1 Introduction
Millipede is storage technology developed by IBM.Millipede is a non-volatile computer memory stored on nanoscopic pits.
It promises a data density of more than 1 terabit per square inch (1 gigabit per square millimeter), about 4 times the density of magnetic storage available today.
Millipede storage technology is being pursued as a potential replacement for magnetic recording in hard drives, at the same time reducing the form-factor to that of Flash media.
IBM says flash memory probably won’t surpass 1GB to 2GB of capacity in the near term, but Millipede technology could pack 10GB to 15GB of data into the same small format without requiring additional power for device operation.
Working procedure:
Thousands of extremely fine tips “write” tiny pits representing individual bits into a thin film of highly specific polymer.
Bits are written by heating the tip to a temperature above the glass transition temperature of the polymer by means of the heating resistor integrated in the cantilever.
The principle is comparable with the old punch cards, but now with structural dimensions in the nanometer scale and the ability to erase data and rewrite the medium.
1.1 What is IBM Millipede?
Millipede is a nano-storage prototype developed by IBM that can store data at a density of a trillion bits per square inch: 20 times more than any currently available magnetic storage medium. The prototype’s capacity would enable the storage of 25 DVDs or 25 million pages of text on a postage-stamp sized surface, and could enable 10 gigabytes (GB) of storage capacity on a cell phone.
1.2 MOTIVATION AND OBJECTIVES
In the 21stcentury, the nanometer will very likely play a role similar to the
one played by the micrometer in the 20thcentury. The nanometer scale will presumably
pervade the field of data storage. In magnetic storage today, there is no clear-cut way to
achieve the nanometer scale in all three dimensions. The basis for storage in the 21st
century might still be magnetism. Within a few years, however, magnetic storage
technology will arrive at a stage of its exciting and successful evolution at which
fundamental changes are likely to occur when current storage technology hits the well-
known superparamagnetic limit. Several ideas have been proposed on how to overcome
this limit. One such proposal involves the use of patterned magnetic media, for which the
ideal write/read concept must still be demonstrated, but the biggest challenge remains the
patterning of the magnetic disk in a cost-effective way. Other proposals call for totally
different media and techniques such as local probes or holographic methods.
In general,if an existing technology reaches its limits in the course of its evolution & newalternatives are emerging in parallel, two things usually happen: First, the existing andwell-established technology will be explored further and everything possible done topush its limits to take maximum advantage of the considerable investments made. Then,when the possibilities for improvements have been exhausted, the technology may still
survive for certain niche applications, but the emerging technology will take over,
opening up new perspectives and new directions.
Consider, for example, the vacuum electronic tube, which was replaced by
the transistor. The tube still exists for a very few applications, whereas the transistor
evolved into today’s microelectronics with very large scale integration (VLSI) of
microprocessors and memories. Optical lithography may become another example:
Although still the predominant technology, it will soon reach its fundamental limits and
be replaced by a technology yet unknown. Today we are witnessing in many fields the
transition from structures of the micrometer scale to those of the nanometer scale, a
dimension at which nature has long been building the finest devices with a high degree of
local functionality. Many of the techniques we use today are not suitable for the coming
nanometer age; some will require minor or major modifications, and others will be
partially or entirely replaced. It is certainly difficult to predict which techniques will fall
into which category. For key areas in information technology hardware, it is not yet
obvious which technology and materials will be used for nanoelectronics and data
storage.
In any case, an emerging technology being considered as a serious
candidate to replace an existing but limited technology must offer long-term perspectives.
For instance, the silicon microelectronics and storage industries are huge and require
correspondingly enormous investments, which makes them long-term-oriented by nature.
The consequence for storage is that any new technique with better areal storage density
than today’s magnetic recording should have long-term potential for further scaling,
desirably down to the nanometer or even atomic scale.
The only available tool known today that is simple and yet provides these
very long-term perspectives is a nanometer sharp tip. Such tips are now used in every
atomic force microscope (AFM) and scanning tunneling microscope (STM) for imaging
and structuring down to the atomic scale. The simple tip is a very reliable tool that
concentrates on one functionality: the ultimate local confinement of interaction.
In the early 1990’s, Mamin and Rugar at the IBM Almaden Research
Center pioneered the possibility of using an AFM tip for readback and writing of
topographic features for the purposes of data storage. In one scheme developed by them,
reading and writing were demonstrated with a single AFM tip in contact with a rotating
polycarbonate substrate. The data were written thermo mechanically via heating of the
tip. In this way, densities of up to 30 Gb/in.were achieved, representing a significant
advance compared to the densities of that day. Later refinements included increasing
readback speeds to a data rate of 10 Mb/s and implementation of track servoing.
In making use of single tips in AFM or STM operation for storage, one
must deal with their fundamental limits for high data rates. At present, the mechanical
resonant frequencies of the AFM cantilevers limit the data rates of a single cantilever to a
few Mb/s for AFM data storage, and the feedback speed and low tunneling currents limit
STM-based storage approaches to even lower data rates. 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. The objectives of our research activities within the Micro- and
Nanomechanics Project at the IBM Zurich Research Laboratory are to explore highly
parallel AFM data storage with areal storage densities far beyond the expected
superparamagnetic limit (60100 Gb/in.) and data rates comparable to those of today’s
magnetic recording.
