IEEE Spectrum December, 2012 - 12

milliseconds long. Researchers found
that temperatures exceeded 800 °C
but that the hot spot was restricted to
the area near the gate. The chips were
able to heal themselves through this
onboard annealing to the point that even
after 100 million cycles, the researchers
claim, the memories held data well.
You might think all this heating
would be bad for battery life. The
"refreshing" does require a substantial
amount of power, Lue concedes, but
the annealing can be done infrequently
and one sector at a time, while the
device is inactive but still connected
to the power source. "It's not going to
drain your cellphone battery," he says.
The experimental memory yielded
another surprise: The heating allowed
faster erasing, something that was
thought to be independent of temperature.
"Further down, this may evolve into a
'thermally assisted' mode of operation
that gives both better performance-
such as the faster erasing-and better
endurance flash memory," Lue says.
Faster erasing could eventually result in
flash taking over from dynamic RAM,
the volatile but fast computer memory,
but it would take a lot to get there. "Flash
is not a random access memory, and the
architecture will need to be
completely different," he says.
Lue says Macronix intends
to
capitalize
on the self-healing
Heat plate
Heat plate
(negative)
flash breakthrough, but he
(positive)
would not give details about
how and when. He was more
Diode
forthcoming about when the
Low-resistance
Diode
flash
industry should have
gate
worked in this technology.
Oxide
"It took a leap of imagination
Floating
to jump into a completely
gate
different regime...very high
temperature and in a very short
time," says Lue. "Afterward,
Silicon substrate
we realized that there was
no new physics principle
invented here, and we could
hot stuff: Current flowing through the memory
have done this 10 years ago."
cells' gate heats and heals the oxide layers.
Macronix engineers saw a solution in
a competing technology, phase change
RAM. In PCRAM, a bit is stored in a
material called a chalcogenide glass,
which can be either conductive or
insulating. The bit's material switches
between those states when briefly
heated in a particular way. According
to Lue, Macronix researchers noticed
that heating the glass to its melting point
had a kind of healing effect on their
PCRAM. (They reported those results
at the IEEE International Reliability
Physics Symposium in April 2012.)
Macronix engineers figured that a
similar trick would work for flash, so
they redesigned a flash memory chip
to include tiny onboard heaters that
could anneal small groups of memory
cells. The redesign required quite a few
changes. The biggest one, says Lue, was
altering the gate electrode so that it
could carry current to heat the memory
cells. That alteration, which included
the addition of diodes, took up so much
space that the Macronix team had to
devise a new architecture for its memory
arrays to squeeze it all in, he adds.
The modified structure enables
current to pass through the transistor's
gate to generate pulses of heat a few

IllustratIon: emIly cooPer

12

NA * Ieee Spectrum * december 2012

-Yu-Tzu Chiu

New King
of Security
Algorithms
Crowned

A five-year hunt for a
new cryptographic hash
scheme leads to the
discovery that the old
one was really good

A

new cryptographic
hash algorithm, proclaimed the winner of a
five-year competition in October,
is literally a solution in search of
a problem.
Basically, cryptographic
hash algorithms are a way to
ensure information security.
They are typically used in digital
signatures and in establishing
connections with secure websites.
Data that is run through a hash
algorithm produces a shorter
string of bits; this string acts as
a kind of digest of the data. Any
change to the data changes the
bit string. The resulting string,
therefore, acts as a fingerprint
of the data, guaranteeing that
no one has tampered with it.
The winning algorithm,
called Keccak, was submitted
by a team of four researchers
from European semiconductor
company STMicroelectronics:
Guido Bertoni, Joan Daemen,
Gilles Van Assche, and Michaël
Peeters (who now works at
NXP Semiconductors).
Keccak is the culmination of a
contest launched in 2007 by the
National Institute of Standards
and Technology (NIST). At the
spectrum.ieee.org
http://spectrum.ieee.org
Table of Contents for the Digital Edition of IEEE Spectrum December, 2012
