millipede- nano technology by mithun d'souza

8/14/2019 Millipede- Nano Technology by mithun d'souza

1/26

Millipede- Nano Technology

1. INTRODUCTION

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 Arial density has

increased by 6 orders of magnitude. But there is a physical limit. It has been predicted that super paramagnetic 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/cm .

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 Arial 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 nano meter 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 themicrosecond 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.

GRNICA, Shivamogga 2009 Page 1


2/26


1.1 MOTIVATION AND OBJECTIVES

In the 21 st century, the nanometer will very likely play a role similar to the one played by

the micrometer in the 20 th century. 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 allthree 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 super paramagnetic 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 and new alternatives are emerging in parallel, two things usually happen: First,

the existing and well established technology will be explored further and everything

possible done to push 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



3/26


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, thesilicon 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 Arial 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 1990s, 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. 2 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 serving .

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 storageapproaches 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 Nan mechanics Project at the IBM Zurich



4/26


Research Laboratory are to explore highly parallel AFM data storage with Arial storage

densities far beyond the expected uppers paramagnetic limit (60100 Gb/in. 2) and data

rates comparable to those of today's magnetic recording.

1.2 MILLIPEDE MEMORY

Millipede is a nonvolatile 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 lash 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 mega byte 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 Arial densities up to 500

GB/in. 2 and parallel operation of very large 2D (32 32) AFM cantilever arrays with integrated

tips and write/read storage functionality.

1.3 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



5/26


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.

1.4 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, therebydramatically 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



6/26


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 32x32 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.



7/26




8/26


9/26


uniformities.

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 Jm. Each cantilever/tip of the array writes and reads data only in its ownstorage 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 Arial density of

500 Gb/in. 2, one storage field of 92 Jm 92 Jm 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 Arial 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 1C 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. DATA STORAGE

Each probe in the cantilever array stores and reads data thermo to 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



10/26


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. 2

storage density and data rates of a few Mb/s for reading and 100 Kb/s for writing have been

demonstrated.

2.1 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, photo thermal micro spectroscopy, etc.). 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 interferometer, capacitive sensing

or piezoresistive AFM cantilevers. These cantilevers are fabricated with piezoresistiveelements 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

interferometer.

2.2 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



11/26


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.

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 temperature2dependent resistance. The resistance

( R) increases nonlinearly with heating power/temperature from room temperature to a peak

value of 5002700C. The peak temperature is determined by the doping concentration of

the heater platform, which ranges from 1 10 17 to 2 10 18. Above the peak

temperature, the resistance drops as the number of intrinsic carriers increases because of

thermal excitation.



12/26


For sensing, the resistor is operated at about 300C, 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 andthe storage substrate changes according to the distance between them. The medium between a

cantilever and the storage substratein our case airtransports 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 piezo resistive strain sensing, which is not surprising

because thermal effects in semiconductors are stronger than strain effects.



13/26


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.

Thermo mechanical writing is a combination of applying a local force by thecantilever/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

400C) 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 increasesthe 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 2) 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



14/26


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 photo resist.

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 402nm film of polymethylmethacrylate (PMMA) and achieved bit sizes ranging between



15/26


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 702nm layer of cross SS linked

photo resist (SU28) between the Si substrate and the PMMA film to act as a softer penetration

stop that avoids tip wear but remains thermally stable.

2.4 ARRAY DESIGN, TECHNOLOGY AND FABRICATION

As a first step, a 5 5 array chip was designed and fabricated to test the basic Millipede

concept. All 25 cantilevers had integrated tip heating for thermo mechanical writing and

piezoresistive deflection sensing for readback. No time to multiplexing addressing scheme

was used for this test vehicle; rather, each cantilever was individually addressable for both

thermo mechanical 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 a 2D AFM array chip with

integrated piezoresistive deflection sensing.

The imaging results also confirmed the global chip to approaching and leveling

scheme, since all 25 tips approached the medium within less than 1 Jm of z actuation.

Unfortunately, the chip was not able to demonstrate parallel writing because of electro migration

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 electro migration

and cantilever deflection due to bimorph effects while heating.

Since the heater platform functions as a write/read element and no individual cantilever

actuation is 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 Jm 92

Jm, which results in a total array size of less than 3 mm 3 mm for the 1024 cantilevers. The



16/26


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 electro migration 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 400N and the heater platform is 11 kN (at 4 V reading bias).

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 designconsiderations led to an array cantilever with 502Jm long, 102Jm to wide, 0.52Jm thick

legs, and a 52Jm2wide, 102Jm2long, 0.52Jm2thick 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 stress2controlled

plasma2enhanced chemical vapor deposition (PECVD) silicon2nitride 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.

2.5 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



17/26


over the wafer; they are equivalent to but independent of the array cantilevers. In the

low2power, low2temperature 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 resistively drop due to the increasing number of carriers.

