MEMORY Non-volatile Memory Technology Continue to Scale Up The emergence of new Non-Volatile memory technology promises to improve performance, efficiency that matches with ideal characteristics. The article reviews some of new non-volatile memory technologies. PADAM RAO RAM, DRAM and Flash are three Landscape dominating memory technologies and With respect to ideal characteristics of memory each technology has some advantages technology described above, till now all known and disadvantages. The ideal memory S memory/storage technologies address only some technology should have low power of the characteristics. Static RAM (SRAM) is very consumption, high performance, high reliability, fast, but it's expensive, has low density and is not high density, low cost, and the ability to be used in any semiconductor memory application. persistent. Dynamic RAM (DRAM) has better Besides computers, today’s portable electronics densities and is cheaper (but still expensive) at the have become computationally intensive devices as cost of being a little slower, and it's not persistent as the user interface has migrated to a fully well. Disks are cheap, highly dense and persistent, multimedia experience. To provide the but very slow. Flash memory is between DRAM and performance required for these applications, the disk; it is a persistent solid-state memory with higher portable electronics designer uses multiple types of densities than DRAM, but its write latency is much memories: a medium-speed random access higher than the latter. memory for continuously changing data, a high- Fortunately, it is possible to design a memory system speed memory for caching instructions to the CPU, that incorporates all these different technologies in and a slower, non-volatile memory for long-term a single memory hierarchy. Such hierarchy allows information storage when the power is removed. the creation of a system that approaches the Combining all of these memory types into a single performance of the fastest component, the cost of memory has been a long-standing goal of the the cheapest component and energy consumption semiconductor industry. Today we watch the of the most power-efficient component. This is emergence of new non-volatile memory technologies, such as Phase-Change RAM (PCRAM), Magnetoresistive RAM (MRAM) and Resistive RAM RRAM), that promise to radically change the landscape of memory systems. The introduction of these new technologies will probably result in more complex memory hierarchies, but is likely to allow the construction of memory chips that are non-volatile, low-energy and have density and latency close to or better than current DRAM chips, improving performance/ efficiency and allowing memory systems to continue to scale up. Fig. 1 - Diagram illustrating the concept of memory hierarchy MEMORY possible due to a phenomenon known as locality of reference, so named because memory references Bitline tend to be localized in time and space. This can be Phase-change material so summarized: Contact Access device - If you use something once, you are likely to use it Insulator again (temporal locality). Wordline - If you use something once, you are likely to use Fig. 2 - Example of a phase-change memory cell. The current its neighbor (spatial locality) This phenomenon can is forced to pass through the phase-change material. Image be exploited by creating different memory levels; obtained from the first levels are smaller and more expensive, but amorphous, it is melt-quenched using a high- have fastest access, and the other layers are power electrical pulse that is abruptly cut of. To progressively bigger and cheaper, but have slower crystallize the material, it is heated above its access times. Appropriate heuristics are then crystallization temperature using a moderate power applied to decide which data will be accessed more pulse with a longer duration. Since the duration of often and place them at the first levels, and move this pulse varies according to the crystallization the data down on the hierarchy as it ceases to be speed of the material being used, this operation used frequently. Figure 1 depicts the concept of tends to dictate the writing speed of PCRAM. memory hierarchy. Reflectivity can vary up to 30%, but resistivity changes can be as large as five orders of Emerging Memory Technologies magnitude. There are several new Non-Volatile Memory (NVM) The principle of phase-change memory is known technologies under research. One study lists 13 since the 1960s, but only recent discoveries of such technologies: FRAM, MRAM, STTRAM, phase-change materials with faster crystallization PCRAM, NRAM, RRAM, CBRAM, SEM, Polymer, speeds led to the possibility of commercially Molecular, Racetrack, Holographic and Probe. feasible memory technology. The most important Most these technologies are in different stages of materials are chalcogenides such as Ge2Sb2Te5 maturity. Some of them are still in early research (GST), that can crystallize in less than 100 ns. stages, others