High aspect ratio silicon etching used for DRAM manufacturing still remains as one of the biggest challenges in semiconductor fabrication, requiring well understood and characterized process fundamentals. In this study, physical and chemical interactions during etching silicon in capacitively coupled plasma discharges were investigated in detail for different HBr/NF3 mixed chemistries for single frequency as well as dual frequency operation and medium discharge pressures inside an industrial MERIE CCP reactor typically used for DRAM fabrication. Utilization of the dual frequency concept for separate control of ion energy and ion flux, as well as the impact on discharge properties and finally on etching at relevant substrate surfaces were studied systematically. The complex nature of multi frequency rf sheaths was both analyzed experimentally by applying mass resolved ion energy analysis, and from simulation of ion energy distributions by using a Hybrid Plasma Sheath Model. Discharge composition and etch processes were investigated by employing standard mass spectrometry, Appearance Potential Mass Spectrometry, Quantum Cascade Laser Absorption Spectroscopy, rf probe measurements, gravimetry and ellipsometry. An etch model is developed to explain limitations of silicon etching in HBr/NF3 discharges to achieve highly aniostropic etching. / Siliziumätzen mit hohen Aspektverhältnissen zur Herstellung von DRAM-Speicherstrukturen stellt nach wie vor eine der größten Herausforderungen in der Halbleiterherstellung dar und erfordert ein grundlegendes Prozessverständnis. Diese Studie beinhaltet eine umfassende und detaillierte Untersuchung physikalischer und chemischer Wechselwirkungen von Siliziumätzprozessen in kapazitiv gekoppelten HBr/NF3-Gasentladungen in einem kommerziellen, typischerweise für die DRAM-Fertigung eingesetzten MERIE CCP Reaktor mit Ein- und Zweifrequenzanregung bei mittleren Entladungsdrücken. Die Anwendung eines Zweifrequenzkonzeptes zur separaten Kontrolle von Ionenenergie und Ionenstromdichte, als auch deren Einfluss auf die Entladungseigenschaften und letztendlich auf das Ätzverhalten auf relevanten Substratoberflächen wurden systematisch untersucht. Die komplexe Natur von mehrfrequenzangeregten HF-Randschichten wurde sowohl experimentell über eine Anwendung von massenaufgelöster Ionenenergieanalyse als auch rechnerisch über Simulationen von Ionenenergieverteilungsfunktionen mit Hilfe eines hybriden Plasmarandschichtmodells analysiert. Gaszusammensetzungen verschiedener Entladungen und Ätzprozesse wurden mit Hilfe von Standard-Massenspektrometrie, Schwellwert-Massenspektrometrie, Quantenkaskaden-Laserabsorptionsspektroskopie, HF-Sondenmessungen, Gravimetrie und Ellipsometrie charakterisiert. Eine neuartige Modellvorstellung zum Siliziumätzen in HBr/NF3-Entladungsgemischen liefert eine plausible Erklärung für die Limitierung der Ätzrate zum Erreichen eines hoch anisotropen Ätzverhaltens.

