  We survey the current state of phase change memory (PCM), a non-volatile
solid-state memory technology built around the large electrical contrast
between the highly-resistive amorphous and highly-conductive crystalline states
in so-called phase change materials. PCM technology has made rapid progress in
a short time, having passed older technologies in terms of both sophisticated
demonstrations of scaling to small device dimensions, as well as integrated
large-array demonstrators with impressive retention, endurance, performance and
yield characteristics.
  We introduce the physics behind PCM technology, assess how its
characteristics match up with various potential applications across the
memory-storage hierarchy, and discuss its strengths including scalability and
rapid switching speed. We then address challenges for the technology, including
the design of PCM cells for low RESET current, the need to control
device-to-device variability, and undesirable changes in the phase change
material that can be induced by the fabrication procedure. We then turn to
issues related to operation of PCM devices, including retention,
device-to-device thermal crosstalk, endurance, and bias-polarity effects.
Several factors that can be expected to enhance PCM in the future are
addressed, including Multi-Level Cell technology for PCM (which offers higher
density through the use of intermediate resistance states), the role of coding,
and possible routes to an ultra-high density PCM technology.