1.3 MILLIPEDE MEMORY
Millipede is a non-volatile computer memory stored on nanoscopic pits
burned into the surface of a thin polymer layer, read and written by a MEMS-based
probe. It promises a data density of more than 1 terabit per square inch (1 gigabit per
square millimeter), about 4 times the density of magnetic storage available today.
Millipede storage technology is being pursued as a potential replacement
for magnetic recording in hard drives, at the same time reducing the form-factor to that of
Flash media. IBM demonstrated a prototype s Millipede storage device at CeBIT 2005,
and is trying to make the technology commercially available by the end of 2007. At
launch, it will probably be more expensive per-megabyte than prevailing technologies,
but this disadvantage is hoped to be offset by the sheer storage capacity that technology
Millipede technology would offer.
The Millipede concept presented here is a new approach for storing data at
high speed and with an ultrahigh density. It is not a modification of an existing storage
technology, although the use of magnetic materials as storage media is not excluded. The
ultimate locality is given by a tip, and high data rates are a result of massive parallel
operation of such tips. Our current effort is focused on demonstrating the Millipede
concept with areal densities up to 500 Gb/in.and parallel operation of very large 2D (32
× 32) AFM cantilever arrays with integrated tips and write/read storage functionality.
1.4 THE NAME MILLIPEDE
The name Millipede came from the way the technology works. It consists
of a thin, organic polymer on which sit thousands of fine silicon tips that can punch
information into the polymer surface, leaving pits and creating a way of storing data.
Each tip is very small, with 4,000 fitting onto a 6.4 mm square.
The unveiling at the CeBIT event was not only to show off the tech but
also to try to get a manufacturing partner on board. IBM does not have the facilities to
manufacture MEMS systems, and needs another interested party to come on board that
has those facilities available. Big Blue also admits that the technology is nowhere near
ready for a release, as researchers still need to sort out the speed that data can be
transferred to and from the memory. IBM does hope, however, that Millipede will form a
future alternative to current flash memory technologies used in consumer digital
equipment.
1.5 BASIC CONCEPT
The main memory of modern computers is constructed from one of a
number of DRAM-related devices. DRAM basically consists of a series of capacitors, which store data as the presence or absence of electrical charge. Each capacitor and its
associated control circuitry, referred to as a cell, holds one bit, and bits can be read or
written in large blocks at the same time.
In contrast, hard drives store data on a metal disk that is covered with a
magnetic material; data is represented as local magnetization of this material. Reading
and writing are accomplished by a single “head”, which waits for the requested memory
location to pass under the head while the disk spins. As a result, the drive’s performance
is limited by the mechanical speed of the motor, and is generally hundreds of thousands
of times slower than DRAM. However, since the “cells” in a hard drive are much smaller,
the storage density is much higher than DRAM.
Millipede storage attempts to combine the best features of both. Like the
hard drive, Millipede stores data in a “dumb” medium that is simpler and smaller than
any cell used in an electronic medium. It accesses the data by moving the medium under
the “head” as well. However, Millipede uses many nanoscopic heads that can read and
write in parallel, thereby dramatically increasing the throughput to the point where it can
compete with some forms of electronic memory. Additionally, millipede’s physical media
stores a bit in a very small area, leading to densities even higher than current hard drives.
Mechanically, Millipede uses numerous atomic force probes, each of
which is responsible for reading and writing a large number of bits associated with it. Bits
are stored as a pit, or the absence of one, in the surface of a thermo-active polymer
deposited as a thin film on a carrier known as the sled. Any one probe can only read or
write a fairly small area of the sled available to it, a storage field. Normally the sled is
moved to position the selected bits under the probe using electromechanical actuators
similar to those that position the read/write head in a typical hard drive, although the
actual distance moved is tiny. The sled is moved in a scanning pattern to bring the
requested bits under the probe, a process known as x/y scan.
The amount of memory serviced by any one field/probe pair is fairly
small, but so is its physical size. Many such field/probe pairs are used to make up a
memory device. Data reads and writes can be spread across many fields in parallel,
increasing the throughput and improving the access times. For instance, a single 32-bit
value would normally be written as a set of single bits sent to 32 different fields. In the
initial experimental devices, the probes were mounted in a 32×32 grid for a total of 1,024
probes. Their layout looked like the legs on a Millipede and the name stuck.
The design of the cantilever array is the trickiest part, as it involves
making numerous mechanical cantilevers, on which a probe has to be mounted. All the
cantilevers are made entirely out of silicon, using surface micromachining at the wafer
surface.
The Millipede concept: for operation of the device, the storage medium – a
thin film of organic material deposited on a silicon “table” – is brought into contact with
the array of silicon tips and moved in x- and y-direction for reading and writing.
Multiplex drivers allow addressing of each tip individually.
The 2D AFM cantilever array storage technique called “Millipede” is
illustrated in figure. It 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. This tip medium contact is maintained and controlled while x/y
scanning is performed for write/read. It is important to note that 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 stringent
control and uniformity of tip height and cantilever bending. Chip approach and leveling
make use of four integrated approaching cantilever sensors in the corners of the array
chip to control the approach of the chip to the storage medium. Signals from three sensors
(the fourth being a spare) provide feedback signals to adjust three magnetic z-actuators
until the three approaching sensors are in contact with the medium. The three sensors
with the individual feedback loop maintain the chip leveled and in contact with the
surface while x/y scanning is performed for write/read operations. The system is thus
leveled in a manner similar to an antivibration air table. This basic concept of the entire
chip approach/leveling has been tested and demonstrated for the first time by parallel
imaging with a 5 × 5 array chip . These parallel imaging results have shown that all 25
cantilever tips have approached the substrate within less than 1 m of z-activation. This
promising result has led us to believe that chips with a tip-apex height control of less than
500 nm are feasible. This stringent requirement for tip-apex uniformity over the entire
chip is a consequence of the uniform force needed to minimize or eliminate tip and
medium wear due to large force variations resulting from large tip-height
nonuniformities.
