IBM Millipede
Encyclopedia
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 millipede storage device at CeBIT 2005, and was trying to make the technology commercially available by the end of 2007. At launch, it would probably be more expensive per-megabyte than prevailing technologies, but this disadvantage is hoped to be offset by the sheer storage capacity that Millipede technology would offer.
-related devices. DRAM basically consists of a series of capacitor
s, 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 disk that is covered with a magnetic material; data is represented as local magnetisation 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 medium 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.
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 cantilever will push it into the hole, increasing the surface area in contact with the sled, and in turn increasing the cooling as heat leaks into the sled from the probe. In the case where there is no pit at that location, only the very tip of the probe remains in contact with the sled, and the heat leaks away more slowly. The electrical resistance of the probe is a function of its temperature, rising with increasing 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. While reading an entire storage field, the tip is dragged over the entire surface and the resistance changes are constantly monitored.
To write a bit, the tip of the probe is heated to a temperature above the glass transition temperature of the polymer used to manufacture the data sled, which is generally acrylic glass
. In this case the transition temperature is around 400 °C. 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. Older experimental systems used a variety of erasure techniques that were generally more time consuming and less successful. These older systems offered around 100,000 erases, but the available references do not contain enough information to say if this has been improved with the newer techniques.
As one might expect, the need to heat the probes requires a fairly large amount of power for general operation. However, the exact amount is dependent on the speed that data is being accessed; at slower rates the cooling during read is smaller, as is the number of times the probe has to be heated to a higher temperature to write. When operated at data rates of a few megabits per second, Millipede is expected to consume about 100 milliwatts, which is in the range of flash memory technology and considerably below hard drives. However, one of the main advantages of the Millipede design is that it is highly parallel, allowing it to run at much higher speeds into the GB
/s. At these sorts of speeds one might expect power requirements more closely matching current hard drives.
Data transfer speed is limited to the kilobits-per-second range for an individual probe, which amounts to a few megabits for an entire array. Experiments done at IBM's Almaden Research Center
showed that individual tips could support data rates as high as 1 - 2 megabits per second, potentially offering aggregate speeds in the GB/s range.
The earliest 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. Arranged in a 32 x 32 grid, the resulting 3 mm x 3 mm chip stores 500 megabits of data or 62.5 MB, resulting in an areal density, the number of bits per square inch, on the order of 200 Gbit/in². IBM initially demonstrated this device in 2003, planning to introduce it commercially in 2005. By that point hard drives were approaching 150 Gbit/in², and have since surpassed it.
More recent devices demonstrated at CeBIT
in 2005 have improved on the basic design, using a 64 x 64 cantilever chips with a 7 mm x 7 mm data sled, boosting the data storage capacity to 800 Gbit/in² using smaller pits. It appears the pit size can scale to about 10 nm, resulting in a theoretical areal density just over 1Tbit/in². IBM planned to introduce devices based on this sort of density in 2007. For comparison, perpendicular recording
hard drives featured areal densities on the order of 329 Gbit/in² at the beginning of 2009, and has been predicted to top out at about 1 Tbit/in². Flash reached almost 250 Gbit/in² in early 2010.
Non-volatile memory
Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, in the most basic sense, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM, most types of magnetic computer...
computer memory
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
stored on nanoscopic pits burned into the surface of a thin polymer layer, read and written by a MEMS
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
-based probe. It promises a data density of more than 1 terabit
Terabit
The terabit is a multiple of the unit bit for digital information or computer storage. The prefix tera is defined in the International System of Units as a multiplier of 1012 , and therefore...
per square inch (1 gigabit
Gigabit
The gigabit is a multiple of the unit bit for digital information or computer storage. The prefix giga is defined in the International System of Units as a multiplier of 109 , and therefore...
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
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...
media. IBM demonstrated a prototype millipede storage device at CeBIT 2005, and was trying to make the technology commercially available by the end of 2007. At launch, it would probably be more expensive per-megabyte than prevailing technologies, but this disadvantage is hoped to be offset by the sheer storage capacity that Millipede technology would offer.
Basic concept
The main memory of modern computers is constructed from one of a number of DRAMDram
Dram or DRAM may refer to:As a unit of measure:* Dram , an imperial unit of mass and volume* Armenian dram, a monetary unit* Dirham, a unit of currency in several Arab nationsOther uses:...
-related devices. DRAM basically consists of a series of capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s, 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 disk that is covered with a magnetic material; data is represented as local magnetisation 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 medium stores a bit in a very small area, leading to densities even higher than current hard drives.
Mechanically, millipede uses numerous atomic force probes
Atomic force microscope
Atomic force microscopy or scanning force microscopy is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit...
