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The Future of Data Storage

by Andrew Jennings

CIT 595, Spring 2007

Abstract: Computer users need to store more data every day. Businesses are required by law to hang

on to records for years to satisfy laws created in the wake of corporate scandals. Consumers are buying

more of their music and movies online and media providers are looking to push bandwidth heavy high

definition content to users' media devices. To store this torrent of data, old technologies will need to be

updated and radical new ideas will have to be brought from engineering labs to the market.

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Table of Contents

Introduction: Revolutionary Random Access............................................................................................3

Perpendicular Storage vs. the Superparamagnetic Effect..........................................................................3

Micromechanical Storage: The Millipede..................................................................................................5

Blu Ray and HD DVD Vie for the Living Room.............................................................. ........................7

Holographic Storage Breaks into the Third Dimension...........................................................................10

Conclusion: More than Technology.........................................................................................................12

Bibliography.............................................................................................................................................13

Image Copyrights......................................................................................................................................14

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Introduction: Revolutionary Random Access

IBM introduced the first computer with a hard disk drive in 1956: the RAMAC 305. The drive

weighed a full ton, used a noisy air compressor to move a pair of read/write heads among its 50 platters,

and stored only 5 MB of data (Hoagland, 1871). Its full name revealed the technology that would make

RAMAC revolutionary: Random Access Method of Accounting and Control. RAMAC's read arm could

reach any bit of data in a short amount of time and didn't have to go through every bit between “here” and

“there” in order to do so. This random access was a huge breakthrough that brought data seek times

down to 600 milliseconds (IBM Archives). Before the introduction of the RAMAC, data was often stored

on reels of magnetic tape that needed to be wound to a specific point before data could be read. Seek

times for sequential access on a magnetic tape could be measured in seconds rather than milliseconds. If

a tape was fully wound and you wanted to read data at the end, you would have to wait until the entire

tape unspooled to do so. With random access, data became quickly accessible regardless of what had just

been read.

As the computer became ubiquitous in the modern world, hard drives have increased drastically in

speed and storage space while shrinking to pocket size. They still rely on the same principles that

RAMAC brought to the market fifty years ago: a read/write head gliding above platters coated in

magnetic material. Although RAMAC had only a pair of data reading heads, a modern hard drive will

have a pair floating on an arm between every two platters, each tied to the actuator that positions the

heads in parallel over a specific track. Data is read by examining the magnetic transitions among the

magnetic grains coating each platter.

Perpendicular Storage vs. the Superparamagnetic Effect

Continual improvements in the miniaturization and speed of the underlying mechanical devices that

make up the hard drive have allowed the number of bits contained within a drive to increase exponentially

(see Figure 1). But there is a limit to the amount of data that can be packed into a drive because of the

physical properties of the hard disk drive platter, the spinning disc that holds the magnetic bits that make

up data.

Data is stored on a hard drive in grains of magnetic material that are laid out on the platters. The

grains are distributed into “islands” of about a thousand grains, with each island making up a bit of

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information stored on a hard drive. The read and write heads on each actuator arm of the hard drive run

above these grains of magnetic material and detect their magnetic orientations, which represent their

status as storing a 1 or a 0. By making the grains of magnetic material smaller and creating more

sensitive read and write heads, manufacturers have been able to increase the number of bits stored on hard

drives. Current disk drives can use this longitudinal storage of grains to reach an areal density of 120

Gb/in2 (Currie et al., 153). RAMAC's disk drive had an areal density of 2 Kb/in2.

But there is a minimum size that each

magnetic grain cannot sink below if it is to

usefully hold a charge. As the grains get

shorter, they reach a point where the thermal

energy around the platters is enough to flip

the magnetic orientation of a grain. This isthe superparamagnetic effect. One way to

get around this is using magnetic materials

that have a higher coercity, the resistance to

change in magnetization. Although such

materials are less likely to unexpectedly

change orientation, higher coercity also

makes reading and writing information moredifficult.

