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Random Access Memory (RRAM) has emerged as a leading candidate for nonvolatile memory storage. RRAM devices typically consist of a metal/insulator/metal (MIM) structure and exhibit switching of the device resistivity state (low-to-high, highto- low) by application of electrical bias. It is now widely accepted that shunting and rupturing of local conductive paths (filaments) directly determines the resistance state. The size and composition of these filaments are very much an open question, but are usually attributed to high local concentrations of oxygen vacancies. Although there has been a huge body of research conducted in this field, the fundamental nature of the conductive path and basic switching/failure mechanisms are still under debate. This is largely due to a lack of structural analysis of existing filament size and composition in actual devices. Since the non-volatile nature and device reliability issues (i.e. retention and endurance) are directly related to the irreversible structural transformations in the device, microstructural characterization is essential for eventual commercialization of RRAM. In this study, I investigated oxygen vacancy defect dynamics under electric filed essential for resistive switching and aim to identify size, location, and chemical nature of the conductive filaments in RRAM devices by using a variety of devices and materials characterization methods: in situ transmission electron microscopy (TEM), highresolution TEM (HRTEM), scanning TEM (STEM)-electron energy loss spectroscopy (EELS), electron holography, rapid thermal annealing (RTA), transient thermometry, and electro-thermal simulation. I adopt an in situ electrical biasing TEM technique to study microstructural changes occurring during resistive switching using a model TiO2-based RRAM device, and confirmed the device is switchable inside of the TEM column. I observed extension and contraction of {011} and {121}-type Wadsley defects, crystallographic shear faults, associated with resistive switching. More specifically, emission and adsorption of oxygen vacancies under different polarity of electrical biases at the fault bounding dislocations were identified. The motion of Wadsley defects was used to track oxygen vacancy migration under electric field. Also, the microstructural changes that occur when the device experiences low electric field (~104 V/cm) was reported, akin to read disturb. Crossbar type RRAM device stacks consisting of TiN/a-HfAlOx/Hf/TiN were investigated to estimate filament size, filament temperature, and its chemical footprint using HRTEM, transient thermometry and numerical simulation. In each of the switched devices, a single crystallite ~ 8-16 nm in size embedded in an amorphous HfAlOx matrix was found. The HfAlOx crystallization temperature (Tc) of 850 K was determined by combining RTA and HRTEM imaging. In parallel, the filament size has been determined by transient thermometry. The temperature profile extracted from these measurements suggested that the peak filament temperature was > 1500 K at the center, with the hot zone (T > Tc = 850 K) extending to a radius of 7 nm around the filament. These results were consistent with the HRTEM observations of the crystallite size. The potential filament location (crystallite) in the switching devices was analyzed by STEM-EELS and identification of the filament chemical nature identification has been attempted.

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Thesis (Ph. D.)--Florida State University, 2003. / Advisor: Dr. Michael S. Chapman, Florida State University, College of Arts and Sciences, Dept. of Chemistry and Biochemistry. Title and description from dissertation home page (viewed Feb. 24, 2003). Includes bibliographical references.

Section to section alignment is a preliminary step to the creation of three dimensional reconstructions from serial section electron micrographs. Typically, the micrograph of one section is aligned to its neighbors by analyzing a set of fiducial points to calculate an appropriate polynomial transform. This transform is then used to map all of the pixels of the micrograph into alignment. Such transforms are usually linear or piecewise linear in order to limit the accumulation of small errors, which may occur with the use of higher–order approximations. Linear alignment is unable to correct common higher order geometric distortions, such as lens distortion in the case of TEM, and scan distortion in the case of transmission-mode SEM. Here, we attempt to show that standard calibration replicas may be used to calculate a high order distortion model despite the irregularities that are often present in them. We show that SEM scan distortion has much less of an effect than TEM lens distortion; however, the effect of TEM distortion on prior geometric measurements made over three-dimensional reconstructions of dendrites, axons, and synapses and their subcellular compartments is negligible. / text

