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Today data storage is dominated by the use of magnetic disks. Storage densities of about more than 5 Gb/cmhave been achieved. In the past 40 years areal density has increased by 6 orders of magnitude. But there is a physical limit. It has been predicted that superparamagnetic effects- the bit size at which stored information become volatile as a function of time- will limit the densities of current longitudinal recording media to about 15.5 Gb/cm2.In the near future century nanometer scale will presumably pervade the field of data storage. In magnetic storage used today, there is no clear-cut way to achieve the nanometer scale in all three dimensions. So new techniques like holographic memory and probe based data storage are emerging. If an emerging technology is to be considered as a serious candidate to replace an existing technology, it should offer long-term perspectives. Any new technology with better areal density than today's magnetic storage should have long-term potential for further scaling, desirably down to nanometer or even atomic scale.
The only available tool known today that is simple and yet offer these long-term perspectives is a nanometer-sharp tip like in atomic force microscope (AFM) and scanning tunneling microscope (STM). The simple tip is a very reliable tool that concentrates on one functionality: the ultimate local confinement of interaction. In local probe based data storage we have a cantilever that has a very small tip at its end. Small indentations are made in a polymer medium laid over a silicon substrate. These indentations serve as data storage locations. A single AFM operates best on the microsecond time scale. Conventional magnetic storage, however, operates at best on the nanosecond time scale, making it clear that AFM data rates have to be improved by at least three orders of magnitude to be competitive with current and future magnetic recording. The "millipede" concept is a new approach for storing data at high speed and with an ultrahigh density.
1.1 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 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 oflocal 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 requirecorrespondingly enormous investments, which makes them long-term-oriented by nature.The consequence for storage is that any new technique with better areal storage densityhan 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 1990â„¢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.2were 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.
1.2 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.2 and parallel operation of very large 2D (32 Ãƒâ€” 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 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, 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 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.
Figure 1.4: Architecture Of Millipede
The Millipede concept: for operation of the device, the storage medium â€œ 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 Ãƒâ€” 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 non 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 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 Ãƒâ€” 32 array chip will generate 32 Ãƒâ€” 32 (1024) storage fields on an area of less than 3 mm Ãƒâ€” 3 mm. Assuming an areal density of 500 Gb/in.2, one storage field of 92 m Ãƒâ€” 92 m has a capacity of about 10 Mb, and the entire 32 Ãƒâ€” 32 array with 1024 storage fields has a capacity of about 10 Gb on 3 mm Ãƒâ€” 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 1Ã‚Â°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. 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.
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, photothermal microspectroscopy, 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 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.
Figure 2.1:Microscopic probes
2.2 READING DATA
To accomplish a read, the probe tip is heated to around 300 Ã‚Â°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 ools, 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. 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 Ã‚Â® increases nonlinearly with heating power/temperature from room temperature to a peak value of 500-700Ã‚Â°C. The peak temperature is determined by the doping concentration of the heater platform, which ranges from 1Ãƒâ€”1017 to 2Ãƒâ€”1018.Above the peak temperature, the resistance drops as the number of intrinsic carriers increases because of thermal excitation
Figure 2.2:Mechanism of Reading Data
For sensing, the resistor is operated at about 300Ã‚Â°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 substrateâ€in our case airâ€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.
2.3 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 Ã‚Â°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 400Ã‚Â°C) to initiate the melting process.
Figure 2.3: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 (1040nm2) 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.
2.4 ARRAY DESIGN, TECHNOLOGY AND FABRICATION
As a first step, a 5 Ãƒâ€” 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 read-back. No time- 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-approaching and - leveling scheme, since all 25 tips approached the medium within less than 1 m 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 Ãƒâ€” 5 test vehicle that 1) global chip approaching and leveling is possible and promising, and 2) metal (Al) wiring on the cantilevers shouldPage 24IBM Millipede Division of Computer Science, SOE, CUSAT17 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 pitchis 92 m Ãƒâ€” 92 m, which results in a total array size of less than 3 mm Ãƒâ€” 3 mm 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 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 400 and the heater platform is 11 k (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 design considerations led to an array cantilever with 50- m-long, 10- m- wide, 0.5- m-thick legs, and a 5- m-wide, 10- m-long, 0.5- m-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 storagemedium 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
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 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 tha 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.
1. Storage capacity â€œ 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
3.1 AREAL DENSITY
DRAM10 Gb/ Sq inch
Flash Drive25 Gb/ Sq inch
Hard Drive250 Gb/ Sq inch
Millipede1 Tb/ Sq inch
Millipede systems can be used for micro drives, which will feature very
mall 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.
4.1 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 mall 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.
4.2 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 106 cantilevers) 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.
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 -- 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 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/inÃ‚Â². IBM initially demonstrated this device in 2003, planning to introduce it commercially in 2005. By that point hard drives were approaching 150 Gbit/inÃ‚Â², 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/inÃ‚Â² using smaller pits. It appears the pit size can scale to about 10 nm, resulting in a theoretical areal density just over 1Tbit/inÃ‚Â². 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/inÃ‚Â², and appear to top out at about 1 Tbit/inÃ‚Â². Semiconductor-based memories offer much lower density, 10 Gbit/inÃ‚Â² for DRAM and about 250 Mbit/inÃ‚Â² 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 - 500-Gb/in.2 storage density we have demonstrated with single cantilevers is among the highest reported so far. The initial densities of 100 â€œ 200 Gb/in.2 achieved with the 32 Ãƒâ€” 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 withCMOS 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:
Â¢ Overall system reliability, including bit stability, tip and medium wea erasing/rewriting.
Â¢ Limits of data rate (S/N ratio), areal density, array and cantilever size.
Â¢ CMOS integration.
Â¢ Optimization of write/read multiplexing scheme.
Â¢ 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.
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.