Nanoscale Nonvolatile Memory Circuit Design using Emerging Spin Transfer Torque Magnetic Random Access Memory

Title from PDF of title page, viewed August 25, 2017Thesis advisor: Masud H ChowdhuryVitaIncludes bibliographical references (pages 67-71)Thesis (M.S.)--School of Computing and Engineering. University of Missouri--Kansas City, 2016The spin transfer torque magnetic random access memory (STT-MRAM) is suitable for embedded and second level cache memories in the mobile CPUs. STT-MRAM is a highly potential nonvolatile memory (NVM) technology. There has been a growing demand to improve the efficiency and reliability of the NVM circuits and architectures. we present a modified STT MRAM cell design, where each cell is comprised of one magnetic tunneling junction (MTJ) device and a regular access transistor. We provide analysis of device, circuit and memory architecture level issues of STT-MRAM. The Modified 1M1T STT-MRAM bit cell circuit offers simpler and more area- and power- efficient design compared to the existing STT-MRAM cell design. Some device-circuit co-design issues are investigated to demonstrate ways to reduce delay in MRAM circuits based on MTJ. An 8x8 conventional MRAM array is implemented using the existing 2M2T cell and the Modified 1M1T cell to perform a comparative analysis at the architecture level. The non-volatile nature of the proposed STT-MRAM is verified through SPICE simulation. The circuit implementations and simulations are performed for 45nm technology node. As the transistor scales down it is prone to subthreshold leakage, gate-dielectric leakage, Short channel effect and drain induced barrier lowering. Now alternative of Access transistor is needed. We are using FinFET as access transistor in the STT-MRAM bit cell. FinFET based bit cell is designed to get an advantage of scaling down. Analysis is done and proven that the power consumption, standalone leakage current is less when compared to NMOS based STT-MRAM bit cell. Also determined FinFET based bit cell produces less access time to access the logic value from MTJ. Now, Industry is looking to have computational and storage capability together and that can be achieved through STT-MRAM. Addition to that there is a possibility to reduce power consumption and leakage more. So replacing FinFET technology with Carbon Nano Tube Field Effect Transistor (CNTFET) is required. As the conventional STT-MRAM requires certain current to reverse the magnetization of MTJ and one CNTFET alone cannot produce sufficient current required to store the logic value into MTJ. So new Bit cell is proposed using 3 CNTFET and 1 MTJ, this bit cell is capable of storing 3 logic values at a time that is capable of doing computation and act as AND gate. Also it utilizes less power to be in active region. Sensing of any memory system is one of the main challenge in industry to get better performance with less resources. Conventional Sense Amplifier (SA) used to sense the value from SRAM, DRAM memory system is also used to sense the STT-MRAM memory. But use of conventional SA is prone to some error. Modified Sense Amplifier is designed to overcome the error produced from the conventional SA. It is compared with all the existing SA to get the performance details of the modified SA.Introduction -- Planar NMOS based STT-MRAM bit cell analysis and circuit designing -- Performance improvement using FINFET based STT-MRAM circuit design -- Logic-in-memory using CNT-FET based STT-MRAM bit cell and optimization -- Error free sense amplifier design for STT-MRAM nonvolatile memor

Spin-Transfer-Torque (STT) Devices for On-chip Memory and Their Applications to Low-standby Power Systems