Another way to get around this

limitation is stacking up the data carrying

magnetic grains so they are magnetically

oriented perpendicular, rather than parallel,

to each platter. By orienting the data carrying islands this way, the volume of each magnetic unit is

increased and the superparamagnetic barrier is pushed back. Although the platters are using a thickercoating of magnetic material, it's possible to coat the surface area of the disk with more bits of

information. The platters are also specialized, with a soft magnetic underlayer that helps orient the

grains' charge vertically. Hitachi currently sells disk drives using this technology and believes that areal

densities using perpendicular recording can extend beyond 500 Gb/ in2 (Currie et al., 153).

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Figure 1: Areal Density Over Time

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Although the superparamagnetic barrier looms in the hard drive's future, perpendicular recording has

pushed it back so that manufacturers have more time to develop new technologies to replace (or

revolutionize) the hard disk drive. For example, IBM is using new platter materials to further push back

the barrier and research in “patterned media” may allow for hard drives that store a bit in a single grain of

magnetic material.

Micromechanical Storage: The Millipede

Another way to get around the superparamagnetic barrier is to use non magnetic materials to store

data. IBM is developing a storage device that employs heated, micromechanical cantilevers to jab tiny

pits into a polymer medium. Initial experiments with the cantilever technology happened in the early

1990s. Scientists were able to successfully write data with an areal density of 30 Gb /in2 by pulling the

tiny tip across a rotating circular medium (Vettiger et al, 2000, p 324). The density was impressive for the

time but read and write operations were slow with access to only one bit at a time. So scientists

developed a chip that can control an array of cantilevers for fast access in a small package. The

appearance of these tiny levers hanging from their controller gives the device its name: Millipede.

The Millipede uses technology developed for the atomic force microscope, a device that drags a tiny

cantilever over a surface to detect its shape. As the cantilever bounces through the peaks and valleys of

the material, its vertical movement is tracked by a photodiode that observers a laser reflecting off the end

of the cantilever (University of Bristol SPM). Although the Millipede uses micromechanical cantilevers

like the AFM, the storage device uses heat rather than light to detect the movement of the lever.

The Millipede's read/write mechanism contains a two dimensional array of cantilevers, each 70

micrometers long. Each lever has a writing tip, a point with a 2 micrometer base and an apex only 20

nanometers wide. These tips write on a three layer storage medium. The bottom of the medium is a base

of hard silicon to dissipate heat. On top of the silicon, a buffer layer of a soft polymer called

“photoresist” keeps the lever tips from reaching the hard silicon layer. The Millipede depends on

uniformity of the size of the writing tips for accuracy and contact with the silicon underlayer would wear

down the tips. The third layer is a coating of the polymer marked by the writing tips,

polymethylmethacrylate (PMMA).

To write one bit of information, a lever is heated to 400o Celsius right above the melting point of the

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PMMA and pressed into the “sled” that holds the media. As the lever gets closer to the medium and

the tips makes surface contact, polymer melts and a tiny bit is formed. When enough heat has been

transferred from the lever to the storage media, that indicates a proper bit has been inscribed and the lever

is pulled away from the medium.

Reading data uses a similar heat detecting process. To read the data underneath the storage array,

each of the tips is heated to 350o, not hot enough to melt the medium. As the tips move over the storage

medium, the temperatures of the levers floating above the storage medium are monitored. As a tip dips

into a bit indentation, the amount of heat transferred from the lever to the medium will increase,

indicating that a mark representing a “1” is present.

Using these methods, scientists at IBM have been able to create patterns from bit indicators only

40nm in diameter, 100 nm apart.

Initial Millipedes had a data array of

only 5x5 levers but recently, 64x64

arrays of cantilevers have been

created, packing data with an areal

density of more than 1 Tb/in2 (IBM

Zurich Research Lab).

The Millipede manufacturing

process involves silicon etching and

surface micromachining, the same

methods used to make microchips.

The Millipede's basic shape is first

etched from a silicon wafer. Then layers of silicon are deposited onto the tips of the cantilevers and

etched into the writing tips. After the array is complete, the Millipede's data array is joined to the CMOS

circuits that control its mechanical movement. There are no additional wires needed to transfer electrical

signal through the Millipede because it's created from doped silicon.

