The millipede project

A nanomechanical AFM-based data storage system

Read/write/erase process Thermo-mechanical writing is performed by locally softening the polymer medium using the heated tip of a cantilever and simultaneously applying a force to create a nano-scale indentation in the polymer film. The tip is heated by applying a current pulse to a resistive heater integrated into the cantilever, directly behind the tip. During writing, the tip is heated to a temperature above the glass transition temperature of the polymer and forces on the order of a few hundred nanonewtons are applied to the tip for a few microseconds.		Images, click to enlarge Thermomechanical writing.
Read-back of the data is performed using a thermo-mechanical sensing technique in which a second resistive heater integrated into the cantilever is heated to a temperature of around 200 °C while the polymer medium is scanned under the tip. The principle of thermal sensing is based on the fact that the rate of cooling of this heater increases as the distance between it and the substrate carrying the polymer medium is reduced. Thus, when the tip moves into an indentation, the distance between the heater and substrate is reduced, resulting in an increase in cooling and hence a decrease in the temperature of the heater. This change in temperature results in an increase in resistance of the heater, which is easily detected by monitoring the current flowing through it.		Thermomechanical reading.
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 "meta-stable" 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.
Ongoing research on the write, read and erase operations concentrates on reducing the energy required to form an indentation, on the effects of repeated erasing and re-writing on tip and medium wear and on extending the lifetime of written indentations.