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IBM MILLIPEDE
ABSTRACT
Using innovative nanotechnology, IBM scientists have demonstrated a data storage density of a trillion bits per square inch -- 20 times higher than the densest magnetic storage available today. Rather than using traditional magnetic or electronic means to store data, Millipede uses thousands of nano-sharp tips to punch indentations representing individual bits into a thin plastic film. The result is akin to a nanotech version of the venerable data processing punch card developed more than 110 years ago, but with two crucial differences: the Millipede technology is re-writeable (meaning it can be used over and over again), and may be able to store more than 3 billion bits of data in the space occupied by just hole in a standard punch card. While flash memory is not expected to surpass 1-2 gigabytes of capacity in the near term, Millipede technology could pack 10 - 15 gigabytes of data into the same tiny format, without requiring more power for device operation.
Millipede" is a new (AFM)-based data storage concept that has a potentially ultrahigh density, terabit capacity, small form factor, and high data rate. Its potential for ultrahigh storage density has been demonstrated by a new thermo mechanical local-probe technique to store and read back data in very thin polymer films. With this new technique, 3040-nm-sized bit indentations of similar pitch size have been made by a single cantilever/tip in a thin (50-nm) polymethylmethacrylate (PMMA) layer, resulting in a data storage density of 400500 Gb/in.2
High data rates are achieved by parallel operation of large two-dimensional (2D) AFM arrays that have been batch-fabricated by silicon surface-nMcromachining techniques. The very large scale integration (VLSI) of micro/nanomechanical devices (cantilevers/tips) on a single chip leads to the largest and densest 2D array of 32 x 32 (1024) AFM cantilevers with integrated write/read storage functionality ever built. Initial areal densities of 100200 Gb/in.2 have been achieved with the 32 x 32 array chip, which has potential for further improvements.
In addition to data storage in polymers or other media, and not excluding magnetics, we envision areas in nanoscale science and technology such as lithography, high-speed/large-scale imaging, molecular and atomic manipulation, and many others in which Millipede may open up new perspectives and opportunities.
The Millipede project could bring tremendous data capacity to mobile devices such as personal digital assistants, cellular phones, and multifunctional watches. In addition, we are also exploring the use of this concept in a variety of other applications, such as large-area microscopic imaging, nanoscale lithography or atomic and molecular manipulation.
1 Introduction
Millipede is storage technology developed by IBM.Millipede is a non-volatile computer memory stored on nanoscopic pits.
It promises a data density of more than 1 terabit per square inch (1 gigabit per square millimeter), about 4 times the density of magnetic storage available today.
Millipede storage technology is being pursued as a potential replacement for magnetic recording in hard drives, at the same time reducing the form-factor to that of Flash media.
IBM says flash memory probably won't surpass 1GB to 2GB of capacity in the near term, but Millipede technology could pack 10GB to 15GB of data into the same small format without requiring additional power for device operation.
Working procedure:
Thousands of extremely fine tips "write" tiny pits representing individual bits into a thin film of highly specific polymer.
Bits are written by heating the tip to a temperature above the glass transition temperature of the polymer by means of the heating resistor integrated in the cantilever.
The principle is comparable with the old punch cards, but now with structural dimensions in the nanometer scale and the ability to erase data and rewrite the medium.
1.1 What is IBM Millipede?
Millipede is a nano-storage prototype developed by IBM that can store data at a density of a trillion bits per square inch: 20 times more than any currently available magnetic storage medium. The prototype's capacity would enable the storage of 25 DVDs or 25 million pages of text on a postage-stamp sized surface, and could enable 10 gigabytes (GB) of storage capacity on a cell phone.
1.2 MOTIVATION AND OBJECTIVES
In the 21stcentury, the nanometer will very likely play a role similar to the one played by the micrometer in the 20thcentury. The nanometer scale will presumably pervade the field of data storage. In magnetic storage today, there is no clear-cut way to achieve the nanometer scale in all three dimensions. The basis for storage in the 21st century might still be magnetism. Within a few years, however, magnetic storage technology will arrive at a stage of its exciting and successful evolution at which fundamental changes are likely to occur when current storage technology hits the well- known superparamagnetic limit. Several ideas have been proposed on how to overcome this limit. One such proposal involves the use of patterned magnetic media, for which the ideal write/read concept must still be demonstrated, but the biggest challenge remains the patterning of the magnetic disk in a cost-effective way. Other proposals call for totally different media and techniques such as local probes or holographic methods.
