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New hopes for nitride-based non-volatile Flash memory |
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Oct 18, 2005 at 11:40 AM |
By Dr Mike Cooke
IMEC and Infineon Technologies have jointly developed a profiling
technique for the accurate extraction of charge profiles in
nitride-based memory to allow the optimization of write/erase voltages
in a novel operating mode. It is hoped that this result may overcome
major reliability problems of nitride-based memory that have hampered
their competitiveness in the non-volatile Flash market.
The profiling technique developed by IMEC and Infineon may allow
nitride technology to compete with (conventional) Flash over its full
application range of both stand-alone code and data storage Flash, as
well as in the embedded arena such as automotive applications.
Nitride memory promises process simplicity. The memory structure uses a
highly localized charge trapping mechanism in nanoscale material
defects. The partners hope that this mechanism could turn out to be
more scalable (shrinkable) than the conventional floating gate approach
that is widely used in Flash memories today.
However, the nitride dual bit storage requires hot holes for the erase
step. This results in an accumulation of residual charges over a number
of write/erase cycles. New reliability issues include memory window
walk-out (unstable threshold voltage) and degradation of the retention
after cycling (RAC). These problems impede the further dissemination of
nitride technology into the wide variety of Flash application areas. As
a consequence, the local charge storage concept is only competitive
within a small segment of the Flash market today.
The new profiling technique enables the accurate and independent
extraction of both electron and hole distributions for the first time.
Based on this technique, write and erase operations have been optimised
and a new operating mode has been found which leads to a 100% matching
of the carrier profiles. In the case of complete matching after one
write/erase cycle (i.e. the injected holes exactly compensate for the
first injected electrons), the window walk-out effect disappears
completely resulting in a stable threshold voltage. Also retention
after cycling remains identical to the retention of a fresh device.
These results open perspectives for both high cycle applications
(1,000,000 cycles have been demonstrated without verify) as well as for
high retention applications (such as stand-alone code storage and
automotive microcontrollers). Additional benefits of the technique are
the smaller periphery (because verify may be skipped and erase voltage
is lower) and the sharper distributions, which allow for further
channel length scaling.
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