Phase Change Memory

Phase change memory (PcRAM) is a type of non-volatile random-access memory. The mechanism of data saving is melt-quenching and crystallization process of phase change materials through controlling temperature range for some time, thereby switching amorphous to crystalline state. Amongst the many materials systems, chalcogenide exhibit extremely fast crystallization rate (few nanoseconds) at the same time significant changes in optical and electrical properties associated with the crystallization. In addition, the crystallization behavior can be controlled to occur only in a very small area of a few nanometers level.

There are still many reports on many reliability issues of PCRAM such as resistance drift and retention issues.

Loss of the information stored in the high-resistance state, so called data retention loss inhibits the reliability, thus limits the competitiveness of PcRAM. Such problem occurs because the crystallization behavior is a kinetic behavior not only subject to the rule of temperature, but also the rule of time. Sometimes the crystallization behavior happen for a very short time (~few nanoseconds) at a high temperature, but also occurs for a very long time (~few decades) at a low temperature even within the same system. Fast crystallization behavior at elevated temperatures is to determine the SET speed of the phase change memory, so that the faster the crystallization behavior is advantageous in the operation speed of PcRAM. In contrast, the crystallization behavior occurring in a low temperature is a major cause of inducing data retention loss, which is disadvantageous in the reliability of PcRAM.

The instability of amorphous state is defined as vulnerability against phase and structural changes which include three behaviors, i) the structural relaxation; structural change to reach more stable state, ii) the glass transition; 2nd order phase transition to super-cooled liquid, and iii) the crystallization; phase transition to crystalline phase. It is hard to detect the slight signal changes associated with the glass transition and the structural relaxation.

PcRAM suffered from harsh operating conditions such as high temperature and current density. These severe conditions can cause reliability problems like void formation or compositional variation, which has been known to current-induced migration called electromigration (EM). There are many previous research about migration behavior of GST induced by electric current, while most failure mechanism has focused only on liquid phase. As diffusivity of liquid phase is much faster than that of crystalline phase, migration-induced failure have been known to occur in the liquid phase dominantly and crystalline phase has been viewed as the stable state for EM so far.