With the scaling of CMOS technology, the proportion of the leakage power to total power consumption increases. Leakage may account for almost half of total power consumption in high performance processors. In order to reduce the leakage power, there is an increasing interest in using nonvolatile storage devices for memory applications. Among various promising nonvolatile memory elements, spin-transfer torque magnetic RAM (STT-MRAM) is identified as one of the most attractive alternatives to conventional SRAM. However, several design challenges of STT-MRAM such as shared read and write current paths, single-ended sensing, and high dynamic power are major challenges to be overcome to make it suitable for on-chip memories. To mitigate such problems, we propose a domain wall coupling based spin-transfer torque (DWCSTT) device for on-chip caches. Our proposed DWCSTT bit-cell decouples the read and the write current paths by the electrically-insulating magnetic coupling layer so that we can separately optimize read operation without having an impact on write-ability. In addition, the complementary polarizer structure in the read path of the DWCSTT device allows DWCSTT to enable self-referenced differential sensing. DWCSTT bit-cells improve the write power consumption due to the low electrical resistance of the write current path. Furthermore, we also present three different bit-cell level design techniques of Spin-Orbit Torque MRAM (SOT-MRAM) for alleviating some of the inefficiencies of conventional magnetic memories while maintaining the advantages of spin-orbit torque (SOT) based novel switching mechanism such as low write current requirement and decoupled read and write current path. Our proposed SOT-MRAM with supporting dual read/write ports (1R/1W) can address the issue of high-write latency of STT-MRAM by simultaneous 1R/1W accesses. Second, we propose a new type of SOT-MRAM which uses only one access transistor along with a Schottky diode in order to mitigate the area-overhead caused by two access transistors in conventional SOT-MRAM. Finally, a new design technique of SOT-MRAM is presented to improve the integration density by utilizing a shared bit-line structure

Energy efficient hybrid computing systems using spin devices

Emerging spin-devices like magnetic tunnel junctions (MTJ\u27s), spin-valves and domain wall magnets (DWM) have opened new avenues for spin-based logic design. This work explored potential computing applications which can exploit such devices for higher energy-efficiency and performance. The proposed applications involve hybrid design schemes, where charge-based devices supplement the spin-devices, to gain large benefits at the system level. As an example, lateral spin valves (LSV) involve switching of nanomagnets using spin-polarized current injection through a metallic channel such as Cu. Such spin-torque based devices possess several interesting properties that can be exploited for ultra-low power computation. Analog characteristic of spin current facilitate non-Boolean computation like majority evaluation that can be used to model a neuron. The magneto-metallic neurons can operate at ultra-low terminal voltage of ∼20mV, thereby resulting in small computation power. Moreover, since nano-magnets inherently act as memory elements, these devices can facilitate integration of logic and memory in interesting ways. The spin based neurons can be integrated with CMOS and other emerging devices leading to different classes of neuromorphic/non-Von-Neumann architectures. The spin-based designs involve `mixed-mode\u27 processing and hence can provide very compact and ultra-low energy solutions for complex computation blocks, both digital as well as analog. Such low-power, hybrid designs can be suitable for various data processing applications like cognitive computing, associative memory, and currentmode on-chip global interconnects. Simulation results for these applications based on device-circuit co-simulation framework predict more than ∼100x improvement in computation energy as compared to state of the art CMOS design, for optimal spin-device parameters

Exploiting Inter- and Intra-Memory Asymmetries for Data Mapping in Hybrid Tiered-Memories

Modern computing systems are embracing hybrid memory comprising of DRAM and non-volatile memory (NVM) to combine the best properties of both memory technologies, achieving low latency, high reliability, and high density. A prominent characteristic of DRAM-NVM hybrid memory is that it has NVM access latency much higher than DRAM access latency. We call this inter-memory asymmetry. We observe that parasitic components on a long bitline are a major source of high latency in both DRAM and NVM, and a significant factor contributing to high-voltage operations in NVM, which impact their reliability. We propose an architectural change, where each long bitline in DRAM and NVM is split into two segments by an isolation transistor. One segment can be accessed with lower latency and operating voltage than the other. By introducing tiers, we enable non-uniform accesses within each memory type (which we call intra-memory asymmetry), leading to performance and reliability trade-offs in DRAM-NVM hybrid memory. We extend existing NVM-DRAM OS in three ways. First, we exploit both inter- and intra-memory asymmetries to allocate and migrate memory pages between the tiers in DRAM and NVM. Second, we improve the OS's page allocation decisions by predicting the access intensity of a newly-referenced memory page in a program and placing it to a matching tier during its initial allocation. This minimizes page migrations during program execution, lowering the performance overhead. Third, we propose a solution to migrate pages between the tiers of the same memory without transferring data over the memory channel, minimizing channel occupancy and improving performance. Our overall approach, which we call MNEME, to enable and exploit asymmetries in DRAM-NVM hybrid tiered memory improves both performance and reliability for both single-core and multi-programmed workloads.Comment: 15 pages, 29 figures, accepted at ACM SIGPLAN International Symposium on Memory Managemen