Les applications d’électronique de spin telles les mémoires à accès aléatoire (MRAM), les capteurs (e.g.les têtes de lecture des disques durs d’ordinateurs) et les éléments de logique magnétique utilisent les interactionsd’échange ferromagnétique/antiferromagnétique (F/AF) dans le but de définir une direction de référence pour lespin des électrons de conduction. Les MRAM à écriture assistée thermiquement (TA-MRAM) utilisent mêmedeux bicouches F/AF : une pour le stockage de l’information et l’autre comme référence. Ces dernièresapplications technologiques impliquent des étapes de nanofabrication des couches minces continues pour formerune matrice de cellules mémoires individuelles. La qualification industrielle du produit final impose de sérieusescontraintes sur la largeur des distributions des propriétés magnétiques - y compris d’échange F/AF - de cellulemémoire à cellule mémoire. Des phases verres de spin réparties de manière aléatoire sur la couche continue, àl’interface F/AF ou dans le coeur de l’AF pourraient contribuer de manière significative à ces distributionsd’échange F/AF dans les dispositifs, après nanofabrication ; comme supposé il y a de cela quelques années. Lebut de cette thèse est d’étudier factuellement le possible lien entre verre de spin répartis dans des couches mincesF/AF et dispersions de propriétés d’échange de cellule mémoire à cellule mémoire dans les dispositifs TAMRAMcorrespondants. Avant cela, l’origine de ces régions verre de spin a été étudiée et une attention plusparticulière a été portée au rôle joué par les diffusions de Mn. Ces dernières ont été directement observées,comprises et l’utilisation de barrières complexes pour les réduire et par là même pour diminuer la quantité dephases verre de spin a été mise en oeuvre avec succès. En guise d’alternative pour varier la quantité de verres despin, l’utilisation d’AFs composites a été également étudiée dans le cadre de cette thèse. Ce type d’AF permet dumême coup de varier la stabilité thermique des grains AF et de répondre à un autre problème identifié pour lesTA-MRAM qui consiste à trouver des matériaux AF avec des propriétés de rétention et d’écriture intermédiairespar rapport aux matériaux actuellement utilisés. Finalement, ces AFs composites ont été utilisés comme moyende varier la quantité de verres de spin dans des dispositifs TA-MRAM réels et de prouver le lien direct avec ladispersion de propriétés de cellule mémoire à cellule mémoire. / Spintronics applications such as magnetic random access memories (MRAM), sensors (e.g.. hard diskdrive read head) and logic devices use ferromagnetic/antiferromagnetic (F/AF) exchange bias (EB) interactionsto set the reference direction required for the spin of conduction electrons. Thermally-assisted (TA-) MRAMapplications even use two F/AF exchange biased bilayers: one for reference and one for storage. Suchtechnological applications involve patterning full sheet wafers into matrix of individual bit-cells. Industrialproducts qualification imposes stringent requirements on the distributions of the magnetic properties from cell tocell, including those related to EB. It was supposed few years ago that randomly spread spin-glass like phases atthe F/AF interface or within the bulk of the AF layer significantly contribute to the distributions of EB propertiesin devices after processing. This thesis aimed at factually studying the link between spin-glasses spread overF/AF thin films and bit-cell dispersions of EB in corresponding TA-MRAM. Prior to that the origin of the spinglasslike regions and more specifically the role of Mn-diffusion was consolidated. Mn-diffusion was directlyobserved, understood and the use of complex barriers to reduce such diffusion and consequently to minimize theamount of spin-glass was successfully studied. Mixing AFs as another way to tune the amount of spin-glass likephases was also evidenced in the framework of this thesis. All at once, this last solution also tuned the AF grainsthermal stability and this solved another issue related to TA-MRAM, i.e. finding AF-materials with intermediateretention and write properties compared to the AFs presently used. Finally, those mixed antiferromagnets werethe mean chosen to tune the amount of spin-glass like phases in real TA-MRAM devices and to factually provetheir link with bit-cell distributions of EB properties.

Conventionally, to support varying power demand, the utility company must prepare to supply more electricity than actually needed, which causes inefficiency and waste. With the increasing penetration of renewable energy which is intermittent and stochastic, how to balance the power generation and demand becomes even more challenging. Demand response, which reschedules part of the elastic load in users' side, is a promising technology to increase power generation efficiency and reduce costs. However, how to coordinate all the distributed heterogeneous elastic loads efficiently is a major challenge and sparks numerous research efforts. In this thesis, we investigate different methods to provide demand response and improve power grid efficiency. First, we consider how to schedule the charging process of all the Plugged-in Hybrid Electrical Vehicles (PHEVs) so that demand peaks caused by PHEV charging are flattened. Existing solutions are either centralized which may not be scalable, or decentralized based on real-time pricing (RTP) which may not be applicable immediately for many markets. Our proposed PHEV charging approach does not need complicated, centralized control and can be executed online in a distributed manner. In addition, we extend our approach and apply it to the distribution grid to solve the bus congestion and voltage drop problems by controlling the access probability of PHEVs. One of the advantages of our algorithm is that it does not need accurate predictions on base load and future users' behaviors. Furthermore, it is deployable even when the grid size is large. Different from PHEVs, whose future arrivals are hard to predict, there is another category of elastic load, such as Heating Ventilation and Air-Conditioning (HVAC) systems, whose future status can be predicted based on the current status and control actions. How to minimize the power generation cost using this kind of elastic load is also an interesting topic to the power companies. Existing work usually used HVAC to do the load following or load shaping based on given