Fig.1.5 Millipede Conceptual Model
During the storage operation, the chip is raster-scanned over an area called
the storage field by a magnetic x/y scanner. The scanning distance is equivalent to the
cantilever x/y pitch, which is currently 92 m. Each cantilever/tip of the array writes and
reads data only in its own storage field. This eliminates the need for lateral positioning
adjustments of the tip to offset lateral position tolerances in tip fabrication. Consequently,
a 32 × 32 array chip will generate 32 × 32 (1024) storage fields on an area of less than 3
mm × 3 mm. Assuming an areal density of 500 Gb/in.one storage field of 92 m × 92
m has a capacity of about 10 Mb, and the entire 32 × 32 array with 1024 storage fields
has a capacity of about 10 Gb on 3 mm × 3 mm. As shown in Section 7, the storage
capacity scales with the number of elements in the array, cantilever pitch (storage-field
size) and areal density, and depends on the application requirements. Although not yet
investigated in detail, lateral tracking will also be performed for the entire chip, with
integrated tracking sensors at the chip periphery.
This assumes and requires very good temperature control of the array chip
and the medium substrate between write and read cycles. For this reason the array chip
and medium substrate should be held within about 1°C operating temperature for bit sizes
of 30 to 40 nm and array chip sizes of a few millimeters. This will be achieved by using
the same material (silicon) for both the array chip and the medium substrate in
conjunction with four integrated heat sensors that control four heaters on the chip to
maintain a constant array-chip temperature during operation. True parallel operation of
large 2D arrays results in very large chip sizes because of the space required for the
individual write/read wiring to each cantilever and the many I/O pads. The row and
column time-multiplexing addressing scheme implemented successfully in every DRAM
is a very elegant solution to this issue. In the case of Millipede, the time-multiplexed
addressing scheme is used to address the array row by row with full parallel write/read
operation within one row.
2. THERMOMECHANICAL AFM DATA STORAGE
In recent years, AFM thermomechanical recording in polymer storage media has undergone extensive modifications, primarily with respect to the integration of sensors and heaters designed to enhance simplicity and to increase data rate and storage density. Using cantilevers with heaters, mermomechanical recording at 30 Gb/in.2 storage density and data rates of a few Mb/s for reading and 100 Kb/s for writing have been demonstrated. Thermomechanical writing is a combination of applying a local force by the cantilever/tip to the polymer layer and softening it by local heating. Initially, the heat transfer from the tip to the polymer through the small contact area is very poor, improving as the contact area increases. This means that the tip must be heated to a relatively high temperature (about 400°C) to initiate the melting process. Once melting has commenced, the tip is pressed into the polymer, which increases the heat transfer to the polymer, increases the volume of melted polymer, and hence increases the bit size. Our rough estimates indicate that at the beginning of the writing process only about 0.2% of the heating power is used in the very small contact zone (1040 ran2) to melt the polymer locally, whereas about 80% is lost through the cantilever legs to the chip body and about 20% is radiated from the heater platform through the air gap to the medium/substrate. After melting has started and the contact area has increased, the heating power available for generating the indentations increases by at least ten times to become 2% or more of the total heating power. With this highly nonlinear heat-transfer mechanism, it is very difficult to achieve small tip penetration and thus small bit sizes, as well as to control and reproduce the thermomechanical writing process.
This situation can be improved if the thermal conductivity of the substrate is increased, and if the depth of tip penetration is limited. We have explored the use of very thin polymer layers deposited on Si substrates to improve these characteristics. The hard Si substrate prevents the tip from penetrating farther than the film thickness allows, and it enables more rapid transport of heat away from the heated region because Si is a much better conductor of heat than the polymer. We have coated Si substrates with a 40-nm film of
polymethylmethacrylate (PMMA) and achieved bit sizes ranging between 10 and 50 nm. However, we noticed increased tip wear, probably caused by the contact between Si tip and Si substrate during writing. We therefore introduced a 70-nm layer of cross-linked photoresist (SU-8) between the Si substrate and the PMMA film to act as a softer penetration stop that avoids tip wear but remains thermally stable. Using this layered storage medium, data bits 40 nm in diameter have been written, as shown in. These results were obtained using a 1-urn-thick, 70-um-long, two-legged Si cantilever. The cantilever legs are made highly conducting by high-dose ion implantation, whereas the heater region remains low-doped. Electrical pulses 2 us in duration were applied to the cantilever with a period of 50 ps.
