Graduate Student

Title: Flow behavior of an axisymmetric vortex ring impingement on solid three-dimensional axisymmetric hemispherical cavities of different curvatures

Department of Mechanical and Aeronautical Engineering

Email: tanvahm@clarkson.edu

Advisor: Byron Erath

Abstract: Vortex-concave cavity interaction is a fundamental fluid mechanics problem that has great significance for many engineering and biological applications. In earlier literature no experimental study has been done on this topic. In this study an experimental investigation of an axisymmetric vortex rings' normal interaction with a concave hemisphere cavity was performed. A vortex ring of formation number ð¹ = 2.67, and Reynolds number Re_Γ = 1450 was generated utilizing a piston-cylinder vortex generator in a water tank. The vortex ring impinged on axisymmetric hemispherical cavities, where six different ratios, ð›¾, (ratio of vortex ring radius to hemisphere radius) were tested; namely, 𛾠= 0, 1/4, 1/3, 2/5, 1/2, & 2/3. Dye flow visualization, and planar laser-induced fluorescence (PLIF) flow visualization were employed to identify the key phenomena during the interactions. Results showed that the dynamics of the flow field changed drastically as a function of ð›¾. For 𛾠= 0, which is a flat plate, the results replicated earlier literature results of a vortex ring impinging on a flat plate. For ð›¾=1/4, 1/3, and 2/5, following primary vortex rings impact, secondary vortex ring formed, rotate around the primary vortex ring, experience a head-on collision, and are ejected from the hemispherical wall and propagate away from the surface. Azimuthal instabilities in the secondary vortex rings were observed for 𛾠= 1/4, 1/3, & 2/5 interactions, and were found to be critical for the ejection of secondary vortices away from the surface, following the initial collision. For γ =1/2, a separation vortex formed along the lip of the hemisphere and played a critical role in altering the interactions; it canceled most of the secondary vortices generated from the wall via cross sign annihilation. For, γ = 2/3 vortex ring impacts directly on the lip of the hemisphere.

Graduate Student

Title: Tribo-electrochemical investigation of Tungsten CMP

Department of Physics

Email: gamageku@clarkson.edu

Advisor: Dip Roy

Kassapa Gamagedara, Shen Wei and Dipankar Roy

Abstract: In chemically promoted CMP of metals, controlled tribo-corrosion is utilized as an essential component of material removal where the role of mechanical abrasion is minimized. The process of anodic modification in this case is designed to differ from that of traditional corrosion in the following aspects: (A) The corrosion products representing the removable material are chemically controlled as insoluble surface complex films in most part, rather than as (potentially defect-generating) dissolving species, and (B) The corroding surface is continuously renewed by repeated chemical formation and mechanical removal, so that material removal is sustained during CMP. Using a laboratory-scale model test system of tungsten CMP, we demonstrate certain necessary analytical aspects of studying such systems. A tribo-electrochemical test cell and a potentiostat are used to examine the open circuit potentials, electrochemical impedances, and voltametric features of the tungsten CMP system under different tribological (polish and hold) conditions. The test slurry uses potassium nitrate and hydrogen peroxide as a complexing additive and an oxidizer, respectively. The results are analyzed to understand the CMP mechanisms operating in this system.

Graduate Student

Title: Monitoring the Aza-Michael Reaction of Chitosan and Poly(ethylene glycol) Diacrylate using Rheology and NMR Spectroscopy

Department of Chemical and Biomolecular Engineering

Email: lent@clarkson.edu

Advisor: Sitaraman Krishnan

Ngoc-Tram Le, Aswin Prathap Pitchiya, Sitaraman Krishnan

Email: skrishna@clarkson.edu

Abstract: Chitosan, a naturally derived polysaccharide, is attractive for a variety of applications because of its polyelectrolyte nature, biodegradability, and biocompatibility. Toward the goal of preparing chitosan microgel particles without the use of potentially toxic reagents, the synthesis of PEGylated chitosan via aza-Michael addition reaction was investigated. The influence of reaction temperature, pH, and amine-acrylate mole ratio on the extent of reaction was characterized using a combination of NMR spectroscopy and rheology. Polymer solution properties such as complex viscosity, storage modulus, loss modulus, and loss tangent were determined at various reaction times and correlated with the reaction extent obtained using NMR spectroscopy. Rheology was found to be highly useful in acquiring reaction kinetics data that were difficult to obtain using other methods in this complex system where grafting and crosslinking reactions occur simultaneously. The resulting information will be used to synthesize biocompatible polymer microparticles for pulmonary vaccine delivery.

