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IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012
Geometrically Planar Ion-Implant Patterned Magnetic Recording Media
Using Block Copolymer Aided Gold Nanoisland Masks
Chulmin Choi
, Kunbae Noh , Duyoung Choi , Jirapon Khamwannah
Daehoon Hong , Li-han Chen , and Sungho Jin
, Chin-Hung Liu
,
Center for Magnetic Recording Research, University of California at San Diego, La Jolla, CA 92093 USA
Mechanical & Aerospace Engineering, University of California at San Diego, La Jolla, CA 92093 USA
Materials Science & Engineering, University of California at San Diego, La Jolla, CA 92093 USA
We have developed patterned media via ion implantation using Au nano mask approach for local control of coercivity of magnetically
multilayer film. Au nano-islands produced through a di-block copolymer templated technique is used as the mask for
hard
ion implantation. The sputter deposited
multilayer film having vertical magnetic
anisotropy is coated with a diblock copolymer layer, two phase decomposed into vertically pored nanostructure, then chemically processed to nucleate gold nanoislands corresponding to the diblock copolymer nanostructures. Subsequent nitrogen (N) ion implantation,
using these Au islands as implantation-blocking masks, allows a patterned penetration of implanted ions into unmasked portion of the
multilayer film, thus creating invisible but magnetically isolated bit island geometry while maintaining the overall flat configuration of the patterned media.
Index Terms—Bit patterned media, nano-island.
I. INTRODUCTION
A
S IS WELL known, bit patterned media (BPM) is one of
the promising approaches to overcome the limitation of
storage density in magnetic hard disk memory. The major advantages of BPM are firstly, the transition noise is eliminated
because the bits are now defined by the physical location of the
elements and not by the boundary between two oppositely magnetized (but physically in contact) regions of a thin film media
[1]. Secondly, very high data densities can be obtained because
the stability criterion now refers to the volume and anisotropy
of the entire magnetic element, not to the individual grains comprising the conventional granular media. Each of the magnetic
switching elements could therefore become as small as a few
nanometers [2], [3], as compared to tens of nanometers.
However, the formation of high-density, a few nanometers scale dot arrays is a challenging task for achieving a
practical goal of ultrahigh-density data storage [4]–[6]. In
particular, methods for producing isolated high-density magnetic nanoscale arrays with a pitch of 20 nm or less have
been extensively studied with the aim of fabricating the next
generation of patterned magnetic media with a recording
and beyond [7]–[9]. Various fabricadensity of 1
tion techniques have been employed, such as those based on
e-beam lithography (EBL), nano imprinting lithography (NIL),
di-block copolymer (DBCP) and anodized aluminum oxide
(AAO) [10]–[15].
One of the essential requirements for bit patterned media is
the surface planarization so as to overcome the flying instability of the head-slider on topographically patterned surfaces
having valleys and trenches. These extra steps can result in
Manuscript received March 02, 2012; revised April 25, 2012 and May 16,
2012; accepted June 07, 2012. Date of current version October 19, 2012. Corresponding author: S. Jin (e-mail: jin@ucsd.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2012.2204867
significantly increased cost and complexity of processing with
potential implications on structural reproducibility and reliability issues. In order to produce flat patterned media without
topographic features, filling of the recessed trench region
, spin-on-glass,
with non-magnetic material (e.g., with
hydrogen silsesquioxane, etc.) followed by removal of the
excess material (chemical mechanical polishing and reactive
ion etching) has been the commonly used approach [16], [17].
Alternative methods are sought to produce bit patterned
media without topographically protruding island geometry
common to many lithography and self-assembly patterning
techniques. Utilizing ion implantation damage is a promising
technique for creating a topographically flat recording media.
Well controlled ion implantation can cause the magnetization
to be reduced subatantially depending on the dose and energy
utilized to displace atoms by ion irradiation [18]. At suitable
ion implantation levels, the lattice parameter and Curie temperature of the magnetic layer can be altered, which can lead to
a reduction or disappearance of magnetization and anisotropy
[19]. Among the various ions used for trials, nitrogen ion
implantation seems to exhibit a promising trend of reducing the
magnetization and anisotropy of [Co/Pd]n magnetic multilayers
[20].