In general,if an existing technology reaches its limits in the course of its evolution & newalternatives are emerging in parallel, two things usually happen: First, the existing andwell-established technology will be explored further and everything possible done topush its limits to take maximum advantage of the considerable investments made. Then,when the possibilities for improvements have been exhausted, the technology may still survive for certain niche applications, but the emerging technology will take over, opening up new perspectives and new directions.
Consider, for example, the vacuum electronic tube, which was replaced by the transistor. The tube still exists for a very few applications, whereas the transistor evolved into today's microelectronics with very large scale integration (VLSI) of microprocessors and memories. Optical lithography may become another example: Although still the predominant technology, it will soon reach its fundamental limits and be replaced by a technology yet unknown. Today we are witnessing in many fields the transition from structures of the micrometer scale to those of the nanometer scale, a dimension at which nature has long been building the finest devices with a high degree of local functionality. Many of the techniques we use today are not suitable for the coming nanometer age; some will require minor or major modifications, and others will be partially or entirely replaced. It is certainly difficult to predict which techniques will fall into which category. For key areas in information technology hardware, it is not yet obvious which technology and materials will be used for nanoelectronics and data storage.
In any case, an emerging technology being considered as a serious candidate to replace an existing but limited technology must offer long-term perspectives. For instance, the silicon microelectronics and storage industries are huge and require correspondingly enormous investments, which makes them long-term-oriented by nature. The consequence for storage is that any new technique with better areal storage density than today's magnetic recording should have long-term potential for further scaling, desirably down to the nanometer or even atomic scale.
The only available tool known today that is simple and yet provides these very long-term perspectives is a nanometer sharp tip. Such tips are now used in every atomic force microscope (AFM) and scanning tunneling microscope (STM) for imaging and structuring down to the atomic scale. The simple tip is a very reliable tool that concentrates on one functionality: the ultimate local confinement of interaction. In the early 1990’s, Mamin and Rugar at the IBM Almaden Research Center pioneered the possibility of using an AFM tip for readback and writing of topographic features for the purposes of data storage. In one scheme developed by them, reading and writing were demonstrated with a single AFM tip in contact with a rotating polycarbonate substrate. The data were written thermo mechanically via heating of the tip. In this way, densities of up to 30 Gb/in.were achieved, representing a significant advance compared to the densities of that day. Later refinements included increasing readback speeds to a data rate of 10 Mb/s and implementation of track servoing. In making use of single tips in AFM or STM operation for storage, one must deal with their fundamental limits for high data rates. At present, the mechanical resonant frequencies of the AFM cantilevers limit the data rates of a single cantilever to a few Mb/s for AFM data storage, and the feedback speed and low tunneling currents limit STM-based storage approaches to even lower data rates. Currently a single AFM operates at best on the microsecond time scale. Conventional magnetic storage, however, operates at best on the nanosecond time scale, making it clear that AFM data rates have to be improved by at least three orders of magnitude to be competitive with current and future magnetic recording. The objectives of our research activities within the Micro- and Nanomechanics Project at the IBM Zurich Research Laboratory are to explore highly parallel AFM data storage with areal storage densities far beyond the expected superparamagnetic limit (60100 Gb/in.) and data rates comparable to those of today's magnetic recording.
1.3 MILLIPEDE MEMORY
Millipede is a non-volatile computer memory stored on nanoscopic pits burned into the surface of a thin polymer layer, read and written by a MEMS-based probe. It promises a data density of more than 1 terabit per square inch (1 gigabit per square millimeter), about 4 times the density of magnetic storage available today. Millipede storage technology is being pursued as a potential replacement for magnetic recording in hard drives, at the same time reducing the form-factor to that of Flash media. IBM demonstrated a prototype s Millipede storage device at CeBIT 2005, and is trying to make the technology commercially available by the end of 2007. At launch, it will probably be more expensive per-megabyte than prevailing technologies, but this disadvantage is hoped to be offset by the sheer storage capacity that technology Millipede technology would offer.
The Millipede concept presented here is a new approach for storing data at high speed and with an ultrahigh density. It is not a modification of an existing storage technology, although the use of magnetic materials as storage media is not excluded. The ultimate locality is given by a tip, and high data rates are a result of massive parallel operation of such tips. Our current effort is focused on demonstrating the Millipede concept with areal densities up to 500 Gb/in.and parallel operation of very large 2D (32 × 32) AFM cantilever arrays with integrated tips and write/read storage functionality.