The read/write part of the Millipede is attached to magnetic actuators that move it above the sled

containing the storage medium. The magnets keep the pieces together and also protect the Millipede

from vibration. The sled is moved along the X and Y axes to the proper position below the Millipede,

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Figure 2: The Millipede's data array moves along the X

and Y axes above the storage medium

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which can then read and write with its levers in parallel.

The Millipede's read and write speeds are based on the number of cantilevers contained in the storage

array. Each lever is currently capable of more than 1 Kb/s, meaning that a 64 x 64 array can do 4 Mb/s.

The speed limitation is not due to the movement of the levers or the data array but the cycle speed of the

electrical pulses used to heat the levers. Improvements to the Millipede design are aimed at reaching

megabit speeds per lever, enabling the device to reach data access speeds of gigabits per second.

One of the issues facing the Millipede project is the low tolerance for error when creating machines at

the micrometer level. Vettiger states that the Millipede is moving less than 1 micrometer above the media

sled. To ensure accurate reading and writing, the tips at the end of each cantilever must have a “tip

uniformity” of less than 500 nanometers. Although the Milllipede is accurate when all levers are in

working condition, prototypes may have between 20% and 40% damaged cantilevers. In addition to tip

non uniformity, many of these non working levers stem from problems with thermal expansion due to the

intense heat at which the Millipede is working.

Because the devices are so tiny, power consumption for heating the storage array is quite low. The

Millipede is manufactured using already existing processes so they will be manufactured at a relatively

low cost. This makes it an ideal storage device for watches, PDAs and other devices that currently depend

on bulky hard drives or expensive flash media for storage. IBM scientists have proposed to use the

Millipede in a Nanodrive, a centimeter sized device capable of holding a gigabyte of data (Vettiger et al,

2006, p 333). It's also possible to use the Millipede in larger storage devices by leveraging technology

existing in the hard drive, i.e., the Millipede could be used as a read/write head while a disc moved the

polymer coated medium underneath the write head (Vettiger et al, 2000, p 336). Whatever the Millipede

is used for, it's certain that this novel device will allow us to store more data than possible with a hard

drive by sidestepping the superparamagnetic limit.

Blu Ray and HD DVD Vie for the Living Room

One of the other common methods of storage is the optical disc. The CD, introduced in 1982 as a

music storage format, is one of the most popular formats of optical data storage. The CD ROM is capable

of storing 650 MB or 700 MB of data. The CD was succeeded by the DVD, capable of storing six times

the data (4.7 GB). DVDs can also contain two layers of information stored on each side of the disc for a

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grand total of 17.1 GB of storage.

Producers of high definition video decided that a new format was needed to bring their content to

consumers' living rooms. The original DVD standard was hammered out of compromises by the various

members of the DVD Forum. This time, the producers could not decide on a single format. Now there

are two incompatible contenders for the successor of the DVD: Blu Ray Disc (BD) and High Definition

Digital Versatile Disc (HDDVD). BD and HD DVD are both capable of holding a higher density of

information than the DVD due to the use of blue lasers (rather than red) used to read CDs and DVDs.

Information on a BD or HD DVD is stored in a spiral shape on the disc. The data spiral radiates from

the center of the disc toward the outer edge. If the disc contains a second layer, that spiral may start on

the outside of the disc so the laser lens pickup doesn't have to stop and move to the center of the disc

during a layer change. That makes it less noticeable to a movie watcher that the layer change occurred.

As the disc spins, the laser on the read head focuses a beam at the layer being read and a

photosensitive device looks for a reflection of the data from the disc. A reflection signifies a pit was

found, meaning a 1 signal. No reflection means the disc was not pitted at that particular spot, causing the

laser light to be deflected away from the reader, signifying a 0 bit. In this way, digital information can be

read from the disc.

