Design considerations of a nonvolatile accumulator-based 8-bit processor

The rise of the Internet of Things (IoT) and theconstant growth of portable electronics have leveraged the con-cern with energy consumption. Nonvolatile memory (NVM)emerged as a solution to mitigate the problem due to its abilityto retain data on sleep mode without a power supply. Non-volatile processors (NVPs) may further improve energy savingby using nonvolatile flip-flops (NVFFs) to store system state,allowing the device to be turned off when idle and resume ex-ecution instantly after power-on. In view of the potential pre-sented by NVPs, this work describes the initial steps to imple-ment a nonvolatile version of Neander, a hypothetical processorcreated for educational purposes. First, we implemented Ne-ander in Register Transfer Level (RTL), separating the com-binational logic from the sequential elements. Then, the lat-ter was replaced by circuit-level descriptions of volatile flip-flops. We then validated this implementation by employinga mixed-signal simulation over a set of benchmarks. Resultshave shown the expected behavior for the whole instructionset. Then, we implemented circuit-level descriptions of mag-netic tunnel junction (MTJ) based nonvolatile flip-flops, usingan open-source MTJ model. These elements were exhaustivelyvalidated using electrical simulations. With these results, weintend to carry on the implementation and fully equip our pro-cessor with nonvolatile features such as instant wake-up

DEMANDS FOR SPIN-BASED NONVOLATILITY IN EMERGING DIGITAL LOGIC AND MEMORY DEVICES FOR LOW POWER COMPUTING

Miniaturization of semiconductor devices is the main driving force to achieve an outstanding performance of modern integrated circuits. As the industry is focusing on the development of the 3nm technology node, it is apparent that transistor scaling shows signs of saturation. At the same time, the critically high power consumption becomes incompatible with the global demands of sustaining and accelerating the vital industrial growth, prompting an introduction of new solutions for energy efficient computations.Probably the only radically new option to reduce power consumption in novel integrated circuits is to introduce nonvolatility. The data retention without power sources eliminates the leakages and refresh cycles. As the necessity to waste time on initializing the data in temporarily unused parts of the circuit is not needed, nonvolatility also supports an instant-on computing paradigm.The electron spin adds additional functionality to digital switches based on field effect transistors. SpinFETs and SpinMOSFETs are promising devices, with the nonvolatility introduced through relative magnetization orientation between the ferromagnetic source and drain. A successful demonstration of such devices requires resolving several fundamental problems including spin injection from metal ferromagnets to a semiconductor, spin propagation and relaxation, as well as spin manipulation by the gate voltage. However, increasing the spin injection efficiency to boost the magnetoresistance ratio as well as an efficient spin control represent the challenges to be resolved before these devices appear on the market. Magnetic tunnel junctions with large magnetoresistance ratio are perfectly suited as key elements of nonvolatile CMOS-compatible magnetoresistive embedded memory. Purely electrically manipulated spin-transfer torque and spin-orbit torque magnetoresistive memories are superior compared to flash and will potentially compete with DRAM and SRAM. All major foundries announced a near-future production of such memories.Two-terminal magnetic tunnel junctions possess a simple structure, long retention time, high endurance, fast operation speed, and they yield a high integration density. Combining nonvolatile elements with CMOS devices allows for efficient power gating. Shifting data processing capabilities into the nonvolatile segment paves the way for a new low power and high-performance computing paradigm based on an in-memory computing architecture, where the same nonvolatile elements are used to store and to process the information

高密度・高速動作可能なSTT-MRAMへ向けたp-MTJのスイッチング過渡現象とアレイ設計指針に関する研究

Tohoku University遠藤哲郎課

Structural, chemical, and magnetic investigation of a graphene/cobalt/platinum multilayer system on silicon carbide

