Volumetric density trends (TB/in.3) for storage components: TAPE, hard disk drives,NAND, and Blu-rayR. E. Fontana Jr., G. M. Decad, and S. R. Hetzler Citation: Journal of Applied Physics 117, 17E301 (2015); doi: 10.1063/1.4906208 View online: http://dx.doi.org/10.1063/1.4906208 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Extendibility of traditional perpendicular magnetic recording for hard disk drives J. Appl. Phys. 109, 07B774 (2011); 10.1063/1.3563095 Perpendicular recording media for hard disk drives J. Appl. Phys. 102, 011301 (2007); 10.1063/1.2750414 Lubricant transfer from disk to slider in hard disk drives Appl. Phys. Lett. 90, 143516 (2007); 10.1063/1.2721122 Robust design of head interconnect for hard disk drive J. Appl. Phys. 97, 10P102 (2005); 10.1063/1.1850811 Tribocharging of the magnetic hard disk drive head–disk interface J. Appl. Phys. 91, 4631 (2002); 10.1063/1.1455129

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Volumetric density trends (TB/in.3) for storage components: TAPE, hard diskdrives, NAND, and Blu-ray

R. E. Fontana, Jr.,1,a) G. M. Decad,1 and S. R. Hetzler2

1IBM Systems and Technology Group, IBM Almaden Research Center, San Jose, California 95120, USA2IBM Research Division, IBM Almaden Research Center, San Jose, California 95120, USA

(Presented 6 November 2014; received 19 September 2014; accepted 1 October 2014; published

online 15 January 2015)

Memory storage components, i.e., hard disk drives, tape cartridges, solid state drives using Flash

NAND chips, and now optical cartridges using Blu-ray disks, have provided annual increases in

memory capacity by decreasing the area of the memory cell associated with the technology of these

components. The ability to reduce bit cell sizes is now being limited by nano-technology physics so

that in order for component manufacturers to continue to increase component capacity, volumetric

enhancements to the storage component are now being introduced. Volumetric enhancements include

adding more tape per cartridge, more disk platters per drive, and more layers of memory cells on the

silicon NAND substrate or on the optical disk substrate. This paper describes these volumetric strat-

egies, projects density trends at the bit cell level, and projects volumetric trends at the component

level in order to forecast future component capacity trends. VC 2015 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4906208]

I. INTRODUCTION

Present day storage components fall into four major cat-

egories: (1) magnetic based technology for hard disk drives

(HDD), (2) magnetic based technology for tape cartridges

(TAPE), (3) NAND flash technology for solid state drives

(SSD), and now (4) optical technology for multiple Blu-ray

disks (BD) configured into an archival cartridge. Associated

with these technologies is an areal density; the number of

bits per unit area formed on the memory surface substrate.

Component capacity is related to both the areal density of

the technology and the amount of physical media surface

area contained in the component. The annual rate of areal

density improvement has been a standard for evaluating a

storage technology’s potential for increasing component

memory capacity and reducing cost per bit. However, as bit

cell areas are reduced, bit cells contend with nano-scale limi-

tations and are becoming more costly to manufacture.

Hence, areal density improvements are now being coupled

with volumetric improvements, where the surface area of the

media within a component is increased, to sustain growth in

component capacities. The volumetric approach for HDD

and TAPE is adding more media in the form of either more

platters in the HDD space or more tape length in the car-

tridge space. The approach for NAND flash and BD is add-

ing multiple layers of storage media on existing media

substrates; a more 3-D approach.

This paper combines areal density trends and volumetric

strategies to forecast component capacity and associated vol-

umetric bit densities for these components. Table I details

volumetric properties for storage components in 2013. In

Table I, LTO TAPE references the Linear Tape Open con-

sortium where multiple tape media and drive vendors

contribute to a tape system. ENT TAPE references a closed

system where the media and the drive are single sourced. For

NAND SSDs, a 2.500 HDD form factor volume was used.

The paper details the limits of bit cell sizes and will

describe the volumetric strategies the various memory tech-

nologies are adopting to maintain component capacity

growth. Two key observations are (1) annual areal density

increases are characteristic of a Moore’s Law metric, geo-

metrical in nature, and these rates are now lower than the

classical 40%/yr rate that doubles density on a two year

cycle and (2) volumetric enhancements are not a Moore’s

Law metric; the enhancements are one time or two time

increases that cannot be sustained annually since the volume

of the component or the number of 3D layers is constrained

II. AREAL DENSITY LANDSCAPE

Using areal density data from Table I, the data reported

in Refs. 1 and 2 and the density data for BD,3 Figure 1 shows

a six year history of areal density for HDD, NAND, TAPE,

and BD technologies. BD areal density is referenced to a 3-

layer disk and NAND areal density is referenced to a 2 bit/

cell design. Trend lines for areal density increases from

Figure 1 are summarized in Table II along with 5 and 10

year projected density increases if these trend lines are sus-

tained. The assumed trend of areal density doubling every 2

years, 5.4X in 5 years and 29X in 10 years, is not being real-

ized. The issue is the inability to manufacture storage bit

cells at decreasing dimensions. The solution to compensate

for the shortfall in areal density is to place more bits into the

existing component volume by adding more media surface

area, i.e., volumetric storage density increases.

