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Moto planning NV MRAM backup: Sonos floating-gateBy David Lammers
Motorola engineers will describe an embedded 90-nanometer Sonos non-volatile memory at the International Electron Devices Meeting, which is planned for Dec. 9-11 in San Francisco.
Sonos stands for the material stack, silicon-oxide-nitride-oxide-silicon, with memory bits stored in the nitride layer directly beneath a polysilicon gate. Sonos non-volatile memory eventually will replace floating-gate flash because scaling the oxide gate dielectric in floating-gate devices has reached practical limits, and high-k dielectrics leak too much current, said Ko-Min Chang, manager of device development and process integration at Motorola's embedded-memory center.
Thus, the search is on for a new form of non-volatile memory, prompting Texas Instruments Inc.'s pursuit of embedded ferroelectric memory, Intel Corp.'s of Ovonic or chalcogenic memory and Motorola's own MRAM technology.
Motorola currently is shipping automotive-engine controllers with up to 8 Mbits of floating-gate flash. Chang said floating-gate flash probably will be scaled to 32-Mbit densities before the gate dielectric problem stunts further advances.
"The attractive thing about Sonos is that it involves only one poly layer, and can be quite a bit cheaper, with higher yield, than floating-gate flash. That's the motivation," Chang said.
The cell size also can be smaller: 10F (10 times the lithographic feature size) for floating-gate flash, and 8F for Sonos memory. If two bits per cell are stored, that figure of merit drops to 4F. Sonos memory requires four additional mask layers over the base logic process, compared with 11 additional masks for floating-gate flash.
For safety-critical automotive applications, Motorola plans to stick with one bit per cell for Sonos non-volatile memory, and to push two bits per cell for consumer and cell phone baseband applications, Chang said.
While Motorola's Sonos differs in certain respects from the MirrorBit flash technology developed by Advanced Micro Devices Inc. and its partner, Israel-based Saifun Inc., both the Sonos and MirrorBit approaches store charge in the nitride layer.
Motorola's Sonos memory uses uniform channel tunneling for erase and hot-electron injection for programming. While other groups have tried to use hot-hole injection for the erase function, Motorola found that approach to be unreliable. Hot-hole injection can lead to a buildup of residual charge if the injected holes do not completely drive out the stored electrons, Chang said.
Chang said the Sonos memory technology can use ±6 volts for programming and erase, a marked improvement over the 10 V that's required for floating-gate flash.
Motorola is positioning Sonos as a complement to magnetoresistive RAM. For applications requiring very fast access times, MRAM can replace SRAM.
While their technical attributes differ, Sonos and MRAM are on a somewhat similar time scale. Motorola plans to offer MRAM samples at 180-nm design rules starting late in 2003, and then jump to the 90-nm process being jointly developed with Philips and STMicroelectronics in Crolles, France. Motorola plans to introduce Sonos memory at the 90-nm node starting early in 2004, and to replace floating-gate flash gradually as customers become assured of the reliability of Sonos.
Q-flash storage
Chang said Sonos is one thrust in what Motorola calls thin-film storage. Another is Q-flash, in which charge is stored in silicon nanocrystals with a diameter of 5 nm.
"These are silicon dots, an early step into quantum devices, where the conductivity laterally is very low. That is one of the wonderful properties of these silicon dots: Once the electron is in there, it will not hop to the next silicon dot. And the barrier of moving another electron in there is tremendously high. In that sense, silicon dots behave like insulators," Chang said.
Q-flash devices can operate at lower voltages, and have smaller dimensions, than Sonos memory.
Joe Mogab, a vice president at Motorola's advanced-process research and development laboratory here, said Motorola has worked with the University of Texas at Austin on developing rapid thermal chemical vapor deposition techniques for the silicon nanocrystals.
"The key is to get the right density and right size of the isolated nanocrystals, and we believe that we've achieved some breakthroughs in development of the deposition chemistries," said Mogab.
Q-flash devices would offer much greater retention times than other non-volatile memories, Mogab said. If progress continues, Q-flash prototypes could be ready as early as 2005.
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