On a factory pressed disc, the bottom layer of the disc is injection molded and contains a data spiral

written in bumps on the top of the plastic layer. A layer of aluminum is placed on top of the plastic, then

another layer of plastic and a label go on top of the aluminum. The reflective aluminum layer is what is

actually read by the laser. An optical pickup detects whether the laser lens hit a bump (actually seen as a

pit in the aluminum layer from the side of the disc the laser is on) or an absence of a pit (called a land).

The spindle rotates the disc as the laser lens pickup moves toward the end of the spiral, leading to a

stream of pits and lands that can be decoded as 0s and 1s and processed by a computer or video player.

Data can be written to a disc using the same equipment. Instead of an injection molded plastic layer

containing bumps, the writable media contains a layer of photosensitive dye under the aluminum layer. A

writing laser combines with the dye to create patches that take the place of the pits and lands in the

factory pressed disc. Reading the disc uses the same mechanism described above.

The most significant difference between next gen discs and previous generations is the color of laser

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used to read and write data. BD and HD DVDs use blue lasers, which have a shorter wavelength than the

red lasers used to read previous optical storage discs. This shorter wavelength means that the blue laser

has a narrower focus and is able to read smaller pits than a red laser (Piepenburg, 31). Therefore next gen

discs can be the same diameter as a DVD while having many more pits in the spiral track.

BD and HD DVD players use a different aperture width for the laser lens, leading to a disparity in

storage sizes for the two formats (Cyberlink). BD laser lenses have a numerical aperture of .85 while

HD DVD lenses are .65. A higher NA means the laser starts wider but spreads less as it travels. The

creators of the HD DVD wanted to manufacture the discs on current DVD pressing equipment. This lead

to the HD DVD having a plastic layer 0.6 mm thick, just like the DVD. The manufacturers of BD did not

bind themselves to the DVD manufacturing equipment and created discs with a bottom layer of only 0.1

mm. The thicker coating combined with the lesser NA means an HD DVD laser needs to focus on larger

pits than the BD's reader. This is why the BD stores more information than the HD DVD. A single

sided, single layer BD can contain 25 GB of data while a similar HD DVD can hold 15 GB of

information.

Specifications for HD content discs

HDDVD BluRay

Storage Space 15 GB per layer 25 GB per layer

Laser NA .65 .85

Track Pitch .40 micrometers .32 micrometers

Codecs MPEG2

MPEG 4 AVC

VC 1

MPEG 2

MPEG 4 AVC

VC 1

Sound Dolby Digital Plus,

DTS HD

Dolby Digital Plus,

DTS HD

Read Speed (1x) 32.4 Mb/s 36 Mb/s

Exclusive content Universal Sony, Fox, Disney,MGM, Lions Gate

Taken from official Blu Ray and HD DVD web sites

Blu Ray and High Definition DVD are both available for use as computer storage devices. Write

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once and rewritable discs are available. But until writable media drops in price, most consumers will

encounter BD and HD DVD as prerecorded media containing movies. Each of the formats has been

backed by a number of content producers, with Sony being one of the largest BD backers and Universal

Studios providing content exclusively for HD DVD. Part of the conflict will play out on the current

generation of video game consoles, with Sony using its PlayStation 3 as a Trojan horse to get Blu Ray

devices into homes and Microsoft releasing an add on HD DVD player for its Xbox 360 console.

Devices that read both formats are in the works but it's more likely that one format will come out ahead in

a few years and leave the other to become the Betamax of this generation of optical storage.

As Dipert points out in his article, there's also a possibility that consumers will choose neither. There

are many alternatives to watching high definition content that use current red laser DVDs. It's possible to

buy Windows MediaVideo encoded content on a DVD that's playable on a PC. Rather than filling more

space on a disc, content could be encoded in a space saving codec such as MPEG 4 or DivX. It's also

possible that another disc type, such as the Enhanced Versatile Disc (EVD) created in China, could take

market share from BD and HD DVD. Finally, consumers may not be ready for the expense involved in

upgrading their equipment to HD. Dipert mentions the often cited studies that claim people can't

distinguish high resolution content from low resolution on televisions below 50” anyway. So it's quite

possible that both high definition discs will go the way of the laserdisc and become a footnote in AV

history.