Imaging and reading are done using a new thermomechanical-sensing concept. The heater cantilever originally used only for writing was given the additional function of a thermal readback sensor by exploiting its temperature-dependent resistance. The resistance (R) increases nonlinearly with heating power/temperature from room temperature to a peak value of 500700°C. The peak temperature is determined by the doping concentration of the heater platform, which ranges from 1 x 1017 to 2 x 1018. Above the peak temperature, the resistance drops as the number of intrinsic carriers increases because of thermal excitation. For sensing, the resistor is operated at about 350°C, a temperature that is not high enough to soften the polymer, as is necessary for writing. The principle of thermal sensing is based on the fact that the thermal conductance between the heater platform and the storage substrate changes according to the distance between them. The medium between a cantilever and the storage substrate—in our case air—transports heat from one side to the other. When the distance between heater and sample is reduced as the tip moves into a bit indentation, the heat transport through air will be more efficient, and the heater’s temperature and hence its resistance will decrease. Thus, changes in temperature of the continuously heated resistor are monitored while the cantilever is scanned over data bits, providing a means of detecting the bits. Under typical operating conditions, the sensitivity of thermomechanical sensing is even better than that of piezoresistive-strain sensing which is not surprising because thermal effects in semiconductors are stronger than strain effects.
In addition to ultra dense thermomechanical write/read, we have also demonshated for the first time the erasing and rewriting capabilities of polymer storage media. Thermal reflow of storage fields is achieved by heating the medium to about 150°C for a few seconds. The smoothness of the reflowed medium allowed multiple rewriting of the same storage field. This erasing process does not allow bit-level erasing; it will erase larger storage areas. However, in most applications single-bit erasing is not required anyway, because files or records are usually erased as a whole. The erasing and multiple rewriting processes, as well as bit-stability investigations, are topics of ongoing research.
3. DATA STORAGE
Each probe in the cantilever array stores and reads data thermo-
mechanically, handling one bit at a time. In recent years, AFM thermo mechanical
recording in polymer storage media has undergone extensive modifications, primarily
with respect to the integration of sensors and heaters designed to enhance simplicity and
to increase data rate and storage density. Using cantilevers with heaters, thermo
mechanical recording at 30 Gb/in.storage density and data rates of a few Mb/s for
reading and 100 Kb/s for writing have been demonstrated.
3.1 ATOMIC FORCE MICROSCOPE PROBES
The AFM consists of a microscale cantilever with a sharp tip (probe) at its
end that is used to scan the specimen surface. The cantilever is typically silicon or silicon
nitride with a tip radius of curvature on the order of nanometers. When the tip is brought
into proximity of a sample surface, forces between the tip and the sample lead to a
deflection of the cantilever according to Hooke’s law. Depending on the situation, forces
that are measured in AFM include mechanical contact force, Van der Waals forces,
capillary forces, chemical bonding, electrostatic forces, magnetic forces (see Magnetic
force microscope (MFM)), Casimir forces, solvation forces etc. As well as force,
additional quantities may simultaneously be measured through the use of specialized
types of probe (see Scanning thermal microscopy, photothermal microspectroscopy, etc.).
Figure 3.1:Microscopic probes
Typically, the deflection is measured using a laser spot reflected from the top of the
cantilever into an array of photodiodes. Other methods that are used include optical
interferometry, capacitive sensing or piezoresistive AFM cantilevers. These cantilevers
are fabricated with piezoresistive elements that act as a strain gauge. Using a Wheatstone
bridge, strain in the AFM cantilever due to deflection can be measured, but this method is
not as sensitive as laser deflection or interferometry.
3.2 READING DATA
To accomplish a read, the probe tip is heated to around 300 °C and moved
in proximity to the data sled. If the probe is located over a pit the cantilever will push it
into the hole, increasing the surface area in contact with the sled, and in turn increasing
the cooling as heat leaks into the sled from the probe. In the case where there is no pit at
that location, only the very tip of the probe remains in contact with the sled, and the heat
leaks away more slowly. The electrical resistance of the probe is a function of its
temperature, rising with increasing temperature. Thus when the probe drops into a pit and
cools, this registers as a drop in resistance. A low resistance will be translated to a “1″ bit,
or a “0″ bit otherwise. While reading an entire storage field, the tip is dragged over the
entire surface and the resistance changes are constantly monitored.
Figure 3.2:Mechanism of Reading Data
.
Imaging and reading are done using a new thermo mechanical sensing
concept. The heater cantilever originally used only for writing was given the additional
function of a thermal readback sensor by exploiting its temperature-dependent resistance.
The resistance ® increases nonlinearly with heating power/temperature from room
temperature to a peak value of 500-700°C. The peak temperature is determined by the
doping concentration of the heater platform, which ranges from 1 × 10to 2 × 10.
Above the peak temperature, the resistance drops as the number of intrinsic carriers
increases because of thermal excitation
For sensing, the resistor is operated at about 300°C, a temperature that is
not high enough to soften the polymer, as is necessary for writing. The principle of
thermal sensing is based on the fact that the thermal conductance between the heater
platform and the storage substrate changes according to the distance between them. The
medium between a cantilever and the storage substrate—in our case air—transports heat
from one side to the other. When the distance between heater and sample is reduced as
the tip moves into a bit indentation, the heat transport through air will be more efficient,
and the heater’s temperature and hence its resistance will decrease. Thus, changes in
temperature of the continuously heated resistor are monitored while the cantilever is
scanned over data bits, providing a means of detecting the bits. Under typical operating
conditions, the sensitivity of thermo mechanical sensing is even better than that of
piezoresistive-strain sensing, which is not surprising because thermal effects in
semiconductors are stronger than strain effects.
3.3 WRITING DATA
To write a bit, the tip of the probe is heated to a temperature above the
glass transition temperature of the polymer used to manufacture the data sled, which is
generally acrylic glass. In this case the transition temperature is around 400 °C. To write
a “1″, the polymer in proximity to the tip is softened, and then the tip is gently touched to
it, causing a dent. To erase the bit and return it to the zero state, the tip is instead pulled
up from the surface, allowing surface tension to pull the surface flat again. Older
experimental systems used a variety of erasure techniques that were generally more time
consuming and less successful. These older systems offered around 100,000 erases, but
the available references do not contain enough information to say if this has been
improved with the newer technique.
