NXP Taking Phase Change to Embedded
David Lammers, News Editor -- Semiconductor International, 12/17/2007 6:33:00 AM
NXP Semiconductors (Eindhoven, Netherlands) is working to develop embedded phase-change memories (PCMs) for its RFID, smart card and microcontroller products, perhaps as soon as the 45 nm generation, NXP researchers said at the International Electron Devices Meeting (IEDM), held in Washington, D.C., Dec. 10-12.
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| John Schmitz, NXP vice president |
Most PCMs use compounds of germanium (Ge), antimony (Sb) and tellurium (Te). By rapidly applying Joule heating, these GST chalcogenide materials can be flipped from crystalline to amorphous phases, changing the resistivity of the cell.
Several companies, including Samsung Electronics Co. Ltd. (Seoul, South Korea) and the Numonyx NV (Geneva) joint venture between Intel Corp. (Santa Clara, Calif.) and STMicroelectronics (Geneva) are developing PCM standalone memory products, starting at the 90 nm node. While PCMs are CMOS-compatible, using PCM as an embedded memory technology presents additional materials and design challenges.
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| Top-down view of the NXP phase change memory (PCM) array. |
Power consumption can be a factor of 10 lower than NOR flash, and PCMs require only three additional mask layers in the back end of the device, making them cheaper to produce than embedded flash. Also, PCMs can adapt to lower programming voltages (in the range of 1.5 V) more easily than floating gate flash, which can require 15 V to write, he said.
An NXP research paper presented at IEDM considered the impact of the Thompson effect, which occurs when both an electrical current and a temperature gradient are present in a device. The Thompson effect could impose limits on cell layouts, requiring further study on how to shape the reset mechanism, said Erwin Hijzen, head of the silicon process options department at NXP.
Hijzen said the the 45 nm generation could be the turning point toward PCMs and away from embedded floating-gate flash. “Embedded flash technology faces real issues at the 45 nm generation. Getting rid of the charge pumps in flash would be a major benefit.”
NXP has developed a method of resetting the phase of the material by using the heat from a thin wire within the cell. “The heater is the line itself,” he said, rather than creating a larger RESET area at the bottom of the bit cell.
Challenges remain, including “some unknowns on the materials side. From an integration point of view, understanding the materials is the biggest issue.”
Researchers from Hitachi Ltd. (Tokyo) have studied PCMs and raised questions about the retention times at high temperatures. NXP has demonstrated retention times of 10 years at 85°C, and Hijzen said he expects to increase that to 100°C soon. The endurance of the prototype devices exhibit of 109 read cycles.
NXP is conducting much of its PCM R&D at IMEC (Leuven, Belgium), where NXP has 60 researchers on site. NXP also is doing PCM studies with Taiwan Semiconductor Manufacturing Co. Ltd. (Hsinchu, Taiwan) at the NXP-TSMC Research Center’s facilities in both Leuven and Eindhoven.Talkback
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