z-millipede data storage

8/3/2019 Z-Millipede Data Storage

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SREE NARAYANA GURUKULAMCOLLEGE OF ENGINEERINGKOLENCHERY

DEPARTMENT OF COMPUTER SCIENCEAND ENGINEERING

MILLIPEDEDATA STORAGESubni i t t~v l y:

ROOP.4 BABY


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SREE NARAYANA GURUKULAMCOLLEGE O F ENGINEERING

KOLENCHERY

DEPARTMENT OF COMPUTER SCIENCEAND ENGINEERING

CER TIFICATEThis is to certifi* that the Seminar Report entitled MILLIPEDE DATA

STORAGE was preseiltecl bj- ROOPA BABY (Reg. No. 56436) o f fir~nl 13eai-Computer Science, and Et lgirleering , Sree Narayana Gurukularn Co Iege ofEngineering, Kadayiruppu in pnrtialjllfilrnent o f the requirement for the crtt*trr-c/ofDegree in Compzrtei- Scicrlce and Engineering o f Mahatma Gandhi Utl rcr-sibsduring the Academic j3eui-2005-2006.Prof. Dr. Janahan La1Head of the Department

Place: Kadayiruppu

P. S. SmijeshStaff in Charge


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ACKNOWLEDGEMENT

First of all I am thankful to God almighty for the divine grace bestowed on me todo my seminar successfully in time.

I express my sincere gratitude to our respected principal K.Rajendran ,for

granting permission to present the seminar.I express my sincere thanks to Dr. Janahan Lal, Head of the Computer Science

& Engineering department for providing me the guidance and facilities for the seminar.I extend my deep sense of gratitude to our Seminar Co-coordinator ,Prof.

Smijesh P.S. ,Lecturer of Computer Science&Engg.Dept for their valuable guidance aswell as timely advice which helped us a lot in taking seminar successfully.

I also extend my sincere thanks to all other faculty members of Computerscience & Engineering department and my friends for their support and encouragement.


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ABSTRACT

Given the rapidly increasing data volumes that are downloaded ontomobile devices such as cell phones and PDAs, there is a growing demand for suitablestorage media with more and more capacity. At CeBIT, IBM for the first time showsthe prototype of the MEMS- Micro Electrical Mechanical System- assembly of ananomechanical storage system known internally as the "millipede" project. Usingrevolutionary nanotechnology, scientists at the IBM Zurich Research Laboratory,Switzerland, have made it to the millionths of a millimetre range, achieving datastorage densities of more than one terabit (1000 gigabit) per square inch, equivalent tostoring the content of 25 DVDs on an area the size of a postage stamp.

With this new technique, 3040-nm-sized bit indentations of similar pitch sizehave been made by a single cantileverltip in a thin (50-nm) polymethylmethacrylate(PMMA) layer, resulting in a data storage density of 400500 ~ b l i n . *he Millipedeproject could bring tremendous data capacity to mobile devices such as personaldigital assistants, cellular phones, and multifunctional watches, can be used to explorea variety of other applications, such as large-area microscopic imaging, nanoscalelithography or atomic and molecular manipulation.


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TABLE OF CONTENTS

..........................................................................Introduction ..I......................................................... Motivation and Objectives -3

....................................................................Millipede Concept 6..............................................1 Technological Background 6

..............................2 Thermomechanical AFM Data Storage 10...........................3 Array design, technology, and fabrication 12

....................................................4 Array Characterization 163.5 First writelread results with the 32x 32 array chip................ 8

........................................................................... Advantages -20..........................................................................Applications 22

...............................................Conclusion and Future Directions 25............................................................................ References -27


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Millipede 1

I.NTRODUCTION

When we think of data storage, it's common to imagine hard drive platters orsolid-state memory chips. But beyond magnetic fields or electrical charges, a surprisingamount of digital information is also stored in a physical form; punched paper tape andpunched cards are very early examples, but our very latest CD and DVD media representdata as a series of "pits and lands" delivered to a physical surface. When IBM launchedthe Millipede project in 1996, it heralded another data storage effort designed to recorddata through microscopic physical techniques, promising very high storage densities in a

In actual practice the Millipede's thousands of microscopic tips write tiny pits to athin film of special polymer. The sequence of pits corresponds to bits. Unlike punchedcards or tape, however, the data can be erased and rewritten.