Thermomechanical writing is a combination of applying a local force by
the cantilever/tip to the polymer layer and softening it by local heating. Initially, the heat
transfer from the tip to the polymer through the small contact area is very poor,
improving as the contact area increases. This means that the tip must be heated to a
relatively high temperature (about 400°C) to initiate the melting process.
Figure 3.3:Mechanism Of Writing Data
Once melting has commenced, the tip is pressed into the polymer, which
increases the heat transfer to the polymer, increases the volume of melted polymer, and
hence increases the bit size. Our rough estimates indicate that at the beginning of the
writing process only about 0.2% of the heating power is used in the very small contact
zone (1040 nm) to melt the polymer locally, whereas about 80% is lost through the
cantilever legs to the chip body and about 20% is radiated from the heater platform
through the air gap to the medium/substrate. After melting has started and the contact
area has increased, the heating power available for generating the indentations increases
by at least ten times to become 2% or more of the total heating power. With this highly
nonlinear heat-transfer mechanism, it is very difficult to achieve small tip penetration and
thus small bit sizes, as well as to control and reproduce the thermo mechanical writing
process.
This situation can be improved if the thermal conductivity of the substrate
is increased, and if the depth of tip penetration is limited. We have explored the use of
very thin polymer layers deposited on Si substrates to improve these characteristics.
a. Early storage medium consisting of a bulk PMMA.
b. New storage medium for small bit sizes consisting of thin PMMA layer
on top of a Si substrate separated by a cross-linked film of photoresist.
The hard Si substrate prevents the tip from penetrating farther than the
film thickness allows, and it enables more rapid transport of heat away from the heated
region because Si is a much better conductor of heat than the polymer. We have coated Si
substrates with a 40-nm film of polymethylmethacrylate (PMMA) and achieved bit sizes
ranging between 10 and 50 nm. However, we noticed increased tip wear, probably caused
by the contact between Si tip and Si substrate during writing. We therefore introduced a
70-nm layer of cross-linked photoresist (SU-8) between the Si substrate and the PMMA
film to act as a softer penetration stop that avoids tip wear but remains thermally stable.
3.4 ARRAY DESIGN, TECHNOLOGY AND FABRICATION
As a first step, a 5 x 5 array chip was designed and fabricated to test the basic Millipede concept. All 25 cantilevers had integrated tip heating for thermomechanical writing and piezoresistive deflection sensing for read-back. No time-multiplexing addressing scheme was used for this test vehicle; rather, each cantilever was individually addressable for both thermomechanical writing and piezoresistive deflection sensing. A complete resistive bridge for integrated detection has also been incorporated for each cantilever.
The chip has been used to demonstrate x/y/z scanning and approaching of the entire array, as well as parallel operation for imaging. This was the first parallel imaging by 2D AFM array chip with integrated piezoresistive deflection sensing. The imaging results also confirmed the global chip-approaching and -leveling scheme, since all 25 tips approached the medium within less than 1 pm of z-actuation. Unfortunately, the chip was not able to demonstrate parallel writing because of electromigration problems due to temperature and current density in the Al wiring of the heater. However, we learned from this 5×5 test vehicle that 1) global chip approaching and leveling is possible and promising, and 2) metal (Al) wiring on the cantilevers should be avoided to eliminate electromigration and cantilever deflection due to bimorph effects while heating.
Encouraged by the results of the 5 x 5 cantilever array, we designed and fabricated a 32 x 32 array chip. With the findings from the fabrication and operation of the 5 x 5 array and the very dense thermomechanical writing/reading in thin polymers with single cantilevers, we made some important changes in the chip functionality and fabrication processes.
The major differences are:
1) Surface rnicromachining to form cantilevers at the wafer surface
2) All-silicon cantilevers
3) Thermal instead of piezoresistive sensing
4) First- and second-level wiring with an insulating layer for a multiplexed row/column- addressing scheme.
Since the heater platform functions, as a write/read element and no individual cantilever actuation are required, the basic array cantilever cell becomes a simple two-terminal device addressed by multiplexed x/y wiring. The cell area and x/y cantilever pitch is 92-um x 92 um, which results in a total array size of less than 3 mm x 3 nun for the 1024 cantilevers. The cantilever is fabricated entirely of silicon for good thermal and mechanical stability. It consists of the heater platform with the tip on top, the legs acting as a soft mechanical spring and an electrical connection to the heater. They are highly doped to minimize interconnection resistance and replace the metal wiring on the cantilever to eliminate electromigration and parasitic z-actuation of the cantilever due to the bimorph effect. The resistive ratio between the heater and the silicon interconnection sections should be as high as possible; currently the highly doped interconnections are 400 and the heater platform is 11 k (at 4 V reading bias).
3.5 CANTILEVER PROPERTIES
Figure 3.5: Cantilever properties
The cantilever mass must be minimized to obtain soft (flexible), high-resonant-frequency cantilevers. Soft cantilevers are required for a low loading force in order to eliminate or reduce tip and medium wear, whereas a high resonant frequency allows high-speed scanning. In addition, sufficiently wide cantilever legs are required for a small thermal time constant, which is partly determined by cooling via the cantilever legs . These design considerations led to an array cantilever with 50-u.m-long, 10-u.m-wide, 0.5-um-thick legs, and a 5-um-wide, 10-um-long, 0.5-um-thick platform. Such a cantilever has a stiffness of 1 N/m and a resonant frequency of 200 kHz. The heater time constant is a few microseconds, which should allow a multiplexing rate of 100 kHz.