A nanomanipulation system operating inside a scanning electron microscope (SEM) enables visual observation and physical interactions with objects at the nanometer scale. Compared to SEM that is a powerful imaging platform (‘eyes’), the development of nanomanipulation systems (‘hands) and techniques for transporting, modifying, and interacting with micro/nanoscaled objects is lagging behind. Two generations of nanomanipulation systems were developed with high SEM compatibility. The vacuum load-lock feature allows setup/sample/end-tools changes to be made within minutes instead of hours as with existing nanomanipulation systems. The integrated high resolution encoders and automation features significantly ease the skill dependency in nanomanipulation. Its small shape factor minimizes effects on SEM imaging performance, and does not restrict the use of the many detectors inside a SEM. The new nanomanipulation systems were applied to the manipulation of sub-cellular structures and the characterization of nano-structures. The first application involves the development of a technique to surgically extract sub-micrometer-sized subnuclear structures within a single cell’s nucleus, followed by biochemical analysis to amplify and sequence the genes contained within. Enabled by the technique, four novel genomic loci associations with promyelocytic leukemia nuclear bodies (PML NB) were discovered in Jurkat cells. The second application targets automated probing of nanostructures under poor imaging conditions. Through real-time image drift compensation and visual servoing of the nano probes, automated probing of nanostructures was achieved with a high success rate and a speed at least three times higher than skilled operator. To enhance the functions of the nanomanipulation system, new types of end-effectors were also developed. A MEMS tool with changeable tool tips was design and prototyped. In-situ (i.e., inside SEM) tool tip change was demonstrated for gripping objects that vary in size by two orders of magnitude (15 um to 100 nm) with a single microgripper body. Furthermore, a microfabrication process was developed to produce changeable nano-spatulas with tip size less than 10 nm, intended for use in the subnuclear structure extraction work. Finally, a local precursor sublimation technique compatible with the nanomanipulation system was developed for enhancing electron beam induced deposition (EBID) inside the SEM.

A nanomanipulation system operating inside a scanning electron microscope (SEM) enables visual observation and physical interactions with objects at the nanometer scale. Compared to SEM that is a powerful imaging platform (‘eyes’), the development of nanomanipulation systems (‘hands) and techniques for transporting, modifying, and interacting with micro/nanoscaled objects is lagging behind. Two generations of nanomanipulation systems were developed with high SEM compatibility. The vacuum load-lock feature allows setup/sample/end-tools changes to be made within minutes instead of hours as with existing nanomanipulation systems. The integrated high resolution encoders and automation features significantly ease the skill dependency in nanomanipulation. Its small shape factor minimizes effects on SEM imaging performance, and does not restrict the use of the many detectors inside a SEM. The new nanomanipulation systems were applied to the manipulation of sub-cellular structures and the characterization of nano-structures. The first application involves the development of a technique to surgically extract sub-micrometer-sized subnuclear structures within a single cell’s nucleus, followed by biochemical analysis to amplify and sequence the genes contained within. Enabled by the technique, four novel genomic loci associations with promyelocytic leukemia nuclear bodies (PML NB) were discovered in Jurkat cells. The second application targets automated probing of nanostructures under poor imaging conditions. Through real-time image drift compensation and visual servoing of the nano probes, automated probing of nanostructures was achieved with a high success rate and a speed at least three times higher than skilled operator. To enhance the functions of the nanomanipulation system, new types of end-effectors were also developed. A MEMS tool with changeable tool tips was design and prototyped. In-situ (i.e., inside SEM) tool tip change was demonstrated for gripping objects that vary in size by two orders of magnitude (15 um to 100 nm) with a single microgripper body. Furthermore, a microfabrication process was developed to produce changeable nano-spatulas with tip size less than 10 nm, intended for use in the subnuclear structure extraction work. Finally, a local precursor sublimation technique compatible with the nanomanipulation system was developed for enhancing electron beam induced deposition (EBID) inside the SEM.