Drift Phenomena in Phase Change Memories

Phase change memory (PCM) relies on the ability of a phase change material to exist in two distinct phases, namely the stable crystalline (or polycrystalline phase), and the metastable amorphous phase, to store digital data in non-volatile way. The metastability of the amorphous phase is the origin of a few reliability concerns, including thermally activated recrystallization of the amorphous phase, and thermally activated drift phenomena induced by structural relaxation of the short-range order. This chapter will provide an overview of drift phenomena in PCM, addressing the physical and electrical characterization results, the physical interpretation, the modeling, and the impact on the device performance, especially for multi-level cell (MLC) operation and for the instability of the polycrystalline phase. Methodologies to alleviate the drift, such as innovative PCM device structures, will be finally discussed.