The tip height should be as small as possible because the heater platform sensitivity depends strongly on the distance between the platform and the medium. This contradicts the requirement of a large gap between the chip surface and the storage medium to ensure that only the tips, and not the chip surface, are making contact with the medium. Instead of making the tips longer, we purposely bent the cantilevers a few micrometers out of the chip plane by depositing a stress-controlled plasma-enhanced chemical vapor deposition (PECVD) silicon-nitride layer at the base of the cantilever . This bending as well as the tip height must be well controlled in order to maintain an equal loading force for all cantilevers of an array. Cantilevers are released from the crystalline Si substrate by surface micromachining using either plasma or wet chemical etching to form a cavity underneath the cantilever. Compared to a bulk-micromachined through-wafer cantilever-release process, as performed for our 5×5 array [10], the surface-micromachining technique allows an even higher array density and yields better mechanical chip stability and heat sinking. Because the Millipede tracks the entire array without individual lateral cantilever positioning, thermal expansion of the array chip must be either small or well controlled. Because of thermal chip expansion, the lateral tip position must be controlled with better precision than the bit size, which requires array dimensions as small as possible and a well-controlled chip temperature. For a 3 mm x 3 mm silicon array area and 10-nm tip-position accuracy, the chip temperature has to be controlled to about 1°C. This is ensured by four temperature sensors in the corners of the array and heater elements on each side of the array. Thermal expansion considerations were a strong argument for the 2D array arrangement instead of ID, which would have made the chip 32 times longer for the same number of cantilevers.
Integrating Schottky diodes in series with the cantilevers interconnects the cantilevers. The diode is operated in reverse bias (high resistance) if the cantilever is not addressed, thereby greatly reducing crosstalk between cantilevers.
3.6 ARRAY CHARACTERIZATION
The array’s independent cantilevers, which are located in the four corners
of the array and used for approaching and leveling of chip and storage medium, are used
to initially characterize the interconnected array cantilevers. Additional cantilever test
structures are distributed over the wafer; they are equivalent to but independent of the
array cantilevers. In the low-power, low-temperature regime, silicon mobility is affected
by phonon scattering, which depends on temperature, whereas at higher power the
intrinsic temperature of the semiconductor is reached, resulting in a resistivity drop due to
the increasing number of carriers.
The cantilevers within the array are electrically isolated from one another
by integrated Schottky diodes. The tip-apex height uniformity within an array is very
important because it determines the force of each cantilever while in contact with the
medium and hence influences write/read performance as well as medium and tip wear.
Wear investigations suggest that a tip-apex height uniformity across the chip of less than
500 nm is required, with the exact number depending on the spring constant of the
cantilever. In the case of the Millipede, the tip-apex height is determined by the tip height
and the cantilever bending.
4. FEATURES
1. Storage capacity – 1 terabit per square inch
2. Equal to 25 DVD
3. 25 billion texts in a stamp sized surface
4. Enable 10Gb of storage in cell phones
5. Uses atomic force probes
6. Data reads & writes in the storage field
7. Access time is small
8. Data rate is 1Gb/s
9. Needs less power about 100mw
4.1 AREAL DENSITY
| DRAM |
10 Gb/ Sq inch |
| Flash Drive |
25 Gb/ Sq inch |
| Hard Drive |
250 Gb/ Sq inch |
| Millipede |
1 Tb/ Sq inch |
Table 1.1
5. ADVANTAGES
Rather than using traditional magnetic or electronic means to storedata, Millipede uses thousands of nano-sharp tips to punch indentations representing individual bits into a thin plastic film. The result is akin to a nanotech version of the venerable data processing ‘punch card’ developed more than 110 years ago, but with two crucial differences: the ‘Millipede’ technology is re-writeable (meaning it can be used over and over again), and may be able to store more than 3 billion bits of data in the space occupied by just one hole in a standard punch card.
More than 100,000 writelover-write cycles have demonstrated the re-write capability of this concept. While current data rates of individual tips are limited to the kilobits-per-second range, which amounts to a few megabits for an entire array, faster electronics will allow the levers to be operated at considerably higher rates. Initial nanomechanical experiments done at IBM’s Almaden Research Center showed that individual tips could support data rates as high as 1 – 2 megabits per second.
Power consumption greatly depends on the data rate at which the device is operated. When operated at data rates of a few megabits per second, Millipede is expected to consume about 100 milliwatts, which is in the range of flash memory technology and considerably below magnetic recording. The 1,024-tip experiment achieved an areal density of 200 gigabits (billion bits, Gb) per square inch , which Computer Science & Engineering translates to a potential capacity of about 0.5 gigabytes (billion bytes, GB) in an area of 3 mm-square. he next-generation Millipede prototype will have four times more tips: 4,096 in a 7 mm-square (64 by 64) array.
Another more advantages are below:
• High storage capacity (1 Tb/in2).
• Very small form factor.
• Low power consumption (100 milliwatts).
• It is re-writeable.
• High data rate (high as 1 – 2 MB/s).
• Long-term perspectives.