In this work, we have developed a convenient method for
local control of coercivity of flat-surface planar magnetic
recording media of [Co/Pd]n multilayer film. Because the
available ion implantation dose and energy were such that the
nitrogen implanting with Au mask particle alone damaged much
of the magnetic material even with the Au nanoparticles, we
utilized silica buffer layer to adjust the energy of the incoming
ions and them weaker so as to induce more optimal implantation., instead of over-exposure of the magnetic layer to the ions.
buffer layer was added on top of the
20–200 nm thick
magnetic material
multilayer structure and the diblock copolymer was processed
to create Au nanoislands. The material was then ion implanted by nitrogen from above the Au nano metal mask and
0018-9464/$31.00 © 2012 IEEE
CHOI et al.: GEOMETRICALLY PLANAR ION-IMPLANT PATTERNED MAGNETIC RECORDING MEDIA USING MASKS
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Fig. 1. Schematic illustration of the fabrication processes for bit patterned
media (BPM) using nanoparticle-masked ion implantation process.
buffer layer. Patterned regions intended to be magnetic
recording bit islands are blocked from ion implantation due to
the presence of Au islands above while the outside regions get
preferentially ion implanted.
II. EXPERIMENTAL PROCEDURES
A. Magnetic Thin Film Deposition
Prior to magnetic thin film multilayer deposition, a 4 in. (100)
Si substrate was cut into several pieces and cleaned in acetone,
isopropyl alcohol, and methanol successively with ultrasonication followed by a de-ionized (DI) water rinse. Then, specimens were immersed in piranha solution comprised of
for 20 min at 80
followed
by copious DI water rinse. Magnetic recording media with a
layer structure having perpendicular magnetic anisotropy were
sputter deposited on a flat substrate first. To assist selective-area
ion implantation, a
buffer layer was used.
B. Di-Block Copolymer Thin Film Deposition
The patterned structure was made by di-block copolymer
self-assembly followed by RIE process. Poly(styrene-b-4vinylpyridine) (S4VP) (number-average molecular weight,
,
,
, where
is the average molecular weight)
was purchased from Polymer Source (Montreal, Canada) and
used without further purification. 0.5 wt.% S4VP copolymer
solution dissolved in toluene was spin-coated at 2000 rpm for
60 s on
film. This film was then exposed to tetrahydrofuran (THF) vapor in a closed glass vessel for 3 h to induce
Fig. 2. SEM micrographs of PS-b-P4VP thin film. (a) As-spun
PS-b-P4VP and (b) THF-annealed PS-b-P4VP film (c) after
the di-block copolymer process, Au dot arrays are formed on
sub. multilayer thin
film to serve as mask islands.
mobility and allow phase separation to occur. The film was
then immersed in pure ethanol for the poly(4-vinylpyridine)
(P4VP) phase to swell, thus leading to a porous structure.
C. Formation of Au Dot Arrays
A 0.05 wt% gold precursor solution
in ethanol was
spin-coated onto the reconstructed films and then, immediately
exposed to toluene vapors for 20 min. The surface reconstructed
films were treated with toluene vapor to recover the film with
original surface morphology. Subsequent oxygen plasma treatment led to arrays of gold nanoparticles in the same positions
as those of the original nanopores, while the polymer was completely degraded.
D. Nitrogen Ion Implantation
Non-magnetic implantation can lead to magnetic softening
and the small ion size would have minimal damages on various materials characteristics [20]. Nitrogen ion implantation
was carried out by using the varian implanters system. The machine used in the manufacture of BPM use 10 KeV beam energy and
ion dosage irradiation. The overall
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012
Fig. 3. Magnetic hysteresis loops for Au dot masked and nitrogen ion implanted [Co/Pd]n multilayer film with as a function of
process of nitrogen ion implanted Co/Pd magnetic multilayer
were schematically illustrated in Fig. 1.
The sample microstructure was characterized by ultrahigh
resolution scanning electron microscopy (UHR SEM) and the
magnetic properties were evaluated by vibrating sample magnetometers (VSM) before and after non-magnetic ion implantation.
