Multilevel - Cell Phase - Change Memory - Modeling and Reliability Framework Thèse

In themodern digital era of big data applications, there is an ever-increasing demand for higher memory capacity that is both reliable and cost effective. In the domain of non-volatilememory systems, Flash-based storage devices have dominated the consumer space for the past 15 years and have also entered the enterprise storage system in the past 2-3 years. However, with Flash memory devices facing serious scalability limits, there is an imminent need to explore the viability of other non-volatile memory technologies that can replace or complement Flash-based storage in the near future. Significant research efforts have been invested by various universities and research organization across the globe into realizing the so-called next-generation memories (NGMs). Phase-change memory is one such technology, which is viewed as the most promising candidate among the emerging technologies. Phase-ChangeMemory (PCM) technology is not new as the principle of storing information in chalcogenide-based materials was first explored in the 1960s. However, with the predominance of charge-based memory technology (DRAM, Flash storage, etc) thriving thanks to the technological advancements of metal-oxide semiconductor field-effect transistor (MOSFET) based devices, it was not until the late 90s that renewed interest in the class of phase-change materials was ignited. This is the same genre of materials as have been widely used in laserdriven optical storage (rewritable CDs and DVDs) during the past 20 years or so. Although PCM chips have already beenmass-produced by companies like Micron and Samsung, their capacity was limited as they were based on single-level cell (SLC) storage. The primary research focus of this thesis is on increasing the memory capacity by storing more than one bit of information per device, known as multilevel-cell (MLC). Achieving MLC capability is quite challenging, and we disclose some of our significant achievements in the realization of MLC PCM in the past few years. One of the main concerns for the sustainability of PCM technology is the power-hungry RESET process. Thermoelectric physics in nano-scale PCM devices is shown to play a significant role in the programming of these devices. Interestingly, these effects result in better thermal confinement and can even pave the way for more power-efficient devices. Therefore, it is high time that the thermoelectric physics within the devices is completely understood. Reliability is the other major concern for PCM technology, especially in the realization of multilevel-cell storage at highly dense PCM arrays. Several MLC-enabling technological advancements that suppress the drift phenomenon and array variability have been explored in the recent past.