Blast from the Past

As the saying goes, anyone who can remember the '60s probably wasn't there. I don't remember much from that era (which means I must have been there!),
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FIG. 1: IBM''s Millipede project uses tiny pointed tips to poke dimples in a plastic film, representing bits of data that can be read by the same tips.

As the saying goes, anyone who can remember the '60s probably wasn't there. I don't remember much from that era (which means I must have been there!), but I do recall one thing: computer punch cards.

Thankfully, those days are gone forever, but the concept lives on in a project from IBM's Zurich Research Laboratory (www.zurich.ibm.com), code-named “Millipede.” Rather than using traditional magnetic or electronic means to store data, Millipede takes advantage of nanotechnology to represent data mechanically. Tiny pointed tips are mounted at the ends of cantilevers, and individual bits of data are written by poking dimples in a thin sheet of plastic (see Fig. 1). The cantilevers are 70 microns (micrometers) long and 0.5 micron thick, while the tips are 1 micron long, and they punch dimples that are 15 nanometers (nm) in diameter.

This resembles the punch cards of yesteryear, but with two important differences: the Millipede technology is rewritable, and it offers the potential to store a maximum of 3 gigabits (Gb) of data in the same space occupied by a single hole in a standard punch card. Recent experiments used a small part of an array of 4,096 tips (64 × 64) to achieve a data density of 500 Gb per square inch, which translates to a capacity of about 3 GB in an area 6.4 mm square using the entire array. Another demonstration, using a single tip, attained a data density of 1.2 terabits per square inch, which is the equivalent of 25 DVDs on a surface the size of a postage stamp.

Interestingly, the storage medium — a polymer film about 100 nm thick that coats a silicon substrate — is moved in two dimensions beneath the fixed array of tips. Electromagnetic actuation provides extreme precision, enabling each tip to read and write within an area 100 microns square.

Bits are written by heating a resistor in the cantilever to about 400 degrees Celsius. The hot tip softens the polymer and briefly sinks into it, creating an indentation. To read a bit, the resistor is heated to a lower temperature, typically 300 degrees Celsius, which does not soften the polymer. When the tip drops into an indentation, the resistor is cooled by the resulting change in heat-transport characteristics, causing a measurable change in resistance.

To overwrite a bit, the tip makes a series of offset pits whose edges overlap so closely that the old pit “relaxes,” effectively erasing the unwanted data. The IBM researchers have demonstrated more than 100,000 write/overwrite cycles.

Currently, the data rate of individual tips is limited to tens of kilobits per second, though a large number of cantilevers working in parallel can achieve data rates of tens of megabits per second. Recent experiments performed at IBM's Almaden Research Center have shown that individual tips can support data rates as high as 1 to 2 megabits per second.

A Millipede device's power consumption depends on the data rate at which it is operated. At a few megabits per second, a device is expected to consume about 100 milliwatts, which is comparable to flash-memory technology and is considerably below magnetic recording. And while SD-format flash memory isn't expected to exceed 4 to 8 GB in the near term, Millipede could pack 15 to 20 GB into the same form factor without requiring more power.

As I've said many times, any advance in computer technology, be it greater storage capacity, higher data rates, or faster processor speeds, is a potential boon for electronic musicians, who depend on computers to realize their musical dreams. Fortunately, we no longer have to rely on paper punch cards, but the concept of storing data mechanically may see the light of day once again as nanotechnology offers new ways to circumvent the limitations of magnetic and electronic storage.