Resistance switching devices based on amorphous insulator-metal thin films

Nanometallic devices based on amorphous insulator-metal thin films are developed to provide a novel non-volatile resistance-switching random-access memory (RRAM). In these devices, data recording is controlled by a bipolar voltage, which tunes electron localization length, thus resistivity, through electron trapping/detrapping. The low-resistance state is a metallic state while the high-resistance state is an insulating state, as established by conductivity studies from 2K to 300K. The material is exemplified by a Si3N4 thin film with randomly dispersed Pt or Cr. It has been extended to other materials, spanning a large library of oxide and nitride insulator films, dispersed with transition and main-group metal atoms. Nanometallic RRAMs have superior properties that set them apart from other RRAMs. The critical switching voltage is independent of the film thickness/device area/temperature/switching speed. Trapped electrons are relaxed by electron-phonon interaction, adding stability which enables long-term memory retention. As electron-phonon interaction is mechanically altered, trapped electron can be destabilized, and sub-picosecond switching has been demonstrated using an electromagnetically generated stress pulse. AC impedance spectroscopy confirms the resistance state is spatially uniform, providing a capacitance that linearly scales with area and inversely scales with thickness. The spatial uniformity is also manifested in outstanding uniformity of switching properties. Device degradation, due to moisture, electrode oxidation and dielectrophoresis, is minimal when dense thin films are used or when a hermetic seal is provided. The potential for low power operation, multi-bit storage and complementary stacking have been demonstrated in various RRAM configurations.Comment: 523 pages, 215 figures, 10 chapter

Lanthanoid based materials in advanced CMOS technology

Ph.DDOCTOR OF PHILOSOPH

Capteur d’hydrogène mos et méthode d’intégration à une technologie de transistor FDSOI

Abstract: hydrogen can be used as an energy carrier (storage) by the renewable energy industry as well as the automotive industry (fuel cell). Other industries already use hydrogen such, food processing and petroleum refineries. Hydrogen is odorless, transparent, and has a lower explosive limit of 4 %. Reliable, fast sensor are essential tools for a hydrogen safe environment. The work of this thesis provides a semiconductor-based hydrogen sensing solution. A MOS capacitor using a CMOS compatible novel Pt/Ti/ALD-Al2O3/p-Si stack. The Pt/Ti/Al2O3 sensing interface materials thicknesses are 100/5/38 nm respectively. The device can detect very low concentrations < 20 ppm. Furthermore, for a concentration of 500 ppm the response time is 56 s. the impact of testing conditions such temperature, and total gas flow have been studied. Results show that at 60℃ the device does not respond to hydrogen. And at 80℃ or higher the sensing response time is significantly reduced with increasing temperature. Furthermore, the total gas flow has an impact on the device response time and shows that a portion of the time response delay can be attributed to the chamber’s volume. Moreover, a heterogeneous integration method has been designed and presented. The latter represents a great tool for a flexible prototyping of sensors using FDSOI transistor technology. The integration has been simulated and results show promising results. The capacitive coupling feature in the FDSOI between the front and back gate is used to amplify the potential variation at the front gate. For instance, a 0.3 V hydrogen induced dipole potential can be amplified by a factor of 14 x.Le travail de cette thèse comprend la conception et la fabrication d’une technologie de capteur d’hydrogène basée sur une structure MOS. La structure est composée d’un empilement de Pt/Ti/Al2O3/p-Si. Les épaisseurs des matériaux utilisés pour la fabrication sont 100/5/38 nm (Pt/Ti/Al2O3) sur un substrat de silicium. Le capteur est capable de détecter de très faibles concentrations < 20 ppm. De plus, pour une concentration de 500 ppm, le temps de réponse est 56 s. L’impact de plusieurs conditions de test, comprenant la température et le débit total dans la chambre a été évalué. Les résultats montrent qu’à 60℃ le dispositive n’est pas capable de détecter la présence d’hydrogène. Cependant, à partir d’une température de 80℃, la réponse est très importante et le temps diminue pour encore des températures plus élevées. Le débit total dans la chambre a aussi démontré un impact sur le temps de réponse du capteur. Ce qui est aussi relié au volume de la chambre. Une intégration hétérogène ensuite a été conçue et présentée. Cette dernière est un outil flexible pour le prototypage avec des technologies de transistor FDSOI. L’intégration des deux dispositifs a été effectuée et montre de résultats prometteurs. Le couplage capacitif entre la grille avant et la grille arrière du transistor FDSOI permet d’amplifier le signal du capteur. Par exemple, une variation de potentiel de 0.3 V peut être amplifier par un facteur de 14 x, donc 4.19 V