The high storage density of more than a terabit was achieved by using individualsilicon tips to create pits approximately 10 nanometers in diameter, i.e. 50,000 timessmaller than the period at the end of this sentence. Experimental chips have beendesigned comprising more than 4,000 of these tips arrayed in a small 6.4 mm x 6.4 mm2.These dimensions make it possible to pack an entire high-capacity storage system intothe SD flash memory format package.

The project is still in an advanced research state. After a decision has beenmade, it will take another two to three years of development until the product would beavailable on the market. Moreover, the nanomechanical data medium has been optimizedto use a minimum amount of energy. Thus, it is ideally suited for use in mobile devicessuch as digital cameras, cell phones and USB sticks. However, it is likely that IBM's

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Millipede 2

these criteria, mobile storage (for example, for cell phones, USB sticks, and digitalcameras) is ideally suited for the Millipede probe storage technology. In this segmentMillipede is able to compete against flash, which is very costly at capacities between 5GBand 40GB. Millipede's inherent shock resistance and low power requirements also bolsterthese features.

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Millipede 3

2. MOTIVATION& OBJECTIVES

In the 21st century, the nanometer will very likely play a role similar to the oneplayed by the micrometer in the 20th century. The nanometer scale will presumablypervade the field of data storage. In magnetic storage today, there is no clear-cut way toachieve the nanometer scale in all three dimensions. Within a few years, however,magnetic storage technology will arrive at a stage of its exciting and successful evolutionat which fundamental changes are likely to occur when current storage technology hitsthe well-known super paramagnetic limit. Several ideas have been proposed on how toovercome this limit. One such proposal involves the use of patterned magnetic media, forwhich the ideal writelread concept must still be demonstrated, but the biggest challengeremains the patterning of the magnetic disk in a cost-effective way. Other proposals callfor totally different media and techniques such as local probes or holographic methods. Ingeneral, if an existing technology reaches its limits in the course of its evolution and newalternatives are emerging in parallel, two things usually happen: First, the existing andwell-established technology will-be explored further and everything possible done to pushits limits to take maximum advantage of the considerable investments made. Then, whenthe possibilities for improvements have been exhausted, the technology may still survivefor certain niche applications, but the emerging technology will take over, opening up newperspectives and new directions.

1 Today we are witnessing in many fields the transition from structures of themicrometer scale to those of the nanometer scale, a dimension at which nature has long1 been building the finest devices with a high degree of local functionality. Many of the

IIt techniques we use today are not suitable for the coming nanometer age. In any case, anA emerging technology being considered as a serious candidate to replace an existing but

limited technology must offer long-term perspectives. The consequence for storage is that

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Millipede 4

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 forcemicroscope (AFM) and scanning tunneling microscope (STM) for imaging and structuringdown to the atomic scale.

In the early 1990s, Mamin and Rugar at the IBM Almaden Research Centerpioneered the possibility of using an AFM tip for readback and writing of topographicfeatures for the purposes of data storage. In one scheme developed by them, reading andwriting were demonstrated with a single AFM tip in contact with a rotating polycarbonatesubstrate. The data were written thermomechanically via heating of the tip. In this way,densities of up to 30 ~ b / i n . ~ere achieved.

The objectives of the research activities within the Micro- and NanomechanicsProject at the IBM Zurich Research Laboratory are to explore highly parallel AFM datastorage with areal storage densities far beyond the expected super paramagnetic limit(60100 ~ b l i n . ~ )nd data rates comparable to those of today's magnetic recording.

