In a new approach, we have fabricated 6:1 aspect ratio magnetic nanocolumns, 60–250 nm in diameter, embedded in a hard aluminum-oxide/gallium-arsenide (Al2O3/GaAs) substrate. The fabrication technique uses the highly selective etching properties of GaAs and AlAs, and highly efficient masking properties of Al2O3 to create small diameter, high aspect ratio holes. Nickel (Ni) is subsequently electroplated into the holes, followed by polishing, which creates a smooth and hard surface appropriate for future reading and writing of the columns as individual bits for high density information storage. We have used magnetic force microscopy and scanning magneto-resistance microscopy to characterize the resulting magnets. We find the columns more magnetically stable than previously achieved with magnets embedded in a SiO2 substrate. Such stability is necessary before further writing of perpendicular patterned media can be demonstrated

Scherer, Axel

Schultz, Sheldon

Todorovic, Mladen

Wong, Joyce

Caltech Authors - Main

JOURNAL OF APPLIED PHYSICS VOLUME 85, NUMBER 8 15 APRIL 1999Fabrication and characterization of high aspect ratio perpendicular
patterned information storage media in an Al2O3/GaAs substrate
Joyce Wonga) and Axel Scherer
Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125
Mladen Todorovic and Sheldon Schultz
Center for Magnetic Recording Research and Department of Physics, University of California, San Diego,
La Jolla, California 92093-0319
In a new approach, we have fabricated 6:1 aspect ratio magnetic nanocolumns, 60–250 nm in
diameter, embedded in a hard aluminum-oxide/gallium-arsenide (Al2O3 /GaAs) substrate. The
fabrication technique uses the highly selective etching properties of GaAs and AlAs, and highly
efficient masking properties of Al2O3 to create small diameter, high aspect ratio holes. Nickel ~Ni!
is subsequently electroplated into the holes, followed by polishing, which creates a smooth and hard
surface appropriate for future reading and writing of the columns as individual bits for high density
information storage. We have used magnetic force microscopy and scanning magneto-resistance
microscopy to characterize the resulting magnets. We find the columns more magnetically stable
than previously achieved with magnets embedded in a SiO2 substrate. Such stability is necessary
before further writing of perpendicular patterned media can be demonstrated. © 1999 American
Institute of Physics. @S0021-8979~99!63408-9#I. INTRODUCTION
By combining high resolution electron beam lithogra-
phy, etching, and electroplating, it is now possible to micro-
fabricate nanometer scale magnets. Recently there has been
an effort to employ such nanomagnets as individual bits for
perpendicular patterned magnetic storage in order to increase
the bit density for magnetic storage in the range of 20–100
Gbits/in.2 Our initial work has focused on creating free-
standing magnets with lateral dimensions as small as 20 nm.1
More realistic structures made of embedded Ni columns in a
SiO2 substrate, with aspect ratios of 4:1, have also been
demonstrated.2 Our previous studies have shown that this
aspect ratio is not sufficient to achieve the magnetic stability
required for reliable reading and writing of this form of
media.3 In order to achieve a sufficiently high coercivity,
embedded magnets of higher aspect ratio are desirable. We
have re-examined the possible substrate material selection
for the lithographic production of high aspect ratio patterned
media and have now developed a new and more robust fab-
rication procedure suitable for embedding high aspect ratio
nanomagnets into a durable matrix.
Instead of using polymethyl methacrylate ~PMMA!
e-beam resist or silicon dioxide (SiO2), we employ Al2O3
and GaAs. It has been found that the etch rate selectivity of
Al2O3 over GaAs is larger than 30:1.4 Thus, by combining
the robust Al2O3 mask, high resolution electron beam lithog-
raphy, and the directional chemically assisted ion beam etch-
ing ~CAIBE!, it is now possible to achieve small diameter
and high aspect ratio holes in the GaAs substrate, which are
then filled with Ni by electrodeposition.
II. PROCEDURE
A schematic flow diagram of the fabrication procedures
in defining the Ni column arrays embedded in an
a!Electronic mail: joyceyw@its.caltech.edu5480021-8979/99/85(8)/5489/3/$15.00
Downloaded 18 Dec 2005 to 131.215.240.9. Redistribution subject toAl2O3 /GaAs substrate is shown in Fig. 1. A 200 nm layer of
AlAs and a 50 nm GaAs cap layer are grown by molecular
beam epitaxy ~MBE! on top of a GaAs substrate. A 200 nm
layer of SiO2 is then sputter deposited and a 10 nm layer of
Cr and 40 nm layer of Au are subsequently evaporated onto
the sample @Fig. 1~a!#. Dot array patterns are defined on the
PMMA coated sample by vector-scanned electron beam li-
thography, followed by development in a 3:7 cellulose–
methanol mixture @Fig. 1~b!#. The patterns are transferred
