Nanopencil as a wear-tolerant probe for ultrahigh density data storage
Noureddine Tayebi,
1
Yoshie Narui,
2
Robert J. Chen,
1
C. Patrick Collier,
2,
a
Konstantinos P. Giapis,
2
and Yuegang Zhang
1,
b
1
Intel Corporation, 2200 Mission College Boulevard, Santa Clara, California 95054, USA
2
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, USA
Received 17 July 2008; accepted 19 August 2008; published online 11 September 2008
A dielectric-sheathed carbon nanotube probe, resembling a “nanopencil,” has been fabricated by
conformal deposition of silicon-oxide on a carbon nanotube and subsequent “sharpening” to expose
its tip. The high aspect-ratio nanopencil probe takes advantage of the small nanotube electrode size,
while avoiding bending and buckling issues encountered with naked or polymer-coated carbon
nanotube probes. Since the effective electrode diameter of the probe would not change even after
significant wear, it is capable of long-lasting read/write operations in contact mode with a bit size
of several nanometers. ©
2008 American Institute of Physics
.
DOI:
10.1063/1.2981641
Small bit size and fast data rate have made probe-based
seek-and-scan data storage systems the ideal candidate for
future ultrahigh density nonvolatile memories.
1
–
4
In such
systems, a scanning probe
or an array of probes
is used to
write and read data on nonvolatile media; the bit size de-
pends mainly on the radius of the probe tip. Recent studies
have shown that the field confinement of a nanometer-scale
tip radius could locally reverse the polarization of a ferro-
electric thin film and create domain size as small as a few
nanometers in diameter.
1
,
2
,
5
,
6
Such technology could realize
storage density of 10 Tbits
/
in.
2
2
The high-speed contact
mode operation in practical devices, however, can cause
rapid mechanical wear and dulling of conventional scanning
probe tips, thus decreasing resolution. Due to their strong
mechanical and wear-resistant properties, carbon nanotubes
have attracted interest as probe tips in scanning probe lithog-
raphy and data storage.
7
–
9
The combination of their small
diameter and high aspect ratio enable carbon nanotubes to
retain their read/write resolution even after significant wear.
However, carbon nanotubes are prone to bending and buck-
ling, which can be mitigated partially only with short nano-
tubes
100 nm in length
.
7
–
9
Such constraints hinder the
use of carbon nanotube probes in practical data storage de-
vices where contact mode is required for high data rate, and
longer carbon nanotubes are needed to outlast the storage
device lifetime. Here we demonstrate dielectric-coated car-
bon nanotube probes of high aspect ratio with wear resis-
tance and stiffness suitable for long-lasting read/write opera-
tions in contact mode. Moreover, such probes are capable of
writing bit sizes with radii as small as 6.8 nm on ferroelectric
films, and thus allowing for a storage density of 1 Tbit
/
in.
2
Long
1–2
m
single-walled
carbon
nanotubes
SWNTs
or their bundles of 5–10 nm in diameter
are first
attached to metal-coated scanning probe tips.
10
,
11
This is
achieved by imaging isolated and vertically aligned SWNTs
grown on a SiO
2
film using the metal-coated tips, during
which the isolated SWNTs attach to the tips. Note that the
attached SWNTs can either be metallic or semiconducting in
nature because of the lack of a reliable method for nanotube
type selection. However, this is not a problem for the current
study because all chemical vapor deposited semiconducting
SWNTs are
p
-doped when exposed to air. In the bundle
cases, the metallic SWNTs in the bundle normally dominate
electric conductance. The probe system is then coated with a
conformal 65-nm-thick SiO
x
layer using a plasma-assisted
chemical vapor deposition at room temperature.
11
,
12
The re-
sulting SWNT
/
SiO
x
composite structure resembles a nan-
opencil, as shown in Fig.
1
a
. When exposed, the SWNT
core serves as a thin electrode, while the thick SiO
x
sheath
improves its mechanical strength during read/write opera-
tions
Fig.
1
b
. Additionally, the coating reinforces the
electrical and mechanical contact between the base probe and
SWNT, and thus prevents the tube detachment under contact
mode scanning.
7
To expose the insulated SWNT electrode, the nanopencil
is “sharpened” by repetitively scanning a 20
20
m
2
area
of a diamond surface in contact mode at a speed of
100
m
/
s.
11
This is performed using the conductive AFM
module of an Asylum Research MFP-3D scanning probe mi-
croscope operating under contact mode wit
ha5nN
applied
force. By simultaneously recording the height and current
signals, the sharpening process is monitored in real time by
applying a 0.5 V bias between the probe and the conductive
diamond sample until a significant current is detected, as
shown in Fig.
2
. Figure
3
shows a 1
1
m
2
height and
current distribution images, and
I
-
V
curve taken after elec-
trode exposure. The resistance
R
= 3.6 M
corresponds to
a
Present address: Oak Ridge National Laboratory, Center for Nanophase
Materials Sciences, Oak Ridge, TN 37831.
b
Author to whom correspondence should be addressed. Present address:
The Molecular Foundry, Lawrence Berkeley National Laboratory, Berke-
ley, California 94720. Electronic mail: yzhang5@lbl.gov.
FIG. 1. TEM images of the nanopencil.
a
Nanopencil before SWNT elec-
trode exposure
initial length: 980 nm
.
b
Nanopencil after electrode ex-
posure
length: 870 nm
. The inset shows the clean SWNT electrode pro-
truding from the SiO
x
coating.
APPLIED PHYSICS LETTERS
93
, 103112
2008
0003-6951/2008/93
10
/103112/3/$23.00
© 2008 American Institute of Physics
93
, 103112-1
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