1. Volumetric density trends (TB/in.3) for storage components: TAPE, hard disk drives,
NAND, and Blu-ray
R. 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
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2. Volumetric density trends (TB/in.3
) for storage components: TAPE, hard disk
drives, NAND, and Blu-ray
R. E. Fontana, Jr.,1,a)
G. M. Decad,1
and S. R. Hetzler2
1
IBM Systems and Technology Group, IBM Almaden Research Center, San Jose, California 95120, USA
2
IBM 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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3. 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
F ¼ 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
a
2 bit/cell design.
b
Bit 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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4. 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
F ¼ 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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5. 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. SUMMARYAND 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.
1
R. Fontana, S. Hetzler, and G. Decad, IEEE Trans. Magn. 48, 1692 (2012).
2
R. Fontana, S. Hetzler, and G. Decad, in IEEE 2013 Symposium on Mass
Storage Systems and Technologies (MSST) (2013), p. 8.
3
Sony Corporation, see http://www.sony.net/SonyInfo/News/Press/201403/
14-0, 2014.
4
International Semiconductor Road Map, see http://www.itrs.org, 2014.
5
K. Park, J. Han, D. Kim et al., in International Solid State Circuits
Conference (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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