Spintronics offers huge potential for data storage and processing and, thus, for overcoming the challenges arising from the ever-increasing demands in the field of electronics. To fully utilize this potential in real-world applications, appropriate materials are required. Graphene-ferromagnetic interfaces show great promise in this context. Combining graphene with a ferromagnet, such as cobalt, results in a system with many advantageous effects, such as Dzyaloshinskii−Moriya interaction (DMI). These effects allow for the formation of useful spin structures with high stability. Furthermore, such structures can also be induced in cobalt by combining it with a heavy non-magnetic metal such as platinum. In this study, the magnetic interlayer coupling and domain structure of ultra-thin ferromagnetic cobalt (Co) layers embedded between a graphene (G) layer and a platinum (Pt) layer on a silicon carbide (SiC) substrate (G/Co/Pt on SiC) were investigated. The (6\sqrt{3} \times 6\sqrt{3}) \text{R}\ang{30}-reconstruction was prepared on SiC and served as a precursor for graphene. It was prepared using two techniques, namely confinement controlled sublimation (CCS) and polymer assisted sublimation growth (PASG). Consequently, the sample properties varied slightly. Following this, the metal layers were prepared by intercalation. Experimentally, a combination of x-ray photoemission electron microscopy (X-PEEM) with x-ray magnetic circular dichroism (XMCD) was carried out at the Co L-edge to study the system's magnetic structure. Furthermore, structural and chemical properties of the system were investigated using low-energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS). In-situ-LEED patterns revealed the crystalline structure of each layer within the system. Moreover, XPS confirmed the presence of quasi-freestanding graphene and the absence of cobalt silicide. These characteristics of a clean and homogeneous Co-layer provide an excellent foundation for magnetic features to form. The magnetic structure of all samples exhibited numerous spin vortices and anti-vortices. In spintronics, these can be used as data carriers. The general magnetic structure of the material is heavily influenced by the preparation method. Different aspects were identified, which enhance or impede the formation of vortices.Die Spintronik bietet enormes Potenzial im Bereich der Datenspeicherung und -verarbeitung und zur Überwindung der immer weiter steigenden Anforderungen im Bereich der Elektronik. Um dieses Potenzial zu nutzen, werden geeignete Materialien benötigt. Diesbezüglich sind Graphen-Ferromagnet-Grenzschichten besonders vielversprechend. Wird Graphen mit einem Ferromagneten, wie Kobalt, kombiniert, ergibt sich ein System mit vielen vorteilhaften Eigenschaften, wie Dzyaloshinskii−Moriya Wechselwirkung (DMI). Diese ermöglichen die Bildung von stabilen, nützlichen Spin-Strukturen. Solche Strukturen können ebenfalls in Kobalt hervorgerufen werden, wenn es mit einem schweren, nicht magnetischen Metall, wie Platin, in Verbindung steht. In dieser Arbeit wurde die magnetische Grenzschicht-Kopplung und Domänenstruktur von ultradünnen ferromagnetischen Kobaltschichten zwischen Graphen und Platinschichten auf einem Siliziumcarbidsubstrat (SiC) untersucht. Die (6\sqrt{3} \times 6\sqrt{3}) \text{R}\ang{30}-Rekonstruktion von SiC diente als Vorstufe für Graphen. Sie wurde mit zwei Methoden präpariert, der Raumbegrenzten-Sublimation (CCS) und der Polymere-unterstützen Sublimation (PASG). Diese führten zu leicht verschiedenen Probeneigenschaften. Anschließend wurden die Metallschichten mittels Interkalation präpariert. Die magnetische Struktur des Systems wurde mittels Synchrotronstrahlung angeregter Photoemissions-Elektronenmikroskopie (X-PEEM) an der Kobalt-L-Kante unter Nutzung des zirkularen magnetischen Röntgendichroismuses (XMCD) untersucht. Strukturelle Eigenschaften wurden mittels niederenergetischer Elektronenbeugung (LEED) und chemische mittels Röntgen-Photoelektronenspektroskopie (XPS) analysiert. in-situ-LEED-Messungen zeigten die kristalline Struktur jeder Schicht im System. Mittels XPS wurde quasi-freistehendes Graphen und die Abwesenheit von Kobaltsiliziden nachgewiesen. Die somit reine und homogene Kobaltschicht bildet eine hervorragende Grundlage für die Ausbildung von Spin-Strukturen. Die magnetische Struktur aller Proben zeigte eine Vielzahl von Spinwirbeln und -antiwirbeln. In der Spintronik können diese als Datenträger genutzt werden. Die generelle magnetische Struktur wurde stark von der verwendeten Präparationsmethode beeinflusst. Verschiedene Aspekte wurden entdeckt, die die Bildung von Spinwirbeln begünstigen oder erschweren