In recent years, non-volatile storage technology has made some significant progress in many aspects, which has brought new opportunities for the improvement of storage energy efficiency of computer systems. Researchers We suggest using new NVM technology to replace traditional storage technology to meet the demand for high storage energy efficiency in the development of computer technology. A variety of new NVM technologies represented by phase change random access memory (PCRAM) have attracted wide attention from domestic and foreign researchers due to their high integration and low power consumption. In particular, PCRAM has the potential to be both main memory and external memory due to its non-volatile, byte addressable and other characteristics. Under its influence, the boundary between main memory and external memory is gradually becoming Fuzziness may even bring about major changes to the future storage architecture. Therefore, PCRAM is considered to be one of the most promising new NVM technologies that can completely replace DRAM.
The earliest research on PCRAM technology can be traced back to 1917. At that time, Waterman published a paper saying that it could gradually change the conductivity of sulfide and make it different in two ways. In the phase state, it exhibits two different electrical properties, high resistivity and low resistivity, which are important characteristics of some phase change materials. However, due to the limitation of the scientific research conditions at the time and the lack of relevant technology to achieve melting and quenching of phase change materials, Waterman was unable to further reveal the changes in the microscopic lattice structure of phase change materials, and was unable to reverse the conductivity of phase change materials. . Although due to the irreversible conductivity of phase change materials and other factors, this research did not attract enough attention from academia and industry at that time. Researchers chose to use other technologies for storing data (such as mechanical Punched cards, magnetic core memory in the 1950s, etc.), but Waterman's pioneering work has made phase change materials a new branch of storage material science, which has attracted the attention of academia and industry. In 1962, Pearson et al. discovered the phenomenon of phase change in tellurium-arsenic glass, and wrote a paper to formally put forward the "phase change" theory. In 1966, Ovshinsky applied for a patent for phase change storage technology for the first time. In 1968, Ovshinsky’s research results broke the bottleneck of irreversible conductivity in the application of phase-change materials to the field of data storage, and published a paper on phase-change theory research in which he described in detail. Under the action of an electric field, this kind of phase change material can reversibly quickly switch between the amorphous phase state and the crystalline phase state, and it exhibits high resistance characteristics in the amorphous phase state, and exhibits high resistance characteristics in the crystalline phase state. Low resistance characteristics, so the amorphous phase state and the crystalline phase state can be used to represent the data "0" and "1" respectively, which are used to store binary information, which opened a precedent in the research of phase change materials and phase change memory devices. In 1970, Neale and Moore published a paper describing in detail a PCRAM storage device with a 256bit capacity.
Currently, chips that realize all functions of reading, writing and erasing have been developed at home and abroad, and PCRAM has gradually entered the process of industrialization. Substances can exist in a variety of phases, and phase change materials have at least two phases: an amorphous phase (the molecular structure is disordered) and a crystalline phase (the molecular structure is neat and orderly). The phase change materials in the amorphous phase and the crystalline phase have very obvious differences in optical and electrical properties, respectively. Therefore, people use the difference in the characteristics of phase change materials in different phases to achieve data storage, and use the difference in optical properties of phase change materials to achieve data storage is phase change optical discs (CD, DVD, Blu-ray Disc, etc.), and It is PCRAM that uses the difference in electrical properties of phase change materials to achieve data storage. At present, the most researched and relatively mature phase change material in academia and industry is Ge-Sb-Te series alloys. These phase change materials not only have excellent optical properties, but also outstanding electrical properties. The resistivity difference between the amorphous phase and the crystalline phase is very large, and they have the potential for multi-value storage, especially doped with In, Sn, Bi, and Si. After the elements such as C and N, all aspects of the characteristics have been greatly improved, so they have been widely used in PCRAM research and device preparation. In addition, the structure of PCRAM devices is different, and there will be subtle differences in shrinking capabilities and power consumption.
Device structure and principle
The main device structures of PCRAM currently studied include T-type structure (ie mushroom structure), μ-Trench structure, edge contact structure and planar structure. Among them, the T-shaped structure is a device structure widely used in academia and industry, and it is mainly composed of three parts: a bottom electrode, a phase change material layer, and a top electrode. The PCRAM memory cell of the T-shaped structure is shown in Figure 1.