into the Cr/Au layer by Ar ion milling @Fig. 1~c!# and into the
SiO2 layer by C2F6 reactive ion etching @Fig. 1~d!#. The SiO2
FIG. 1. Schematic diagram of the processing sequence for the fabrication of
Ni columns embedded in an Al2O3 /GaAs substrate.9 © 1999 American Institute of Physics
 AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
5490 J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 Wong et al.layer then acts as an etch mask for further pattern transfer
into the AlAs layer by Cl2 assisted ion beam etching @Fig.
1~e!#. In our CAIBE system, a Kauffman Ar1 ion source is
used in conjunction with a gas introduction nozzle to accel-
erate high energy ions towards the substrate covered with an
etch mask. This allows us to achieve a high etching rate and
selectivity of specific materials, as well as the directionality
necessary for defining high aspect ratio structures. Immedi-
ately following CAIBE, the AlAs layer is converted into
Al2O3 by wet thermal oxidation at 380 °C for 15 min @Fig.
1~f!#.5 This robust layer of Al2O3 is the final mask for pattern
amplification in the perpendicular direction into the GaAs
substrate by using further CAIBE @Fig. 1~g!#. Figure 2 shows
a scanning electron microscopy ~SEM! image of an array of
holes ~left! defined by electron beam lithography, produced
by the procedures just described. In the right side of Fig. 2,
we present a 103 enlarged view of a single hole ~right! of
approximately 60 nm in diameter.
Electroplating is then used to deposit Ni into the holes
that have been etched into the GaAs substrate @Fig. 1~h!#.
The plating apparatus is identical to that in Ref. 6, where the
probe contacts the sample outside of the plating bath and
does not disturb the electrodeposition of the Ni columns. In
our case, Ni is used as the anode and the conductive GaAs
substrate is used as the cathode. Nickel sulfamate is used as
the plating medium and a pulse current with a duty cycle of
80% ~2 s on and 0.5 s off! is applied. The plating is con-
ducted under an optical microscope and end point detection
is possible through the change in optical contrast of the
plated Ni columns. Precise end point detection is not critical
in our case as any overplated Ni column can be polished to
the desired height, which is determined by the thickness of
the Al2O3 layer, using a mechanical polish.
Using this new approach, we have fabricated 80380 ar-
rays of Ni columns of 6:1 aspect ratio that are embedded in a
hard Al2O3 /GaAs substrate. Figure 3 shows an SEM image
of an electroplated Ni dot array of 100 nm diameter and 2.5
mm spacing embedded in an Al2O3 /GaAs substrate. The
plating seems to be very uniform over a large area and the
mechanical polishing procedure results in a clean and
FIG. 2. SEM image of an array of etched holes ~left! before filling by
electroplating with Ni, and a 103 enlargement of a single hole ~right! of
approximately 60 nm in diameter.Downloaded 18 Dec 2005 to 131.215.240.9. Redistribution subject tosmooth Al2O3 surface. Atomic force microscopy shows that
the columns protrude 20–50 nm out of the surface, and the
area around the columns has a surface roughness of 5 nm.
We have also fabricated dot arrays of the same diameter ~100
nm! but smaller spacing ~215 nm! using the same technique.
However, we chose a 2–2.5 mm column spacing in our struc-
tures to match the sizes of the magneto-resistive sensors used
in this work. Figures 4~a! and 4~b! show cross-section views
of an etched line of 225 nm width and 1.65 mm depth before
and after electroplating Ni, respectively. It is believed that
the etched holes have similar cross-section profiles as the
etched lines, with the exception of slightly smaller diameters
and depths, which, however, correspond to similar aspect
ratios.
III. MAGNETIC CHARACTERIZATION
In order to characterize the magnetic properties of the
newly developed patterned media structure, we used both
magnetic force microscopy ~MFM!7 and scanning magneto-
resistance microscopy ~SMRM!.8 In the MFM measurement,
FIG. 4. Cross-section views of an etched line of 225 nm width and 1.65 mm
depth ~a! before and ~b! after electroplating Ni.
FIG. 3. SEM image of an electroplated Ni dot array of 100 nm diameter and
2.5 mm spacing embedded in an Al2O3 /GaAs substrate. AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
5491J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 Wong et al.we use a home made MFM utilizing electrochemically
etched nickel tips9 and a fiber-optic interferometer10 as the
vibration detector. We focused our investigation on the cor-
ner pattern of one of the fabricated arrays in order to com-
pare images from the same columns obtained through the
two measurement techniques. Figure 5 shows an MFM im-
age of a 12312 mm scan of plated Ni columns, with diam-
eters of approximately 230 nm, embedded in an Al2O3 /GaAs
substrate. We believe that the observed nonuniformity in the