The "Millipede" concept is a new approach for storing data at high speed and withan ultrahigh density. It is not a modification of an existing storage technology, althoughthe use of magnetic materials as storage media is not excluded. The ultimate locality isgiven by a tip, and high data rates are a result of massive parallel operation of such tips.The current effort is focused on demonstrating the Millipede concept with areal densitiesup to 500 Gblin.* and parallel operation of very large 2D (32 x 32) AFM cantilever arrayswith integrated tips and writelread storage functionality.

The AFM-based data storage concept, Millipede has a potentially ultrahighdensity, terabit capacity, small form factor, and high data rate. Its potential for ultrahighstorage density has been demonstrated by a new thermo mechanical local-probetechnique to store and read back data in very thin polymer films. With this new technique,

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Millipede 5cantileverltip in a thin (50-nm) polymethylmethacrylate (PMMA) layer, resulting in a datastorage density of 400500 ~ b l i n . ~igh data rates are achieved by parallel operation oflarge two-dimensional (2D) AFM arrays that have been batch-fabricated by siliconsurface-micromachining techniques. The very large scale integration (VLSI) ofmicrolnanomechanical devices (cantileversltips) on a single chip leads to the largest anddensest 2D array of 32 x 32 (1024) AFM cantilevers with integrated writelread storagefunctionality ever built. Time-multiplexed electronics control the writelread storage cyclesfor parallel operation of the Millipede array chip. Initial areal densities of 100200 ~ b l i n . ~have been achieved with the 32 x 32 array chip, which has potential for furtherimprovements. In addition to data storage in polymers or other media, and not excludingmagnetics, we envision areas in nanoscale science and technology such as lithography,high-speedllarge-scale imaging, molecular and atomic manipulation.



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Millipede 6

3.MILLIPEDE CONCEPT

3.1. Technological BackgroundThe 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 cantileverarray 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 storagemedium. This tipmedium contact is maintained and controlled while x/y scanning isperformed for writelread. It is important to note that the Millipede approach is not basedon individual z-feedback for each cantilever; rather, it uses a feedback control for theentire chip, which greatly simplifies the system. However, this requires stringent controland uniformity of tip height and cantilever bending. Chip approach and leveling make useof four integrated approaching cantilever sensors in the corners of the array chip tocontrol the approach of the chip to the storage medium. Signals from three sensorsprovide feedback signals to adjust three magnetic z-actuators until the three approachingsensors are in contact with the medium. The three sensors with the individual feedbackloop maintain the chip leveled and in contact with the surface while x/y scanning isperformed for writelread operations.

This basic concept of the entire chip approachlleveling has been tested anddemonstrated for the first time by parallel imaging with a 5 x 5 array chip. These parallelimaging results have shown that all 25 cantilever tips have approached the substratewithin less than 1 pm of z-activation. This promising result has led us to believe that chipswith a tip-apex height control of less than 500 nm are feasible. This stringent requirementfor tip-apex uniformity over the entire chip is a consequence of the uniform force needed



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Millipede

large tip-height nonuniformities.

Cant11evor rray on CFI'BS &lp

T.4---

Storage medtur? anL.lEhgScanner

The millipede concept

During the storage operation, the chip is raster-scanned over an area calledthe storage field by a magnetic x/y scanner. The scanning distance is equivalent to thecantilever x/y pitch, which is currently 92 pm. Each cantileverltip of the array writes andreads data only in its own storage field. This eliminates the need for lateral positioningadjustments of the tip to offset lateral position tolerances in tip fabrication.

Consequently, a 32 x 32 array chip will generate 32 x 32 (1024) storage fields onan area of less than 3 mm x 3 mm. Assuming an areal density of 500 ~ b l i n . ~ ,ne storagefield of 92 pm x 92 pm has a capacity of about 10Mb, and the entire 32 x 32 array with1024 storage fields has a capacity of about 10Gb on 3 mm x 3 mm. As the storagecapacity scales with the number of elements in the array, cantilever pitch (storage-fieldsize) and areal density, and depends on the application requirements. Lateral tracking will

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Millipede 8also be performed for the entire chip, with integrated tracking sensors at the chipperiphery. This assumes and requires very good temperature control of the array chip andthe medium substrate between write and read cycles. For this reason the array chip andmedium substrate should be held within about 1 C operating temperature for bit sizes of30 to 40 nm and array chip sizes of a few millimeters. This will be achieved by using thesame material (silicon) for both the array chip and the medium substrate in conjunctionwith four integrated heat sensors that control four heaters on the chip to maintain aconstant array-chip temperature during operation.

