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  • Development of High-Performance Hafnium Oxide based Non-Volatile Memory Devices on 300mm Wafer Platform for Data Storage and Neuromorphic Applications

    Diebold, Alaine (Committee member); Ventrice, Carl A. Jr. (Committee member); Lloyd, James (Committee member); Kurinec, Santosh (External committee member); Cady, Nathaniel (Dissertation Committee Chair); Hazra, Jubin (2021-08)
    Fundamental limitations associated with scaling and modern von Neumann computing architectures illustrates the need for emerging memory solutions in the semiconductor industry. One such promising non-volatile memory (NVM) solution is resistive random access memory (RRAM), which is seen as a potential candidate that can meet the performance needs of DRAM and the density of NAND Flash in terms of scalability, reliability and switching performance. However, reliable operation of RRAM devices requires further development to remedy device- to-device and cycle-to-cycle uniformity variation, increase the conductance window, and to improve retention, yield and endurance properties. This research work primarily focuses on improving RRAM performance metrics through optimization of processing conditions and programming algorithms for CMOS-integrated nanoscale HfO2 RRAM devices on a full scale 300mm wafer platform. It was observed that tuning of ALD parameters during RRAM switching layer HfO2 deposition had a significant impact on device switching performance. An excellent memory window of >30 with switching yield ~90%, along with low cycle-to-cycle (σ <0.5) and cell-to-cell variability (σ <0.4) were achieved for tested 1 Transistor 1 RRAM (1T1R) cells across full 300mm wafers. The devices demonstrated excellent endurance (>1010 switching cycles) and data retention performance at elevated temperature (105 s at 373K). The fabricated RRAM cells were also optimized for multi-level-cell switching behavior and ~10 distinct resistance levels were obtained through a combined current- and voltage-control based programming approach. An incremental pulse write technique combined with read verification algorithm enabled accurate resistance states programming within a large resistance window along with linear and symmetric potentiation-depression characteristics yielding superior analog synaptic functionality of fabricated RRAM devices. In addition to RRAM devices, hafnium zirconium oxide (HZO) based nanoscale ferroelectric tunnel junction (FTJ) devices were successfully implemented on a 300 mm wafer platform. Current measurement, as a function of voltage for both up and down polarization states, yielded a tunneling electroresistance (TER) ratio of ~5 and switching endurance up to 106 cycles in TiN/ Al2O3/ Hf0.5Zr0.5O2/ TiN FTJ devices distributed across full 300 mm wafer. Investigation of current transport mechanisms showed that the conduction in these FTJ devices is dominated by direct tunneling (DT) at low electric field and by Fowler-Nordheim (F-N) tunneling at high electric field. The realization of CMOS-compatible nanoscale RRAM and FTJ devices on 300mm wafers demonstrates the promising potential of these devices in large scale high-yield NVM manufacturing for high performance embedded memory and mass data storage applications.
  • ULTRATHIN HIGH-K OXIDES FOR AREA-SELECTIVE DEPOSITION AND CHARACTERIZATION BY BALLISTIC ELECTRON EMISSION MICROSCOPY AND X-RAY PHOTOEMISSION SPECTROSCOPY