control signals or objectives. However, optimal external control signals may not always be available. Without such control signals, how to make a tradeoff between the fluctuation of non-renewable power generation and the limited demand response potential of the elastic load, and to guarantee user comfort level, is still an open problem. To solve this problem, we first model the temperature evolution process of a room and propose an approach to estimate the key parameters of the model. Then, based on the model predictive control, a centralized and a distributed algorithm are proposed to minimize the fluctuation and maximize the user comfort level. In addition, we propose a dynamic water level adjustment algorithm to make the demand response always available in two directions. Extensive simulations based on practical data sets show that the proposed algorithms can effectively reduce the load fluctuation. Both randomized PHEV charging and HVAC control algorithms discussed above belong to direct or centralized load shaping, which has been heavily investigated. However, it is usually not clear how the users are compensated by providing load shaping services. In the last part of this thesis, we investigate indirect load shaping in a distributed manner. On one hand, we aim to reduce the users' energy cost by investigating how to fully utilize the battery pack and the water tank for the Combined Heat and Power (CHP) systems. We first formulate the queueing models for the CHP systems, and then propose an algorithm based on the Lyapunov optimization technique which does not need any statistical information about the system dynamics. The optimal control actions can be obtained by solving a non-convex optimization problem. We then discuss when it can be converted into a convex optimization problem. On the other hand, based on the users' reaction model, we propose an algorithm, with a time complexity of O(log n), to determine the RTP for the power company to effectively coordinate all the CHP systems and provide distributed load shaping services. / Graduate

Les applications d’électronique de spin telles les mémoires à accès aléatoire (MRAM), les capteurs (e.g.les têtes de lecture des disques durs d’ordinateurs) et les éléments de logique magnétique utilisent les interactionsd’échange ferromagnétique/antiferromagnétique (F/AF) dans le but de définir une direction de référence pour lespin des électrons de conduction. Les MRAM à écriture assistée thermiquement (TA-MRAM) utilisent mêmedeux bicouches F/AF : une pour le stockage de l’information et l’autre comme référence. Ces dernièresapplications technologiques impliquent des étapes de nanofabrication des couches minces continues pour formerune matrice de cellules mémoires individuelles. La qualification industrielle du produit final impose de sérieusescontraintes sur la largeur des distributions des propriétés magnétiques - y compris d’échange F/AF - de cellulemémoire à cellule mémoire. Des phases verres de spin réparties de manière aléatoire sur la couche continue, àl’interface F/AF ou dans le coeur de l’AF pourraient contribuer de manière significative à ces distributionsd’échange F/AF dans les dispositifs, après nanofabrication ; comme supposé il y a de cela quelques années. Lebut de cette thèse est d’étudier factuellement le possible lien entre verre de spin répartis dans des couches mincesF/AF et dispersions de propriétés d’échange de cellule mémoire à cellule mémoire dans les dispositifs TAMRAMcorrespondants. Avant cela, l’origine de ces régions verre de spin a été étudiée et une attention plusparticulière a été portée au rôle joué par les diffusions de Mn. Ces dernières ont été directement observées,comprises et l’utilisation de barrières complexes pour les réduire et par là même pour diminuer la quantité dephases verre de spin a été mise en oeuvre avec succès. En guise d’alternative pour varier la quantité de verres despin, l’utilisation d’AFs composites a été également étudiée dans le cadre de cette thèse. Ce type d’AF permet dumême coup de varier la stabilité thermique des grains AF et de répondre à un autre problème identifié pour lesTA-MRAM qui consiste à trouver des matériaux AF avec des propriétés de rétention et d’écriture intermédiairespar rapport aux matériaux actuellement utilisés. Finalement, ces AFs composites ont été utilisés comme moyende varier la quantité de verres de spin dans des dispositifs TA-MRAM réels et de prouver le lien direct avec ladispersion de propriétés de cellule mémoire à cellule mémoire. / Spintronics applications such as magnetic random access memories (MRAM), sensors (e.g.. hard diskdrive read head) and logic devices use ferromagnetic/antiferromagnetic (F/AF) exchange bias (EB) interactionsto set the reference direction required for the spin of conduction electrons. Thermally-assisted (TA-) MRAMapplications even use two F/AF exchange biased bilayers: one for reference and one for storage. Suchtechnological applications involve patterning full sheet wafers into matrix of individual bit-cells. Industrialproducts qualification imposes stringent requirements on the distributions of the magnetic properties from cell tocell, including those related to EB. It was supposed few years ago that randomly spread spin-glass like phases atthe F/AF interface or within the bulk of the AF layer significantly contribute to the distributions of EB propertiesin devices after processing. This thesis aimed at factually studying the link between spin-glasses spread overF/AF thin films and bit-cell dispersions of EB in corresponding TA-MRAM. Prior to that the origin of the spinglasslike regions and more specifically the role of Mn-diffusion was consolidated. Mn-diffusion was directlyobserved, understood and the use of complex barriers to reduce such diffusion and consequently to minimize theamount of spin-glass was successfully studied. Mixing AFs as another way to tune the amount of spin-glass likephases was also evidenced in the framework of this thesis. All at once, this last solution also tuned the AF grainsthermal stability and this solved another issue related to TA-MRAM, i.e. finding AF-materials with intermediateretention and write properties compared to the AFs presently used. Finally, those mixed antiferromagnets werethe mean chosen to tune the amount of spin-glass like phases in real TA-MRAM devices and to factually provetheir link with bit-cell distributions of EB properties.

