This research focuses on device design and process integration of high density nonvolatile memory devices. Research was carried out to improve scaling of floating gate memories by increasing charge density as well as spin-based memories by reducing critical switching current. This work demonstrates fabrication of CMOS-compatible nonvolatile hybrid memory device using fullerene molecules as a floating gate. Molecules have dimensions of several Angstroms resulting in an electron density of ~10¹³ cm⁻² or higher. In hybrid MOSCAPs, fullerenes were encapsulated between inorganic oxides, i.e. SiO₂ as a tunnel oxide and HfO₂ as a control oxide. Introduction of a high-k material as a control oxide improves capacitive coupling between control gate and floating gate as well as the program/erase efficiency. The MOS capacitors demonstrate nonvolatile memory operation at room temperature. The device data infers that program/erase mechanism in fullerene devices is Fowler-Nordheim tunneling; however, retention is determined by trap-assisted tunneling. The next part of the work focused on spin-transfer-torque (STT) based magnetic memory. Spin-based memory has the unique potential to be the universal memory because of its high density, fast switching, and nonvolatility. This work presents STT switching of perpendicular magnetic anisotropy (PMA) spin-valves with tilted magnetization using point contact measurement. The PMA materials have high coercivity resulting in good retention and tilted magnetization induces precessional switching resulting in a lower switching current density. First, micromagnetic simulations were performed for spin-valves with tilted magnetization and precessional switching was observed to reduce the switching current. Then, spin-valve structures were fabricated by e-beam evaporation. The structure consisted of Co/Pt and Co/Ni layers, where the thickness of the layers was optimized to obtain different amount of tilt in magnetization. Point contact measurements of tilted spin-valves show STT switching, where the switching field of the free layer varies with the magnitude and sign of the applied current. The observed STT effect is stronger in a 45° tilted spin-valve compared to a 12° tilted device presumably due to the tilted spin polarization. However, tilting introduces nonuniform effective field and canting of the domains which affect the STT. / text

Non-volatile memories (NVM) including flash memories and resistive memories have attracted significant interest as data storage media. Flash memories are widely employed in mobile devices and solid-state drives (SSD). Resistive memories are promising as storage class memory and embedded memory applications. Data reliability is the fundamental requirement of NVM as data storage media. However, modern nano-scale NVM suffers from challenges of inter-cell interference (ICI), charge leakage, and write endurance, which threaten the reliability of stored data. In order to cope with these adverse effects, advanced coding techniques including soft decision decoding have been investigated actively. However, current coding techniques do not capture the physical properties of NVM well, so the improvement of data reliability is limited. Although soft decision decoding improves the data reliability by using soft decision values, it degrades read speed performance due to multiple read operations needed to obtain soft decision values. In this dissertation, we explore coding schemes that use side information corresponding to the physical phenomena to improve the data reliability significantly. The side information is obtained before writing data into memory and incorporated during the encoding stage. Hence, the proposed coding schemes maintain the read speed whereas the write speed performance would be degraded. It is a big advantage from the perspective of speed performance since the read speed is more critical than the write speed in many memory applications. First, this dissertation investigates the coding techniques for memory with stuckat defects. The idea of coding techniques for memory with stuck-at defects is employed to handle critical problems of flash memories and resistive memories. For 2D planar flash memories, we propose a coding scheme that combats the ICI, which is a primary challenge of 2D planar flash memories. Also, we propose a coding scheme that reduces the effect of fast detrapping, a degradation factor in 3D vertical flash memories. Finally, we investigate the coding techniques that improve write endurance and power consumption of resistive memories.

