Product category:
Memory Devices and Modules
News Release from: STMicroelectronics | Subject: Phase-change memory
Edited by the Electronicstalk Editorial
Team on 22 June 2004
Phase-change memory promises Flash
replacement
STMicroelectronics is claiming significant progress in the development of a new type of electronic memory that could eventually replace Flash memory technology.
STMicroelectronics is claiming significant progress in the development of a new type of electronic memory that could eventually replace Flash memory technology The new technology, called phase-change memory (PCM), potentially offers better performance than Flash, with benefits such as faster read and write times, greater endurance, and the ability to write to individual memory addresses
This article was originally published on Electronicstalk on 8 Nov 2004 at 8.00am (UK)
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Most importantly, it is also inherently more scalable than Flash and other nonvolatile memory technologies in use today.
ST's progress towards developing a commercial PCM technology was described in two papers presented at one of the semiconductor industry's most important annual forums, the VLSI Technology and Circuits Symposia held in Honolulu last week.
In one technology paper, ST showed for the first time an innovative micro-trench cell structure that reduces cell current, can be easily integrated into the CMOS base process, and which gives good indications of its manufacturability and cost.
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In a second design paper, ST described the implementation of the vertical bipolar selector cell into an 8Mbit demonstrator designed to assess the feasibility of cost-effective large nonvolatile memories.
The new memory technology exploits the fact that certain so-called "chalcogenide" materials, in this case an alloy of germanium, antimony and tellurium called GST, 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 memory cell in the new technology consists essentially of a variable resistance formed by the GST and a heater, along with a selection transistor used for the read/write operations.
A key innovation in the new technology is that the contact area between the GST and the heater is defined by the intersection of a thin vertical semi-metallic heater and a trench (the "micro-trench") in which the GST is deposited.
This results in a compact vertical integration and a low programming current.
The selection transistor is a PNP bipolar transistor directly underneath the heater.
The result is a cell area (0.32um2 in 180nm CMOS) that is comparable with the best NOR Flash cells in the same technology, while offering advantages such as single-bit granularity, faster writing, far greater endurance and better scalability.
Scalability is particularly important because semiconductor manufacturers foresee considerable difficulties in developing Flash technology beyond the 45nm node due to physical limitations such as the unacceptably high charge leakage that occurs with tunnel-oxide thicknesses below 8-9nm.
In contrast, there are no physical limitations to the scalability of the PCM storage element, the principle challenge being one of reducing the operating current.
This makes it a leading candidate for the post-Flash era that will be necessary in a few years.
Although the demonstrator was based on a PCM cell with a bipolar selection transistor, the PCM architecture is fully compatible with the use of a MOSFET selector.
In this case, cell area would be about four times larger but fewer masks would be needed.
Therefore, MOSFET selectors would provide benefits in embedded NVM applications.
Based on the results obtained to date, ST already envisages the PCM technology being used in medium-density stand-alone memories and embedded applications.
Moreover, by demonstrating the feasibility of the features that make the PCM cell most attractive, ST has increased its confidence in the long-term scalability of the technology, which will eventually allow PCM to become a mainstream nonvolatile memory technology.
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