6. APPLICATIONS
Millipede systems can be used for micro drives, which will feature very
small form factor, enabling use in small footprint devices like watches, mobile phones
and personal media systems, and at the same time provide high capacity. The very high
data density of Millipede systems makes them a very good candidate to be put to this use.
6.1 SMALL FORM FACTOR STORAGE SYSTEM (NANODRIVE)
IBM’s recent product announcement of the Microdrive represents a first
successful step into miniaturized storage systems. As we enter the age of pervasive
computing, we can assume that computer power is available virtually everywhere.
Miniaturized and low-power storage systems will become crucial, particularly for mobile
applications. The availability of storage devices with gigabyte capacity having a very
small form factor (in the range of centimeters or even millimeters) will open up new
possibilities to integrate such “Nanodrives” into watches, cellular telephones, laptops,
etc., provided such devices have low power consumption.
The array chip with integrated or hybrid electronics and the micro
magnetic scanner are key elements demonstrated for a Millipede -based device called
Nanodrive, which is of course also very interesting for audio and video consumer
applications. All-silicon, batch fabrication, low-cost polymer media, and low power
consumption make Millipede very attractive as a centimeter- or even millimeter-sized
gigabyte storage system
6.2 TERABIT DRIVE
The potential for very high areal density renders the Millipede also very
attractive for high-end terabit storage systems. As mentioned, terabit capacity can be
achieved with three Millipede-based approaches:
1) Very large arrays,
2) Many smaller arrays operating in parallel, and
3) Displacement of small/medium-sized arrays over large media.
Although the fabrication of considerably larger arrays (105 to 106cantilevers) appears to be possible, control of the thermal linear expansion will pose a
considerable challenge as the array chip becomes significantly larger. The second
approach is appealing because the storage system can be upgraded to fulfill application
requirements in a modular fashion by operating many smaller Millipede units in parallel.
The operation of the third approach was described above with the example of a modified
hard disk. This approach combines the advantage of smaller arrays with the displacement
of the entire array chip, as well as repositioning of the polymer-coated disk to a new
storage location on the disk. A storage capacity of several terabits appears to be
achievable on 2.5- and 3.5-in. disks. In addition, this approach is an interesting synergy
of existing, reliable (hard-disk drive) and new (Millipede) technologies.
6.3 HIGH CAPACITY HARD DRIVES
The Millipede system provides high data density, low seek times, low
power consumption and, probably, high reliability. These features make them candidates
for building high capacity hard drives, with storage capacity in the range of terabytes.
Although the data density of a Millipede is high, the capacity of an individual device is
expected to be relatively low — on the order of single gigabytes. Thus replacing hard a
drive probably requires economically collecting around 100 Millipede devices into a
single enclosure.
7. CURRENT STATE OF THE ART
The progress of Millipede storage to a commercially useful product has
been slower than expected. Huge advances in other competing storage systems, notably
Flash and hard drives, has made the existing demonstrators unattractive for commercial
production. Millipede appears to be in a race, attempting to mature quickly enough at a
given technology level that it has not been surpassed by newer generations of the existing
technologies by the time it is ready for production.
The earliest generation Millipede devices used probes 10 nanometers in
diameter and 70 nanometers in length, producing pits about 40 nm in diameter on fields
92 m x 92 m. Arranged in a 32 x 32 grid, the resulting 3 mm x 3 mm chip stores 500
megabits of data or 62.5 MB, resulting in an areal density, the number of bits per square
inch, on the order of 200 Gbit/in². IBM initially demonstrated this device in 2003,
planning to introduce it commercially in 2005. By that point hard drives were
approaching 150 Gbit/in², and have since surpassed it.
More recent devices demonstrated at CeBIT in 2008 have improved on the
basic design, using a 64 x 64 cantilever chips with a 7 mm x 7 mm data sled, boosting the
data storage capacity to 800 Gbit/in² using smaller pits. It appears the pit size can scale to
about 10 nm, resulting in a theoretical areal density just over 1Tbit/in². IBM now plans to
introduce devices based on this sort of density in 2007. For comparison, the very latest
perpendicular recording hard drives feature areal densities on the order of 230 Gbit/in²,
and appear to top out at about 1 Tbit/in². Semiconductor-based memories offer much
lower density, 10 Gbit/in² for DRAM and about 250 Mbit/in² for Flash RAM.
8. ONGOING DEVELOPMENTS
For the first time, it has fabricated and operated large 2D AFM arrays for
thermo mechanical data storage in thin polymer media. In doing so, it has demonstrated
key milestones of the Millipede storage concept. The 400 – 500-Gb/in.storage density
we have demonstrated with single cantilevers is among the highest reported so far. The
initial densities of 100 – 200 Gb/in.achieved with the 32 × 32 array are very
encouraging, with the potential of matching those of single cantilevers. Well-controlled
processing techniques have been developed to fabricate array chips with good yield and
uniformity.
This VLSINEMS chip has the potential to open up new perspectives in many
other applications of scanning probe techniques as well. Millipede is not limited to
storage applications or polymer media. The concept is very general if the required
functionality can be integrated on the cantilever/tip. This of course applies also to any
other storage medium, including magnetic ones, making Millipede a possible universal
parallel write/read head for future storage systems. Besides storage, other Millipede
applications can be envisioned for large-area, high-speed imaging and high-throughput
nanoscalelithography, as well as for atomic and molecular manipulation and
modifications.