In recently years, nonvolatile memory with nanocrystals cell have widely applied to overcome the issue of operation and reliability for conventional floating gate memory. The excellent electrical characteristics of memory device need good endurance, long retention time and small operation voltage. Among numerous memory devices with nanocrystals, the memory device with metal nanocrystals was widely researched. It will be new candidate for flash memory. The advantages of metal nanocrystals has have higher density of states around Fermi level, stronger coupling with conduction channel, wide range of available work functions and smaller energy perturbation due to carrier confinement. So metal nanocrystals can reduce operate voltage, and increase write/erase speed and endurance. Most important of all, we can control the sizes of nanocrystals dot and manufacture at low temperature¡CThis advantage can apply to thin film transistor liquid crystal display; it fabricates driving IC and logical IC on panel for diverseness and adds memory beside switch TFT as image storage to reduce power consumption for portability. In this thesis, we will discuss metal nanocrystals as memory storage medium. And we can use high temperature oxidation, low temperature annealing with oxygen to form nanocrystals. Besides we analyze the effect of electron storage at metal nanocrystals by means of material and electrical analysis.

abstract: Programmable Metallization Cell (PMC) technology has been shown to possess the necessary qualities for it to be considered as a leading contender for the next generation memory. These qualities include high speed and endurance, extreme scalability, ease of fabrication, ultra low power operation, and perhaps most importantly ease of integration with the CMOS back end of line (BEOL) process flow. One area where detailed study is lacking is the reliability of PMC devices. In previous reliability work, the low and high resistance states were monitored for periods of hours to days without any applied voltage and the results were extrapolated to several years (>10) but little has been done to analyze the low resistance state under stress. With or without stress, the low resistance state appears to be highly stable but a gradual increase in resistance with time, less than one order of magnitude after ten years when extrapolated, has been observed. It is important to understand the physics behind this resistance rise mechanism to comprehend the reliability issues associated with the low resistance state. This is also related to the erase process in PMC cells where the transition from the ON to OFF state occurs under a negative voltage. Hence it is important to investigate this erase process in PMC cells under different conditions and to model it. Analyzing the programming and the erase operations separately is important for any memory technology but its ability to cycle efficiently (reliably) at low voltages and for more than 1E4 cycles (without affecting the cells performance) is more critical. Future memory technologies must operate with the low power supply voltages (<1V) required for small geometry nodes. Low voltage programming of PMC memory devices has previously been demonstrated using slow voltage sweeps and small numbers of fast pulses. In this work PMC memory cells were cycled at low voltages using symmetric pulses with different load resistances and the distribution of the ON and OFF resistances was analyzed. The effect of the program current used during the program-erase cycling on the resulting resistance distributions is also investigated. Finally the variation found in the behavior of similar resistance ON states in PMC cells was analyzed more in detail and measures to reduce this variation were looked into. It was found that slow low current programming helped reducing the variation in erase times of similar resistance ON states in PMC cells. This scheme was also used as a pre-conditioning technique and the improvements in subsequent cycling behavior were compared. / Dissertation/Thesis / Ph.D. Electrical Engineering 2011