Defect Engineering in HfO2/TiN-based Resistive Random Access Memory (RRAM) Devices by Reactive Molecular Beam Epitaxy

Recently, there has been huge interest in emerging memory technologies, spurred by the ever increasing demand for storage capacities in various applications like Internet of Things (IoT), Big Data, etc. CMOS based flash memory, the current mainstay of the memory technology, has been able to increase its density by scaling down to a 16 nm node and further implementation of 3D architectures. However, flash memory is expected to soon run into disadvantage due to challenges in further scaling. Therefore, extensive efforts are being made towards developing new devices for the next generation of non-volatile memories with the combined advantages of flash memory like non-volatility, high density, low cost and low power consumption as well as high speed performance of DRAM. Among the many competitors, resistive random access memories (RRAM) based on resistive switching in oxides are promising due to its simple metal-insulator-metal (MIM) structure, fast switching speeds (<10 ns), excellent scalability (<10 nm) and potential for multi-level switching. RRAM devices based on the popular dielectric-metal gate combination of hafnium oxide (HfO2) and titanium nitride (TiN), which is the subject of research in this work, are particularly interesting due to its compatibility with existing CMOS technology in addition to the aforementioned advantages. Though prototype RRAM chips have already been demonstrated, key problems for commercial realization of RRAM include large variability and insufficient understanding of the complex switching physics. Resistive switching mechanism in oxides is generally understood to be mediated via the transport of oxygen ions leading to the formation of a conductive filament composed of oxygen vacancy defects. Appropriate defect engineering approaches offer potential towards tailoring the switching behavior as well as improving the performance and yield of HfO2-RRAM. In this thesis, the impact of pre-induced defects on the resistive switching behavior of HfO2-RRAM is investigated in detail and our results are presented. Defect engineered oxide thin films were deposited using reactive molecular beam epitaxy (RMBE) to fabricate metal oxide/TiN based devices. RMBE technique offers the unique possibility to precisely and reproducibly control the oxygen stoichiometry of the thin films in a wide range. Using RMBE, defects were introduced in polycrystalline HfOx thin films intrinsically by oxygen stoichiometry engineering and extrinsically via impurity doping (trivalent lanthanum and pentavalent tantalum). Both the studies were performed at at CMOS compatible deposition temperatures (< 450 °C) with an eye on practical applications. Prior to tantalum doping in HfO2, oxygen stoichiometry engineering studies were also performed in amorphous tantalum oxide (TaOx) thin films to identify the oxidation conditions of tantalum metal. The density of oxygen stoichiometry engineered thin films of HfOx and TaOx could be tuned in a wide range from that of the bulk oxide density to close to metallic density. High degree of oxygen deficiency in oxides led to the formation of defect states near the Fermi level as well as multiple oxidation states of the metal, as observed by X-ray photoelectron spectroscopy (XPS). The pure stoichiometric hafnium oxide films crystallize as expected in a stable monoclinic structure (m-HfO2) whereas, oxygen deficient HfOx thin films were found to crystallize in vacancy stabilized tetragonal like structure (t-HfO2-x). Impurity doping also led to the stabilization of higher symmetry tetragonal (t-Ta:HfOx) or cubic structures (c-La:HfOx) depending on the ionic radii of the dopant. The growth of TiN thin films was also investigated using RMBE. The devices used for electrical studies in this work mostly involved deposition of oxides by RMBE on polycrystalline TiN/Si electrodes after ex-situ transfer for further deposition. Therefore, RMBE grown TiN thin film electrodes with similar or better quality would allow in-situ uninterrupted deposition of subsequent oxide layers in future to form cleaner interfaces. Optimized conditions for growth of epitaxial TiN films on the commercially relevant (001) oriented silicon and c-cut sapphire substrates were established, with focus on achieving smooth surfaces and low resistivity. High quality epitaxial TiN(111)||Al2O3(0001) and TiN(001)||Si(001) films with a low resistivity (20-200 uOhm.cm) were achieved, in spite of the large lattice mismatch. Very low surface roughness, characterized by a streaky reflection high energy electron diffraction (RHEED) pattern during TiN film growth was additionally obtained, by tuning the Ti/N flux ratios. Oxygen engineered HfOx/TiN devices were further electrically characterized to obtain I-V characteristics during quasi-static DC switching. Usually, an initial electroforming step (high voltages) is required to obtain further reproducible switching operation (at lower voltages). High device to device variability in RRAM is typically associated with the stochastic nature of electroforming process which increases at higher forming voltages. Using highly oxygen deficient HfOx and TaOx films, the forming voltages were found to be reduced to levels close to operating voltages, paving the way for forming-free devices. However, the use of high defect concentration adds to increasing the complexity of the switching mechanism. This is reflected in the rather complex and dissimilar switching behaviors observed in the myriad of similar RRAM devices reported in the rapidly growing literature. Using model Pt/HfOx/TiN-based device stacks; it is shown that a well-controlled oxygen stoichiometry governs the filament formation and the (partial) occurrence of multiple resistive switching modes (bipolar, unipolar, threshold, complementary). These findings fuel a better fundamental understanding of the underlying phenomena for future theoretical considerations. The oxygen vacancy concentration is found to be the key factor in manipulating the balance between electric field and Joule heating during formation, rupture (reset), and reformation (set) of the conductive filaments in the dielectric. While a bipolar switching occurs in all the devices irrespective of defect concentration, switching modes like unipolar and threshold switching is favored only at higher oxygen stoichiometry. This suggests the suppression of thermal effects via higher heat dissipation and lowered concentration gradient of oxygen vacancies in oxygen deficient devices. A qualitative switching model based on the drift, diffusion and thermophoresis of oxygen ions is suggested to account for the partial occurrence of various switching modes depending on the oxygen stoichiometry. Further, the evolution or drift of high resistance states during endurance test of the common bipolar operation is compared for HfO2 and HfO1.5 based devices and interpreted using the quantum point contact (QPC) model. Similar observations regarding switching modes were also obtained in oxygen engineered Pt/TaOx/TiN devices, therefore allowing the findings to be generalized to other filamentary resistive switching oxides and contributing towards developing a unified switching model. Besides finding application as non-volatile memory, RRAM devices are also promising for hardware implementation of neuromorphic computing. This is motivated by the possibility of multi-level switching or gradual (analog) modulation of resistance in an RRAM device which can emulate biological synapses. Defect engineering approaches have thus been investigated in Pt/hafnium oxide/TiN devices for tuning the DC I-V switching dynamics to achieve multi-level or gradual switching electronic synapses. Higher contribution of thermal effects in pure stoichiometric HfO2 typically results in a single sharp set process and abrupt sharp current jumps during the reset process during a conventional bipolar operation. By using ~18% La-doped HfOx based device, a completely gradual reset behavior with a higher ON/OFF ratio could be achieved during the bipolar reset operation. This is likely related to filament stabilization around the dopant sites allowing a uniform rupture during reset. More interestingly, in oxygen deficient HfO1.5 based devices, intermediate conductance states corresponding to integer or half-integer multiples of quantum conductance (G0) was observed during both the set and reset operations at room temperature. These are related to the better stabilization of intermediate atomic size filament constrictions during the switching process. Occurrence of these intermediate quantum conductance states, especially during the typically abrupt set process, is likely aided by a weaker filament and better thermal dissipation in the highly oxygen deficient devices. These results suggest that a combination of doping and high oxygen vacancy concentration may lead to improved synaptic functionality with concurrent gradual set and reset behaviors

