1000 Tips for Ultrahigh-Density Data Storage

Zurich, Switzerland, 11 Oct 1999—Will micro- and nanomechanic systems be able to compete with electronic and magnetic devices? A novel concept developed at IBM Research - Zurich promises high-density storage using mechanical components derived from atomic force microscopy (AFM): tiny indentations poked into a polymer layer by AFM tips represent stored bits that can be read out by the same tip. High data rates can be achieved by parallel operation of a large number of tiny tips in a small area. IBM scientists believe that it is possible to reach storage densities of up to 80 billion bits per square centimeter (80 Gbit/cm²), which is up to five times more than the expected ultimate limit for magnetic storage. A research prototype of the "Millipede", as the scientists nicknamed their novel device, demonstrates the feasibility of this new approach to ultrahigh-density storage.

"Mechanical" scanning probe techniques, specifically the scanning tunneling and the atomic force microscope invented at IBM Research - Zurich, have demonstrated the potential of mechanics in very small dimensions not only for imaging purposes, but also for modifications on the nanometer scale and even for precise positioning of individual atoms and molecules. The movement of tiny mechanical components consumes little energy and can be quite fast, and wear is less of a problem than with larger mechanical systems. Nanomechanical devices, however, became feasible only because they lend themselves to batch fabrication similar to microelectronic chip manufacturing, thus opening up the VLSI (very large scale integration) age of micro- and nanomechanics.

A group at IBM's Almaden Research Center in San Jose, California, pioneered micromechanical data storage based on AFM technology: a fine tip on the free end of a minute bar called a cantilever was operated over a spinning disk, an arrangement similar to that of a magnetic storage disk drive. The read-only system used disks replicated from a master, and had 100-nm-sized features sensed by an AFM tip as zeros and ones at densities of up to 10 Gbit/cm², a hundred times the data density of a CD-ROM. In an experimental write-once/ read-only scheme, an AFM tip heated by electrical pulses poked indentations representing the data bits into a thin polymer layer at densities of more than 5 Gbit/cm². Mechanical response times allowed readback data rates of up to 10 Megabit per second. To push the data rate of AFM-based storage into the range of today's magnetic recording and beyond, the researchers at IBM Research - Zurich used parallel operation of a large number of tips arranged in a two-dimensional array over a non-rotating storage medium. A first experimental device consisted of 25 tips arranged in a 5 x 5 grid on a 25 mm² silicon area. The next generation device, which is operating in the laboratory today, has 1024 tips in a 32 x 32 array in an area of 3 mm x 3 mm. Indentation sizes and spacing as small as 30-40 nanometers (nm) have been demonstrated with single tips, which leads to a storage density of 60 - 80 Gbit/cm².

Parallel operation of the more than 1000 tips is expected to make possible data rates of more than hundred Megabit/second. The team at IBM Research - Zurich also demonstrated that the storage medium can be erased by heating up the polymer and restoring it to its original state by a reflow process. This process does not allow bit-level erasing, which is, however, not required in most applications.

"We have demonstrated for the first time that it is possible to realize arrays of more than one thousand tips on a chip, and that devices like the Millipede may take thermomechanical data storage considerably beyond the density of magnetic storage technology," says Peter Vettiger who leads the Millipede research effort at IBM Research - Zurich. "However, our work is still in the early stages of development, and the use of a polymer as the storage media is only one of several possible solutions. If the required functionality can be integrated into cantilevers and tips, the Millipede concept may become a universal read/write device for future storage systems and even for other applications, such as large-area imaging and nanoscale lithography, as well as atomic and molecular manipulation." The availability of very small storage devices only a few centimeters or even millimeters across will open up new possibilities for integrating computer power into small pervasive devices such as video cameras, mobile phones or even watches.

Technical background

Very small cantilevers with tips at the free end are etched out entirely from silicon using a process adapted from technology currently used in microelectronic chip manufacturing and suited for low-cost batch fabrication. Each tip of the 32 x 32 array covers its own storage field of 92 µm by 92 µm, and has thus—assuming 80 Gbit/cm² areal density—a capacity of ca. 10 Mbit, which leads to ca. 10 Gbit for the 3 mm x 3 mm area of the entire tip array. Terabit capacity may eventually be achieved with larger arrays, arrays operating in parallel, and by displacing arrays over large media, something that in principle could be done by mounting the array on a modified read/write head of a hard disk system.

The tip array as a whole is scanned in x and y directions by magnetic actuation like that used in voice coil drives. Three additional actuators are used for the precise approach and leveling of the tips to ensure that they are brought into contact with the storage medium under light pressure in an exactly controlled way.

The thermomechanical method of writing is a combination of applying a local force by the tip to the polymer and softening the polymer by local heating, which is achieved by sending an electrical pulse through an area of high resistance underneath the tip region. A relatively high temperature of about 400 °C is required to initiate the local melting process of the 40 nm thin polymethylmethacrylate (PMMA) film. A 70-nm layer of photoresist (SU-8) between this polymer and the silicon substrate stops the tip softly and thus avoids tip wear.

For reading, the resistor on the cantilever is operated at ca. 350 °C. The amazingly simple method used to detect individual bit indentations is to sense the lower cantilever resistance caused by a decrease of the tip temperature. When the tip "drops" into an indentation, the closer proximity of the tip to the storage medium leads to increased heat dissipation, and thus to cooling of the heater.

Since the heater platform functions as a read/write element and no individual cantilever actuation is required, every cantilever cell is a simple two-terminal device addressed by a multiplexed x/y wiring like that commonly used for the control of DRAM devices.

Thermal reflow of storage fields for erasing data was achieved by heating the medium to about 150 °C for a few seconds, and the smoothness of the reflowed medium allowed multiple rewriting of the same storage field.