Holographic Storage Breaks into the Third Dimension

Holographic storage as a concept has been examined for decades but only recently has it become

commercially possible. InPhase, a spinoff of Lucent Labs, will likely be the first to market with their

Tapestry line of drives this year. They have managed to bring this product to market by developing a

special media that solves some of the problems encountered with earlier attempts at holographic storage.

Holographic storage is fundamentally different from other optical storage like the DVD because it

stores data throughout the medium instead of on a two dimensional surface. Rather than bouncing a laser

off a disc or reading from the surface of a platter, holographic devices shoot lasers through media to

create and read patterns. This allows for a density of information not possible with two dimensional

storage. The basic unit of data in holographic storage is called a page. A page can contain one million

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bits, arranged in a checkerboard like pattern. Pages are projected by shining a laser through a Spatial

Light Modulator that turns 0s and 1s into a pattern of lights different intensity. The SLM lets through a

beam of light containing the pattern of the data page – the data beam.

In addition to going through the SLM, the device's laser is split and aimed at a small mirror that

projects the beam into the media at an angle intersecting the data beam. This beam is called the reference

beam. The angle of the beams and their relation to the media will be the key to retrieving the page.

The reference beam and the data beam produced by the SLM intersect inside the media, combining to

create an interference pattern where they meet. The interference pattern is recorded by the photosensitive

media and becomes the representation of the data page on the media. To retrieve data, the reference beam

can be aimed at the media at the same angle. The reference beam, shining through the interference

pattern, will recreate the data beam on the

other side of the medium. The data hits a

charge coupled device (CCD), the light

sensing device at the heart of the digital

camera, which can translate the data beam

back into an array of digital bits.

By slightly changing the angle of the

reference and data beams, moving the media

or changing a beam's wavelength, overlapping

pages can be written throughout the medium.

In prototype devices, media filled with

“books” of 100 overlapping pages could store

data at 80 Gb/in2. InPhase has an eye on

improving that density by adjusting laser NA and wavelength to allow for books of more than 600 pages

and a density of 1600 Gb/in2 (Wilson et al, 35).

One of the initial problems with holographic storage was finding a proper medium. Initial polymers

reacted poorly to lasers and deformed, damaging the information stored in the media (Huang, 66).

InPhase claims to have avoided these issues by creating a “two chemistry” media that reacts correctly

when writing but stays stable during read operations. The Tapestry media is created from two polymers,

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Figure 3: Writing to holographic media

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one that keeps the disc stable and another that's photosensitive to record the holograms.

Because an entire megabit of data can be read in parallel, data retrieval from holographic media is

extremely fast. Seek times on the initial Tapestry drives are said to average 250ms and data transfer rates

should be 20 Mb/s. InPhase has created a roadmap (Wilson, 35) of technologies and is shooting for

eventual speeds of 120 Mb/s.

Conclusion: More than Technology

Those are just a few of the storage technologies we may see in the next few years. Despite each being

an advancement over current technologies in speed or space, there's no guarantee that any of them will

catch on enough to become as ubiquitous as the modern hard drive or the DVD. There's more to

technology than efficiency and design; there's marketing, price and ease of use. Whether these devices

become popular will likely depend more on advertisers than engineers but each is a fascinating way to

conquer the issues facing a world that needs to store an ever increasing number of 0s and 1s.

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Image Copyrights

Images reproduced for academic purposes.

Figure 1: IBM 2003, from IBM Systems Journal, vol 42, no 2

Figure 2: IEEE 1999, from Technical Digest, 12th IEEE International Micro Electro Mechanical

Systems Conference, p 564569

Jennings p. 14

http://bookstore.spie.org/index.cfm?fuseaction=DetailVolume&productid=539168&CFID=5356778&CFTOKEN=3832005

http://bookstore.spie.org/index.cfm?fuseaction=DetailVolume&productid=539168&CFID=5356778&CFTOKEN=3832005

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Figure 3: InPhase Technologies 2007, from http://www.inphase

technologies.com/technology/default.asp

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