Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Includes bibliographical references (leaves 157-164). / by Brent Victor Chambers. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2002. / Includes bibliographical references (p. 115-119). / We use what is known about transition metal (TM) defect thermodynamic driving forces and kinetic responses to make predictive simulation of gettering during solar cell fabrication possible. We have developed a simulator to explore the impact of various device and process parameters on gettering effectiveness. The relevant heat treatments are ramps up in temperature, isothermal annealing, and cools from high temperature down to room temperature. We explore a range of surface conditions, density and size of heterogeneous nucleation sites in the bulk, and the degree of contamination in order to create a framework in which to examine these mechanisms acting in concert. Such simulations enable process optimization for gettering. For solar cell processing, segregation to an Al back contact layer is routine. We have estimated the segregation coefficient between a p-type Si wafer and a molten Al layer by the Calphad method and use these results to estimate the thermodynamic driving force for redistribution of Fe into the Al layer. We simulate gettering treatments of supersaturated levels of Fe contamination in Si samples with FeSi2 and Al contacts and compare these results with data at various temperatures. The gettering data for FeSi2 contacts follow a simple exponential decay and can be simulated with appropriate choice of internal gettering time constant. We recognize that radiative heating dominates the temperature ramp for samples in evacuated quartz ampoules and use reasonable parameters to include this effect in our simulations. Fitting parameters for [Fe] data taken from heat treatments at 755Ê»C on samples with FeSi2 and Al contacts successfully predict the gettering data of Al coated samples treated at 810Ê»C. / (cont.) Discrepencies in the data for Al coated samples treated at 6950C and data for Al coated samples treated at 755Ê»C after long times have exposed a new mechanism dominating internal gettering processes. We propose the existence of a silicide precipitate growth retardation mechanism as a result of supersaturation of the Si vacancy (V). Accumulation of V reduces the ability of precipitates to relax strain free-energy ([Delta]g strain) by further V emission. We performed Cu gettering experiments on p/p+ epitaxial wafers. Photoluminescence measurements revealed significant Cu removal from the epitaxial region compared to similarly doped uniformly doped float zone (FZ) Si wafers. Step etching revealed haze, indicating the presence of silicide precipitates below the epitaxial layer in the heavily doped substrates. Uncontaminated heat treated epitaxial wafers did not demonstrate the presence of haze after step etching. This finding demonstrates that redistribution of Cu from the lightly doped expitaxial layer to the heavily doped substrate as predicted by the dopant enhanced solubility model has occurred. The commonly used T3/2 model for effective density of conduction and valence band states (Nc and Nv respectively) is not accurate for Si, even in the device operation regime, and the available experimentally determined relations of Green do not extend past 500K. We have constructed a DOS model using ab initio calculations and temperature appropriate Fermi-Dirac ... / by Aimée Louise Smith. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000. / Includes bibliographical references (leaves 245-251). / The stringent performance and reliability demands that will accompany the development of next-generation circuits and new metallization technologies will require new and more accurate means of assessing interconnect reliability. Reliability assessments based on conventional methodologies are flawed in a number of very important ways, including the disregard of the effects of complex interconnect geometries on reliability. New models, simulations and experimental methodologies are required for the development of tools for circuit-level and process-sensitive reliability assessments. Most modeling and experimental characterization of interconnect reliability has focused on simple straight lines terminating at pads or vias. However, laid-out integrated circuits usually have many interconnects with junctions and wide-to-narrow transitions. In carrying out circuit-level reliability assessments it is important to be able to assess the reliability of these more complex shapes, generally referred to as "trees". An interconnect tree consists of continuously connected high-conductivity metal within one layer of metallization. Trees terminate at diffusion barriers at vias and contacts, and, in the general case, can have more than one terminating branch when the tree includes junctions. We have extended the understanding of "immortality" demonstrated and analyzed for straight stud-to-stud lines, to trees of arbitrary complexity. We verified the concept of immortality in interconnect trees through experiments on simple tree structures. This leads to a hierarchical approach for identifying immortal trees for specific circuit layouts and models for operation. We suggest a computationally efficient and flexible strategy for assessment of the reliability of entire integrated circuits. The proposed hierarchical reliability analysis can provide reliability assessments during the design and layout process (Reliability Computer Aided Design, RCAD). Design rules are suggested based on calculations of the electromigration-induced development of inhomogeneous steadystate mechanical stress states. Failure of interconnects by void nucleation in single-layermetallization, as well as failure by void growth in the presence of refractory metal shunt layers are taken into account. The proposed methodology identifies a large fraction of interconnect trees in a typical design as immune to electromigration-induced failure. To complete a circuit-level-reliability analysis, it is also necessary to estimate the lifetimes of the mortal trees. We have developed simulation tools that allow modeling of stress evolution and failure in arbitrarily complex trees. We have demonstrated the validity of these models and simulations through comparisons with experiments on simple trees, such as "L"- and "T"-shaped trees with different current configurations. Because analyses made using simulations are computationally intensive, simulations should be used for analysis of the least reliable trees. The reliability of the majority of the mortal trees can be assessed using a conservative default model based on nodal reliability analyses for the assessment of electromigration-limited reliability of interconnect trees. The lifetimes of the nodes are calculated by estimating the times for void nucleation, void growth to failure, and formation of extrusions. The differences between straight stud-to-stud lines and interconnect trees are studied by investigating the effects of passive and active reservoirs on electromigration. Models and simulations were validated through comparisons with experiments on simple tree structures, such as lines broken into two limbs with different currents in each limb. Models, simulations and experimental results on the reliability of interconnect trees are shown to yield mutually consistent results. Taken together, the results from this research have provided the basis for the development of the first RCAD tool capable of accurate circuit-level, processing sensitive and layout-specific reliability analyses. / by Stefan P. Hau-Riege. / Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1992. / Includes bibliographical references (leaves 246-262). / by Hai Pham Longworth. / Sc.D.

