ST claims progress in PCM memory
Three years ago, ST reports, it concluded that the PCM technology of Ovonyx could be adapted as a semiconductor memory technology. The material is already used in re-writable CDs. In 2001, ST licensed the technology, also known as Ovonic Unified Memory (OUM), from Ovonyx and the two companies set up a joint-development team, largely based in Agrate Brianza, near Milan, where ST's world wide non-volatile memory (NVM) development is based. Intel is also working with Ovonyx on the technology in an independent agreement.
The chalcogenide materials used in PCM can be reversibly switched between two stable states - one amorphous with a high electrical resistance, the other crystalline with a low resistance - by appropriately heating the material. A chalcogen is any material that contains sulphur, selenium, or tellurium (three elements that have closely related chemical properties) - in ST's new technology, tellurium plays the key role.
A PCM cell consists essentially of a variable resistance formed by the chalcogenide material and its tiny electrical heater, along with a selection transistor used for the read/write operations.
ST's progress towards developing a commercial PCM technology was described in two papers presented at the VLSI Technology and Circuits Symposia in Hawaii, June 15-19, 2004. In one paper, ST presented a cell structure that can be integrated into mainstream chip manufacturing processes, along with indications of its manufacturability and cost. In a second paper, ST described the practical implementation of the technology in the form of an 8Mbit demonstrator chip designed to assess the feasibility of cost-effective large non-volatile memories.
Based on the highly-promising results it has obtained to date, ST already envisages PCM memory being used in medium-density, stand-alone memories and embedded applications. Moreover, by demonstrating the feasibility of the key features that make the PCM cell most attractive, ST has increased its confidence in the long-term scalability of the technology with the prospect of its becoming a mainstream non-volatile memory.
Specialists currently see barriers to pushing Flash memory technology beyond about 20nm. The problem arises because Flash depends on a high voltage (about 20V) for write/erase operations. Insulating barriers that can sustain these levels of potential difference need to be a certain thickness, any smaller and charge leaks out losing the memory value, any thicker and the write/erase voltage increases further.
Flash memory has been one of the industry high points in the last year or two, being one of the fastest growing sectors. ST estimates that the Flash market will show a compound annual growth rate (CAGR) of 19% over the 2003-2008 period. In the ten years from 1990 to 2000, the size of an individual Flash cell was reduced by a factor of 30.
