Phase change memory (PCM) is an emerging non-volatile solid-state memory technology employing phase change materials. These materials are among the most ubiquitous materials in information storage, as they are already mass-deployed in rewriteable optical discs such as CDs and DVDs. In PCM, use is made of the pronounced change in electrical resistivity when the material changes between its two stable states, namely the amorphous and poly-crystalline phases.
PCM exhibits highly desirable characteristics, such as rapid state transition, good data retention and cyclability performance, as well as future scaling to ultra-small device dimensions. Because of the above properties, PCM has been touted to succeed or even replace the ubiquitous flash memory at embedded or stand-alone applications. However, several practical challenges remain to be resolved before PCM becomes a commercial reality. Among other things, multilevel functionality (multiple bits per unit memory cell) is considered to be one way to increase capacity and thereby reduce cost, making PCM competitive with flash memory in the marketplace.
We are investigating key issues of multilevel functionality in PCM, such as reliability, endurance, as well as high-speed and low-power programming through device characterization, modeling, definition and optimization of write strategy, and coding/signal processing [1,2].
At the heart of every PCM device is the phase change material, the properties of which determine, to a large extent, the operation of the device. Characterization of material characteristics is thus a critical step towards obtaining desirable device properties. Scanning probes are powerful tools for the characterization of phase change materials and the study of the phase change mechanism. The conductive AFM, in particular, offers highly-localized, nondestructive sample profiling as well as modification with very-high spatial resolution and is thus well-suited for in-situ, high-throughput characterization of phase change materials at the nanometer-scale. Leveraging our long-standing experience in scanning probe technology, we embark on the characterization of phase change materials and the modeling of their properties using conductive AFM probes [3-5].