The drivers of human health and changing climate are important areas of environmental and atmospheric studies. Among many environmental factors present in our biosphere, small particles, also known as ultrafine particles or UFPs, have direct and indirect pathways to affect human health and climatic processes. The rapid change in their properties makes UFPs dynamic and often challenging to quantify their effect on health and radiative forcing. To reduce uncertainty in the climate effects of UFPs and to strengthen the evidence on health effects, accurate characterizations of physical and chemical properties of UFPs are needed. In this thesis, two broad aspects of UFPs were investigated: (1) the development of particle instrumentation to study particle properties; and (2) measurement of physical and chemical properties of UFPs relevant to human health and climate. These two broad aspects are divided into four specific aims in this thesis. The measurement of UFP concentration at different locations in an urban location, from roadside to various residential areas, can be improved by using a mobile particle counter. A TSI 3786 Condensation Particle Counter (CPC) was modified for mobile battery-power operation. This design provided high-frequency, one second time resolution measurements of particle number and carbon dioxide (CO2). An independent electric power system, a central controller and robust data acquisition system, and a GPS system are the major components of this mobile unit. These capabilities make the system remotely deployable, and also offer flexibility to integrate other analog and digital sensors. A Volatility Tandem Differential Mobility Analyzer (V-TDMA) system was designed and built to characterize the volatility behavior of UFPs. The physical and chemical properties of UFPs are often challenging to measure due to limited availability of instruments, detection limit in terms of particle size and concentration, and sampling frequency. Indirect methods such as V-TDMA are useful, for small mass (<1 µg/m3), and nuclei mode particles (<30nm). Another advantage of V-TDMA is its fast response in terms of sampling frequency. A secondary motivation for building a V-TDMA system was to improve instrumentation capability of our group, thus enabling study of kinetic and thermodynamic properties of novel aerosols. Chapter four describes the design detail of the built V-TDMA system, which measures the change in UFP size and concentration during heated and non-heated (or ambient) condition. The V-TDMA system has an acceptable penetration efficiency of 85% for 10 nm and maintains a uniform temperature profile in the heating system. Calibration of V-TDMA using ammonium sulfate particles indicated that the system produces comparable evaporation curves (in terms of volatilization temperature) or volatility profiles to other published V-TDMA designs. Additionally the system is fully programmable with respect to particle size, temperature and sampling frequency and can be run autonomously after initial set up. The thesis describes a part of yearlong study to provide a complete perspective on particle formation and growth in a rural and agricultural Midwestern site. Volatility characterizations of UFPs were conducted to enable inference about particle chemistry, and formation of low volatile core or evaporation resistant residue in the UFP in the Midwest. This study addresses identification of the volatility signature of particles in the UFP size range, quantification of physical differences of UFPs between NPF1 and non-NPF events and relation of evaporation resistant residue with particle size, seasonality and mixing state. K-means clustering was applied to determine three unique volatility clusters in 15, 30, 50 and 80 nm particle sizes. Based on the proposed average volatility, the identified volatility clusters were classified into high volatile, intermediate volatile and least volatile group. Although VFR alone is insufficient to establish chemical composition definitively, least volatile cluster based on average volatility may be characteristically similar to the pure ammonium sulfate. The amount of evaporation residue at 200 °C was positively correlated with particle size and showed significant correlation with ozone, sulfur dioxide and solar radiation. Residue also indicated the presence of external mixture, often during morning and night time. Air quality science and management of an accidental urban tire fire occurring in Iowa City in May and June of 2012 were investigated. Urban air quality emergencies near populated areas are difficult to evaluate without a proper air quality management and response system. To support the development of an appropriate air quality system, this thesis identified and created a rank for health-related acute and chronic compounds in the tire smoke. For health risk assessment, the study proposed an empirical equation for estimating multi-pollutant air quality index. Using mobile measurements and a dispersion model in conjunction with the proposed air quality index, smoke concentrations and likely health impact were evaluated for Iowa City and surrounding areas. It was concluded that the smoke levels reached unhealthy outdoor levels for sensitive groups out to distances of 3.1 km and 18 km at 24 h and 1h average times. Tire smoke characterization was another important aspect of this study which provided important and new information about tire smoke. Revised emission factors for coarse particle mass and aerosol-PAH and new emission factors and enhancement ratios values for a wide range of fine particulate mass, particle size (0.001-2.5 µm), and trace gas were estimated. Overall the thesis added new instrumentation in our research group to measure various physical properties such as size, concentration, and volatility UFP. The built instruments, data processing algorithm and visualization tools will be useful in estimation of accurate concentration and emission factors of UFP for health exposure studies, and generate a fast response measurement of kinetic and thermodynamics properties of ambient particles. This thesis also makes a strong case for the development of an air quality emergency system for accidental fires for urban location. It provides useful evaluation and estimation of many aspects of such system such as smoke characterization, method of air quality monitoring and impact assessment, and develops communicable method of exposure risk assessment.