III. BIT CELLS

Table III shows the sizes of present day bit cells and

how these dimensions change if the areal density trends of

a)Author to whom correspondence should be addressed. Electronic mail:

rfontan@us.ibm.com.

0021-8979/2015/117(17)/17E301/4/$30.00 VC 2015 AIP Publishing LLC117, 17E301-1

JOURNAL OF APPLIED PHYSICS 117, 17E301 (2015)

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Table II are maintained and no new volumetric strategies are

introduced. NAND bit cell size for a two bit/cell design is

calculated as 4F2/2, where F is the lithographic minimum

feature. Optical bit cell is the area occupied in one layer of a

three-layer disk structure.

Each of these technologies must confront length scale

limitations should the areal density goals be met. Optical BD

technology is constrained by the laser spot size which for a

405 nm source with a 0.85 NA lens is 476 nm and this limits

track pitch to typically no more than 1=2 the beam diameter.

This places an upper limit on density increases in the BD disk

tracks per inch (TPI). NAND technology is limited today by

F¼ 16 nm for line/space features. More critically, the bit cell

area limits the charge in the cell that can be distributed among

4 levels for two bit/cell designs and introduces nearest neigh-

bor interactions during the read and write current selection

processes. The 2019 and 2024 bit cell dimensions would

require minimum features of 8 nm and 4 nm, respectively, and

these dimensions exceed present day lithography roadmaps of

11 nm and 8 nm.4 For HDD, two length scale issues arise.

First, the nature of magnetic recording is to utilize a read

transducer that is smaller than the written track by a factor of

two so that tracking margins in the rotating media can be

achieved. This implies that today’s 35 nm sensors will be

reduced by a factor of 2 in 5 years and a factor of 4 in 10 years

moving sensor dimensions to the 10 nm region. Second, the

recording media is composed of grains that are today typically

�7 nm in diameter so that a bit contains from 10 to 20 grains.

Unless the grain size of the media can be reduced by factors

of 2–4 over the next 5–10 years, signal to noise issues associ-

ated with the statistics of a small number of grains per bit cell

will reduced bit cell stability. Tape, with the lowest areal den-

sity has the largest bit cell area of these technologies and not

surprisingly is not impacted by length scale issues in sensor

dimension. The 2024 TAPE bit cell dimensions are larger

than the associated cell dimensions for HDD, NAND, and BD

in 2014. The issue for TAPE is, like HDD, transitioning to

media that support higher bit density with smaller particles or,

as was done 30 years ago with HDD, a transition from particu-

late media to sputter deposited media.

The net of these nano technology challenges is that each

technology must pursue alternative strategies to maintain an

increase in component capacity that is now not satisfied by

areal density alone.

IV. VOLUMETRIC STRATEGIES

With apparent limits to areal density growth, storage

technologies are developing volumetric strategies to deliver

annual increases in component capacity. HDD and TAPE are

adding more media into the component and are continuing

with conventional bit cell designs and the associated nano

technology issues cited in Sec. III. NAND is radically chang-

ing the cell to a larger 3D design and adding more media

layers onto the basic silicon substrate. BD is adding more

media layers on the disk substrate and reducing the bit cell

size, while retaining the same wavelength source.

A. HDD volumetrics

The HDD volumetric strategy is to add more platters to

the drive coupled with gradual increases in areal density by

TABLE I. YE 2013 memory component characteristics.

NAND SSD HDD LTO TAPE ENT TAPE Optical BD

Component 2.500 drive 3.500 drive LTO cartridge Enterprise cartridge 12 disk cartridge

Volume (in.3) 3.0 23.7 20.4 20.4 21.3

Volumetric Strategy 2 bits/cell 5 platters 840 m tape 840 m tape 3 layer disk

Areal density 900 Gb/in.2 860 Gb/in.2 2.1 Gb/in.2 3.1 Gb/in.2 75 Gb/in.2

Capacity (TB) 1 5 2.5 4 1.2

Storage density (TB/in.3) 0.33 0.21 0.12 0.20 0.06

FIG. 1. Areal density (AD) 5 year and 10 year trends.

