“MILLIPEDE” - STORAGE TECHNOLOGY
Sameer Tayal (sameer.tayal@gmail.com)

ABSTRACT
Through this paper we report a new atomic force microscope (AFM)-based data storage concept called the “Millipede” that has a potentially ultrahigh density, terabit capacity, small form factor, and high data rate. Its potential for ultrahigh storage density can be used as a technique to store and read back data in very thin polymer films. With this new technique, 3040-nm-sized bit indentations of similar pitch size have been made by a single cantilever/tip in a thin (50-nm) polymethylmethacrylate (PMMA) layer, resulting in a data storage density of 400-500 Gb/in.2 High data rates are achieved by parallel operation of large two-dimensional (2D) AFM arrays that have been batch-fabricated by silicon surface-micromachining techniques. The very large scale integration (VLSI) of micro devices (cantilevers/tips) on a single chip leads to the largest and densest 2D array of 32 x 32 (1024) AFM cantilevers with integrated write/read storage functionality ever built. Timemultiplexed electronics control the write/read storage cycles for parallel operation of the Millipede array chip. Initial areal densities of 100-200 Gb/in.2 have been achieved with the 32 × 32 array chip, which has potential for further improvements. In addition to data storage in polymers or other media, and not excluding magnetics, we envision areas in nanoscale science and technology such as lithography, high-speed/large-scale imaging, molecular and atomic manipulation, and many others in which Millipede may open up new perspectives and opportunities.
KEYWORDS: Atomic Force Microscope (AFM), Cantilevers, Thermo-mechanical, x/y/z scanning, biomorph effects, plasma-enhanced chemical vapor deposition (PECVD) silicon-nitride layer

1. INTRODUCTION:
In the 21st century, the nanometer will very likely play a role similar to the one played by the micrometer in the 20th century. 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 wellknown 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 costeffective 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. The earliest approach was in the form of a Scanning tunnel microscope (STM). The STM is based on the concept of quantum tunneling. When a conducting tip is brought very near to a metallic or semiconducting surface, a bias between the two can allow electrons to tunnel through the vacuum between them. The data is stored in the atoms and for the purpose of reading the recorded information, the tip of a scanning tunneling microscope is passed over the information layer, said scanning tunneling microscope being of sufficient voltage to excite the fluorescent dye material to an excited state. Upon return of the dye material to ground state, fluorescence results. By measuring the

fluorescence, or absence thereof, the recorded information can be deciphered.

The objectives of our research activities are to explore highly parallel AFM data storage with areal storage densities far beyond the expected super paramagnetic limit (60¬100 Gb/in.2) and data rates comparable to those of today's magnetic recording. 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 × 32) AFM cantilever arrays with integrated tips and write/read storage functionality. The fabrication and integration of such a large number of mechanical devices (cantilever beams) will lead to what we envision as the VLSI age of microand nano-mechanics.

1. THE MILLIPEDE CONCEPT: The 2D AFM cantilever array storage technique called “Millipede” is illustrated in Figure 2.1 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 anti-vibration air table. The stringent requirement for tip-apex uniformity over the entire chip is a consequence of the uniform force needed to minimize or eliminate tip and medium wear due to large force variations resulting from large tip-height nonuniformities.

During the storage operation, the chip is rasterscanned over an area called the storage field by a magnetic x/y scanner. 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. 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 can 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. Using the same material (silicon) for both the array chip and the medium substrate in conjunction with four integrated heat sensors should be done 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. 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. The current Millipede storage approach is based on a new thermo-mechanical write/read process in nanometer-thick polymer films. As previously noted, thermo-mechanical writing in polycarbonate films and optical readback were first investigated and demonstrated with a single cantilever.

2. Thermo-mechanical storage:

AFM

data

Fig2.1-The pictorial view of a Millipede

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. Thermomechanical writing is a combination of applying a local force by the cantilever/tip to the polymer layer and softening it by local heating. Initially, the heat transfer from the tip to the polymer through the small contact area is

very poor, improving as the contact area increases. This means that the tip must be heated to a relatively high temperature (about 400°C) to initiate the melting process. Once melting has commenced, the tip is pressed into the polymer, which increases the heat transfer to the polymer, increases the volume of melted polymer, and hence increases the bit size. 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 heattransfer 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, 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. Then the following step was to coat Si substrates with a 40-nm film of polymethylmethacrylate (PMMA) and achieved bit sizes ranging between 10 and 50 nm. However, what was noticed was an increased tip wear, probably caused by the contact between Si tip and Si substrate during writing. Therefore introducing 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 was done. This avoids tip wear but remains thermally stable.