The data storage function of PCRAM is to apply electric pulses of different intensities and different durations to one or more chalcogenide films in the device, so that they can be presented under the action of different degrees of current Joule heat. Different resistance characteristics to achieve. After the phase change material absorbs a certain amount of heat energy, it will realize rapid mutual transition between the amorphous phase and the crystalline phase, and show a very obvious difference in resistivity. The phase change material exhibits semiconductor characteristics in the amorphous phase state and has a higher resistance value; in the crystalline phase state, it exhibits semi-metallic characteristics and has a lower resistance value. Therefore, the data to be stored can be represented by the different resistance characteristics of the phase change material in the amorphous phase and the crystalline phase.
PCRAM has a better miniaturization capability. Research shows that based on the 20nm process node, the storage density of PCRAM technology is approximately 16 times that of DRAM technology. In particular, ordinary PCRAM memory cells realize binary data storage according to the different resistance characteristics of the phase change material in the two phases of the amorphous phase and the crystalline phase. However, some phase change materials have excellent electrical properties. The difference in resistance between the amorphous phase and the crystalline phase can reach 5 orders of magnitude, and it has the potential for multi-value storage. PCRAM multi-value storage technology is to use the huge difference between the high and low resistance of phase change materials in different phase states to achieve two or more storage states on a single memory cell, which can make storage without changing the process technology. The effect of multiplying the cell capacity, which can significantly improve the integration of the device and greatly reduce the storage cost, is one of the important research directions of the current PCRAM storage technology. The PCRAM memory cell realizes the resistance distribution of single-level storage (ie, binary storage) and second-level storage (quaternary storage), and the conversion between single-level storage cells and second-level storage cells is shown in Figure 2.
PCARM has good non-volatility. Studies have shown that PCRAM not only has very low leakage power consumption, but also the state of the phase change material is also very stable. Loss of sex.
Research shows that PCRAM has the advantages of large capacity, high integration, fast speed, low function and low cost, especially compatible with the new CMOS process, at the 5nm technology node There are no physical limitations before, and it has a wide range of application values. It is one of the new types of NVM with great development potential. At present, PCRAM has been gradually mass-produced and put into commercial application. However, the preparation of higher-quality phase change materials and the high power consumption generated by the conversion between the crystalline phase and the amorphous phase of the phase change material are constraints. The bottleneck of its commercial applications. Especially the large write operation delay (60ns~120ns, about 5-10 times the read operation delay), high write operation power consumption (approximately 10 times the read operation power consumption) and poor write endurance are urgent for the further development of PCRAM Problems to be solved.
To sum up, compared with SRAM and DRAM, PCRAM has the advantages of non-volatile, higher storage density, no need to refresh, and anti-radiation interference. Compared with Flash, PCRAM It has the advantages of fast read and write speed, long service life, and bit operations; compared with other new non-volatile memory, PCRAM has strong durability, low power consumption or large capacity, simple manufacturing process, relatively more mature research, etc. Advantages: Compared with FeRAM, it has the advantages of high storage density and non-destructive read operation; Compared with STT-MRAM, it has the characteristics of low power consumption and strong stability; Compared with RRAM, it has the advantages of mature material preparation and storage density Higher merits. In particular, further shrinking of DRAM and FLASH will have a large technical bottleneck for compatibility with the new CMOS technology, while PCRAM is compatible with the new CMOS technology and has good scaling capabilities. Studies have shown that it can continue for more than four generations after the 40nm technology node. The advantages of PCRAM have laid a solid foundation for its large-scale application in the storage field. It is one of the most promising new storage technologies and the most likely to completely replace DRAM. However, PCRAM also has some obvious shortcomings, especially the write operation speed is not comparable to DRAM, and the write endurance is also quite different from DRAM. In particular, poor write durability is one of the main obstacles facing its large-scale application in computer systems. At present, domestic and foreign researchers are studying some solutions to deal with it.
Compared with traditional storage devices, new storage technologies such as FeRAM, MRAM, RRAM and PCRAM have high integration, low power consumption, fast read and write access speed, and Volatile, zero or low idle energy consumption, byte-level addressing, small size and earthquake resistance, and many other excellent characteristics, have brought new opportunities for the development of computers and the improvement of storage energy efficiency, and promoted the transformation of computer system structure. In particular, because of its unique advantages, PCRAM has taken the lead in mass production and commercial applications. It occupies a very important position in the new data center and consumer electronics market, and is very likely to become the next-generation mainstream storage technology.