    Rogers, Jack (2021-05)
    Insulators play an important role in the architecture and resulting performance of semiconductor devices manufactured today. Materials such as HfO2 and Al2O3 are utilized as gate oxides and spacers to control leakage current and enable bottom-up self-aligned patterning of device features. Understanding the electrostatic barrier that forms at the metal-oxide-semiconductor (MOS) interface is crucial in the development of field effect transistors and other devices, especially as the scaling of device features continues to shrink into the nanoscale. Characterization of the barrier height using current-voltage (IV) and capacitance-voltage (CV) techniques provides only a spatially averaged view of the interface, and is incapable of accounting for local nonuniformity which arises at nanoscale dimensions. Additionally, common lithographic strategies for patterning small feature oxides are limited by printing misalignments such as edge placement error (EPE), and in order to achieve smaller pitch sizes lithography steps must be repeated multiple times which adds time and cost to the process. The feasibility of uniform, cost-effective insulator films at the 5 nm technology node and beyond relies on the development of new deposition strategies. In this thesis, hafnium oxide grown using atomic layer deposition (ALD) is examined with ballistic electron emission microscopy (BEEM). Localized nonuniformities in the barrier height are found to exist for two identically prepared samples which reveal three distinct electrostatic barriers at the buried Au/HfO2/SiO2/Si-p interface, including a novel barrier found at 0.45 eV due to ultrathin HfO2. The results uncover changes in electrostatic behavior of the film which are otherwise impossible to detect using spatially averaged techniques. These variations in barrier height are visualized in a novel way that produces spatial maps showing transitions between high energy and lower energy barriers across a few nanometers. The resolution of this mapping technique is determined by comparing the measured barrier heights of Au/Si(001) and Au/Si(111) interfaces. Momentum conservation and electron scattering result in slightly different barrier heights for both interfaces that depends on metal thickness. The Rayleigh criterion is applied to the barrier height distributions as a function of metal thickness, resulting in a 10 meV resolution. Both aluminum oxide and hafnium oxide are also selectively grown on patterned metal / low-k silicon wafers using ALD. Self-assembled monolayer (SAM) materials such as octodecanethiol (ODT) and dodecanethiol (DDT) -which are functionalized to metal -are first deposited on the copper lines in order to block high-k film deposi¬tion on metal. Both HfO2 and Al2O3 are shown to selectively cover the low-k lines for linespace pitches greater than 100 nm and 5 mM concentration of SAM, and better selectivity is achieved for smaller pitches using lower SAM concentrations. Selectivity is measured qualitatively and quantitatively using x-ray photoemission spectroscopy and confirmed with transmission electron microscopy.
  • Bioengineered Platforms for Human Stem Cell-Based Diagnostic and Therapeutic Interventions

    Paluh, Janet L.; Thesis Advisor; Sharfstein, Susan T.; Committee Member; Xie, Yubing; Committee Member; Wang, Jun; Outside Committee Member; Amini, Nooshin (2020-08)
    Human stem cells offer an unprecedented ability to restore function lost through disease or injury by providing options for cell therapies and regenerative medicine. Two hurdles that delay greater clinical use of stem cells are production of differentiated therapeutic cells in large-scale platforms and the challenge of choosing the optimum cell type and delivery method for cell therapy that is optimized for cell-cell signaling in the therapeutic microenvironment. In my thesis work I investigated different bioengineered platforms in combination with human stem cell technology to mass produce functional hiPSC-derived beta islets in a miniature bioreactor and study cytokine release from multipotent and differentiated hiPSC-derived neural stem cells as neural rosettes and their dissociated cells or differentiating inhibitory and excitatory neurons alone and in mixed cultures applying a neural cell-cell interaction microchip (NCCIM) with features developed specifically for these studies. My work has further expanded the application of hiPSC-derived neurons in an in vitro model of traumatic brain injury. In this study a hybrid culture of hiPSC-derived excitatory pyramidal neurons, inhibitory GABAergic interneurons and immortalized human microglia are being evaluated for secreted cytokines under healthy and stretch injured induced conditions. One of the challenges of TBI is the inability to yet effectively and with minimal invasiveness track changes following injury that may indicate healing or deterioration and an in vitro model is one important contribution to identifying biomarkers.
  • MAXIMIZING THE CHEMICAL REMOVAL OF CERIA ABRASIVES IN CMP FOR SILICON OXIDE AND METAL POLISHING

    Thiel, Brad; Defense Committee; Carpenter, Michael; Defense Committee; Borst, Christopher; Defense Committee; Hatzistergos, Michael; Defense Committee; Dunn, Kathleen; Committee Chair; Netzband, Christopher M. (2020-08)
    Cerium oxide or ceria has garnered a wide range of applications due to its redox active nature. This redox activity is due to oxygen vacancies on the surface of the ceria creating a layer of mixed oxide with the unstable oxide Ce2O3 (Ce[superscript 3+]) present at the same time as the bulk oxide CeO2 (Ce[superscript 4+]). Possible applications for ceria include water splitting, oxidation of carbon monoxide, oxidation of reactive oxygen species and polishing of glass films. In recent years, ceria nanoparticles have been used for polishing thermal silicon oxide during the early steps of semiconductor fabrication in a process referred to as chemical mechanical planarization (CMP). The advantage of these particles is their ability to abrade an oxide surface chemically using the aforementioned redox properties, as well as mechanically. To meet the needs of manufacturing, mainly removal rate and surface roughness, the particles used must have well controlled physical properties such as size and shape for mechanical removal and ratio of cerium oxidation state for chemical removal. This study encompasses three parts following the design of ceria slurries, their implementation in the existing silicon oxide polish and applying these findings to create novel slurries for polishing metals. To design ceria slurry, the ratio of Ce[superscript 3+]/Ce[superscript 4+] on the surface of abrasive was maximized by altering the slurries’ chemical environment. Maximizing this ratio increases the proportion of active Ce[superscript 3+] sites which participate in removal reactions. The effect of chemical environment on the Ce[superscript 3+]/Ce[superscript 4+] ratio was determined through XPS analysis of the Ce 3d spectrum. The knowledge gained in this first section informed the design of ceria slurries for the following two parts to maximize their effectiveness. The second part of this thesis applies this knowledge to create ceria iv slurries that polished thermal oxide with higher material removal rate (MRR) and lower postpolish roughness than slurries that are currently being used in industry. The basis of ceria polishing is known as the tooth-comb model. In this model oxygen at Ce[superscript 3+] sites will undergo a condensation reaction with oxygen on the surface to be polished. As the particle leaves this will rip material off of the wafer surface. While the tooth-comb model was proposed for polishing silica, the final part of this thesis seeks to generalize it to encompass polishing any oxide given the correct conditions. To demonstrate this, I created ceria slurries to polish metals relevant to the semiconductor industry (copper, tungsten and ruthenium) with polishing metrics that equal or exceed those of industry standard slurries.
  • BIOENGINEERED, STEM CELL DERIVED OCULAR OUTFLOW TISSUE

