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Abstract
Ultrahigh storage densities of up to 1 Tb/in.2
or
more can be achieved by using local-probe techniques
to write, read back, and erase data in very
thin polymer films. The thermomechanical scanning-probe-based
data-storage concept, internally
dubbed ìmillipedeî, combines ultrahigh density,
small form factor, and high data rates. High data
rates are achieved by parallel operation of large
2D arrays with thousands micro/nanomechanical
cantilevers/tips that can be batch-fabricated by
silicon surface-micromachining techniques. The
inherent parallelism, the ultrahigh areal densities
and the small form factor may open up new
perspectives and opportunities for application in
areas beyond those envisaged today
Introduction
Data storage is one of the key elements in information
technology. The ever increasing demand for more storage
capacity in an ever shrinking form factor as well as the
pressure to decrease the price per storage unit in $/Gbyte
have been a major driving force for substantial worldwide
research and development activities to increase
storage densities by various means.
For many decades, silicon-based semiconductor
memory chips and magnetic hard drives (HDD) have been
dominating the data-storage market. So far, both technologies
have improved their storage densities by about 60ñ 100% per year, while reducing the cost per gigabyte.
However, the areal densities that todayís magnetic
recording technologies can achieve will eventually reach a
limit imposed by the well-known superparamagnetic
effect, which today is conjectured to be on the order of
250 Gbit/in.2
for longitudinal recording. Several proposals
have been formulated to overcome this limit, for
example, the adoption of patterned magnetic media,
where the biggest challenge remains the patterning of the
magnetic disk in a cost-effective manner. In the case of
semiconductor memories, such as DRAM, SRAM, Flash
etc., the challenges are predominately in lithography to
define and fabricate sub-100-nm FET gates as well as
very thin gate-oxide materials.
Techniques that use nanometer-sharp tips for imaging
and investigating the structure of materials down to the
atomic scale, such as the atomic force (AFM) and the
scanning tunneling microscope (STM), are suitable for the
development of ultrahigh-density storage devices [1-3].
As the simple tip is a very reliable tool for the ultimate
local confinement of interaction, tip-based storage technologies
can be regarded as natural candidates for extending
the physical limits that are being approached by conventional
magnetic and semiconductor storage.
Currently a single AFM operates at 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 technologies.
One solution to achieve such a substantial increase in the
data rates of tip-based storage devices is to employ microelectro-mechanical-system
(MEMS)-based arrays of cantilevers
operating in parallel, with each cantilever performing
write/read/erase operations on an individual
storage field. We believe that very-large-scale integrated
(VLSI) micro/nanomechanics will provide an ideal complement
to future micro- and nanoelectronics (integrated or hybrid), and may generate hitherto unheard of VLSIMEMS
application opportunities.
Various efforts are under way to develop MEMSbased
storage devices. For example, in [4], a MEMSactuated
magnetic-probe-based storage device that should
be capable of storing 2 Gbyte of data on 2 cm2
of die area
and whose fabrication is compatible with a standard integrated
circuit manufacturing process is proposed. With
this approach, a magnetic storage medium is positioned in
the x/y plane, and writing is achieved magnetically by
means of an array of probe tips, each tip being actuated in
the z-direction. Another approach is the storage concept
described in [5], where electron field emitters are
employed to change the state of a phase-change medium
in a bit-wise fashion from polycrystalline to amorphous
and vice versa. Reading is then done with lower currents
by detecting either back-scattered electrons or changes in
the semiconductor properties in the medium.
The thermomechanical probe-based data-storage concept,
our ìmillipedeî, combines ultrahigh density, small
form factor, and high data rates by means of highly
parallel operation of a large number of probes [6-10]. This
device stores digital information in a completely different
way from magnetic hard disks, optical disks, and
transistor-based memory chips. The ultimate locality is
provided by a tip, and high data rates result from the
massively parallel operation of such tips. As storage
medium, polymer films are being considered, although the
use of other media, in particular magnetic materials, is not
ruled out. Our current effort focuses on demonstrating the
concept with areal densities of up to 0.5ñ1 Tbit/in.2
and
parallel operation of very large 2D AFM cantilever (up to
64×64) arrays with integrated tips and write/read/erase
storage functionality. While a MEMS-based electromagnetically-activated
microscanner moves the polymer
medium in the x/y directions underneath the array chip,
the individual tips can be addressed for parallel write/read
operations.