A channel change in digital television is usually conducted at a reference frame, which are sent at certain intervals. A higher compression ratio could however be obtained by sending reference frames at arbitrary long intervals. This would on the other hand increase the average channel change time for the end user. This thesis investigates various approaches for reducing the average channel change time while using arbitrary long intervals between reference frames, and presents an implementation and evaluation of one of these methods, called Baydec. The approach of Baydec for solving the channel switch problem is to statistically estimate what the original image looked like, starting with an incoming P-frame and estimate an image between the original and current image. Baydec gathers statistical data from typical video sequences and calculates expected likelihood for estimation. Further on it uses the Simulated Annealing search method to maximise the likelihood function. This method is more general than the requirements of this thesis. It is not only applicable to channel switches between video streams, but can also be used for random access in general. Baydec could also be used if an I-frame is dropped in a video stream. However, Baydec has so far shown only theoretical result, but very small visual improvements. Baydec produces images with better PSNR than without the method in some cases, but the visual impression is not better than for the motion compensated residual images. Some examples of future work to improve Baydec is also presented.

Ferroelectricity is a material property were a remanent polarization exists under zero electrical field that can be reversed by applying an electrical field [1]. As consequence, two nonvolatile states exist that can be switched by an electrical field. This feature makes ferroelectrics ideally suited for nonvolatile memories with low write energy. Therefore, already in the 1950s first attempts have been made to realize ferroelectric nonvolatile memories based on ferroelectric barium titanate (BTO) crystals having evaporated electrodes on both sides [2]. The success of this approach was hindered by disturb issues that could be solved in the early 1990s by adding a transistor device as a selector [3]. Such a memory is referred to as a ferroelectric random access memory (FeRAM). Since reading of the ferroelectric polarization from a capacitor requires switching of the ferroelectric [1], the information will be destroyed and a write back is necessary. This can be avoided if the ferroelectric is placed inside of the gate stack of a MOS transistor resulting in a ferroelectric field effect transistor (FeFET) [1]. Conventional ferroelectric materials like BTO or lead- zirconium titanate (PZT) cannot be placed directly on silicon since unwanted interface reactions will occur. The necessary interface layer together with the space charge region of the transistor device leads to a rather low capacitance in series with the ferroelectric dielectric and consequently results in a strong depolarization field that has destroyed the nonvolatility of the FeFET device for many years and hinters scaling as well [4]. Today FeRAM devices are established on the market [3,5], but are limited to niche application since scaling is hindered by many integration problems associated to materials like PZT.

