Scaling Path and Issues in Various Emerging Architectures

The current NVM mainstream is based on flash technology, and it is expected that flash will be the high-volume NVM in production for the next years. Flash technology is characterized by a compact structure in which the selecting element and the storage element are merged in a MOS-like architecture. The resulting, full compatibility with the CMOS technology and the compact device size have made flash technology the cheapest solution for stand-alone and embedded-memory applications. However, discordant requirements to shrink the MOS structure, while preserving good selection and storage capabilities, are making flash scaling more and more difficult. Moreover, even if in the long term the cost advantage is important, better performance can speed up a novel technology introduction. In fact, a NVM with low-power and low-voltage capabilities, bit granularity, fast operations, and higher endurance would be a potential game changer for system designers.

To enlarge application segments, offering better performance and scalability, new materials, and alternative memory concepts are mandatory to boost the NVM industry. During recent decades, a total of more than 30 NVM technologies and technology variations have been competing for a piece of the fast growing NVM market, many of them aiming to replace also DRAMs. Although the planar-NAND scaling is becoming harder and it is obtained with several compromises in term of reliability, state-of-the-art NAND technology is 3 bits-per-cell 16 nm. Moreover, 3D NAND is becoming a mature technology. It follows that any NVM development must be able to provide the same capabilities at higher density. If this is not achieved in the next generation, technology scaling will not result in a cost reduction, thus eliminating the interest to continue along this path. Cost structure is therefore a fundamental parameter for benchmarking novel NVM concepts, in particular for those that want to compete for data-storage solutions, where MLC capabilities and/or 3D-stacking are mandatory.

On the other hand, the actual development efforts of alternative NVM concepts are demonstrating that disruptive innovation takes a long time. Figure 6 recounts schematically the development history of PCM technology, the only emerging memory concept that has reached the volume production maturity for large-density arrays. It is worth noting that the continuous need to stay close to the state-of-the-art lithographic node, combined with the necessary learning cycles, necessitated a decade of effort to evolve from concept to mass production. Moreover, we need also to consider the time-to-market, i.e., the time needed to get a significant profit from a novel NVM technology. For the flash NOR, about four years were needed to reach the break-even with the EEPROM in terms of profit for the main semiconductor industries involved in this market. The scaling lesson was also clearly demonstrated by the MRAM developments. MRAM products reached product maturity simultaneously with the last feasible technology node for the conventional toggle-MRAM, thus limiting the commercial success of MRAM technology and reducing it to a niche market. Only the discovery and exploitation of the STT-MRAM concept enabled a better scalability below the 90 nm node, thus renewing the interest in this technology.

In this perspective, there are two major aspects that must be considered for evaluating the potentials of any emerging memory concept, namely the readiness for moving beyond the leading—edge technology node and the scalability perspective. If we combine the above two statements, it follows that any realistic proposal for a novel NVM technology must prove its feasibility for the sub-1X-nm-technology node.

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