Über die Entwicklung von Memsensoren

Since the postulation of the experimental realization of memristive devices in 2008, a broad variety of concepts for the fabrication of memristive devices has been pursued and the underlying switching mechanisms have been studied in detail. The unique electronic properties of memristive devices inspire applications that go beyond conventional electronics, such as using memristive devices as programmable interconnects, to realize logics for in-array-computing or in neuromorphic engineering. A particularly interesting aspect of biological neural networks is the close connection between signal detection and processing at the neuron level, which is an essential contribution to their outstanding efficiency. This work evolves around the concept of memsensors, which unify the characteristic features of memristive devices and sensor devices and as such appear as promising candidates to realize a close connection between signal detection and processing on the device level. Memsensors are a highly interdisciplinary topic, bridging research in the fields of material science and electrical engineering and relating to insights from biology and medicine through neuromorphic engineering. The major objective of this thesis is to provide tools and building blocks and showcase pathways to incorporate memristive and sensitive properties into memsensor devices. For this purpose, motivated by an experimental point of view, a nanoparticle-based memristive device with diffusive memristive switching characteristics was developed and characterised in detail and sensors relying on semiconducting metal oxide thin films and nanostructures were thoroughly studied. In addition, in terms of modelling of memsensor circuits, emerging features such as amplitude adaptation are discussed, showcasing the particular eligibility of memsensors in the context of neuromorphic engineering