have working prototypes, and some Figure 2 shows a memory cell based on phase- of them are already entering into commercial change principles. The SET operation is achieved manufacturing. by crystallizing the material and RESET by making it The article reviews three of these technologies: amorphous. Unlike Flash memory, PCRAM can be Phase-Change RAM, Resistive RAM (including switched from 0 to 1 and vice-versa without an Memristors) and Magnetoresistive RAM (including ERASE operation. Spin-Torque Transfer RAM). All these fall into the Given the great difference in resistance, phase- category of the most actively researched today, are change materials can be easily used to store binary backed by solid companies of the technology states per cell (single-level cell) and even more industry, and considered most promising of being states (multi-level cell). commercially feasible. PCRAM is argued to be a scalable technology. As the feature density increases, phase change Phase-Change RAM (PCRAM) material layers become thinner and need less Phase-Change Random Access Memory (also current for the programming operations. called PCRAM, PRAM or PCM) is currently the most It has been demonstrated to work in 20nm device mature of the new memory technologies under prototype and is projected to scale down to 9nm. research. It relies on some materials, called phase- , change materials, that exist in two different phases PCRAM is argued to be a scalable with distinct properties: technology. It has been 1. An amorphous phase, characterized by high demonstrated to work in 20nm electrical resistivity; device prototype and is projected 2. A crystalline phase, characterized by low electrical resistivity. to scale down to 9nm. DRAM These two phases can be repeatedly and rapidly probably will not able to scale cycled by applying heat to the material. To make it down beyond 40nm., MEMORY DRAM probably will not able to scale down beyond that voltage has been applied. When you turn off 40nm. Today PCRAM is positioned as a Flash the voltage, the memristor remembers its most replacement. It offers great advantages over Flash, recent resistance until the next time you turn it on, but given the current limitations of access latency, whether that happens a day later or a year later. (...) energy consumption and endurance, further The ability to indefinitely store resistance values development is required in order to employ it as a means that a memristor can be used as a replacement for DRAM. nonvolatile memory.” This memristor device consisted of a crossbar of Resistive RAM (RRAM) platinum wires with titanium dioxide (TiO2) Despite the fact that PCRAM also uses resistance switches, as shown in Figure 4. Each switch consists variations to store bit values, the term Resistive RAM of a lower layer of perfect titanium dioxide (TiO2), (RRAM or ReRAM) has been applied to a distinct set which is electrically insulating, and an upper layer of technologies that explore the same of oxygen deficient titanium dioxide (TiO2 - x), phenomenon. Essentially these technologies fall which is conductive. The size of each layer can be into one of two categories: changed by applying voltage to the top electrode. If 1. Insulator resistive memories: based on bipolar a positive voltage is applied, the TiO2 – x ayer resistance switching properties of some metal thickness increases and the switch becomes oxides. The most important example is the conductive (ON state). A negative voltage has the memristor memory device, which will be further opposite effect (OFF state). This behavior matches described in more detail. the hysteresis loop previously shown in Figure 3. 2. Solid-electrolyte memories: based on solid- Titanium dioxide was used in the mentioned electrolyte containing mobile metal ions prototype, but several other oxides are known to sandwiched between a cathode and an anode. present similar bipolar resistive switching, and there Also known as Programmable Metallization Cell are multiple research projects in motion to explore (PMC) or Conductive Bridge RAM (CBRAM). these other materials for similar memory device There is a long list of RRAM technologies. This article will concentrate attention on the memristor, which is currently the most promising RRAM 1.0 technology under research. 0.5 Memristor t n e r Since the 19th century, three fundamental passive r 0.0 u circuit elements were known: the resistor, the c inductor and the capacitor. In 1971, Leon Chua -0.5 theorized the existence of a fourth passive circuit element, which he called the memristor, but no -1.0 actual physical device with memristive properties -1.0 -0.5 0.0 0.5 1.0 could be constructed. In 2008, a group of scientists voltage reported the invention of a device that behaved as Fig. 3 - Relationship between current and voltage in a predicted for a memristor. Later the same year, an memristor, known as a ‘hysteresis loop’. Extracted from Wang, article detailed how that device could be used to 2008 create nonvolatile memories.