Thesis (M.Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2002. / Includes bibliographical references (p. 45-47). / Magnetic Random Access Memory (MRAM) is considered to be the most viable option for nonvolatile memory in the computer industry. This need for nonvolatile computer memory has resulted in the dramatic evolution of MRAM technology in the past ten years. Currently in the latter stages of development, emphasis is being put on experiments concerning optimization of density and reduction of the switching fields of the magnetic elements. Applications of MRAM technology are currently being explored by companies who seek to obtain relevant intellectual property in those areas. Once research is completed, companies must create a business plan that recognizes the initial, breakthrough markets and implement technology integration accordingly. / by Allison Hernandez. / M.Eng.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 123-139). / Developing new cathode materials is key to improving the energy density of rechargeable batteries and enabling new applications of energy storage. In this thesis, two families of materials were explored as candidate cathode materials: the dirutile and rutile polymorphs of LiMnF4, and layered lithium-excess . . . Dirutile LiMnF4 was identified from high-throughput computation as a promising conversion cathode. The dirutile polymorph was synthesized through a new low temperature route, and the rutile polymorph was discovered upon mechanical milling. With simple synthesis and electrode preparation methods, both dirutile and rutile polymorphs of LiMnF4 showed electrochemical activity. Electron diffraction confirmed both polymorphs to convert upon lithiation along different reaction paths. As with other fluorides, specific capacity was strongly linked with processing conditions. The layered lithium-excess . . . compounds were designed from recent understanding of diffusion channels in lithium-excess materials. Increasing lithium content was found to improve both discharge capacity and capacity retention. Structural studies revealed a complex nanostructure pattern of Li-Sb and Ni-Sb ordering where the interface between these domains formed the correct local configuration for good lithium mobility. The < 1nm Li-Sb stripe domains enable percolation of the low barrier lithium diffusion channels at lower lithium excess levels. The redox mechanisms of the lithium-excess . . . materials were then studied as a function of lithium content and rate. . . . surprisingly exhibited higher discharge capacities at faster rates, and traversed distinct voltage curves at different rates. Characterization of redox processes confirmed nickel redox and oxygen loss, with oxygen redox proposed to account for the balance of the capacity. Finally, irreversible nickel migration is suggested as an explanation for the rate-dependent voltage curve features. / by Nancy Twu. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 99-106). / The primary objective of this work is to establish an innovative and broad platform to engineer living materials and assemble them into functional devices. First, we assemble bacterial sensor communities into core-shell hydrogel structures to address the major challenge of biocontainment. Biosafety has become a major challenge for synthetic biology tools to transition from laboratory experiments to real applications and prevent potential negative impacts. Genetic and chemical containment strategies have been implemented to restrict the growth and replication of genetically modified organisms while no robust physical containment has been proposed. We developed a hydrogel-based encapsulation technique by leveraging a tough biocompatible shell and genetically recoded organisms to achieve unprecedented containment performance. Then, we implemented biocontainment into wearable hydrogel devices. We use stretchable, robust, and biocompatible hydrogel-elastomer hybrids to host genetically programed bacteria, thus creating a set of stretchable and wearable living materials and devices that possess unprecedented functions and capabilities. Lastly, we genetically encode the formation of biological polymers in E.coli to achieve the self-assembly of bacterial devices. Generating complex biomaterials often requires the coordinated and precise expression of several genes and light induction of biological material formation and patterning offer a powerful toolkit to achieve the necessary degree of precision and control. We leveraged a multichromatic optogenetic control in the bacterium Escherichia coli to express the principal structural component biological nanowires. / by Eleonore Tham. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1988. / Includes bibliographical references. / by James Spiros Nakos. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 185-192). / Thin films are the fundamental building blocks of many micro- and nano-scale devices. However, their high surface-area-to-volume ratio makes them unstable due to excess surface free energy. Capillarity drives a process known as dewetting, during which holes form, the film edges retract, and a thickened rim of material accumulates at the edges. Various shape instabilities can occur on the film edge, resulting in complicated morphologies and break-up of the film into isolated particles. Dewetting occurs in the solid state by surface self-diffusion. In this work, a variety of models are presented to gain insights into the mechanisms that control the shape evolution of thin films. A combination of thermodynamic study, stability analyses, analytical models, explicit interface-tracking simulations, and phase-field simulations reveal the underlying driving forces and mass flows, explain observed morphologies and instabilities, and offer insights into how to manipulate the final structure. These pathways to control dewetting are applicable in two areas: to design micro- and nano-scale devices that are resistant to thermal degradation, and to use dewetting as a new patterning method to generate stable, complex, small-scale geometries. / by Rachel Victoria Zucker. / Ph. D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000. / Includes bibliographical references (p. 183-190). / by Scott A. Uhland. / Ph.D.