Keywords: reaction kinetics, biocompatible polymer gels, rheology

Graduate Student

Title: In-Situ Electrochemical Investigation of Post-CMP Cleaning with the Incorporation of Tribology

Department of Physics

Email: reffcm@clarkson.edu

Advisor: Dipankar Roy

Collin Reff, David Santefort, Cody Johnson, and Dipankar Roy*

*Contact: David Santefort, e-mail: santefdr@clarkson.edu

Abstract: We demonstrate how the task of designing and optimizing post-CMP cleaning (PCMPC) solutions for chemically complex CMP systems can be assisted in a cost-effective approach by using tribo-electrochemical techniques. With these experiments, separate cleaning solutions of malonic acid and DL-tartaric acid were compared to further understand the removal mechanism of surface contaminants typically encountered in PCMPC of Cu. We focus specifically on the oxide removal feature of PCMPC, using a prototype system consisting of atomic layer deposited films of Cu (used for wiring of interconnects). Removal of residual surface oxides is critical in PCMPC, since this step also eliminates organic residues adsorbed at the oxide sites. Malonic acid and DL-tartaric acid are employed here as experimental oxide removers in the form of neutral pH solutions. The Cu film sample is pre- treated in a CMP slurry, and the resulting chemical residues on this sample are then removed by brushing the sample surface with a commercial poly(vinyl) alcohol brush in the cleaning solution. The PCMPC-enabling interfacial reactions are simultaneously probed in the presence (as well as in the absence) of brushing by employing tribo-controlled voltammetry and impedance spectroscopy.

Graduate Student

Title: Investigating Kinetic Pre-Alignment with Naphthalene Derivatives

Department of Chemistry and Biomolecular Science

Email: roysj@clarkson.edu

Advisor: Ryan Brown

Abstract: This study explores surface-confined reactions and the viability of kinetic pre-alignment in product selectivity, through observation with a home-built scanning tunneling microscope (STM). Condensation reactions are promising alternatives to irreversible mechanisms such as coupling reactions, as they open the opportunity for recrystallization of the surface product. This research targets an assembly method applicable to systems regardless of the desired reversibility by utilizing condensation reactions with kinetic control over the hydrolytic pathway. Variation of growth parameters will establish the kinetic pre-alignment of molecules at the surface, foregoing the need for post-deposition product refinement methods. The amide bonding reaction was chosen for study due to its inherent reversibility at surfaces and will be applied to a simple and versatile system of naphthalene derivatives. Preliminary data will be presented on the initial analysis of 2,6-naphthalenedicarboxylic acid (NDCA) at gold and graphite surfaces, providing a starting point to test kinetic control parameters before moving on to molecular co-deposition with 1,5-diaminonaphthalene and subsequent amide bond formation.

Research Assistant Professor

Title: Challenges and solutions for post STI-CMP cleaning

Department of Chemical and Biomolecular Engineering

Email: jseo@clarkson.edu

Abstract: Chemical mechanical planarization (CMP) process has been widely used to planarize a variety of materials including dielectrics, metal, and semiconductors in Si-based semiconductor devices. It is one of the most critical steps to achieve the nanolevel wafer and die scale planarity. However, various contaminants are observed on the wafer surfaces after the CMP process, and they become the most critical yield detractor over many generations of rapidly diminishing feature sizes because they have the most direct impacts on device performance and reliability.

Devices at 7 nm node and beyond present more stringent requirements for the level of acceptable defects along the entire manufacturing process. Thus, post-CMP cleaning has become a crucial step to eliminate many of these contaminants. In this talk, I will briefly present the challenges and our solutions in developing post-CMP cleaning solutions for the shallow trench isolation (STI) process.

Graduate Student

Title: Dynamic Covalent Exchange in Poly(thioether anhydrides)

Department of Chemistry & Biomolecular Science

Email: tillmakr@clarkson.edu

Advisor: Devon Shipp

Abstract: There has recently been much focus on integrating dynamic functionality into polymer networks to produce new polymer-based materials with recycling, reconfiguring, and self-healing capabilities. Our research focuses on the dynamic covalent exchange (DCE) of anhydride moieties, conducting experiments on model compounds and network polymers. Specifically, thermodynamic and kinetic analysis of the exchange process in symmetric model compounds methacrylic anhydride and 4-pentenoic anhydride, which produce an asymmetric anhydride upon exchange, was undertaken using 13C NMR spectroscopy. The equilibrium constant (Keq) was determined to be approximately 2-3, and the activation energy (Ea) for the production of the asymmetric anhydride was 178 kJ mol-1. The rate of exchange was shown to be increased catalytically in the presence of carboxylic acid. Poly(thioether anhydrides) were made using 4-pentenoic anhydride and mixtures of dithiol (1,6-hexanedithiol) and tetrathiol (pentaerythritol tetrakis(3-mercaptopropionate)) via radical-mediated thiol-ene polymerizations, and network relaxation rates studied using dynamic mechanical analysis. The activation energy and stress relaxation times for various polymer compositions were correlated to crosslink density and polymer composition. It was determined that cross-link density of the polymer system does not significantly impact Ea; however, relaxation time (ï´) nearly quadrupled from the least cross-linked system to the most cross-linked system. The network polymers exhibited self-healing and facile recycling behavior, which we attributed to the anhydride DCE process that occurs readily in these materials at elevated temperatures. Using the composition with the fastest relaxation, complete visible recovery of damage (notch) within a 4 hour duration at 90 oC. Recycling under compression molding for 15 minutes using 76 MPa hydrostatic pressure at 90 oC was successful. Both healed and recycled samples were successful in recovering or surpassing their original mechanical properties.