III. RESULTS AND DISCUSSION
As-spun S4VP type DBCP thin film can be further ordered
by THF (tetrahydrofuran) vapor annealing treatment and
thus leading to improved close-packed cylindrical P4VP microdomains [21]. However, annealing in THF longer than the
optimized time can readily result in a detachment of the DBCP
film from the substrate, which deteriorates local ordering of
DBCP. In this paper, optimum annealing time was 3 h. When
the solventannealed film is immersed in ethanol, the S4VP
film reconstruction occurs due to its solvating power difference on PS and P4VP phases; thus leading to a nanoporous
film [22]. The representative SEM images before and after
solvent annealing treatment are seen in Figs. 2(a) and (b),
which show that highly self-ordered S4VP film is developed
on
substrate multilayer thin film. The diblock copolymer interpore
(pore-to-pore) distance, an important parameter that determines
the applied voltage for anodization, was 48.3 nm
2.7 nm,
based on SEM micrograph analysis. After solvent annealing,
Au
solution was spin-coated onto the
reconstructed films. Subsequent oxygen plasma treatment led
to a more or less periodic array of gold nano-particles in the
same positions as those of the original nanopores, while the
polystyrene (PS) was completely removed. These Au nano
dots from spontaneous self-assembly of PS-b-P4VP diblock
copolymer are utilized in this study as a metal mask for ion
implantation (Fig. 2(c)). After solvent annealing process and
oxygen plasma treatment, the sample is covered with Au
nano-island array with a good periodicity and high density. A
buffer layer thickness.
further improvement in Au island periodicity, density and long
range ordering would be beneficial for BPM.
Fig. 3 shows the magnetic hysteresis loop of nitrogen ion
implanted Co/Pd multilayer film prepared using Au island
mask. The M-H loop properties for different
buffer layer
thicknesses (0, 20, 100 nm thickness) are shown in the figure.
Because of the particular ion energy and dose utilized for
these sets of implantation experiments, the Au nano-island
mask alone was not enough to block the impinging ions with
dosage at 10 keV. Thus a
buffer
layer was added to absorb some of the ions and help to create
well-defined selective-area ion implantation. With just 20 nm
thick
buffer layer, the saturation magnetization of the
magnetic layer, even in the supposedly masked regions, is significantly reduced and the perpendicular anisotropy is mostly
destroyed. The M-H loop with a thicker 100 nm
buffer
layer, on the other hand, is improved to a higher coercivity
value indicating a substantial blocking of implanted ions by
buffer layer, the successful isolation of magnetic islands
and the excellent exchange decoupling between the magnetic
islands enabled by ion implantation to the in-between regions.
Those regions outside the magnetic recording bit islands are not
masked by Au metal mask dots. using DBCP technique, which
was then nitrogen ion implanted to allow penetration into the
Co/Pd multilayer film, thus creating magnetically isolated bit
island geometry while maintaining the overall flat geometry
of the patterned media. However, as can be seen in Fig. 3, the
M-H loop of nitrogen ion implanted Co/Pd magnetic multilayer
still shows the presence of in-plane magnetized regions in the
outer core of the bit patterned area, which indicates that, the
processing needs to be improved. At 200 nm thickness
buffer layer, there is no difference between the M-H loop
of implanted Co/Pd magnetic multilayer and un-implanted
sample. In the case of the nitrogen ions with
dosage at 10 keV, they are too weak to get through
layer
having a 200 nm thickness. Various types of non-magnetic ions
(e.g., oxygen, argon or fluorine) and different process/materials
parameters (e.g., ion beam energy (keV), implantation dose
CHOI et al.: GEOMETRICALLY PLANAR ION-IMPLANT PATTERNED MAGNETIC RECORDING MEDIA USING MASKS
, beam angle and the thickness of the buffer layer)
can be studied for ion implantations for patterned media.
IV. SUMMARY AND CONCLUSIONS
We have developed a convenient method for local control
of coercivity of geometrically planar magnetic recording media
of a Co/Pd multilayer magnetically hard film by ion implantation with Au nanoisland metal mask and
buffer layer.
The Au nano dot arrays fabricated using spontaneous self-assembly of PS-b-P4VP diblock copolymer has been utilized here
as an ion-blocking metal mask for selective area patterned ion
implantation.
buffer layer has been found to be useful in
terms of reducing the implanting ion energy to avoid excessive
ion damage to the magnetic layer. Optimization of the
buffer layer dimension needs to be studied further. It is expected
an improved processing specifics will be useful for developing
more practical fabrication techniques for bit-patterned magnetic
media.
ACKNOWLEDGMENT
The authors wish to acknowledge the financial support of
this work by the NSF-Nanomanufacturing Division under
Grant CMMI #0856674, the Center for Magnetic Recording
Research at UC San Diego, a National Research Foundation
Grant through World Class University Program under Grant
R33-2008-000-10025-0, and the Center for Nanostructured
Materials Technology under the Grant Frontier R&D Program.
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