True parallel operation of large2D arrays results in very large chip sizes because ofthe space required for the individual writelread wiring to each cantilever and the many 110pads. The row and column time-multiplexing addressing scheme implementedsuccessfully in every DRAM is a very elegant solution to this issue. In the case ofMillipede, the time-multiplexed addressing scheme is used to address the array row byrow with full parallel writelread operation within one row.

Temperature plays a critical role in every part of the device's operation oncethe tips contact the polymer surface. Bits are written by heating the tip to a temperatureabove the glass transition temperature of the polymer by means of the heating resistorintegrated in the cantilever. The polymer in close proximity to the tip is heated andbecomes softer allowing the tip to indent a few nanometers into the film, mechanicallystressing the material. For reading the cantilever's reading sensor, which is separate fromthe tip, is heated slightly. As the polymer film is scanned under the tip, the tip moves inand out of the written indentations. When the tip moves into an indent, it cools downbecause of the reduced distance to the substrate. This cooling results in a measurablechange in electrical conductivity of the sensor. To ovetwrite data, thermo-mechanicaleffects are used. They cause the stressed polymer material closely around a newlycreated bit to relax.

The current Millipede storage approach is based on a new thermomechanicalwritehead process in nanometer-thick polymer films. Thermomechanical writing inpolycarbonate films and optical readback were first investigated and demonstrated with a



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Millipede 9

single cantilever by Mamin and Rugar. Although the storage density of 30 ~ b l i n . ~obtained original& was not overwhelming, the results encouraged to use polymer films aswell to achieve density improvements.



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Millipede 10

3.2. Thermomechanical AFM data storage

Thermomechanical writing is a combination of applying a local force by thecantileverltip to the polymer layer and softening it by local heating. Initially, the heattransfer from the tip to the polymer through the small contact area is very poor, improvingas the contact area increases. This means that the tip must be heated to a relatively hightemperature (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 increases the bit size.

It is estimated that at the beginning of the writing process only about 0.2% of theheating power is used in the very small contact zone (1040 nm2) to melt the polymerlocally, whereas about 80% is lost through the cantilever legs to the chip body and about20% 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 availablefor generating the indentations increases by at least ten times to become 2% or more ofthe total heating power. With this highly nonlinear heat-transfer mechanism, it is verydifficult to achieve small tip penetration and thus small bit sizes, as well as to control andreproduce the thermomechanical writing process.

This situation can be improved if the thermal conductivity of the substrate isincreased, and if the depth of tip penetration is limited. They have explored the use ofvery thin polymer layers deposited on Si substrates to improve these characteristics.

The hard Si substrate prevents the tip from penetrating farther than the filmthickness allows, and it enables more rapid transport of heat away from the heated regionbecause Si is a much better conductor of heat than the polymer. Si substrates are coatedwith a 40-nm film of polymethylmethacrylate (PMMA) to achieve bit sizes rangingbetween 10 and 50 nm. However there is increased tip wear, probably caused by thecontact between Si tip and Si substrate during writing. So introduced a 70-nm layer of

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Millipede 1 1

cross-linked photoresist (SU-8) between the Si substrate and the PMMA film to act as asofter penetration stop that avoids tip wear but remains thermally stable.