1.4 THE NAME MILLIPEDE
The name Millipede came from the way the technology works. It consists of a thin, organic polymer on which sit thousands of fine silicon tips that can punch information into the polymer surface, leaving pits and creating a way of storing data. Each tip is very small, with 4,000 fitting onto a 6.4 mm square. The unveiling at the CeBIT event was not only to show off the tech but also to try to get a manufacturing partner on board. IBM does not have the facilities to manufacture MEMS systems, and needs another interested party to come on board that has those facilities available. Big Blue also admits that the technology is nowhere near ready for a release, as researchers still need to sort out the speed that data can be transferred to and from the memory. IBM does hope, however, that Millipede will form a future alternative to current flash memory technologies used in consumer digital equipment.
1.5 BASIC CONCEPT
The main memory of modern computers is constructed from one of a number of DRAM-related devices. DRAM basically consists of a series of capacitors, which store data as the presence or absence of electrical charge. Each capacitor and its associated control circuitry, referred to as a cell, holds one bit, and bits can be read or written in large blocks at the same time.
In contrast, hard drives store data on a metal disk that is covered with a magnetic material; data is represented as local magnetization of this material. Reading and writing are accomplished by a single "head", which waits for the requested memory location to pass under the head while the disk spins. As a result, the drive's performance is limited by the mechanical speed of the motor, and is generally hundreds of thousands of times slower than DRAM. However, since the "cells" in a hard drive are much smaller, the storage density is much higher than DRAM.
Millipede storage attempts to combine the best features of both. Like the hard drive, Millipede stores data in a "dumb" medium that is simpler and smaller than any cell used in an electronic medium. It accesses the data by moving the medium under the "head" as well. However, Millipede uses many nanoscopic heads that can read and write in parallel, thereby dramatically increasing the throughput to the point where it can compete with some forms of electronic memory. Additionally, millipede's physical media stores a bit in a very small area, leading to densities even higher than current hard drives. Mechanically, Millipede uses numerous atomic force probes, each of which is responsible for reading and writing a large number of bits associated with it. Bits are stored as a pit, or the absence of one, in the surface of a thermo-active polymer deposited as a thin film on a carrier known as the sled. Any one probe can only read or write a fairly small area of the sled available to it, a storage field. Normally the sled is moved to position the selected bits under the probe using electromechanical actuators similar to those that position the read/write head in a typical hard drive, although the actual distance moved is tiny. The sled is moved in a scanning pattern to bring the requested bits under the probe, a process known as x/y scan.
The amount of memory serviced by any one field/probe pair is fairly small, but so is its physical size. Many such field/probe pairs are used to make up a memory device. Data reads and writes can be spread across many fields in parallel, increasing the throughput and improving the access times. For instance, a single 32-bit value would normally be written as a set of single bits sent to 32 different fields. In the initial experimental devices, the probes were mounted in a 32x32 grid for a total of 1,024 probes. Their layout looked like the legs on a Millipede and the name stuck. The design of the cantilever array is the trickiest part, as it involves making numerous mechanical cantilevers, on which a probe has to be mounted. All the cantilevers are made entirely out of silicon, using surface micromachining at the wafer surface.
The Millipede concept: for operation of the device, the storage medium - a thin film of organic material deposited on a silicon "table" - is brought into contact with the array of silicon tips and moved in x- and y-direction for reading and writing. Multiplex drivers allow addressing of each tip individually. The 2D AFM cantilever array storage technique called “Millipede” is illustrated in figure. It is based on a mechanical parallel x/y scanning of either the entire cantilever array chip or the storage medium. In addition, a feedback-controlled z- approaching and -leveling scheme brings the entire cantilever array chip into contact with the storage medium. This tip medium contact is maintained and controlled while x/y scanning is performed for write/read. It is important to note that 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 stringent control and uniformity of tip height and cantilever bending. Chip approach and leveling make use of four integrated approaching cantilever sensors in the corners of the array chip to control the approach of the chip to the storage medium. Signals from three sensors (the fourth being a spare) provide feedback signals to adjust three magnetic z-actuators until the three approaching sensors are in contact with the medium. The three sensors with the individual feedback loop maintain the chip leveled and in contact with the surface while x/y scanning is performed for write/read operations. The system is thus leveled in a manner similar to an antivibration air table. This basic concept of the entire chip approach/leveling has been tested and demonstrated for the first time by parallel imaging with a 5 × 5 array chip . These parallel imaging results have shown that all 25 cantilever tips have approached the substrate within less than 1 m of z-activation. This promising result has led us to believe that chips with a tip-apex height control of less than 500 nm are feasible. This stringent requirement for tip-apex uniformity over the entire chip is a consequence of the uniform force needed to minimize or eliminate tip and medium wear due to large force variations resulting from large tip-height nonuniformities.