Seminar on Millipede

Ram Pothuraju



Millipede is a highly parallel scanning probe based data storage that has a real storage densities far beyond superparamagnetic limits and data rates comparable to today's magnetic recording. At the first glance, millipede looks like a conventional 14 X 7 mm 2 silicon chip. Mounted at the center of the chip is a miniature two-dimensional array of 1024 'v'-shaped cantilevered arms that are 70 µm long and 0.5 µm thick. A nano-sharp fang-like tip, only 20 nm in diameter, hangs from the apex of each cantilever. The multiplex drivers, allow addressing of each tip individually. Beneath the cantilever array, is a thin layer of polymer film deposited on a movable, three-axis silicon table. The 2-D AFM cantilever array storage technique called "millipede" is based on a mechanical parallel x/y scanning of either the entire cantilever array chip or the storage medium.
Description of Millipede

In addition, a feedback-controlled z-approaching and leveling scheme brings the entire cantilever array chip into contact with the storage medium. The tip-medium contact is maintained and controlled while x/y scanning is performed for read/write. The millipede approach is not based on individual z-feedback for each cantilever ; rather it uses a feedback control for the entire chip, which greatly simplifies the system. However this requires very good control and uniformity of tip height and cantilever bending.

Chip approach/leveling makes use of additionally integrated approaching cantilever sensors in the corners of the array chip to control the approach of the chip to the storage medium. Signals from these sensors provide feedback signals to adjust the z-actuators until contact with the medium is established. Feedback loops maintain the chip leveled and in contact with the surface while x/y scanning is performed for write/read operations.

Today data storage is dominated by the use of magnetic disks. Storage densities of about more than 5 Gb/cm2 have been achieved. In the past 40 years areal density has increased by 6 orders of magnitude. But there is a physical limit. It has been predicted that super paramagnetic effects- the bit size at which stored information become volatile as a function of time- will limit the densities of current longitudinal recording media to about 15.5 Gb/cm2.

In the near future century nanometer scale will presumably pervade the field of data storage. In magnetic storage used today, there is no clear-cut way to achieve the nanometer scale in all three dimensions. So new techniques like holographic memory and probe based data storage are emerging. If an emerging technology is to be considered as a serious candidate to replace an existing technology, it should offer long-term perspectives. Any new technology with better areal density than today's magnetic storage should have long-term potential for further scaling, desirably down to nanometer or even atomic scale.


The prototype built by IBM includes all the basic building blocks of the millipede concept. A 3 X 3 mm2 silicon substrate is spin-coated with the SU-8/ PMMA polymer medium. This storage medium is attached to the x/y/z microscanner and approaching device. The magnetic z-approaching actuators bring the medium into contact with the tips of the array chip. The z-distance between medium and the millipede chip is controlled by the approaching sensors in the corners of the array. The signals from these cantilevers are use to determine the forces on the z-actuators and, hence, also the forces of the cantilever while it is in contact with the medium. This sensing/actuation feedback loop continues to operate during x/y scanning of the medium. The PC-controlled write/read scheme addresses the 32 cantilevers of one row in parallel.

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