    Xie, Yubing; Advisor; Torrejon, Karen; Thesis committee; Cady, Nate; Thesis committee; Danias, John; Thesis committee; Sharfstein, Susan; Thesis committee; Tian, Yangzi Isabel (2018-10)
    Glaucoma is one of the leading causes of irreversible blindness in the world. Despite decades of research, intraocular pressure (IOP) is the only known treatable risk factor. IOP is affected by the timely removal of aqueous humor through the conventional outflow track, which is made up of the trabecular meshwork and adjacent Schlemm’s canal. Dysfunction in these tissues due to aging, oxidative stress, metabolic or pathological changes lead to increased flow resistance, elevated IOP, and ultimately glaucoma. Recent advances in ocular regenerative therapy have the potential to rescue glaucomatous tissue function and restore its delicate microenvironment. The possibility of using stem cell-derived trabecular meshwork and Schlemm’s canal cells to recreate a functional outflow tissue are explored in this thesis. Previously, our lab developed a well-defined, micro-porous substrate that promotes in vivo-like physiology and outflow function in primary trabecular meshwork and Schlemm’s canal cell cultures. Using these primary cell cultures as controls, we have created 3D stem cell-derived outflow tissues, evaluated and compared their morphology, expression, outflow facility, and drug responsiveness. To explore the importance of the dynamic microenvironment in outflow function, we developed a dual-flow microfluidic chamber that mimics the basal-to-apical and shear flow of aqueous humor through the conventional outflow track. Overall, this dissertation demonstrates the promising application of stem cells in future glaucoma drug screening and treatment.
  • Mapping, Implementing, and Programming Spiking Neural Networks

    Cady, Nathaniel; Chair; Cafaro, Carlo; LaBella, Vincent; Oktyabrsky, Serge; Plank, James; External Committee Member; Olin-Ammentorp, Wilkie (2019-03)
    Computer architectures inspired by biological neural networks are currently an area of growing interest, due to immense utility of these systems which is shown by their near-ubiquity within animals. An essential aspect of these systems is their ability to compute through the exchange of temporal events called ‘spikes.’ However, many aspects of biological computation remain unknown. To improve our ability to measure neural systems, we create an efficient implementation and statistical testing method to calculate an information-theory based metric, transfer entropy, on signals recorded from cultures of neurons. Taking inspiration from established knowledge regarding biological neurons, we investigate the impact which stochastic behavior has on the robustness of spiking networks when their synaptic weights are inaccurate. We find that a level of stochasticity can help improve this robustness. Lastly, we investigate methods of creating programs for spike-based computation through evolutionary optimization methods, and identify opportunities and challenges in this area.
  • Assessing a Multi-Electron Beam Application Approach for Semiconductor Process Metrology

