Via ars technica
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Flash memory is the dominant nonvolatile (retaining information when
unpowered) memory thanks to its appearance in solid-state drives (SSDs)
and USB flash drives. Despite its popularity, it has issues when feature
sizes are scaled down to 30nm and below. In addition, flash has a
finite number of write-erase cycles and slow write speeds (on the order
of ms). Because of these shortcomings, researchers have been searching
for a successor even as consumers snap up flash-based SSDs.
There are currently a variety of alternative technologies competing to replace silicon-based flash memory, such as phase-change RAM (PRAM),
ferroelectric RAM (FERAM), magnetoresistive RAM (MRAM), and
resistance-change RAM (RRAM). So far, though, these approaches fail to
scale down to current process technologies well—either the switching
mechanism or switching current perform poorly at the nanoscale. All of
them, at least in their current state of development, also lack some
commercially-important properties such as write-cycle endurance,
long-term data retention, and fast switching speed. Fixing these issues
will be a basic requirement for next-gen non-volatile memory.
Or,
as an alternative, we might end up replacing this tech entirely.
Researchers from Samsung and Sejong University in Korea have published a
paper in Nature Materials that describes tanatalum oxide-based (TaOx) resistance-RAM (RRAM), which shows large improvements over current technology in nearly every respect.
RRAM devices work by applying a large enough voltage to switch material
that normally acts as an insulator (high-resistance state) into a
low-resistance state. In this case, the device is a sandwich structure
with a TaO2-x base layer and a thinner Ta2O5-x
insulating layer, surrounded by platinum (Pt) electrodes. This
configuration, known as metal-insulator-base-metal (MIMB), starts as an
insulator, but it can be switched to a low resistance, metal-metal
(filament)-base-metal (MMBM) state.
The nature of the switching process is not well understood in this case,
but the authors describe it as relying on the creation of conducting
filaments that extend through the Ta2O5-x layer.
These paths are created by applying sufficiently large voltages, which
drive the movement of oxygen ions through a redox (reduction-oxidaton)
process.
When in the MIMB state, the interface between the Pt electrode and the Ta2O5-x forms a metal-semiconductor junction known as a Schottky barrier, while the MMBM state forms an ohmic contact.
The main difference between these two is that the current-voltage
profile is linear and symmetric for ohmic but nonlinear and asymmetric
for Schottky. The presence of Schottky barriers is a benefit, as it
prevents stray current leakage through an array of multiple devices
(important for high-density storage).
The results presented by the authors appear to blow other memory
technologies out of the water, in pretty much every way we care about.
The devices presented here are 30nm thick, and the switching current is
50 ?A—an order of magnitude smaller than that of PRAM. They also
demonstrated an endurance of greater than 1012 switching cycles (higher than the previous best of 1010 and six orders of magnitude higher than that of flash memory at 104-106).
The device has a switching time of 10ns, and a data retention time
that's estimated to be 10 years operating at 85°C. This type of RRAM
also appears to work without problems in a vacuum, unlike
previously-demonstrated devices.
This may all seem too good to be true—it should be emphasized that this
was only a laboratory-scale demonstration, with 64 devices in an array
(therefore capable of storing only 64 bits). There will still be a few
years of development needed before we see gigabyte-size drives based on
this RRAM memory.
Like all semiconductor device fabrication, advances will be needed to
improve nanoscale lithography techniques for large-scale manufacturing
and, in this particular case, a better understanding of the basic
switching mechanism is also needed. However, based on the results shown
here, this new memory technology shows promise for use as a universal
memory storage: the same type could be used for storage and working
memory.