magnetic strength of the columns is due to the graininess
during growth of the plated structures.
In the SMRM technique,8 a commercial magneto-
resistance ~MR! head is raster scanned in contact with the
sample and the detected MR voltage is recorded with respect
to the position over the sample. In our investigation, we fo-
cused on determining the stability of magnetic columns
when imaged by this technique. We believe that it is impor-
tant to demonstrate reliable reading with this type of sensor,
since future magnetic storage systems using patterned media
are likely to employ a similar read method. Previous work3
has shown that the perpendicular component of the magnetic
field from the bias current in the MR element was sufficient
to switch the column magnetization direction, thus eliminat-
ing the possibility of reliably writing the perpendicular pat-
terned media without erasing it through the reading process.
The MR signal from the columns embedded in a SiO2 sub-
strate was therefore a ‘‘dibit’’ type signal, and only with the
application of the external bias field was a single pole re-
sponse obtained.3
We found that in our current structures, the columns do
provide at least a 23 larger signal when biased with an ex-
ternally applied magnetic field, which indicates that they re-
lax into a magnetic state that deviates from a uniform mag-
netization throughout the particle. But more importantly,
85% of the particles remain magnetized in the direction of
the original saturation field, and the MR current is not suffi-
cient to switch them when imaging in zero field. Figures
6~a!–6~d! show four 12312 mm SMRM images of the same
535 corner array in different external fields, and Fig. 6~e!
shows the single line scans that represent the MR voltage
signals from each of the images. Figures 6~a! and 6~c! show
the different saturation magnetization states of the columns
FIG. 5. 12312 mm MFM image of Ni columns of 2 mm spacing embedded
in an Al2O3 /GaAs substrate.Downloaded 18 Dec 2005 to 131.215.240.9. Redistribution subject toachieved with 626 Oe of applied field, while Figs. 6~b! and
6~d! show the remanent zero field states approached from the
positive and negative saturation fields respectively. An im-
portant comparison is to be made between images of Figs.
6~b! and 6~d!. Even though a few columns provide almost no
contrast in zero field, most of the columns still provide large
signals and are oriented in the ‘‘correct’’ direction. In the
SMRM scans in zero field, we always obtain the single pole
response and have not observed a dibit type response, which
would have indicated a low switching field magnetic particle.
The increase in Hc of the individual columns is critical for
further improvement in the stability of such media, where the
written information must be read nondestructively. We find
the width of the column MR response to be 600–700 nm,
which is much larger than the column diameter. Such a wide
response from a single column has been observed
previously.3 To our knowledge, the spatial MR response of a
perpendicularly magnetized cylinder has not been modeled,
and we are currently doing a theoretical investigation of this
experimental observation.
ACKNOWLEDGMENTS
The authors gratefully acknowledge C. C. Cheng and O.
J. Painter for many helpful discussions. They thank F. Spada,
R. O’Barr, S. Y. Yamamoto, M. Re, and V. Prabhakaran for
helpful suggestions, J. F. Smyth for providing them with the
MR heads, and R. K. Lee for the growth of the MBE wafers.
This work was funded by the National Science Foundation
and the Army Research Office.
1 W. Xu, J. Wong, C. C. Cheng, R. Johnson, and A. Scherer, J. Vac. Sci.
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2 P. R. Krauss and S. Y. Chou, J. Vac. Sci. Technol. B 13, 2850 ~1995!.
3 S. Y. Yamamoto, R. O’Barr, S. Schultz, and A. Scherer, IEEE Trans.
Magn. 33, 3016 ~1997!.
4 O. J. Painter, C. C. Cheng, and A. Scherer ~unpublished!.
5 K. D. Choquette et al., IEEE J. Sel. Top. Quantum Electron. 3, 916
~1997!.
6 R. O’Barr, S. Y. Yamamoto, S. Schultz, W. Xu, and A. Scherer, J. Appl.
Phys. 81, 4730 ~1997!.
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8 S. Y. Yamamoto and S. Schultz, Appl. Phys. Lett. 69, 3263 ~1996!.
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FIG. 6. 12312 mm SMRM image of ~a! positive saturation field of 126 Oe,
~b! 0 Oe positive remanent state, ~c! negative saturation field of 226 Oe, ~d!
0 Oe negative remanent state, and ~e! MR voltage vs position of ~a!, ~b!, ~c!,
and ~d!. AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp


Fabrication and characterization of high aspect ratio perpendicular patterned information storage media in an Al2O3/GaAs substrate

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Fabrication and characterization of high aspect ratio perpendicular patterned information storage media in an Al2O3/GaAs substrate

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