    Mukhtar, Maseeh; Thiel, Bradley; Dissertation Committee Chair; Bello, Abner; Dissertation Committee; Diebold, Alain; Dissertation Committee; Cady, Nathan; Dissertation Committee; Geer, Robert; Dissertation Committee; Sung, Woongje; Dissertation Committee (2018)
    Radical and disruptive technological approaches regularly require experimental prototypes be built, which is difficult to justify considering their oft-prohibitive requirements in terms of financial and/or time commitments. It is also frequently the situation that use cases for new technologies are not entirely worked out precisely which in turn make it even more difficult to build prototypes but the analysis of simulation data sets from virtual samples can be used to predict sensitivity to the devised signal, detection limits, and impact of design rules and material sets. The results can thus be used to guide prototype design. The aim of this work is to develop and demonstrate a predictive approach to technology assessment and prototype design. This work will focus on two such disruptive technology concepts: electron beam defect inspection and critical dimension measurement. These two concepts are based on the transfer from conventional process metrology technologies i.e., brightfield inspection and optical critical dimension scatterometry to multi-electron beam approaches. Here, a multi-scale modeling approach is used to simulate data streams nominally generated by virtual tools inspecting virtual wafers. To this end, Java Monte Carlo Simulator for Secondary Electrons (JMONSEL) simulations are used to generate expected imaging responses of chosen test cases of patterns and defects with ability to vary parameters for beam energy, spot size, pixel size, and/or defect material and form factor. The patterns are representative of the design rules for aggressively-scaled FinFET-type designs. With these simulated images and resulting shot noise, a signal-to-noise framework is developed, which relates to defect detection probabilities. Additionally, with this infrastructure the effect of detection chain noise and frequency dependent system response can be made, allowing for targeting of best recipe parameters for multi-electron beam inspection validation experiments. Ultimately, leading to insights into how such parameters will impact tool design, including necessary doses for defect detection and estimations of scanning speeds for achieving high throughput for high-volume manufacturing. Simulated images are also executed for measurement of critical dimensions of the abovementioned class of FinFETs. Similarly, validation experiments for multi-electron critical dimension measurements may use the information extracted for development of volume manufacturing metrology systems.
  • Nanoscale Schottky Barrier Visualization Utilizing Computational Modeling and Ballistic Electron Emission Microscopy

    Nolting, Westly; LaBella, Vincent; Advisor (2018-05)
    Understanding the properties and performance of semiconductor interfaces on the nanoscale advances semiconductor device technology which has had tremendous impact on nearly all aspects of our daily lives. Investigating the nanoscale fluctuations in the electrostatics of metal-semiconductor, or Schottky, interfaces is crucial. However, techniques for directly measuring the electrostatics at an interface are limited. Current state-of-the-art finFETs use metal-semiconductor silicides, such as Ti-Si/Si, for Schottky source-drain contacts. Studying the underlying physics of the Schottky barrier interface of silicides and other metal-semiconductor systems is critical for measuring the Schottky barrier accurately, which can be accomplished with ballistic electron emission microscopy (BEEM), a scanning tunneling microscopy (STM) based technique. In this work, the visualization of the interface to nanoscale dimensions is enhanced by computational modelling of threshold histograms acquired by the BEEM measurement technique. Modelling using a kinetic Monte-Carlo approach is utilized to simulate the distributions of barrier heights that includes effects from the interface and transport of the hot electrons as well as indication of a multi-barrier heights present at the interface. The aid of this modelling enables the discovery of several underlying properties of the interface. Analyzing the parameters of the modelling and comparing to measured data provides detailed insight into the effects that both electron scattering and incomplete silicide formation in W/Si(001) and WSi2/Si(001) have upon the transport of electrons through these structures, which is difficult to detect with conventional current-voltage measurements. The modelling also includes simulation of multiple barriers present at the interface due to the intermixing of similar metals such as Au and Ag at the interface of Si(001) In this regard, Schottky barrier visualization as the combination of histograms, mapping, and modelling provides a new insight into the local nanoscale phenomenon of the Schottky barrier. This thesis investigates the modelling of these metal-semiconductor systems and uses modelling to look at metal thickness dependent effects on the Schottky barrier from Fermi-level pinning in Au/Cr-Si/Si(001) and Au/Cr-Si/Si(111) silicide.
  • Characterization and Control of the Surface of the Topological Insulator Bi2Se3