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 temperaturedependent resistance. The resistance (R) increases nonlinearly with heating power/temperature from room temperature to a peak value of 500-700°C. Above the peak temperature, the resistance drops as the number of intrinsic carriers increases because of thermal excitation. The principle of thermal sensing is based on the fact that the thermal conductance between the heater platform and the storage substrate changes according to the distance between them. The medium between a cantilever and the storage substrate—in our case air—transports heat from one side to the other. When the distance between heater and sample is reduced as the tip moves into a bit indentation, the heat transport through air will be more efficient, and the heater's temperature and hence its resistance will decrease. Thus, changes in temperature of the continuously heated resistor are monitored while the cantilever is scanned over data bits, providing a means of detecting the bits. Figure 3.3 illustrates this concept. Under typical operating conditions, the sensitivity of thermomechanical 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.

Fig 3.3- Principal of AFM thermal sensing. The heater cantilever is continuously heated by a DC power supply while it is being scanned and the heater resistivity measured. Thermal reflow of storage fields is achieved by heating the medium to about 150°C for a few

Fig3.2- Prevention of merging

seconds. The smoothness of the reflowed medium allowed multiple rewriting of the same storage field. This erasing process does not allow bit-level erasing; it will erase larger storage areas. However, in most applications single-bit erasing is not required anyway, because files or records are usually erased as a whole. The erasing and multiple rewriting processes, as well as bit-stability investigations, are topics of ongoing research. 3. 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 piezo-resistive deflection sensing for readback. 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 piezo-resistive deflection sensing. The imaging results also confirmed the global chip-approaching and leveling scheme. 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. Encouraged by the results of the 5 × 5 cantilever array, designing and fabrication of a 32 × 32 array chip was carried out. With the findings from the fabrication and operation of the 5 × 5 array and the very dense thermomechanical writing/reading in thin polymers with single cantilevers, some important changes in the chip functionality and fabrication processes were done. The major differences were:

1) Surface micromachining cantilevers at the wafer surface, 2) All-silicon cantilevers,

to

form

3) Thermal instead of piezo-resistive sensing, and 4) First- and second-level wiring with an insulating layer for a multiplexed row/columnaddressing scheme. 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, as shown in figure 4.1. 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 zactuation 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.

Fig 4.1-Layout and cross-section of one cantilever cell. 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. The tip height should be as small as possible because the heater platform sensitivity depends strongly on the distance between the platform and the medium. This contradicts the requirement of a large gap between the chip surface and the storage medium to ensure that only the tips, and not the chip surface, are making contact with the medium. Instead of making the tips longer, we purposely bent the cantilevers a few micrometers out of the chip plane by depositing a stress-controlled plasmaenhanced chemical vapor deposition (PECVD) silicon-nitride layer at the base of the cantilever (see Figure 4.1). 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. Because the Millipede tracks the entire array without individual lateral cantilever positioning, thermal expansion of the array chip must be either small or well-controlled. Because of thermal chip expansion, the lateral tip position must be controlled with better precision than the bit size, which requires array dimensions as small as possible and a wellcontrolled chip temperature. This is ensured by four temperature sensors in the corners of the array and heater elements on each side of the array.

Fig 4.3- SEM images of the cantilever array section with approaching and thermal sensors in the corners, array and single cantilever details, and tip apex. The cantilevers are interconnected by integrating Schottky diodes in series with the cantilevers. The diode is operated in reverse bias (high resistance) if the cantilever is not addressed, thereby greatly reducing crosstalk between cantilevers.

4.

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. Figure 5.1 shows an I/V curve of such a cantilever; note the nonlinearity of the resistance. In the low-power part of the curve, the resistance increases as a function of heating power, whereas in the high-power regime, it decreases.

Fig 4.2-Photograph of fabricated chip (14mm x 7mm).The 32 x 32 cantilever array is located at the centre, with bond pads distributed on either side.