Graduate Student

Title: Post CMP Cleaning Solutions for Removal of Organic Contaminants with Reduced Galvanic Corrosion at Copper/Cobalt Interface for Advanced Cu Interconnect Applications

Department of Chemical Engineering

Email: vegis@clarkson.edu

Advisor: SV Babu

Abstract: Post chemical mechanical planarization (CMP) cleaning has been challenging with shrinking feature size, especially below 14nm. During polishing, the wafer surface is contaminated with various defects such as residual particles, foreign materials, metallic impurities, etc. One of the main challenges in post-CMP cleaning is the removal of the organic contaminants (e.g. metal complexes like Cu-BTA and Co-BTA) formed during polishing as a result of the presence of benzotriazole (BTA) in CMP slurries. In this study, we focus on the adsorption and the removal of these metal complexes from Cu, Co, TaN and SiO2 films for advanced Cu interconnect applications. Cleaning mechanism is explained based on the ligand exchange happening between the metal complex and the chelating agent present in the cleaning solution. Additionally, the proposed cleaning solution employs suitable corrosion inhibitors to control both individual corrosion currents of both Cu and Co films and galvanic corrosion of the Cu-Co couple.

Graduate Student

Title: Electrodeposited Co-Mo-P-TiO2 Electrocatalysts for Hydrogen Evolution Reaction

Department of Chemical Engineering

Email: wangc4@clarkson.edu

Advisor: Elizabeth Podlaha-Murphy

Abstract: Water electrolysis is one method to produce high purity dihydrogen (H2) for energy storage and chemical synthesis. The process can achieve zero emissions if electricity is provided by renewable sources such as hydro, wind and solar. An important factor that limits its energy efficiency is associated with the sluggish hydrogen evolution reaction (HER), which requires an electrocatalyst where reaction kinetics and electrode stability are essential factors to reduce cost. Platinum is a benchmark, but the high price of Pt hinders its large-scale utilization. In this work, Co-Mo-P-TiO2 composite electrocatalysts were fabricated for the first time by electrodeposition, and the hydrogen evolution reaction (HER) catalytic performance was characterized. The deposits were galvanostatically deposited onto rotating cylinder electrodes from citrate-boric acid electrolytes. The HER performance of the novel composites were compared with pure Co, Co-Mo, Co-Mo-TiO2 and Co-Mo-P deposits prepared under comparable galvanostatic conditions. The Co-Mo-P-TiO2 composites exhibited a very low overpotential at -10 mA/cm2 of only 31 mV in a 1 M NaOH electrolyte, superior to the other thin films having similar elemental Co/Mo composition, suggesting that the addition of TiO2 particles into the electrocatalysts can enhance the HER activity, even in the presence of phosphorous. The electrochemical active surface area was characterized, to show that the improvement is not merely due to surface roughness but an intrinsic effect. The Tafel slope was not altered with the embedded TiO2 particles by comparing the phosphorous-free deposits. However, by adding phosphorus, the Tafel slope was favorably reduced, and was stable after electrolysis for 48 hr.

Graduate Student

Title: Investigation of the Mechanism of Copper CMP in Abrasive-Free Slurry Solutions of Malonic Acid

Department of Physics

Email: shwei@clarkson.edu

Advisor: Dip Roy

Abstract: The effects of slurry-pH on the CMP of copper (Cu) are investigated by tribo-electrochemical measurements performed in a series of experimental slurries with malonic acid (MA) used as a surface complex agent. The results indicate that the material removal rates (MRRs) of Cu in these silica-free slurries are strongly pH dependent, and that the MRRs are closely correlated to the pH dependent tribo-corrosion rates of the Cu surface. The results also suggest that, in the presence of H2O2 (an oxidizer) the Cu surface is mostly covered with passive films of Cu2O in moderately acidic conditions, and by CuO under alkaline conditions. In the acidic solutions, the formation of a soluble complex CuMal22- is favored, which in turn enhances the MRRs. The individual and synergistic roles of MA, silica abrasives and the slurry-pH in determining the Cu removal process are examined using a proposed model of chemically prevailed CMP.

Graduate Student

Title: Thiol-containing copolymers: Synthesis and characterization

Department of Chemistry and Biomolecular Science

Email: yonwu@clarkson.edu

Advisor: Devon Shipp

Abstract: Thiols are extremely reactive functional groups toward a large variety of substrates to form a covalent bond. In many polymeric material design and fabrication applications, thiol-X reactions such as thiol-ene reaction play a significant role. However, including monomers that contain thiol functional groups in radical polymerizations is not possible due to the chain transfer activity of the thiol group. To obtain thiol-containing polymers, one strategy is applying protected thiol method. Thiolactone derivative molecules can open the ring and release thiol when they react with primary amine. Incorporating thiolactone functional group into polymer and treating with primary amine can yield thiol-containing polymer. In this work, two thiolactone monomers were synthesized and were copolymerized with either N,N-dimethylacrylamide (DMA) or tert-butylacrylate (tBA). The hydrophobicity of copolymers can be tuned by adjusting monomer type and initial monomer feeding ratio. Benzylamine and propylamine were used to ring-open the thiolactone to yield thiol-containing polymer. Nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) were used for kinetic study and copolymer characterization.