    Green, Avery James; Diebold, Alain; Advisor (2017-12)
    The field of topological insulator (TI) materials is new. The ideal TI contains surface states in helical Dirac cones that can be used for spintronics or interconnect applications. Of the TI class, Bi2Se3 is the most promising for applications due to its stoichiometric composition, its relatively large band gap (0.3 eV), and the central (??-point) position of the Dirac cone in its 2D surface band structure. Although the theoretical solid-state models that the TI field has produced are powerful and unique, their novel emergent physical properties are not universally observed in every sample. These materials are difficult to grow and maintain under ambient conditions. Growths tend to either not be applicable to wafer-scale production or produce high polycrystallinity, and all samples experience natural oxidation, band bending, and intrinsic n-doping, which generates spin-degenerate or bulk conduction. This thesis contains a primer on topologically non-trivial materials, and two studies aimed at understanding and minimizing defects at the surface of Bi2Se3. In the first, the aging process of Bi2Se3 when exposed to air at room temperature is investigated. The time scale and topographic changes of the oxidation process at micromechanically exfoliated surfaces are measured, and an optical model of the bulk and oxide layers are developed. The surface appears to oxidize starting at 2 hours after exfoliation, and continuing through 1.5 weeks, by which time, the oxide layer growth has reached an asymptote of 1.9 nm. New optical characterization methods are developed to monitor the orientation of the crystal (via second harmonic generation) and to measure the oxide growth at the surface (using spectroscopic ellipsometry and the derived dielectric functions of the bulk and oxide layers). The goal of the second study is to assess the use of Se capping and subsequent thermal decapping to preserve a pristine surface and maintain a constant Fermi level. This was measured by annealing samples in a UHV environment to successively higher temperatures until the Bi2Se3 film decomposed, and measuring the surface crystallinity, topography, surface chemistry, and Fermi level between each anneal. Thermally decapping samples has no measurable effect on crystallinity, minimal effect on surface topography, reveals the expected Bi-Se surface bonds, and retains a mid-gap Fermi level. This may serve as a reference to improve the fabrication process of devices that include Bi2Se3.
  • Biomimetic Scaffolds Using Natural/Synthetic Polymers for Salivary Gland Regeneration

    Sfakis, Lauren; Castracane, James; Advisor (2017-06-01)
    Salivary glands are essential in maintaining oral cavity homeostasis. This tissue can become impaired by chemotherapy/radiotherapy given to head and neck cancer patients, as well as systemic diseases. Once this gland is damaged, it has limited ability to regenerate, and so the need for potential biodegradable/biocompatible scaffolds to aid in the growth and repair is of great interest. This soft tissue is made up of multiple cell populations that contribute to the function of the gland. Creating an environment that can recapitulate the one seen in vivo will promote the functionality of the engineered tissue. This research aims to investigate: (1) cell-substrate interactions with salivary gland epithelial cells and nanofiber scaffolds, (2) cell-cell interactions via incorporation of a second native cell population to further enhance epithelial differentiation, mimicking the in vivo microenvironment and (3) the development of engineering a three-dimensional scaffold that will better facilitate the two interactions described above. The hypothesis is that sponge scaffolds that mimic the mechanical properties and architecture of the tissue observed in vivo will provide a platform for future implantation and regeneration strategies. Bio-mimetically engineered scaffold systems for the growth of organs, such as the one described here, yield novel tools for studying organ development in applications for regenerative medicine.
  • Development of Novel Technologies for Direct Cellular Patterning for the Establishment of Well Controlled Microenvironments to Facilitate Studies on Cellular Signaling, Sensing, and Other Diffusion-Based Phenomena

    Hynes, William (2016-05-07)
    This work focuses on the utilization of novel bioprinting technologies for the investigation of cellular signaling, sensing, and other diffusion-based phenomena with spatiotemporal dependencies. Two different printing techniques were developed for the purpose of fabricating controlled microenvironments comprised of cells, nutrients, hydrogels, and soluble signaling molecules in a repeatable fashion. The first application explored was the development of a novel, bioprinted, cell-based biosensor as a nondestructive method for the monitoring of the cellular redox environment. Mammalian cells were engineered to express a redox sensitive protein and were patterned and immobilized within a photopolymerizing hydrogel matrix, resulting in biocompatible, three-dimensional microenvironments which supported cell growth and facilitated small molecule sensing. Exposure of the printed, redox sensitive cells to oxidative and reductive compounds and monitoring via confocal microscopy demonstrated proper and reversible functioning of the living biosensor. Bioprinting was also used to generate complex, micro-scale, multi-species populations of bacteria in order to evaluate the effects of distance and various forms of competition on syntrophic relationships. An artificial, syntrophic bacterial consortium was printed within controlled microenvironments confined by geometry and nutrient availability. The growth of the printed strains was monitored, analyzed, and compared to the predictions of an experimental, computational bacterial growth model known as COMETS. Results indicated that the general trends exhibited in vitro by most of the examined micro-scale interactions can be predicted in silico, and that the effects of microbial interactions on the micro-scale can differ considerably than those observed at the macro-scale.