, 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
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
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
Millipede
Millipedes are arthropods that have two pairs of legs per segment . Each segment that has two pairs of legs is a result of two single segments fused together as one...
, and the name stuck.
The design of the cantilever
Cantilever
A cantilever is a beam anchored at only one end. The beam carries the load to the support where it is resisted by moment and shear stress. Cantilever construction allows for overhanging structures without external bracing. Cantilevers can also be constructed with trusses or slabs.This is in...
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
Surface micromachining
Unlike Bulk micromachining, where a silicon substrate is selectively etched to produce structures, surface micromachining builds microstructures by deposition and etching of different structural layers on top of the substrate....
at the wafer surface.
Reading and writing data
Each probe in the cantilever array stores and reads data thermo-mechanically, handling one bit at a time.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 cantilever will push it into the hole, increasing the surface area in contact with the sled, and in turn increasing the cooling as heat leaks into the sled from the probe. In the case where there is no pit at that location, only the very tip of the probe remains in contact with the sled, and the heat leaks away more slowly. The electrical resistance of the probe is a function of its temperature, rising with increasing 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. While reading an entire storage field, the tip is dragged over the entire surface and the resistance changes are constantly monitored.
To write a bit, the tip of the probe is heated to a temperature above the glass transition temperature of the polymer used to manufacture the data sled, which is generally acrylic glass
Acrylic glass
Poly is a transparent thermoplastic, often used as a light or shatter-resistant alternative to glass. It is sometimes called acrylic glass. Chemically, it is the synthetic polymer of methyl methacrylate...
. In this case the transition temperature is around 400 °C. 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
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
to pull the surface flat again. Older experimental systems used a variety of erasure techniques that were generally more time consuming and less successful. These older systems offered around 100,000 erases, but the available references do not contain enough information to say if this has been improved with the newer techniques.
As one might expect, the need to heat the probes requires a fairly large amount of power for general operation. However, the exact amount is dependent on the speed that data is being accessed; at slower rates the cooling during read is smaller, as is the number of times the probe has to be heated to a higher temperature to write. When operated at data rates of a few megabits per second, Millipede is expected to consume about 100 milliwatts, which is in the range of flash memory technology and considerably below hard drives. However, one of the main advantages of the Millipede design is that it is highly parallel, allowing it to run at much higher speeds into the GB
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
/s. At these sorts of speeds one might expect power requirements more closely matching current hard drives.
Data transfer speed is limited to the kilobits-per-second range for an individual probe, which amounts to a few megabits for an entire array. Experiments done at IBM's Almaden Research Center
Almaden Research Center
The IBM Almaden Research Center is in San Jose, California, and is one of IBM's nine worldwide research labs. Its scientists perform basic and applied research in computer science, services, storage systems, physical sciences, and materials science and technology. The center opened in 1986, and...
showed that individual tips could support data rates as high as 1 - 2 megabits per second, potentially offering aggregate speeds in the GB/s range.
Current state of the art
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, has made the existing demonstrators unattractive for commercial production. Millipede appears to be in a race, attempting to mature quickly enough at a given technology level that it has not been surpassed by newer generations of the existing technologies by the time it is ready for production.The earliest 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. Arranged in a 32 x 32 grid, the resulting 3 mm x 3 mm chip stores 500 megabits of data or 62.5 MB, resulting in an areal density, the number of bits per square inch, on the order of 200 Gbit/in². IBM initially demonstrated this device in 2003, planning to introduce it commercially in 2005. By that point hard drives were approaching 150 Gbit/in², and have since surpassed it.
More recent devices demonstrated at CeBIT
CeBIT
CeBIT is the world's largest and most international computer expo. CeBIT is held each year on the world's largest fairground in Hanover, Germany, and is a barometer of the state of the art in information technology...
in 2005 have improved on the basic design, using a 64 x 64 cantilever chips with a 7 mm x 7 mm data sled, boosting the data storage capacity to 800 Gbit/in² using smaller pits. It appears the pit size can scale to about 10 nm, resulting in a theoretical areal density just over 1Tbit/in². IBM planned to introduce devices based on this sort of density in 2007. For comparison, perpendicular recording
Perpendicular recording
Perpendicular recording is a technology for data recording on hard disks. It was first proven advantageous in 1976 by Shun-ichi Iwasaki, then professor of the Tohoku University in Japan, and first commercially implemented in 2005.-Advantages:Perpendicular recording can deliver more than three...
hard drives featured areal densities on the order of 329 Gbit/in² at the beginning of 2009, and has been predicted to top out at about 1 Tbit/in². Flash reached almost 250 Gbit/in² in early 2010.