TABLE II. Areal density (AD) 5 and 10 year trends.

AD increase

(2008–2013) (%/yr)

5 YR AD

increase (2019)

10 YR AD

increase (2024)

TAPE 28 3.4X 11.8 X

NAND 35 4.5X 20.1X

Optical 12 (18) 2.3X 5.2X

HDD 18 2.3X 5.2X

TABLE III. Bit cell evolution, 5 and 10 year trends.

2014 bit cell 2019 bit cell 2024 bit cell

TAPE 4000 nm � 65 nm 1400 nm � 55 nm 600 nm � 50 nm

HDD 70 nm � 13 nm 40 nm � 10 nm 25 nm � 7 nm

NANDa 22 nm � 22 nm 10 nm � 10 nm 5 nm � 5 nm

Opticalb 320 nm � 80 nm 225 nm � 50 nm 225 nm � 22 nm

a2 bit/cell design.bBit cell size on one of three media layers on substrate.

17E301-2 Fontana, Jr., Decad, and Hetzler J. Appl. Phys. 117, 17E301 (2015)

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transitioning to a now well-documented heat assisted mag-

netic recording approach (HAMR). HDD product families

routinely contain 2, 3, or 4 disks all using the same areal den-

sity. The addition of an extra disk adds 1.2 mm of space for

the disk and about 0.8 mm of space for the read/write trans-

ducers on the two surfaces of the new disk. The art in this

strategy is moving the discs closer together, thinning the

disks and maintaining mechanical integrity at rotation rates.

In early 2014, 6 platter enterprise disks were introduced and

the likely strategy is to move to 7 or 8 platters in the near

future implying one time storage density increases between

26% and 33% in addition to any areal density improvement

from HAMR. However, the platter strategy does not support

continued annual increases of platter number.

B. TAPE volumetrics

LTO tape length is �840 m. Tape thickness is �5.5 lm,

the bulk being the substrate thickness, not the magnetically

active media which is �50 nm thick. The tape volumetric

strategy is two fold. Maintain areal density growth at 28%

annually and move to thinner media to allow for increases in

tape length per cartridge. For example, 4 lm tape media

allow for 1140 m tape length, an increase of 35%. This is

also a one-time event since mechanical stability of the tape

media diminishes with thinner media. Areal density growth

will continue since the transducer and media strategies used

by tape would replicate technologies practiced in manufac-

turing volumes by the HDD industry for the last 10 years.

C. Optical volumetrics

Unlike NAND, HDD, and Tape, BD disks already

employ a volumetric strategy at the substrate level. Present

day BD 100 GB disks use three optically sensitive phase

change material layers, each separated by 25 lm, on one side

of plastic substrate. The disk surface is grooved at a pitch of

320 nm to form a track pitch and the bit length on a track is

80 nm giving an areal density on each layer of 25 Gbit/in.2

with an effective areal density of 75 Gbit/in.2 on the disk sur-

face. An aggressive, unproven BD disk capacity roadmap

strategy3 was recently announced using four volumetric strat-

egies, all achieved with the same optical source, 405 nm, and

same optics, NA¼ 0.85. The first strategy, one time only, is to

use media on the top and bottom of the substrate, i.e., like

HDD media, with a 2X enhancement in volumetric density.

The second enhancement, areal density oriented, is moving to

plateau and valley recording, i.e., the groove pitch is increased

to 0.45 lm but bits are recorded on both the upper and lower

regions of the grooved structure. This provides an effective

track pitch of 225 nm or an areal density enhancement of 1.4X

but assumes that heat transfer to adjacent tracks will be mini-

mal. The third strategy is a density enhancement along the

track, i.e., bit length, where reflection edges are moved closer

to decrease the effective bit length by 40% for an additional

density of 1.66X that relies on signal processing. The caution-

ary notes for BD areal density changes are that the strategies

are unproven and must be reliably manufactured in volume. In

principle, these optical strategies could yield 500 GB BD

disks in a 5-year time frame. The last volumetric change,

highly speculative, is to move to two bits/cell. This requires 4

reflectivity levels in the phase change media. Figure 2 illus-

trates the bit cell roadmap for optical. Each of the optical

enhancements is different one time improvements but of suffi-

cient magnitude to increase the BD disk capacity by �5X

while increasing areal density by a factor of �2.5X.