Microelectromechanical Systems and Devices

The advances of microelectromechanical systems (MEMS) and devices have been instrumental in the demonstration of new devices and applications, and even in the creation of new fields of research and development: bioMEMS, actuators, microfluidic devices, RF and optical MEMS. Experience indicates a need for MEMS book covering these materials as well as the most important process steps in bulk micro-machining and modeling. We are very pleased to present this book that contains 18 chapters, written by the experts in the field of MEMS. These chapters are groups into four broad sections of BioMEMS Devices, MEMS characterization and micromachining, RF and Optical MEMS, and MEMS based Actuators. The book starts with the emerging field of bioMEMS, including MEMS coil for retinal prostheses, DNA extraction by micro/bio-fluidics devices and acoustic biosensors. MEMS characterization, micromachining, macromodels, RF and Optical MEMS switches are discussed in next sections. The book concludes with the emphasis on MEMS based actuators

Towards Oxide Electronics:a Roadmap

At the end of a rush lasting over half a century, in which CMOS technology has been experiencing a constant and breathtaking increase of device speed and density, Moore's law is approaching the insurmountable barrier given by the ultimate atomic nature of matter. A major challenge for 21st century scientists is finding novel strategies, concepts and materials for replacing silicon-based CMOS semiconductor technologies and guaranteeing a continued and steady technological progress in next decades. Among the materials classes candidate to contribute to this momentous challenge, oxide films and heterostructures are a particularly appealing hunting ground. The vastity, intended in pure chemical terms, of this class of compounds, the complexity of their correlated behaviour, and the wealth of functional properties they display, has already made these systems the subject of choice, worldwide, of a strongly networked, dynamic and interdisciplinary research community. Oxide science and technology has been the target of a wide four-year project, named Towards Oxide-Based Electronics (TO-BE), that has been recently running in Europe and has involved as participants several hundred scientists from 29 EU countries. In this review and perspective paper, published as a final deliverable of the TO-BE Action, the opportunities of oxides as future electronic materials for Information and Communication Technologies ICT and Energy are discussed. The paper is organized as a set of contributions, all selected and ordered as individual building blocks of a wider general scheme. After a brief preface by the editors and an introductory contribution, two sections follow. The first is mainly devoted to providing a perspective on the latest theoretical and experimental methods that are employed to investigate oxides and to produce oxide-based films, heterostructures and devices. In the second, all contributions are dedicated to different specific fields of applications of oxide thin films and heterostructures, in sectors as data storage and computing, optics and plasmonics, magnonics, energy conversion and harvesting, and power electronics

Characterization of the doped silicon dioxide and its implications on the resistive switching phenomena in the electrochemical metallization cells

In this Master's thesis, the switching behavior of the doped and undoped SiO2-based memory cells was compared. The aim of doping was to enhance the switching behavior of the ECM memory cells. About 270 samples were sputtered using the CT1000 cluster deposition tool in the IWE2 of RWTH Aachen University. For the deposition of the thin films, the platinum, titanium nitride and Al2O3 substrates were used. The deposition was performed by using three differently doped targets. The physical characterization of the thin films was done using SEM, XRR, XRD, and EDX. Electroforming and electric characterization of the fabricated memory cells were made in the probe station with the light microscope and the Keithley electrometer. The results of the physical and electrical characterization were analyzed using the principle of Exploratory Data Analysis (EDA). The analysis of the result shows that two undoped samples on the platinum substrate and some doped samples exhibit the unexpected volatile threshold switching of metallic and semiconductive origin, respectively. Linear fitting of the measurement data in a logarithmic scale suggests that Schottky- and Frenkel- Poole conduction mechanisms are not dominant

Cutting Edge Nanotechnology

The main purpose of this book is to describe important issues in various types of devices ranging from conventional transistors (opening chapters of the book) to molecular electronic devices whose fabrication and operation is discussed in the last few chapters of the book. As such, this book can serve as a guide for identifications of important areas of research in micro, nano and molecular electronics. We deeply acknowledge valuable contributions that each of the authors made in writing these excellent chapters