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Definition
Today data storage is dominated by the use of magnetic disks. Storage densities
of about more than 5 Gb/cm 2 have been achieved. In the past 40 years areal density
has increased by 6 orders of magnitude. But there is a physical limit. It has
been predicted that superparamagnetic effects- the bit size at which stored information
become volatile as a function of time- will limit the densities of current longitudinal
recording media to about 15.5 Gb/cm2 . In the near future century nanometer scale
will presumably pervade the field of data storage. In magnetic storage used today,
there is no clear-cut way to achieve the nanometer scale in all three dimensions.
So new techniques like holographic memory and probe based data storage are emerging.
If an emerging technology is to be considered as a serious candidate to replace
an existing technology, it should offer long-term perspectives. Any new technology
with better areal density than today's magnetic storage should have long-term
potential for further scaling, desirably down to nanometer or even atomic scale.
The
only available tool known today that is simple and yet offer these long-term perspectives
is a nanometer-sharp tip like in atomic force microscope (AFM) and scanning tunneling
microscope (STM). The simple tip is a very reliable tool that concentrates on
one functionality: the ultimate local confinement of interaction. In local probe
based data storage we have a cantilever that has a very small tip at its end.
Small indentations are made in a polymer medium laid over a silicon substrate.
These indentations serve as data storage locations. A single AFM operates best
on the microsecond time scale. Conventional magnetic storage, however, operates
at best on the nanosecond time scale, making it clear that AFM data rates have
to be improved by at least three orders of magnitude to be competitive with current
and future magnetic recording. The "millipede" concept is a new approach
for storing data at high speed and with an ultrahigh density.
Millipede
Concept
Millipede is a highly parallel scanning probe based data storage
that has a real storage densities far beyond superparamagnetic limits and data
rates comparable to today's magnetic recording. At the first glance, millipede
looks like a conventional 14 X 7 mm 2 silicon chip. Mounted at the center of the
chip is a miniature two-dimensional array of 1024 'v'-shaped cantilevered arms
that are 70 µm long and 0.5 µm thick. A nano-sharp fang-like tip,
only 20 nm in diameter, hangs from the apex of each cantilever. The multiplex
drivers, allow addressing of each tip individually. Beneath the cantilever array,
is a thin layer of polymer film deposited on a movable, three-axis silicon table.
The 2-D AFM cantilever array storage technique called "millipede" is
based on a mechanical parallel x/y scanning of either the entire cantilever array
chip or the storage medium. In
addition, a feedback-controlled z-approaching and leveling scheme brings the entire
cantilever array chip into contact with the storage medium. The tip-medium contact
is maintained and controlled while x/y scanning is performed for read/write. The
millipede approach is not based on individual z-feedback for each cantilever ;
rather it uses a feedback control for the entire chip, which greatly simplifies
the system. However this requires very good control and uniformity of tip height
and cantilever bending. Chip approach/leveling makes use of additionally integrated
approaching cantilever sensors in the corners of the array chip to control the
approach of the chip to the storage medium. Signals from these sensors provide
feedback signals to adjust the z-actuators until contact with the medium is established.
Feedback loops maintain the chip leveled and in contact with the surface while
x/y scanning is performed for write/read operations.
Millipede
Is Unique
Conventional data storage devices, such as disk drives and CD/DVDs,
are based on systems that sense changes in magnetic fields or light to perform
the read/write/store/erase functions. Millipede is unique both in form and the
way it performs data storage tasks; it is based on a chip-mounted, mechanical
system that senses a physical change in the storage media. The millipede's technology
is actually closer to, although on an atomic scale, the archaic punched card than
the more recent magnetic media. Using millipede, the IBM scientists have demonstrated
a data storage density of a trillion bits per square inch -20 times higher than
the densest magnetic storage available today. Millipede is dense enough to store
the equivalent of 25 DVDs on a surface of the size of a postage stamp. This technology
may boost the storage capacity of handheld devices - personal digital assistants
(PDAs) and cell phones - often criticized for their low storage capabilities.
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