Using this layered storage medium, data bits 40 nm in diameter have been written.These results were obtained using a I-pm-thick, 70-pm-long, two-legged Si cantilever.The cantilever legs are made highly conducting by high-dose ion implantation, whereasthe heater region remains low-doped. Electrical pulses 2 ps in duration were applied tothe 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 athermal readback sensor by exploiting its temperature-dependent resistance. Theresistance (R) increases nonlinearly with heating powerltemperature from roomtemperature to a peak value of 500700C.For sensing, the resistor is operated at about350C, a temperature that is not high enough to soften the polymer, as is necessary forwriting.. When the distance between heater and sample is reduced as the tip moves intoa bit indentation, the heat transport through air will be more efficient, and the heater'stemperature and hence its resistance will decrease. Thus, changes in temperature of thecontinuously heated resistor are monitored while the cantilever is scanned over data bits,providing a means of detecting the bits.

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Millipede 12

3.3. Array design, technology, and fabricationAs 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 forthermomechanical writing and piezoresistive deflection sensing for read-back. No time-multiplexing addressing scheme was used for this test vehicle; rather, each cantileverwas individually addressable for both thermomechanical writing and piezoresistivedeflection sensing. A complete resistive bridge for integrated detection has also beenincorporated for each cantilever.

The array of tiny levers at the heart of the M~ l l ~ p edeystem

The chip has been used to demonstrate x/y/z scanning and approaching of the entirearray, as well as parallel operation for imaging. This was the first parallel imaging by a 2DAFM array chip with integrated piezoresistive deflection sensing. The imaging results alsoconfirmed the global chip-approaching and -leveling scheme, since all 25 tips approachedthe medium within less than 1 pm of z-actuation. Unfortunately, the chip was not able todemonstrate parallel writing because of electro migration problems due to temperatureand current density in the Al wiring of the heater. However, the results got from 5 x 5 testvehicle are I)lobal chip approaching and leveling is possible and promising, and 2)metal (Al) wiring on the cantilevers should be avoided to eliminate electromigration andcantilever deflection due to bimorph effects while heating.

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Millipede 13

designed and fabricated. With the findings from the fabrication and operation of the 5 x 5array and the very dense thermomechanical writinglreading in thin polymers with singlecantilevers, they made some important changes in the chip functionality and fabricationprocesses. The major differences are 1) surface micromachining to form cantilevers at thewafer surface, 2) all-silicon cantilevers, 3) thermal instead of piezoresistive sensing, and4) first- and second-level wiring with an insulating layer for a multiplexed row/column-addressing scheme.

Since the heater platform functions as a writelread element and no individualcantilever actuation is required, the basic array cantilever cell becomes a simple two-terminal device addressed by multiplexed x/ywiring. The cell area and x/y cantilever pitchis 92 pm x 92 pm, which results in a total array size of less than 3 mm x 3 mm for the1024 cantilevers. The cantilever is fabricated entirely of silicon for good thermal andmechanical stability. It consists of the heater platform with the tip on top, the legs actingas a soft mechanical spring, and an electrical connection to the heater. They are highlydoped to minimize interconnection resistance and replace the metal wiring on thecantilever to eliminate electromigration and parasitic z-actuation of the cantilever due tothe bimorph effect. The resistive ratio between the heater and the silicon interconnectionsections should be as high as possible; currently the highly doped interconnections are400 aand the heater platform is 11 krm. (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 toeliminate or reduce tip and medium wear, whereas a high resonant frequency allowshigh-speed scanning. In addition, sufficiently wide cantilever legs are required for a smallthermal time constant, which is partly determined by cooling via the cantilever legs. Thesedesign considerations led to an array cantilever with 50-pm-long, 10-pm-wide, 0.5-pm-thick legs, and a 5-pm-wide, 10-pm-long, 0.5-pm-thick platform. Such a cantilever has astiffness of 1 Nlm and a resonant frequency of 200 kHz. The heater time constant is a few

i microseconds, which should allow a multiplexing rate of 100 kHz.



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Millipede 14

sensitivity depends strongly on the distance between the platform and the medium. Thiscontradicts the requirement of a large gap between the chip surface and the storagemedium to ensure that only the tips, and not the chip surface, are making contact with themedium. Instead of making the tips longer, bent the cantilevers a few micrometers out ofthe chip plane by depositing a stress-controlled plasma-enhanced chemical vapordeposition (PECVD) silicon-nitride layer at the base of the cantilever.