In recent years, the conventional Flash memory with floating structure is expected to reach physical limits as devices scaling down in near future. In order to overcome this problem, alternative memory technologies have been widely investigated. And the resistance random access memory (RRAM) has attracted extensive attention for the application in next generation nonvolatile memory, due to the excellent memory property including lower consumption of energy, lower operating voltage, higher density, fast operating speed, simple structure, higher endurance, retention and process compatibility with CMOS. In this study, the tungsten-based oxide is chosen as RRAM switching layer because the tungsten is compatible with the present complementary metal oxide semiconductor (CMOS) process. The Pt/WOX/TiN structure device cells had the resistance switching property successfully. However, the experiment result revealed the inferior resistance switching property. The resistance switching characteristic of the WOX thin film is extremely unstable, it is impossible to become the products. Compared with WOX, the resistance switching property of WSiOX RRAM device is improved substantially such as stability of resistance states and reliability of device. In second parts, we purposed two methods to enhance the device switching characteristic, including controlling the filament formation/ interruption in the W doped SiOX layer and restricting oxygen movement in the WSiON layer. Finally, the transport mechanisms of carrier is analyzed and researched from the current-voltage (I-V) switching characteristic of the device. A designed circuit was used in this study to accurately observe the resistance switching process with a pulse generator and oscilloscope, which reveals that the switching process is related to both time and voltage. The oxygen movement will drift in the low temperature due to the electrical field and restricted the crystal lattice vibration. But, it will diffuse through thermal dynamics in the high temperature.

Power consumption is one of the most important design factors in modern electronic design. With a large market increase in portable battery-operated devices and push for environmental focus, it is of interest for the industry to decrease the power consumption of modern chips as much as possible. However, as circuits scale down in size the leakage current increases. This increases the static power consumption, and in future technologies the static power is expected to make up most of the overall power consumption. Power gating can decrease static power by isolating a circuit block from the power supply. In large chips, this requires state-retention flip flops and non-volatile memories in order to keep the circuit functioning continuously between power gating sequences. A design concept utilizing this is a Normally Off computer, which is in an off-state with no static power for the majority of the time. This is achieved by using non-volatile logic and memories. This concept has been realized by using a new semiconductor technology developed at Semiconductor Energy Laboratories Corporation Ltd., which is known as crystalline In-Ga-Zn oxide semiconductor material. This technology realizes transistors with an ultra-low off-state current, and enables several novel designs of state-retention circuits suitable for Normally-Off computers. This thesis presents two different architectures of state retention flip flops utilizing In-Ga-Zn oxide semiconductor transistors, which are produced and compared to determine their tradeoffs and effectiveness. These flip flops are then implemented in a 32-bit Normally-Off microprocessor to determine the performance of each implementation. This is evaluated by calculating the energy break-even time, which is the power gating time required to overcome the power overhead introduced by the state-retention flip flops. The resulting circuits and the work in this thesis has been presented at two conferences and submitted for publication in one scientific journal.

The evolution of the MOSFET technology has been driven by the aggressive shrinkage of the device size to improve the device performance and to increase the circuit density. Currently, many research demonstrated that the continuous polycrystalline silicon film in the floating-gate dielectric could be replaced with nanocrystal (nc) embedded high-k thin film to minimize the charge loss due to the defective thin tunnel dielectric layer. This research deals with both the statistical aspect of reliability and electrical aspect of reliability characterization as well. In this study, the Zr-doped HfO2 (ZrHfO) high-k MOS capacitors, which separately contain the nanocrystalline zinc oxide (nc-ZnO), silicon (nc-Si), Indium Tin Oxide (nc-ITO) and ruthenium (nc-Ru) are studied on their memory properties, charge transportation mechanism, ramp-relax test, accelerated life tests, failure rate estimation and thermal effect on the above reliability properties. C-V hysteresis result show that the amount of charges trapped in nanocrystal embedded films is in the order of nc-ZnO>nc-Ru>nc-Si~nc-ITO, which might probably be influenced by the EOT of each sample. In addition, all the results show that the nc-ZnO embedded ZrHfO non-volatile memory capacitor has the best memory property and reliability. In this study, the optimal burn-in time for this kind of device has been also investigated with nonparametric Bayesian analysis. The results show the optimal burn-in period for nc-ZnO embedded high-k device is 5470s with the maximum one-year mission reliability.

