Cantilevers for AFM data storage
Cantilevers with Integrated Heaters and Integrated Piezoresistive Sensors for
AFM Thermomechanical Data Storage
Benjamin W. Chui, H. Jonathon Mamin*, Thomas W. Kenny
Bruce D. Terris*, Robert Ried*, Dan Rugar*
Timothy D. Stowe, Yongho S. Ju, Kenneth E. Goodson
Terman 551, Stanford University, California 94305-4021
*IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099
A Stanford-IBM collaborative project funded in part by an
IBM Cooperative Fellowship
AFM Thermo-mechanical Data Storage is a data storage scheme developed at IBM in which tiny pits on a plastic disk represent digital data. ("AFM" stands for "Atomic Force Microscope.") At present, bit densities of 45 gigabits per square inch are achievable. In this technology, a single-crystal silicon micromachined cantilever with a sharp tip can be used to write and read data 0's and 1's. For reading, we are fabricating such a cantilever with an integrated deflection sensor. For writing, we are incorporating a resistive heating element into the cantilever.
As can be seen from the diagram above, the tip of a cantilever is positioned over a spinning polycarbonate disk which has a glass transition point of about 120-140 degrees Celsius. When the tip is heated (as has been demonstrated with a laser pulse), it melts the polycarbonate upon contact, creating a tiny indentation. Heating can also be achieved with an integrated resistive heater, eliminating the need for an external laser. The tip of the cantilever is kept in contact with the spinning disk by means of a very small loading force applied to the base of the cantilever. At a sufficiently small loading force, no wear occurs on either the tip or the disk. The indentations on the polycarbonate disk can be used to represent encoded digital data.
Shown above is an SEM image of a integrated heater cantilever. It is in the form of a two-legged cantilever with a constriction at the end where the legs join. The cantilever is doped heavily with phosphorus to provide electrical conduction paths to the constriction, which is lightly doped itself. When a current flows through the cantilever, localized heating occurs at the constriction, allowing the tip to become hot enough to write on the polycarbonate substrate. Using this type of heater, data has been written at the rate of 100 kbit/s. Voltage pulses 0.2 microseconds in duration and 30 volts in amplitude were used.
Bit reading depends on the piezoresistivity of the silicon cantilever. Heavily doped silicon exhibits piezoresistivity in that its resistance changes slightly under stress. Therefore a heavily doped cantilever can be used to sense indentations on a disk: whenever the tip rides over an indentation, the cantilever flexes one way or the other. The stress in the cantilever varies accordingly, and so does the resistance. These slight changes in resistance can be converted to voltage signals, amplified and processed to regenerate digital data.
It is desirable to restrict the doped piezoresistive layer to less than one-half the thickness of the cantilever. This is because the tensile stress experienced by the cantilever on one surface cancels the compressive stress on the other. We have developed a special doping and annealing process for the cantilever that yields shallow, boron-doped piezoresistive layers less than half a micron deep.
This is a top-view photo of a piezoresistive cantilever. The cantilever is made of single-crystal silicon and is about 50 microns in length. It has an integrated piezoresistive deflection sensor.
This is an SEM close-up image of the cantilever tip (courtesy of B.D.Terris, IBM Almaden Research Center).
Active research is continuing in AFM thermo-mechanical data storage. Collaboration on this program between Stanford and IBM is a good example of a partnership between academia and industry. Under this arrangement, Stanford's sophisticated semiconductor fabrication facilities (such as those in the Stanford Nanofabrication Facility) can be used to make advanced components for exploratory studies at IBM.
It is possible that very high bit densities at a low cost per bit can be achieved with AFM thermo-mechanical data storage. Furthermore, the use of cantilever arrays (see photo below) could directly improve the speed of data reading and writing through parallel operation. Finally, the basic technology behind micromachined cantilevers with integrated tips can be applied to other high-density data storage schemes as well.
1. "Improved cantilevers for AFM thermomechanical data storage," Proceedings of Solid-state Actuators and Sensors, Hilton Head, South Carolina, June 2-6, 1996, pp. 219-224.
2. "Low-stiffness silicon cantilevers for thermal writing and piezoresistive readback with the atomic force microscope," Applied Physics Letters vol. 69, no. 18 (Oct. 1996) pp. 2767-9.