The high areal storage density and small form factor
make this concept very attractive as a potential future
storage technology in mobile applications, offering gigabytes
of capacity and low power consumption at data rates
of megabytes per second. Moreover, these features,
coupled with the inherent massive parallelism, may open
up new perspectives and opportunities for application in
areas beyond those envisaged today.
2. Principles of operation
Our AFM cantilever-array storage technique is illustrated
in Fig. 1. Information is stored as sequences of indentations
and no indentations written in nanometer-thick
polymer films using the array of AFM cantilevers. The
presence and absence of indentations will also be referred
to as logical marks. Each cantilever performs write/read/
erase operations within an individual storage field with an
area on the order of 100%100 µm2
. Write/read operations depend on a mechanical x/y scanning of either the entire
cantilever array chip or the storage medium. The tipmedium
spacing can be either controlled globally by a
single z-actuation system for the entire array, or by simply
assembling the device with a well-controlled z-position of
the components such that the z-position of each tip falls
within a predetermined range.
Efficient parallel operations of large 2D arrays can be
achieved by a row/column time-multiplexed addressing
scheme similar to that implemented in DRAMs. In our
device, the multiplexing scheme could be used to address
the array column by column with full parallel write/read
operation within one column. The time between two
pulses being applied to the cantilevers of the same column
corresponds to the time it takes for a cantilever to move
from one logical-mark position to the next. An alternative
approach is to access all or a subset of the cantilevers
simultaneously without resorting to the row/column
multiplexing scheme. Clearly, the latter solution yields
higher data rates, whereas the former leads to a lower
implementtation complexity of the electronics.
Thermomechanical writing is achieved by applying a
local force through the cantilever/tip to the polymer layer
and simultaneously softening the polymer layer by local
heating. The tip is heated by application of a current pulse
to a resistive heater integrated in the cantilever directly
above the tip. Initially, the heat transfer from the tip to the
polymer through the small contact area is very poor, but it
improves as the contact area increases. This means that
the tip must be heated to a relatively high temperature of
about 400ºC to initiate softening. Once softening has been
initiated, the tip is pressed into the polymer, and hence the
indentation size is increased.
Imaging and reading are done using a thermomechanical
sensing concept. To read the written information, the
heater cantilever originally used for writing is given the
additional function of a thermal readback sensor by exploiting
its temperature-dependent resistance. For readback
sensing, the resistor is operated at a temperature in
the range of 150ñ300ºC, which is not high enough to
soften the polymer as in the case of writing. The principle
of thermal sensing is based on the fact that the thermal
conductance between heater platform and storage substrate
changes as a function of the distance between them.
The medium between the heater platform and the storage substrate, in our case air, transports heat from the
cantilever to the substrate. When the distance between
cantilever and substrate decreases as the tip moves into a
bit indentation, the heat transport through the air becomes
more efficient. As a result, the evolution of the heater
temperature differs in response to a pulse being applied to
the cantilever. In particular, the maximum value achieved
by the temperature is higher in the absence of an
indentation. As the value of the variable resistance
depends on the temperature of the cantilever, the
maximum value achieved by the resistance will be lower
as the tip moves into an indentation: During the read
process, the cantilever resistance reaches different values,
depending on whether the tip moves into an indentation
(logical bit ì1î) or over a region without an indentation
(logical bit ì0î). Under typical operating conditions, the
sensitivity of thermomechanical sensing exceeds that of
piezoresistive-strain sensing, which is not surprising
because in semiconductors thermal effects are stronger
than strain effects. The good sensitivity is demonstrated
by the images in Fig. 2, which were obtained using the
thermal-sensing technique described.