Close-up of a lever's tiny tip

Cantilevers are released from the crystalline Si substrate by surfacemicromachining using either plasma or wet chemical etching to form a cavity underneaththe cantilever. Compared to a bulk-micromachined through-wafer cantilever-releaseprocess, as performed for 5 x 5 array, the surface-micromachining technique allows aneven higher array density and yields better mechanical chip stability and heat sinking.Because the Millipede tracks the entire array without individual lateral cantileverpositioning, 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 betterprecision than the bit size, which requires array dimensions as small as possible and awell-controlled chip temperature.



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Millipede 15

For a 3 mm x 3 mm silicon array area and 10-nm tip-position accuracy, the chiptemperature has to be controlled to about 1 C .This is ensured by four temperaturesensors in the comers of the array and heater elements on each side of the array.Thermal expansion considerations were a strong argument for the 2D array arrangementinstead of 1D, which would have made the chip 32 times longer for the same number of

The cantilevers are interconnected by integrating Schottky diodes in series with thecantilevers. The diode is operated in reverse bias (high resistance) if the cantilever is notaddressed, thereby greatly reducing crosstalk between cantilevers.



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Millipede 16

3.4. Array characterization

The array's independent cantilevers, which are located in the four corners of thearray and used for approaching and leveling of chip and storage medium, are used toinitially characterize the interconnected array cantilevers. Additional cantilever teststructures are distributed over the wafer; they are equivalent to but independent of thearray cantilevers. Figure shows an IN curve of such a cantilever; note the nonlinearity ofthe resistance. In the low-power part of the curve, the resistance increases as a functionof heating power, whereas in the high-power regime, it decreases.

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--."xi"--

In the low-power, low-temperature regime, silicon mobility is affected by phononscattering, which depends on temperature, whereas at higher power the intrinsic

increasing number of carriers. Depending on the heater-platform doping concentration of



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Millipede 171 x l o f 7 o 2 x l o f 8at./cm3, calculations estimate a resistance maximum at temperaturesof 500C and 700C, respectively.

The cantilevers within the array are electrically isolated from one another byintegrated Schottky diodes. Because every parasitic path in the array to the addressedcantilever of interest contains a reverse-biased diode, the crosstalk current is drasticallyreduced. Thus, the current response to an addressed cantilever in an array is nearlyindependent of the size of the array. Hence, the power applied to address a cantilever isnot shunted by other cantilevers, and the reading sensitivity is not degraded-not even forvery large arrays (32 x 32). The introduction of the electrical isolation using integratedSchottky diodes turned out to be crucial for the successful operation of interconnectedcantilever arrays with a simple time-multiplexed addressing scheme.

The tip-apex height uniformity within an array is very important because itdetermines the force of each cantilever while in contact with the medium and henceinfluences writelread performance as well as medium and tip wear. Wear investigationssuggest 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 ofthe Millipede, the tip-apex height is determined by the tip height and the cantilever



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- - - - --- --Millipede 18

3.5. First writelread results with the 32 x 32 array chip

IBM has explored two x/y/z scanning approaching schemes to operate thearray for writinglreading. The first one is based closely on the Millipede basic concept. A 3mm x 3 mm silicon substrate is spin-coated with the SU-81PMMA polymer mediumstructure. This storage medium is attached to a small magnetic x/y/z scanner andapproaching device. The three magnetic z-approaching actuators bring the medium intocontact with the tips of the array chip. The z-distance between the medium and theMillipede chip is controlled by the approaching sensors (additional cantilevers) in thecorners of the array. The signals from these cantilevers are used to determine the forceson the z-actuators and, hence, also the forces of the cantilever while it is in contact withthe medium. This sensing and actuation feedback loop continues to operate during x/yscanning of the medium. The PC-controlled writelread scheme addresses the 32cantilevers of one row in parallel. Writing is performed by connecting the addressed rowfor 20 ps to a high, negative voltage and simultaneously applying data inputs ("0" or "1")to the 32 column lines. The data input is a high, positive voltage for a "1" and ground for a"0." This row-enabling and column-addressing scheme supplies a heater current to allcantilevers, but only those cantilevers with high, positive voltage generate an indentation("1"). Those with ground are not hot enough to make an indentation, and thus write a "0."When the scan stage has moved to the next bit position, the process is repeated, and thisis continued until the line scan is finished. In the read process, the selected row line isconnected to a moderate negative voltage, and the column lines are grounded via aprotection resistor of about 10 kit, which keeps the cantilevers warm. During scanning,the voltages across the resistors are measured. If one of the cantilevers falls into a "1"indentation, it cools, thus changing the resistance and voltage across the series resistor.The written data bit is sensed in this manner.

