Reminder: Wei Wang Ph.D Oral Defense 08/07/2008 (Tomorrow)

Wei Wang mailvv at stanford.edu
Wed Aug 6 22:01:47 PDT 2008


Ph.D Oral Defense Wei Wang
----- Original Message ----- 
From: Diane Shankle 
To: ee-students at lists.stanford.edu 
Sent: Friday, August 01, 2008 11:59 AM
Subject: Ph.D Oral Defense Wei Wang




Title:       Titanium Oxide Nonvolatile Memory Device and its Application


Name:     Wei Wang


Advisor:  Prof.  Simon Wong


Time:      2pm, Thursday, August 7th, 2008 (refreshments at 1:45pm)


Place:     CISX Auditorium


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With the memory cost per bit reducing at very aggressive rates, nonvolatile memories (NVM) have seen explosive growth in the last few years in electronic applications such as memory cards, cell phones, and other consumer electronic devices. On the other hand, FLASH as the current mainstream nonvolatile memory, is facing major scaling difficulties due to non-scalability of dielectric and cell to cell interference. While FLASH is predicted to reach its definite limit beyond 22nm, non-charge-storage nonvolatile memories are being researched as alternatives.





Resistive switching in transition metal oxide (TMO) thin films, such as TiOx and NiO, has raised great interests for possible applications in nonvolatile memories due to their excellent CMOS process compatibility, promising scalability and low manufacturing cost.





One extremely undesirable characteristic of TiOx NVM devices is that all fresh devices have to go through a "forming process" before they can be further switched on and off. The forming process is a high-voltage stress (~2Vdd), which is a major obstacle for widespread adoption in products. We pioneered a fabrication process which allows in-situ deposition of the bottom electrode, metal oxide and top electrode The process has been demonstrated to eliminate high voltage forming in TiOx resistive switching devices.


TiOx NVM device can switch in both unipolar and bipolar modes. A novel cross-point structure is built to understand the physics of resistive switching in both modes.  Our experiments reveal that unipolar and bipolar switching have different working mechanisms: unipolar switching can be explained by thermal dissolution model, and bipolar switching by local redox reaction model. Conductive filament is destroyed during unipolar switching, but can be reused during bipolar switching, which suggests that bipolar switching may have better endurance.


One possible application of TiOx NVM device is nonvolatile SRAM (NVSRAM), a technology seeking to solve the problem of rapidly increasing SRAM leakage power in embedded systems. To reduce SRAM leakage power, we propose a nonvolatile SRAM cell with two back-up nonvolatile memory devices. This novel cell offers non-volatile storage, thus allowing selected blocks of SRAM to be powered down during operation, which completely eliminates their leakage power. There is no area penalty in this approach and only a slight performance penalty of less than 15% is anticipated. It is also shown that power savings resulting from the NVSRAM approach increase with technology scaling, for both high speed and general purpose processors.









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