Challenges of Patterned Media
S.N. Piramanayagam**, **-Senior Member, IEEE,
1
Abstract—Hard disk drive industry has been
looking at a successor for the granular perpendicular
recording media technology for more than 5 years.
Patterned media technology, in the form of discrete
tracks and subsequently in discrete bits, was believed
to be one of the potential candidates. However, the
interest in patterned media has been declining
because of certain challenges. This paper discusses
the technological and economical challenges
associated with patterned media and some potential
solutions.
Index Terms—patterned media, electron beam
lithography, nano-imprint lithography, areal density
I. INTRODUCTION
Hard disk drives have been increasingly using
granular perpendicular media for the past 5 years.
The current areal density stands at around 500
Gbpsi, roughly indicating a 4-fold increase
compared to the initial products [1-4]. It is widely
believed that the areal density progress beyond 1
Tbpsi is very difficult without introducing
alternative technologies such as energy-assisted
magnetic recording or patterned media recording.
Energy-assisted magnetic recording (EAMR)
makes use of media materials with a high
anisotropy constant (Ku) to increase thermal
stability of smaller grains. The writability on such
high Ku materials is enabled from the assistance of
external energy, which may be in the form of heat.
Patterned media, using discrete bits, is on the other
side of the technology spectrum, where media
materials with a Ku smaller than the current ones
can be used. Thermal stability is rather achieved by
increasing the volume, V, of the magnetic units.
While challenges of EAMR are in obtaining highanisotropy media with smaller grain size, high
temperature lubricants and in fabricating integrated
heads with energy delivery, bit-patterned media
faces severe lithography challenges. Patterned
media using discrete-track was expected to provide
a smooth transition from granular to bit-patterned
media, as it involves granular media and
Manuscript received 29 August, 2010 (date on which paper
is submitted for review).
All the authors are with Data Storage Institute, A*STAR
(Agency for Science, Technology and Research) Singapore.
Corresponding author: S.N. Piramanayagam (Phone: +6568748550, email: prem_SN@dsi.a-star.edu.sg)
All rights reserved - © Copyright 2014 IEEE
lithography. However, the progress in both the
technologies is not sufficient to make a transition.
This paper will focus on the challenges of
patterned media in both forms.
II DISCRETE TRACK MEDIA
Discrete track media extends the granular
perpendicular recording media technology by
isolating neighboring tracks by a non-magnetic
material. The non-magnetic material may be
obtained by ion-implantation to convert a magnetic
material into non-magnetic or by physically
removing the magnetic material and filling it back
with a non-magnetic material [5-8]. The
advantages are gained from reduced track-edge
noise. Areal density demonstrations have been
made at close to 900 Gbpsi, indicating the potential
of this technology. However, commercial products
with DT media are not available.
Although the technology challenges introduced by
lithography resolution are not severe, the main
challenges arise from a commercial perspective.
For example, the throughput of commercial nanoimprint machines has been estimated to be about
300 disks per hour, whereas the throughput of the
granular perpendicular media is roughly 3 times
higher. If a switch is made to discrete-track media,
3 units of nano-imprint machines have to be bought
to match the throughput which makes the needed
investment very huge. This is a significant
problem, especially considering the fact that the
hard disk media industry made huge investments
only in the past 6 years to make the transition from
longitudinal to perpendicular recording media
technology.
TABLE I
MAJOR CHARACTERISTICS OF DISCRETE TRACK AND DISCRETE
BIT PATTERNED MEDIA
Lithography
Type of
Technological
half-pitch
Media
Challenge
(nm)
Discrete
25-40
Granular
Same as
Track
granular media
Discrete
Below 12
Continuous
Lithography
Bit
and writeSynchronization
Another example of cost-forbidden technology
transition is the cost of a mold and imprinting on
media manufacturing. For example, if the cost of a
mold is $400000, the cost per disk arising from the
mold is 16 cents, if the number of imprints that can
be made from a mold is only 5000 (assuming 5000
daughter molds from a master and 5000 disks from
each daughter mold). However, if the number of
successful imprints is only 500, the cost climbs up
to $1.6 per piece, just from the cost of the mold.
Considering that lithography is a new process in
media manufacturing, achieving high yield using
lithography based media manufacturing is another
effect of staggered bits is lacking in the literature
[8].
In addition, synchronization of the writing head
over the magnetic islands during the writing
process is a technical challenge that needs to be
solved prior to the introduction of bit-patterned
media.
REFERENCES
Fig. 1. The dependence of cost per disk, incurred by the mold
alone, on the number of successful imprints
challenge.
III DISCRETE BIT PATTERNED MEDIA
At areal densities in the range of 1-2 Tbpsi, bitpatterned media help to overcome certain
challenges, such as grain size reduction and
writabilitiy, associated with the conventional
granular media. On the other hand, they bring
several new challenges.
Areal density of 1 Tbpsi requires features of
about 12 nm in size and 2 Tbpsi requires a feature
size less than 9 nm. These features need to be
achieved at a pitch of only double these values.
There are several lithography challenges associated
with achieving these small dimensions. For
example, the e-beam resist commonly used for the
manufacturing of the discrete-track media molds
cannot be used at this resolution. The resists that
can provide higher resolution have a larger dose
requirement, resulting in a larger exposure time
and the associated increase in the cost of mold.
Therefore, in the current scenario, the cost of a
patterned media disk (just from the mold
perspective) will be even larger.
In addition to cost, there are also several
technical challenges associated with the transition
to bit-patterned media recording, which gains areal
density advantage by relaxing the linear density
and aggressively increasing the track density.
Besides the challenges in achieving a high track
density from the servo perspective, the head noise
and the writability of the narrower heads will be
other sources of problems. Staggered bit-patterned
media have been proposed to overcome some of
these challenges; however a detailed study on the
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