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Millipede

The SEM and the AFM image show IBM's first parallel writingtreading results.SEM image of a large area of the polymer medium contain many small bright spots whichindicate the location of storage fields with data written by the corresponding cantilevers.The data written consisted of an IBM logo composed of indentations ("1"s) and clearseparations ("0"s). The dots are about 50 nm in diameter, which results in areal densitiesof 100200 ~ b t i n . ~epending on the ability to separate the bit indentations by adistinguishable amount. A first successful attempt demonstrates the read-back of thestored data by the integrated thermomechanical sensing

The second x/y/z scanning and approaching system explored makes use of amodified magnetic hard-disk drive. The array chip replaced the magnetic writetread headslider and was mechanically leveled and fixed on the suspension arm. The z-approachingand -contacting procedure was performed by a piezoelectric actuator mounted on top ofthe suspension, which brought the array chip into contact with the medium andmaintained it there. The 92-pm scanning in x was achieved by the standard voice-coilactuator of the suspension arm, whereas the y-scanning was performed by the slowlymoving disk. The row/column-addressing scheme is very similar to the one used for thex / y z scanner. The bright lines identify the locations of the storage fields written by manycantilevers with 92-pm pitches in each direction. This lower areal density is primarily dueto the thicker PMMA layer (100 nm) used for this experiment. Reading operation iscurrently being investigated.



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Millipede 20

4. ADVANTAGES

Rather than using traditional magnetic or electronic means to storedata, Millipede uses thousands of nano-sharp tips to punch indentations representingindividual bits into a thin plastic film. The result is akin to a nanotech version of thevenerable data processing punch card' developed more than 110 years ago, but with twocrucial differences: the 'Millipede' technology is re-writeable (meaning it can be used overand over again), and may be able to store more than 3 billion bits of data in the spaceoccupied by just one hole in a standard punch card.

Over-writing the data

More than 100,000 writelover-write cycles have demonstrated the re-writecapability of this concept. While current data rates of individual tips are limited to thekilobits-per-second range, which amounts to a few megabits for an entire array, fasterelectronics will allow the levers to be operated at considerably higher rates. Initialnanomechanical experiments done at IBM's Almaden Research Center showed thatindividual tips could support data rates as high as 1 - 2 megabits per second.

Power Consumption

Power consumption greatly depends on the data rate at which the device isoperated. When operated at data rates of a few megabits per second, Millipede isexpected to consume about 100milliwatts, which is in the range of flash memorytechnology and considerably below magnetic recording. The 1,024-tip experimentachieved an areal density of 200 gigabits (billion bits, Gb) per square inch , which

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Millipede 2 1translates to a potential capacity of about 0.5 gigabytes (billion bytes, GB) in an area of 3mm-square. he next-generation Millipede prototype will have four times more tips: 4,096in a 7 mm-square (64 by 64) array.

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Millipede 2 2

5. APPLICATIONS

Current 32 x 32 array chip is just one example of the many possible designs ofa data-storage system; the design and concept depend strongly on the intendedapplication. Out of the wide range of design and application scenarios, consider twocases of particular interest.