High aspect ratio silicon etching used for DRAM manufacturing still remains as one of the biggest challenges in semiconductor fabrication, requiring well understood and characterized process fundamentals. In this study, physical and chemical interactions during etching silicon in capacitively coupled plasma discharges were investigated in detail for different HBr/NF3 mixed chemistries for single frequency as well as dual frequency operation and medium discharge pressures inside an industrial MERIE CCP reactor typically used for DRAM fabrication. Utilization of the dual frequency concept for separate control of ion energy and ion flux, as well as the impact on discharge properties and finally on etching at relevant substrate surfaces were studied systematically. The complex nature of multi frequency rf sheaths was both analyzed experimentally by applying mass resolved ion energy analysis, and from simulation of ion energy distributions by using a Hybrid Plasma Sheath Model. Discharge composition and etch processes were investigated by employing standard mass spectrometry, Appearance Potential Mass Spectrometry, Quantum Cascade Laser Absorption Spectroscopy, rf probe measurements, gravimetry and ellipsometry. An etch model is developed to explain limitations of silicon etching in HBr/NF3 discharges to achieve highly aniostropic etching. / Siliziumätzen mit hohen Aspektverhältnissen zur Herstellung von DRAM-Speicherstrukturen stellt nach wie vor eine der größten Herausforderungen in der Halbleiterherstellung dar und erfordert ein grundlegendes Prozessverständnis. Diese Studie beinhaltet eine umfassende und detaillierte Untersuchung physikalischer und chemischer Wechselwirkungen von Siliziumätzprozessen in kapazitiv gekoppelten HBr/NF3-Gasentladungen in einem kommerziellen, typischerweise für die DRAM-Fertigung eingesetzten MERIE CCP Reaktor mit Ein- und Zweifrequenzanregung bei mittleren Entladungsdrücken. Die Anwendung eines Zweifrequenzkonzeptes zur separaten Kontrolle von Ionenenergie und Ionenstromdichte, als auch deren Einfluss auf die Entladungseigenschaften und letztendlich auf das Ätzverhalten auf relevanten Substratoberflächen wurden systematisch untersucht. Die komplexe Natur von mehrfrequenzangeregten HF-Randschichten wurde sowohl experimentell über eine Anwendung von massenaufgelöster Ionenenergieanalyse als auch rechnerisch über Simulationen von Ionenenergieverteilungsfunktionen mit Hilfe eines hybriden Plasmarandschichtmodells analysiert. Gaszusammensetzungen verschiedener Entladungen und Ätzprozesse wurden mit Hilfe von Standard-Massenspektrometrie, Schwellwert-Massenspektrometrie, Quantenkaskaden-Laserabsorptionsspektroskopie, HF-Sondenmessungen, Gravimetrie und Ellipsometrie charakterisiert. Eine neuartige Modellvorstellung zum Siliziumätzen in HBr/NF3-Entladungsgemischen liefert eine plausible Erklärung für die Limitierung der Ätzrate zum Erreichen eines hoch anisotropen Ätzverhaltens.

<div>The Internet-of-Things (IoT) is one of the fastest-growing technologies in computing, revolutionizing several application domains such as wearable computing, home automation, industrial manufacturing, <i>etc</i>. This rapid proliferation, however, has given rise to a plethora of new security and privacy concerns. For example, IoT devices frequently access sensitive and confidential information (<i>e.g.,</i> physiological signals), which has made them attractive targets for various security attacks. Moreover, with the hardware components in these systems sourced from manufacturers across the globe, instances of counterfeiting and piracy have increased steadily. Security mechanisms such as device authentication and key exchange are attractive options for alleviating these challenges.</div><div><br></div><div>In this dissertation, we address the challenge of enabling low-cost and low-overhead device authentication and key exchange in off-the-shelf embedded systems. The first part of the dissertation focuses on a hardware-intrinsic mechanism and proposes the design of two Physically Unclonable Functions (PUFs), which leverage the memory (DRAM, SRAM) in the system, thus, requiring minimal (or no) additional hardware for operation. Two lightweight authentication and error-correction techniques, which ensure robust operation under wide environmental and temporal variations, are also presented. Experimental results obtained from prototype implementations demonstrate the effectiveness of the design. The second part of the dissertation focuses on the application of these techniques in real-world systems through a new end-to-end authentication and key-exchange protocol in the context of an Implantable Medical Device (IMD) ecosystem. Prototype implementations exhibit an energy-efficient design that guards against security and privacy attacks, thereby making it suitable for resource-constrained devices such as IMDs.</div><div><br></div>