The cantilevers within the array are electrically isolated from one another by integrated Scotty

diodes. The tip2apex 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 tip2apex 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 tip2apex height is determined by the tip height and the cantilever bending.

2.6 Millipede chip

Fig: Photograph of fabricated chip (14 X 7 mm 2). The 32 X 32 cantilever array is located at

the center, with bond pads distributed on either side.



18/26


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 theread/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 millipedes 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.



19/26


CHALLENGES TO COMMERCIAL PRODUCTION

The researchers admit that the technology is a number of years away from being ready for

practical applications. As a prototype, millipede has proven the design concept works; however,

several challenges remain that must be resolved prior to marketing the storage devicecommercially. The millipede design team listed concerns about overall system reliability, bit

stability, tip and medium wear, limits of data rate, CMOS integration, optimization of the multiplexing

scheme, array chip tracking and data rate versus power consumption tradeoffs. Other questions

are, whether millipede has overcome the write slow/read fast problem of flash technology and

how stable millipede data is at various temperatures. All are significant valid concerns that must

be addressed prior to full-scale production.

MANUFACTURABILITY

There is not a single step in fabrication that needs to be invented.

Millipede would be relatively inexpensive to manufacture because chips can be made using

IBMs existing manufacturing equipments and techniques.

HEAT DISSIPATION AND THERMAL STABILITY

Ultra dense computer architectures intensify some of the same issues facing conventional

computer hardware, such as heat dissipation. Heat dissipation is one of the several reasons that

the dense interconnects ( wires ) are approaching a limit. Quantum effects dominate in very smalltransistors. Unlike the nano-scale quantum-dot cellular automata, millipede does not require

cryogenic temperatures for operation. How closely bits are packed without interacting in a way

that degrades the data is unknown. There is a cost for pushing the density to its limit while

neglecting speed, reliability and ease of use. Results of testing to determine the stability of

millipedes polymer medium have not been released; however adjustments to the formulation of

the film could alter its chemical properties, if necessary. Minimum temperature of 400 oC is

required to cause the film to flow and thus minimize the likelihood of thermally damaging the

data.

3. FEATURES

1. Storage capacity 1 terabit per square inch

2. Equal to 25 DVD



20/26


21/26


of Millipede systems makes them a very good candidate to be put to this use.

4.1 SMALL FORM FACTOR STORAGE SYSTEM (NANODRIVE)

IBM's recent product announcement of the Micro drive 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 2based device called Nanodrive,

which is of course also very interesting for audio and video consumer applications. All to

silicon, batch fabrication, low2cost polymer media, and low power consumption make

Millipede very attractive as a centimeter or even millimeter sized gigabyte storage system.

4.2 TERABIT DRIVE

The potential for very high Arial 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/medium2sized arrays over large media.

Although the fabrication of considerably larger arrays (10 5 to 10 6cantilevers)

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



22/26


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 theentire array chip, as well as repositioning of the polymer2coated disk to a new storage

location on the disk. A storage capacity of several terabits appears to be achievable on 2.52

and 3.52in. Disks. In addition, this approach is an interesting synergy of existing, reliable

(hard2disk drive) and new (Millipede) technologies.

4.3 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 22 on the order of single gigabytes. Thus replacing hard a drive probably

requires economically collecting around 100 Millipede devices into a single enclosure.

5. 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



23/26


and 70 nanometers in length, producing pits about 40 nm in diameter on fields 92 Jm x 92 Jm.

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 Arial density, the number of bits per square inch, on the order of 200

Gbit/in. IBM initially demonstrated this device in 2003, planed to introduce it commercially in

2005. By that point hard drives were approaching 150 Gbit/in, 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/in using smaller pits. It appears the pit size can scale to about 10 nm,

resulting in a theoretical Arial density just over 1Tbit/in. IBM now plans to introduce devices

based on this sort of density in 2007. For comparison, the very latest perpendicular recording

hard drives feature Arial densities on the order of 230 Gbit/in, and appear to top out at

about 1 Tbit/in. Semiconductor based memories offer much lower density, 10 Gbit/in for

DRAM and about 250 Mbit/in for Flash RAM.

6. 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 2 5002Gb/in. 2 storage density we have

demonstrated with single cantilevers is among the highest reported so far. The initial densities

of 100 2 200 Gb/in. 2 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 probetechniques 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



24/26


area, high speed imaging and high throughput nano scale lithography, 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 storageareas. 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), Arial density, and 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 transistortomicroprocessor 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



25/26


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 VLSI nano mechanical 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.

7. CONCLUSION



26/26


Millipede is a nanostorage 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 10nanometer 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 Vet tiger, head of the

Millipede project, "There is not a single step in fabrication that needs to be invented." Vet tiger

predicts that a nanostorage device based on IBM's technology could be available as early as

2009.

millipede- nano technology by mithun d'souza

Documents