By:- Varun Khandelwal

Content Millipede memory Introduction. The Millipede concept. Cantilever Structure. Reading data. Writing data. Stored bits. Usage Scenarios. Current state of the art. Future challenges. Conclusion.

What is millipede ? Millipede is a non-volatile computer memory. IBM demonstrated a prototype of millipede at CeBIT Data is stored as pits burned into the surface of a thin polymer layer. Read and write is done by a MEMS-based probe. Seen as potential replacement for magnetic recording in hard drives.

How it is ? Millipede uses thousands of nano-sharp tips to punch indentations representing individual bits into a thin polymer film. The 'Millipede' technology is re-writeable. Can store more than 3 billion bits of data in the space occupied by just one hole in a standard punch card. Their layout looked like the legs of a millipede, and hence the name.

Need of Millipede ? Flash memory cannot surpass 1-2 gigabytes of capacity, whereas Millipede technology could pack gigabytes of data into the same tiny format. Power required will be same as that of flash memory. The Millipede can add tremendous data capacity to mobile devices such as PDA, cellular phones. Data storage density is 1 Tb/inch^2, equivalent to storing the content of 25 DVDs on the size of a postal stamp.

The Millipede concept The main memory of modern computers is constructed from number of DRAM-related devices. DRAM store data as the presence or absence of electrical charge. Hard drive store data on a metal disk. Data is represented as local magnetization of the material. Reading and writing is accomplished by a single “head”. The drive's performance is dependent on how fast the disk spins.

Millipede storage combine the best features of both. Millipede uses numerous probes,for reading and writing. Bits are stored as a pit, in the surface of a thermo-active polymer known as the “sled”. The sled is moved in a scanning pattern to bring the requested bits under the probe, a process known as x/y scan. The core components of probe storage system are (1) A two-dimensional array of silicon probes (cantilevers) (2) A micro-mechanical scanner which moves the storage medium relative to the array.

Animated View of Millipede An animated view of the Millipede storage device illustrates how an individual tip creates an indentation in a polymer surface (bottom) and how a large number of such tips are operated in parallel (top).

What is a Cantilever ? Millipede has cantilever array, on which a probe has to be mounted,which is made of silicon. For reading, writing and erasing functions, the cantilever tips are brought into contact with the storage medium. Most recent array design consists of an array of 64 × 64 cantilevers (4096). The storage medium is positioned relative to the cantilever array.

Reading Data To accomplish a read, the probe tip is heated to around 300°C and moved in proximity to the data sled. If the probe is located over a pit,the surface area increases in contact with the sled, and cools as heat leaks into the sled from the probe. The electrical resistance of the probe is proportional of its temperature. Thus when the probe drops into a pit and cools, this registers as a drop in resistance. A low resistance will be translated to a "1" bit, or a "0" bit otherwise.

Thermo-mechanical reading

Writing data To write a bit, the tip of the probe is heated to a temperature above the glass transition temperature of the polymer, which is generally acrylic glass. The transition temperature is around 400 K. To write a "1", the polymer in proximity to the tip is softened, and then the tip is gently touched to it, causing a dent. To erase the bit and return it to the zero state, the tip is instead pulled up from the surface, allowing surface tension to pull the surface flat again

Thermo-mechanical writing

Overwriting Data To over-write data, the tip makes a series of offset pits that overlap so closely that their edges fill in the old pits, effectively erasing the unwanted data. The write or overwrite cycles are limited to 1,00,000 cycles. Current data rates of individual tips is limited to kilobits/sec which is few Mbits/sec for entire array.

Stored bits Fig. shows that more than 80 percent of the 1,024 cantilevers of an experimental setup were able to write data (12 storage areas at right).

Usage Scenarios Micro Drives Millipede systems can be used for micro drives, like watches, mobile phones and personal media systems. The very high data density of millipede systems makes them a very good candidate to be put to this use. High-capacity hard drives The Millipede system provides  high data density,  low seek times,  low power consumption and,  high reliability. These features make them candidates for building high capacity hard drives.

Current state of the art The Earlier generation millipede devices used probes 10 nanometers in diameter and 70 nanometers in length, producing pits about 40 nm in diameter on fields 92 µm x 92 µm. This is arranged in a 32 x 32 grid, the resulting 3 mm x 3 mm chip storing 500 megabits of data or 62.5 MB, resulting in an areal density of 200 Gbit/in². But recent devices have used a 64 x 64 cantilever chips with a 7 mm x 7 mm data sled, the pit size is about 10 nm, resulting in a areal density just over 1Tbit/in².

Challenges The progress of millipede storage to a commercially useful product has been slower than expected. Huge advances in other competing storage systems, notably Flash and hard drives. More expensive per megabyte then current technology. It has not been surpassed by newer generations of the existing technologies but this can be a great challenge.

Conclusion Today there are many emerging markets for nanotechnology where high density nano storage devices are required. It is a nano version of punch card but rewriteable. Can be used in micro devices as well as in hard drive manufacturing. Millipedes read and write data parallely. High storage density of 1Tb/square inch.

References IBM Zurich Research Millipede project T-bit demonstration Millipede animation IBM Journal Res. Dev. paper

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