized transport, SLoT) is derived and applied to literature studies. The utility of the SLoT model is its ability to quantify both localized (hopping-like) and delocalized (metal-like) contributions to the observable transport properties and quantify key transport parameters such as the localization energy in the dilute doping limit, the carrier density need for delocalized transport, and the maximum hypothetical electrical conductivity. The SLoT model is then used to contextualize the transport properties in several chemically doped semiconducting polymer systems, that have systematic changes to the polymer chemistry, dopant chemistry, and doping level. Although the SLoT model captures the transport properties of several systems, it has shortcomings with electro-thermal transport modeling, so one chapter is devoted to developing models that capture this phenomenon. This work is concluded by detailing future experimental methods, transport models, and material systems that ought to be explored for the rational advancement of organic electronics and thermoelectrics.
Vapor Phase Infiltration: Sorption Thermodynamics, Chemical Entrapment Mechanisms, and Hybrid Material Structure-Property Relations

(Georgia Institute of Technology, 2021-08-23) McGuinness, Emily K.

Vapor phase infiltration (VPI) creates hybrid organic-inorganic materials by infusing the sub-surface of polymers with vapor phase, metal containing precursors. These volatile materials are often then co-reacted with an oxidant to form a final state (commonly a metal oxide) that is incorporated within the polymer at the molecular to nanoscale level. The chemistries and processes used in VPI direct how the inorganic is included within the polymer and therefore dictate the hybrid material’s ultimate properties. Generally, the whole of the properties evoked via VPI are inaccessible by the organic and inorganic portions alone. In addition to nanoscale incorporation, VPI offers the advantage of leaving the macroscale form of the polymer unchanged, allowing for the post-fabrication modification of polymeric materials such as membranes, fabrics, etc. In this thesis, VPI is explored from a materials science and engineering perspective where characterizing the influence of processing parameters on material structure opens opportunities for new application spaces. Following an introduction to VPI and an assessment of the state of the art in VPI literature, this work explores the influence of VPI processing parameters (temperature, precursor exposure times, precursor dose pressures, etc.) and system chemistries on thermodynamic and kinetic principles. This exploration culminates in a proposed model for mathematically describing VPI processes that feature reactions between polymer functional groups and precursors. This knowledge is then employed to broaden the application space of VPI by studying how VPI can improve the solvent stability of both commodity polymers and polymer membranes. Finally, an exploration of the durability of the hybrid materials created via VPI is conducted. The work is concluded by looking into with future prospects and considerations for the VPI process.
An Investigation into the Glass Transition Temperature of Vapor Phase Infiltrated Organic-Inorganic Hybrid Materials

(Georgia Institute of Technology, 2021-05) Bamford, James

Glass transition temperature (Tg) is a fundamental property of a polymer that defines its upper service temperature for structural applications and is reflective of its physicochemical features. We are interested in how vapor phase infiltration (VPI), which infuses polymers with inorganic species to create hybrid materials, affects the glass transition temperature of a material. We examine Al2O3 VPI into poly(styrene-co-2-hydroxyethyl methacrylate) (PS-r-PHEMA) using trimethylaluminum (TMA) and water precursors. Our VPI precursors are selected to be unreactive towards the styrene monomer units and highly reactive towards the HEMA monomer units. Experiments were conducted on PS-r-PHEMA thin films (200 nm) spun-cast onto silicon wafers and infiltrated at 100°C with 4 hr. exposure times. Copolymers with varying fractions of HEMA units were investigated, from 0 mole % to 20.2 mole % HEMA. Volumetric swelling of the films after VPI and aluminum oxide film thicknesses after pyrolysis both confirmed higher metal oxide loading with higher fraction HEMA units. Tg was measured using a spectroscopic ellipsometer with a heating unit. We find that the glass transition temperature increases significantly with metal oxide loading. Copolymers with 0.0%, 3.0%, 7.7%, 11.5%, and 20.2% HEMA units experienced 6°C, 8°C, 22°C, 37°C, and 46°C increases in Tg respectively. Changes in Tg at low HEMA composition fit the Fox-Loshaek model for crosslinking phenomena which, along with a dissolution study on these materials, suggests that VPI alumina crosslinks PS-r-PHEMA. We conclude that VPI may be useful as a crosslinking process for designing the thermophysical and thermochemical properties of polymer thin films, fibers, and fabrics.
INVESTIGATION OF PROCESSING-STRUCTURE-PROPERTY RELATIONS IN VAPOR PHASE MODIFIED CELLULOSIC MATERIALS