Fig 5.1- I/V curve of one cantilever. The curve is non-linear owing to the heating of the

platform as the power and temperature are increased. 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. Because every parasitic path in the array to the addressed cantilever of interest contains a reverse-biased diode, the crosstalk current is drastically reduced. Thus, the current response to an addressed cantilever in an array is nearly independent of the size of the array. Hence, the power applied to address a cantilever is not shunted by other cantilevers, and the reading sensitivity is not degraded— not even for very large arrays (32 × 32). The introduction of the electrical isolation using integrated Schottky diodes turned out to be crucial for the successful operation of interconnected cantilever arrays with a simple time-multiplexed addressing scheme. 6. First read/write and erase results with the 32x32 array chip: We have explored two x/y/z scanning approaching schemes to operate the array for writing/reading. The first one is based closely on the Millipede basic concept. A 3 mm × 3 mm silicon substrate is spin-coated with the SU-8/PMMA polymer medium structure. This storage medium is attached to a small magnetic x/y/z scanner and approaching device. The three magnetic z-approaching actuators bring the medium into contact with the tips of the array chip. The z-distance between the medium and the Millipede chip is controlled by the approaching sensors (additional cantilevers) in the corners of the array. The signals from these cantilevers are used to determine the forces on the z-actuators and, hence, also the forces of the cantilever while it is in contact with the medium. This sensing and actuation feedback loop continues to operate during x/y scanning of the medium. The PC-controlled write/read scheme addresses the 32 cantilevers of one row in parallel. Writing is performed by connecting

the addressed row for 20 µs 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 all cantilevers, 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 this is continued until the line scan is finished. In the read process, the selected row line is connected to a moderate negative voltage, and the column lines are grounded via a protection resistor of about 10 k , 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. In the thermal-mechanical writing process described above, indentations are created by elastically straining the locally softened polymer by applying a force. This stress is then frozen into the film by rapidly cooling the material, resulting in the creation of a "metastable" indentation. If the polymer is reheated, the polymer softens, thereby allowing the stored elastic strain to relax, and hence erasing the indentation. This can be done at the individual bit level by using the tip as a localized heat source. For example, writing a new indention very close to a previous one results in the erasure of the old indentation and the creation of a new one. Hence, a previously written data track can be erased simply by overwriting it with a series of closely spaced indentations. In other words, erasing is essentially similar to writing at a narrower pitch between indentations. The SEM and the AFM image in Figure 6.1 show our first parallel writing/reading results. Figure 6.1(a) shows a SEM image of a large area of the polymer medium, in which the many small bright spots indicate 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 clear separations

(“0”s). Figure 6.1(b) shows magnified images of two storage fields. Figure 6.1(c) shows the raw read-back data of two different storage fields with areal density similar to that in 6.1(b).

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. Figure 7.1(a) shows the basic principles of the integrated micro-magnetic scanner concept, whereas Figure 7.1(b) is a photograph of the first micro-machined silicon scanner.

Fig6.1- SEM images of x/y/z scanner millipede writing results. The second x/y/z scanning and approaching system we explored, makes use of a modified magnetic hard-disk drive. The array chip replaced the magnetic write/read head slider and was mechanically leveled and fixed on the suspension arm. The z-approaching and -contacting procedure was performed by a piezoelectric actuator mounted on top of the suspension, which brought the array chip into contact with the medium and maintained it there. Reading operation is currently being investigated. 7. MILLIPEDE STORAGE: APPLICATIONS IN DATA 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.  TERABIT DRIVE:

FIG 7.1- Integrated micro-magnetic and micromachined x/y/z scanner.

It is important to note that the same data capacity as the 32x32 array chip can be achieved, for example, using large arrays with small cantilever pitch/scan range or, conversely, using small arrays with a larger scan range. In addition, terabit data capacity can be achieved by one large array, by many identical small ones operating in parallel, or by displacing a small array on a large medium. Out of this wide range of design and application scenarios, we would like to explain two cases of particular interest.  SMALL FORM SYSTEM: FACTOR STORAGE

The potential for very high areal density renders the Millipede also very attractive for high-end terabit storage systems. As mentioned above, 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. 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

As we enter the age of pervasive computing, we can assume that computer power is available

application requirements in a modular fashion by operating many smaller Millipede units in parallel. 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. 8. FUTURE SCOPE OF IMPROVEMENT: (OUR CONTRIBUTION TO THE PAPER) Although for the first time, we have fabricated and operated large 2D AFM arrays for thermomechanical data storage in thin polymer media. Still there are some scope of improvement:  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. We are looking for much more complex level of micromechanical/electronic integration to make sensing and actuation faster and more reliable. 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. Building a smart Millipede could possibly find useful pieces of information very quickly by built-in complex pattern recognition ability, e.g., by ignoring information when certain bit patterns occur within the array. Overall system reliability, including bit stability, tip and medium wear, erasing/rewriting. Limits of data rate (S/N ratio), areal density, array and cantilever size. CMOS integration. Optimization of write/read multiplexing scheme. Array-chip tracking.

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10. REFRENCES:      Wilkepedia.com Zdnet.com Whatis.com PRdomain.com IBM press journals