Small-form-factor storage system (Nanodrive)

IBM's recent product announcement of the Microdrive represents a firstsuccessful step into miniaturized storage systems. As we enter the age of pervasivecomputing, we can assume that computer power is available virtually everywhere.Miniaturized and low-power storage systems will become crucial, particularly for mobileapplications. The availability of storage devices with gigabyte capacity having a very smallform factor (in the range of centimeters or even millimeters) will open up new possibilitiesto integrate such "Nanodrives" into watches, cellular telephones, laptops, etc., providedsuch devices have low power consumption.

The array chip with integrated or hybrid electronics and the micromagneticscanner 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 makeMillipede very attractive as a centimeter- or even millimeter-sized gigabyte storage



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Terabit drive

The potential for very high areal density renders the Millipede also veryattractive for high-end terabit storage systems. As mentioned above, terabit capacity canbe achieved with three Millipede-based approaches: I ) very large arrays, 2) many smallerarrays operating in parallel, and 3) displacement of small/medium-sized arrays over large

Although the fabrication of considerably larger arrays ( l o5 o l o 6cantilevers)appears to be possible, control of the thermal linear expansion will pose a considerablechallenge as the array chip becomes significantly larger. The second approach isappealing because the storage system can be upgraded to fulfill application requirementsin a modular fashion by operating many smaller Millipede units in parallel. The operationof the third approach was described above with the example of a modified hard disk. Thisapproach combines the advantage of smaller arrays with the displacement of the entirearray chip, as well as repositioning of the polymer-coated disk to a new storage locationon the disk. A storage capacity of several terabits appears to be achievable on 2.5- and3.5-in. disks.

Big applications

Prototype versions of Millipede have gone through more than 100,000 write anderase cycles to prove the durability of the system. Complex electronics on two sides ofthe array make it possible to heat individual tips. Currently, data can be read and writtenfrom the device at a rate of a few kilobits per second, but the IBM researchers estimatethat with refinement the system could boost this to megabits per second. The plastic filmthat data is written to and from is moved around beneath the lever array so eachindividual tip addresses an area 100 micrometers square.

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Using this set-up, the IBM researchers managed to cram 500 megabits of data intoeach three-millimeter square. This is approximately 20 times denser than can beachieved with the best magnetic storage systems today. Peter Vettiger, Millipede projectleader, said the technology could mean mobile phones; watches and handheld computerscould carry around vast amounts of data.



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6. CONCLUSION AND FUTURE DIRECTIONS

The current Millipede array chip fabrication technique is compatible withCMOS circuits, which will allow future microelectronics integration. This is expected toproduce better performance (speed) and smaller system form factors, as well as lowercosts.

Although IBM have demonstrated the first high-density storage operations withthe largest 2D AFM array chip ever built, a number of issues must be addressed beforethe Millipede can be considered for commercial applications; a few of these are listedbelow:

Overall system reliability, including bit stability, tip and medium wear, erasinglrewriting.Limits of data rate (SIN ratio), areal density, array and cantilever size.CMOS integration.Optimization of writelread multiplexing scheme.Array-chip tracking.

IBM's near-term future activities are focused on these important aspects.

The thoughts and visions on the long-term outlook for the Millipede conceptare given below. There is at least one feature of the Millipede that we have not yetexploited. With integrated Schottky diodes and the temperature-sensitive resistors on thecurrent version of the Millipede array chip, IBM have already achieved the first andsimplest level of micromechanical/electronicintegration, but looking for much morecomplex ones to make sensing and actuation faster and more reliable. Whenever there isparallel operation of functional units, there is the opportunity for sophisticatedcommunication or logical interconnections between these units. The topology of such anetwork carries its own functionality and intelligence that goes beyond that of the



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mean that a processor and VLSlnanomechanical device may be merged to form a "smart"Millipede.. A smart Millipede could possibly find useful pieces of information very quicklyby a built-in complex pattern recognition ability, e.g., by ignoring information when certainbit patterns occur within the array. The bit patterns are recognized instantaneously bylogical interconnections of the cantilevers.

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z-millipede data storage

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