(Georgia Institute of Technology, 2021-04-22) Li, Yi

Cellulosic materials are widely used in our daily lives for paper products and functional polymers. The cellulose molecule has a high density of hydroxyl groups, which causes strong intra-/inter- fiber hydrogen bonding. These abundant hydroxyl groups make cellulose super-hydrophilic and difficult to disperse or dissolve in nonpolar organic solvents or polymers. The traditional methods to functionalize cellulose is either surface modification or regeneration. Vapor phase modification of cellulose has gained interest in recent years. Instead of using liquid phase precursor solutions, vapor phase processing uses gas molecules as precursors to realize surface coatings with better uniformity and consistency amongst batches. Atomic layer deposition (ALD) technique can be conducted at relative lower reaction temperatures (25 – 300 ℃) and realize a conformal coating on substrates with high aspect ratios. Reported literatures on ALD modified cellulose are more focus on functional coatings, replicas, physicochemical property of new generated materials, while less focus has been given to studying the reaction mechanisms and underlying physics of the physicochemical property changes. This work focuses on the study of the initial cycle’s reaction mechanism and process-structure-property relation for TMA and water reacting with cellulose substrates. Different cellulose products (chromatography paper, cotton ball and cellulose free-standing film made from cellulose nanofibrils) were investigated for their corresponding properties after ALD reaction. Specifically, this work contains three sections, which are (1) Investigating the heat stimulated surface wettability transition after “low” cycle ALD reaction on cellulosic materials, then apply different wetting models to explain this wettability transition. (2) Investigating the reaction mechanism and resulting physicochemical property difference for low cycle of TMA and water ALD reacted cellulose nanofilms. (3) Developing new ALD processing recipes to modify cellulose nanofilms by exploring the effect of the TMA exposure time to alter the cellulose’s chemistry and physical microstructure This study provides a novel insight of surface property and mechanical property control for cellulosic materials through ALD process parameters development and will offer guideline information for future process development and substrate selection to achieve designed material property.
ULTRA-LOW DIELECTRIC CONSTANT AND ULTRA-THIN POLYMER DIELECTRIC MATERIALS, PROCESSES AND RELIABILITY FOR ULTRA-HIGH BANDWIDTH COMPUTING APPLICATIONS

(Georgia Institute of Technology, 2020-04-15) Dwarakanath, Shreya

The increase in the number of connected devices in homes, cars and offices coupled with the growth of advanced data processing algorithms enabled by artificial intelligence (AI) has been driving an unprecedented need for ultra-high bandwidth computing. At the package level, the bandwidth increase can be achieved by increasing the number of input-output (I/O) connections or by increasing the data rate for each connection. The number of I/Os depends on the wiring density supported by each layer and the number of layers. These layers have to be vertically spaced at ultra-small distances to enable high-wiring density. The data rate is primarily influenced by the dielectric constant or Dk. Hence, the focus of this research is to develop ultra-low Dk and ultra-thin polymer dielectric materials, processes and reliability to meet the next-generation computing needs of ultra-high bandwidth. Silicon back-end-of line (BEOL) wiring has significant limitations such as high RC delays because of the choice of dielectric materials and cost. Current organic materials and processes are limited by their incapacity to scale to fine-features because of thick dielectric materials and poor dimensional stability of the core. This research is focused on overcoming the limitations of current approaches and demonstrating the potential for ultra-low Dk, ultra-thin polymer dielectrics to signal at higher data rates, establishing process guidelines for panel-scalable and low-cost processes and investigating the reliability of ultra-thin, ultra-low Dk dielectrics/copper interfaces, thus leading to enhanced electrical performance and lower cost compared to silicon BEOL and current organic RDL. The specific objectives of this thesis are to a) develop ultra-low-Dk (< 3.0) and ultra-thin (2-5 µm) polymer dielectric materials with optimal properties for high-signal speed, b) develop panel-scale processes for ultra-thin dielectrics with high surface planarity capable of supporting fine line/spaces of < 2 µm line width/space and < 5 µm diameter vias and, c) investigate the thermo-mechanical and chemical reliability of polymer/copper interfaces. In summary, this thesis explores polymer material classes in their compatibility for high-density RDL wiring in terms of their material properties, ease of fabricating fine-pitch features on planar and smooth surfaces and finally, in creating reliable copper/polymer interfaces with good adhesion.