Large Resistance Ratio for High Reliability of Multi-Level Storage in Phase-Change Memory

Reliability (or stability) of multi-level storage (MLS) is the critical characteristics for multi-level cells. In order to improve reliability of MLS of phase-change memory, there are two effective approaches, (i) enlargement of the ratio between resistance levels and (ii) reduction of scattering of resistance level. On the basis of our experimental results, it is demonstrated that the Ge2Sb2Te5-based double-layered cell has a high ratio of highest to lowest levels up to two-to-three orders of magnitude, implying high reliability. The cells exhibit the possibility of stable switching for four-level storage.