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  • 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.