The smoothness of the reflowed medium allowed multiple rewriting of the
same storage field. This erasing process does not allow bit-level erasing; it will erase
larger storage areas. However, in most applications single-bit erasing is not required
anyway, because files or records are usually erased as a whole. The erasing and multiple
rewriting processes, as well as bit-stability investigations, are topics of ongoing research.
The current Millipede array chip fabrication technique is compatible with
CMOS circuits, which will allow future microelectronics integration. This is expected to
produce better performance and smaller system form factors, as well as lower costs.
Although it has demonstrated the first high-density storage operations with
the largest 2D AFM array chip ever built, a number of issues must be addressed before
the Millipede can be considered for commercial applications; a few of these are listed
below:
• Overall system reliability, including bit stability, tip and medium wear,
erasing/rewriting.
• Limits of data rate (S/N ratio), areal density, array and cantilever size.
• CMOS integration.
• Optimization of write/read multiplexing scheme.
• Array-chip tracking.
The near-term future activities are focused on these important aspects.
The highly parallel nanomechanical approach is novel in many respects.
Recalling the transistor-to-microprocessor story mentioned at the beginning, we might
ask whether a new device of a yet inconceivable level of novelty could possibly emerge
from the Millipede. There is at least one feature of the Millipede that we have not yet
exploited. With integrated Schottky diodes and the temperature-sensitive resistors on the
current version of the Millipede array chip, we have already achieved the first and
simplest level of micromechanical/electronic integration, but we are looking for much
more complex ones to make sensing and actuation faster and more reliable. However, we
envision something very much beyond this. Whenever there is parallel operation of
functional units, there is the opportunity for sophisticated communication or logical
interconnections between these units. The topology of such a network carries its own
functionality and intelligence that goes beyond that of the individual devices. It could, for
example, act as a processor. For the Millipede this could mean that a processor and
VLSInanomechanical device may be merged to form a “smart” Millipede.
If the Millipede is used, for example, as an imaging device, let us say for
quality control in silicon chip fabrication, the amount of information it can generate is so
huge that it is difficult to transmit these data to a computer to store and process them.
Furthermore, most of the data are not of interest at all, so it would make sense if only the
pertinent parts were predigested by the specialized smart Millipede and then transmitted.
For this purpose, communication between the cantilevers is helpful because a certain
local pattern detected by a single tip can mean something in one context and something
else or even nothing in another context. The context might be derived from the patterns
observed by other tips. A similar philosophy could apply to the Millipede as a storage
device. A smart Millipede could possibly find useful pieces of information very quickly
by a built-in complex pattern recognition ability, e.g., by ignoring information when
certain bit patterns occur within the array. The bit patterns are recognized instantaneously
by logical interconnections of the cantilevers.
9. CONCLUSION
Millipede is a nano-storage prototype developed by IBM that can store
data at a density of a trillion bits per square inch: 20 times more than any currently
available magnetic storage medium. The prototype’s capacity would enable the storage of
25 DVDs or 25 million pages of text on a postage-stamp sized surface, and could enable
10 gigabytes (GB) of storage capacity on a cell phone.
Millipede uses thousands of tiny sharp points (hence the name) to punch
holes into a thin plastic film. Each of the 10-nanometer holes represents a single bit. The
pattern of indentations is a digitized version of the data. According to IBM, Millipede can
be thought of as a nanotechnology version of the punch card data processing technology
developed in the late 19th century. However, there are significant differences: Millipede
is rewritable, and it may eventually enable storage of over 1.5 GB of data in a space no
larger than a single hole in the punch card. Storage devices based on IBM’s technology
can be made with existing manufacturing techniques, so they will not be expensive to
make. According to Peter Vettiger, head of the Millipede project, “There is not a single
step in fabrication that needs to be invented.” Vettiger predicts that a nano-storage device
based on IBM’s technology could be available as early as 2009.
10. FUTURE SCOPE
In future, whenever there is parallel operation of functional units, there is the opportunity for sophisticated communication or logical interconnections between these units. The topology of such a network carries its own functionality and intelligence that goes beyond mat of the individual devices. It could, for example, act as a processor. For the Millipede this could mean that a processor and VLSI-nanomechanical device may be merged to form a “smart” Millipede. If the Millipede is used, for example, as an imaging device, let us say for quality control in silicon chip fabrication, the amount of information it can generate is so huge that it is difficult to transmit these data to a computer to store and process them.
Furthermore, most of the data are not of interest at alt so it would make sense if only the pertinent parts were predigested by the specialized smart Millipede and then transmitted. The bit patterns are recognized instantaneously by logical interconnections of the cantilevers. Even with this somewhat vague vision, we are very confident that the “smart” Millipede will have interesting long-term prospects in many application fields, possibly in fields that we cannot even envision today.
11. REFERENCES
1. http://www.research.ibm.com/journal/rd/4…tiger.html
2. http://en.wikipedia.org/wiki/IBM_Millipede
3. http://www.domino.research.ibm.com/comm/…ipede.html
4. http://www.news.zdnet.co.uk/hardware/0,1…254,00.htm
5. http://www.news.cnet.com/Photos-IBMs-Millipede-packs-
apunch/20091015_35615611.html
6. http://www.searchstorage.techtarget.com/…97,00.html
CONTENTS
v Introduction
v Thermo Mechanical AFM Data storage
v Data Storage
v Features
v Advantages
v Application
v Current state of the art
v Ongoing development
v Conclusion
v Future scope
v References
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