Le marché mondial des semi-conducteurs connait une croissance continue due à l'essor de l'électronique grand public et entraîne dans son sillage le marché des mémoires non volatiles. L'importance de ces produits mémoires est accentuée depuis le début des années 2000 par la mise sur le marché de produits nomades tels que les smartphones ou plus récemment les produits de l’internet des objets. De par leurs performances et leur fiabilité, la technologie Flash constitue, à l'heure actuelle, la référence en matière de mémoire non volatile. Cependant, le coût élevé des équipements en microélectronique rend impossible leur amortissement sur une génération technologique. Ceci incite l’industriel à adapter des équipements d’ancienne génération à des procédés de fabrication plus exigeants. Cette stratégie n’est pas sans conséquence sur la dispersion des caractéristiques physiques (dimension géométrique, épaisseur…) et électriques (courant, tension…) des dispositifs. Dans ce contexte, le sujet de ma thèse est d’optimiser et de réduire la variabilité d’une nouvelle architecture mémoire non volatile ultra basse consommation.Cette étude vise à poursuivre les travaux entamés par STMicroelectronics sur le développement, l’étude et la mise en œuvre de boucles de contrôle de type Run-to-Run (R2R) sur une nouvelle cellule mémoire ultra basse consommation. Afin d’assurer la mise en place d’une régulation pertinente, il est indispensable de pouvoir simuler l’influence des étapes du procédé de fabrication sur le comportement électrique des cellules en s’appuyant sur l’utilisation d’outils statistiques ainsi que sur une caractérisation électrique pointue. / The global semiconductor market is experiencing steady growth due to the development of consumer electronics and the wake of the non-volatile memory market. The importance of these memory products has been accentuated since the beginning of the 2000s by the introduction of nomadic products such as smartphones or, more recently, the Internet of things. Because of their performance and reliability, Flash technology is currently the standard for non-volatile memory. However, the high cost of microelectronic equipment makes it impossible to depreciate them on a technological generation. This encourages industry to adapt equipment from an older generation to more demanding manufacturing processes. This strategy is not without consequence on the spread of the physical characteristics (geometric dimension, thickness ...) and electrical (current, voltage ...) of the devices. In this context, the subject of my thesis is “Optimization and reduction of the variability of a new architecture ultra-low power non-volatile memory”.This study aims to continue the work begun by STMicroelectronics on the improvement, study and implementation of Run-to-Run (R2R) control loops on a new ultra-low power memory cell. In order to ensure the implementation of a relevant regulation, it is essential to be able to simulate the process manufacturing influence on the electrical behavior of the cells, using statistical tools as well as the electric characterization.

abstract: Lateral programmable metallization cells (PMC) utilize the properties of electrodeposits grown over a solid electrolyte channel. Such devices have an active anode and an inert cathode separated by a long electrodeposit channel in a coplanar arrangement. The ability to transport large amount of metallic mass across the channel makes these devices attractive for various More-Than-Moore applications. Existing literature lacks a comprehensive study of electrodeposit growth kinetics in lateral PMCs. Moreover, the morphology of electrodeposit growth in larger, planar devices is also not understood. Despite the variety of applications, lateral PMCs are not embraced by the semiconductor industry due to incompatible materials and high operating voltages needed for such devices. In this work, a numerical model based on the basic processes in PMCs – cation drift and redox reactions – is proposed, and the effect of various materials parameters on the electrodeposit growth kinetics is reported. The morphology of the electrodeposit growth and kinetics of the electrodeposition process are also studied in devices based on Ag-Ge30Se70 materials system. It was observed that the electrodeposition process mainly consists of two regimes of growth – cation drift limited regime and mixed regime. The electrodeposition starts in cation drift limited regime at low electric fields and transitions into mixed regime as the field increases. The onset of mixed regime can be controlled by applied voltage which also affects the morphology of electrodeposit growth. The numerical model was then used to successfully predict the device kinetics and onset of mixed regime. The problem of materials incompatibility with semiconductor manufacturing was solved by proposing a novel device structure. A bilayer structure using semiconductor foundry friendly materials was suggested as a candidate for solid electrolyte. The bilayer structure consists of a low resistivity oxide shunt layer on top of a high resistivity ion carrying oxide layer. Devices using Cu2O as the low resistivity shunt on top of Cu doped WO3 oxide were fabricated. The bilayer devices provided orders of magnitude improvement in device performance in the context of operating voltage and switching time. Electrical and materials characterization revealed the structure of bilayers and the mechanism of electrodeposition in these devices. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020