Indiana University-Purdue University Indianapolis (IUPUI) / This thesis presents the development, experimentation and validation of a reliable and robust system to monitor the injector pulse generated by an Engine Control Module (ECM) and send the corresponding fueling quantity to the real-time computer in a closed loop Hardware In the Loop (HIL) bench. The system can be easily calibrated for different engine platforms as well. The fueling quantity that is being injected by the injectors is a crucial variable to run closed loop HIL simulation to carry out the performance testing of engine, aftertreatment and other components of the vehicle. This research utilized Field Programmable Gate Arrays (FPGA) and Direct Memory Access (DMA) transfer capability offered by National Instruments (NI) Compact Reconfigurable Input-Output (cRIO) to achieve high speed data acquisition and delivery. The research was conducted in three stages. The first stage was to develop the HIL bench for the research. The second stage was to determine the performance of the system with different threshold methods and different sampling speeds necessary to satisfy the required accuracy of the fueling quantity being monitored. The third stage was to study the error and its variability involved in the injected fueling quantity from pulse to pulse, from injector to injector, between real injector stators and cheaper inductor load cells emulating the injectors, over different operating conditions with full factorial design of experimentation and mixed model Analysis Of Variance (ANOVA). Different thresholds were experimented to find out the best thresholds, the Start of Injection (SOI) threshold and the End of Injection (EOI) threshold that captured the injector “ontime” with best reliability and accuracy. Experimentation has been carried out at various data acquisition rates to find out the optimum speed of data sampling rate, trading off the accuracy of fueling quantity. The experimentation found out the expected error with a system with cheaper solution as well, so that, if a test application is not sensitive to error in fueling quantity, a cheaper solution with lower sampling rate and inductors as load cells can be used. The statistical analysis was carried out at highest available sampling rate on both injectors and inductors with the best threshold method found in previous studies. The result clearly shows the factors that affect the error and the variability in the standard deviations in error; it also shows the relation with the fixed and random factors. The real-time application developed for the HIL bench is capable of monitoring the injector waveform, using any fueling ontime table corresponding to the platform being tested, and delivering the fueling quantity in real-time. The test bench made for this research is also capable of studying injectors of different types with the automated test sequence, without occupying the resource of fully capable closed loop test benches for testing the ECM unctionality.

[pt] Esse trabalho realiza uma extensa revisão bibliográfica da literatura, onde pontos de inovação são observados e novas soluções para os principais problemas no uplink são propostas. Baseados no algoritmo adaptativo Recursive Least Squares (RLS), são apresentadas duas variações regularizadas que conjuntamente detectam a atividade e decodificam os sinais dos dispositivos, sem a necessidade de estimação explícita do canal. Além disso, duas técnicas de detecção por listas são desenvolvidas de modo a refinar o processo de ajuste dos pesos do algoritmo RLS e assim mitigar possíveis propagações de erros. Dividindo o problema em grafos fatoriais, são propostos algoritmos baseados em troca de mensagens de modo a realizar conjuntamente a estimação de canal e detecção de atividade. Além de uma nova derivação das mensagens, são exploradas técnicas de agendamento dinâmico, com base na convergência do algoritmo. Por fim, é apresentada uma solução completa baseada na troca de mensagens, que realiza conjuntamente a estimação de canal, detecção de atividade e decodificação dos dados transmitidos. Resultados numéricos são fornecidos com o objetivo de comparar o desempenho dos algoritmos propostos aos existentes na literatura. Análises de complexidade computacional, evolução dos estados, convergência e ordem de diversidade também são realizadas, assim como a derivação da soma das taxas para o uplink. Por fim, são apresentadas as conclusões obtidas dos trabalhos realizados e discutidas direções para trabalhos futuros. / [en] This work, presents an extensive literature review that highlights innovation opportunities and presents novel solutions for the main uplink mMTC problems. Based on the adaptive Recursive Least Squares (RLS) algorithm, the proposed regularized techniques jointly performs activity detection and signal decoding, without the need to perform explicit channel estimation. In order to improve the detection performance, a list detection technique that uses two candidate-list schemes is developed. Rewriting the problem with factor-graphs, novel message-passing algorithms with dynamic scheduling that jointly estimates the channels and detects devices activity are proposed. Lastly, a complete message-passing solution is presented, where LDPC decoding beliefs are introduced in the system, in a way that the algorithm besides the channel estimation and activity detection, also jointly decodes the signals. In order to evaluate the proposed techniques, numerical results are provided as well as a computational complexity, state-evolution, convergence and a diversity analysis. Uplink sum-rate expressions that take into account metadata collisions, interference and a variable activity probability for each user are also derived. Finally, conclusions and future directions are discussed.