D. NAND volumetrics

The present NAND cell structure shown in Fig. 3 con-

tains 2 bits in an area of 4F2, where F is the linewidth of the

word and bit lines. Four levels of charge can be sustained in

the floating gate structure. Present cells are built with 16 nm

linewidths yielding 512 nm2 per bit. This traditional planar or

2D cell is possibly scalable to the 13 nm linewidth node but

adjacent cells will be 25% closer to each other and there will

be 34% less charge that can be distributed among the 4 dis-

crete charge levels. For this reason, NAND manufacturers are

moving to single bit/cell 3D designs as pictured in Figure 3.

The 3D cell strategy addresses the charge issue by making the

cell �20X larger using a single bit/cell area of 6F2 with

F¼ 40 nm or 9600 nm2 area but layering �20 layers sequen-

tially of these cells on the substrate for an effective area of

480 nm2, equivalent to the 16 nm 2D cell. The net result is

that a 3D single bit/cell design eliminates the nano technology

issues of insufficient charge, nearest neighbor interactions,

and lithography minimum features. The device functionality

is moved to processing issues that are the core strengths of the

semiconductor industry. With this 3D strategy, a path to

equivalency for the 13 nm lithography node is assured by

FIG. 2. BD bit cell evolution (to scale) from traditional 0.32 lm track pitch

to 0.225 lm track pitch to enhanced bit per inch (BPI) signal processing.

FIG. 3. NAND design cells (to scale in device plane). Pictured 2D design

supports 18 bits. Pictured 3D design (with 3 layers) supports 12 bits.

17E301-3 Fontana, Jr., Decad, and Hetzler J. Appl. Phys. 117, 17E301 (2015)

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moving to 30 layers of cells (1.5X). Moving to a 2 bit/cell

design would further double density (2X). Increasing layers to

90, an aggressive and unproven goal, triples density again

(3X). Density increases of 10X to 12X are potentially achieva-

ble. The issue is the cost of the novel processing requirements

for the 3D designs. However, 32 layer structures are being

sampled.5 The NAND strategy increases the density of cells

on the surface of the silicon wafer by applying volumetrics at

the substrate level, comparable to the optical strategy of multi-

ple layers on the disk substrate.

V. VOLUMETRIC TRENDS

Using the discussion in Sec. IV and the areal density

trends in Table I, one can forecast best case areal density

trends, volumetric trends, and component capacity trends for

the various storage technologies over a 10 year period.

Figure 4 shows areal density trends. The steps in the optical

curve represent the transition to plateau and valley recording,

bit length reduction, and 2 bit per cell recording. The open

circles are likely scenarios from the optimum density trends

based on NAND limitations with the number of 3D layers,

slow progress in HDD HAMR, and the unlikelihood of 2 bit/

cell optical recording.

Figure 5 shows best-case component capacity trends for

a ten year period along with notations on volumetric

enhancements for technologies. Another way to view the

capacity data is to translate the data into a storage density

space using component volumes (Table I). This is shown in

Figure 6. Volumetric characteristics of the various technolo-

gies normalize the capacity metric. While any time period in

areal density spans two orders of magnitude, the volumetric

density spans only one half of an order of magnitude.

Notable is that both NAND and TAPE component capacities

will approach HDD capacities due to the lower areal density

growth rate for HDD. Furthermore, HDD has comparable

storage density with TAPE now but this advantage will also

be eroded for the same reason. Finally, BD lags in compo-

nent capacity.

VI. SUMMARY AND OBSERVATIONS

Storage technologies are now incorporating volumetric

strategies to ensure continued increases in component

capacity. While NAND and BD technologies use 3D strat-

egies on the substrate layer to circumvent nano technology

limitations, HDD, lacking 3D layer options on the substrate,

pursues smaller bit cells with one time volumetric additions

of media. TAPE, although faced with the same issue as

HDD, does not yet face these nano issues and continues den-

sity growth with smaller bit cell designs.

1R. Fontana, S. Hetzler, and G. Decad, IEEE Trans. Magn. 48, 1692 (2012).2R. Fontana, S. Hetzler, and G. Decad, in IEEE 2013 Symposium on MassStorage Systems and Technologies (MSST) (2013), p. 8.

3Sony Corporation, see http://www.sony.net/SonyInfo/News/Press/201403/

14-0, 2014.4International Semiconductor Road Map, see http://www.itrs.org, 2014.5K. Park, J. Han, D. Kim et al., in International Solid State CircuitsConference (ISSCC) (2014), p. 334.FIG. 5. 10-year component capacity trends.

FIG. 4. Areal density—10-year trends.

FIG. 6. 10-year volumetric storage density trends.

17E301-4 Fontana, Jr., Decad, and Hetzler J. Appl. Phys. 117, 17E301 (2015)

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