Thermomechanical data storage (Download Full Report And Abstract)
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22-02-2009, 01:57 AM


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 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. Other proposals call for totally different media and techniques such as local probes or holographic methods. Similarly, consider Optical lithography. Although still the predominant technology, it will soon reach its fundamental limits and be replaced by a technology yet unknown. 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.

Today we are witnessing in many fields the transition from structures of the micrometer scale to those of the nanometer scale, a dimension at which nature has long been building the finest devices with a high degree of local functionality. Many of the technologies we use today are not suitable for the coming nanometer age; some will require minor or major modifications, and others will be partially or entirely replaced. It is certainly difficult to predict which techniques will fall into which category. For key areas in information-technology hardware it is not yet obvious which technology and materials will be used for nanoelectronics and data storage.

In any case, an emerging technology being considered as a serious candidate to replace an existing but limited technology must offer long-term perspectives. For instance, the silicon microelectronics and storage industries are huge and require correspondingly enormous investments, which makes them long-term oriented by nature, The consequence for storage is that any new technique with better areal storage density than todayâ„¢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 being 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 90's, Mamin and Rugar at the IBM Almaden Research Center pioneered the possibility of using an AFM tip for read back 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 writing was done thermomechanically via heating of the tip. In this way, storage densities of up to 30Gb/in2 were achieved, representing a significant advance compared to the densities of that day. Later refinements included increasing readback speeds up 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 has to deal with their fundamental limits for high data rates. 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 magnitudes to be competitive with current and future magnetic recording. Later, it was found that by operating the AFM tips in parallel, data storage with areal storage densities far beyond the expected superparamagnetic limit (~100 Gb/in2) and data rates comparable to those of today's magnetic recording can be achieved.

The "Millipede concept which will be discussed 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 medium 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. Using this Millipede concept areal densities up to 0.5-1 Tb/in2 can be achieved by the parallel operation of very large 2D (32 x 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 micro/ nanomechanics. It is our conviction that VLSI micro/nanomechanics will greatly complement future micro and nanoelectronics (integrated or hybrid) and may generate applications of VLSI-MEMS (VLSI-MicroElectroMechanical Systems) not conceived of today.


In recent years, AFM thermomechanical recording in polymer storage media has undergone extensive modifications mainly with respect to the integration of sensors and heaters designed to enhance simplicity and to increase data rate and storage density. Using these heater cantilevers, high storage density and data rates have been achieved. Let us now describe the storage operations in detail.


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 and improves as the contact area increases. This means the tip must be heated to a relatively high temperature (about 400oC) to initiate the softening. Once softening has commenced, the tip is pressed into the polymer, which increases the heat transfer to the polymer, increases the volume of softened 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 (10-40 nm2) to soften 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 softening 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.

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