# High-Resolution Magnetic Force Microscopy Using Carbon Nanotube Probes Fabricated Directly by Microwave Plasma-Enhanced Chemical Vapor Deposition

Authors: Kei Tanaka; Masamichi Yoshimura; Kazuyuki Ueda
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

Carbon nanotubes (CNTs) have been successfully grown on the tip apex of an atomic force microscopy (AFM) cantilever by microwave plasma-enhanced chemical vapor deposition (MPECVD). Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations reveal that the diameter of the CNTs is∼30 nm and the magnetic particles with diameter of ∼20 nm, which was used as catalyst for the CNT growth, exist on the top. This CNT probe has been applied to magnetic force microscopy (MFM) on the ultrahigh-density magnetic recording media with 1200 kilo flux change per inch (kfci).


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## Body

## 1. Introduction

Magnetic  force microscopy (MFM) is a
powerful tool to study the magnetic domain structure at a microscopic level,
and is widely used in the research and development of magnetic nanodevices.
Images with 50 nm resolution are routinely obtained when using commercial
microscope and magnetic probes. However, higher resolution becomes more desired with increased recording density in future magnetic recording media. It is well known that
the MFM resolution is limited by the tip
geometry and height above the sample surface [1]. In particular, decrease in tip diameter is the most significant
factor to improve the resolution.Thus to achieve reedy tips, methods using focused ion beam
(FIB) [2, 3] and the utilization of carbon nanotube (CNT) [4–9] have been
proposed. There have been two methods for the
fabrication of CNT-based MFM probes: a mechanical attachment of metal-filled
CNTs or catalyst-attached CNTs under scanning electron microscopic (SEM)
observation [7–9], and a coating
of magnetic metal over a preattached CNT probe [4–6]. In the latter
technique, Kuramochi et al. reported that the magnetic recording media up to
1100 kilo flux changes per inch (kfci) has been clearly observed with a
resolution of approximately 10 nm [4]. However, the methods described above
require time-consuming mechanical attachment or multiple processes, and seem
inadequate for the large-scale production.In a previous work, we successfully
fabricated CNT probes for scanning tunneling microscopy (STM) by microwave
plasma-enhanced chemical vapor deposition (MPECVD) [10]. This direct growth is quite suitable for the mass-scaled
fabrication. It should be noted that the transmission electron microscopy (TEM)
observation revealed the existence of magnetic particle as a catalyst on the
top of each CNT. This feature can be applicable to MFM probes. Here, we have
performed a growth of CNTs on the AFM cantilever made of Si, and have evaluated
the resolution in the MFM measurement of longitudinal magnetic recording media with
1200 kfci.


## 2. Experimental

The  growth of CNTs on
the Si cantilever was performed by using an MPECVD system
(CVD-CN-100, Ulvac Japan, Ltd., Kanagawa, Japan). A Co thin film 20 nm
thick, as the catalyst, was coated on the Si cantilever (k=2.8N/m) by vacuum deposition. It was then set into the metal
holder with seat mesh on the small hole in order to avoid the electric
discharge and ion etching from the plasma [10], and was placed on the electrode
in the MPECVD system. The mixed gas of CH4 and H2 was
used for the CVD growth. The flow rate of H2 and CH4 was
80 and 20 sccm, respectively. Total gas pressure was set at 226 Pa.
The microwave used was
2.45 GHz, 500 W and the growth time was 7 minutes. During the growth process, a
voltage of 200 V was applied between the electrodes. Prior to the CNT growth,
the cantilever was exposed to hydrogen plasma for cleaning as well as for
activation of catalysts. During CVD process, no
heater was used, but the substrate temperature was raised to approximately 600C° as measured by
thermocouple. The morphology of tip apex after growth was observed by a
field emission scanning electron microscope (FE-SEM, Hitachi, S4700). A high-resolution
transmission electron microscope (TEM, JEOL, JEM2000EX) was used to determine the
atomic structure of the CNT. In order to examine MFM performance, the surface
of longitudinal magnetic recording media with densities of 600, 800, 1000, and 1200 kfci was
observed by Nanoscpoe III (Digital
Instruments, NY, USA) in air.


## 3. Results and Discussion

Figure1 shows an SEM
image of CNTs  grown on the tip apex of the Si cantilever. The CNTs ~30 nm in
diameter are clearly observed at the tip apex. Figure 2 shows a TEM image of a
typical CNT similarly grown on an Si wafer. The catalyst particle exists on the
top of CNT. This
suggests that CNTs grew via the so-called tip growth mechanism [11]. The CNT shows tubular structure with graphene layers (lattice spacing of 0.34 nm) as
resolved in the enlarged TEM image (Figure 2(b)). The magnified image of
catalyst particle is shown in Figure 2(c), where the lattice of 0.19 nm is thoroughly
observed. The lattice spacing corresponds to Co3C(103),
suggesting that the CNT is grown with a nanoparticle of single crystalline  Co3C
at its top [12].Figure 1
SEM image of CNTs on
the tip apex of Si cantilever.(a) TEM images of cocatalyzed CNT that scraped
away from Si substrate. (b) and (c) are enlarged images of (a).Using this fabricated CNT probe
and a conventional probe, MFM observation is performed for the
ultrahigh-density recording media of 1000 and 1200 kfci, as shown in Figure3.
In the case of conventional probe, a stripe pattern of magnetic domain
structure is observed up to 1000 kfci (Figure 3(a)), though the periodicity is not so clear. Irregular pattern appears
appreciably comparable to the periodicity. In the case of 1200 kfci, no stripe
magnetic domain structure is observed, as shown in Figure 3(c). In contrast,
the recording period is clearly resolved up to 1000 kfci using the present CNT
probe (Figure 3(b)), and even for 1200 kfci, the periodicity is visible on the
background of the irregular pattern due to the limitation in the resolution
and/or sample preparation.MFM images of ultrahigh-density recording media of 1000 and
1200 kfci using conventional and CNT probes. (a),
(c) show MFM image obtained by conventional probe. (b), (d) show MFM image
obtained by CNT probe. The recording densities were (a) and (b) 1000 kfci, (c)
and (d) 1200 kfci.
(a)(b)(c)(d)Figure4 shows the power
spectra corresponding to Figures 3(b) and 3(d). These spectra were obtained by
averaging twenty spectra after performing a Fourier transform of a single-line
profile of the cross-section in MFM image. As shown in Figure 4, peaks at 55.1
and 45.2 nm are recognized (which is double the recoding bit size). These values are larger than the calculated values of 50.8
and 43.2 nm for 1000 and 1200 kfci. This may be due to inaccuracies in the
recording process [2]. In these power spectra, strong background is observed on
the low-frequency side. The inverse Fourier analysis indicates that this is
caused by an irregular period due to disorder of the longitudinal recording.
The writing of 1200 kfci is known to be difficult in the case of the
longitudinal recording.Power spectra corresponding to Figures3(b) and 3(d). We have averaged
twenty power spectra after performing a Fourier transform on a single-line
profile obtained from MFM image.
(a)
1000 kfci(b)
1200 kfciIn order to estimate attainable resolution, we follow analysis procedure reported
by Kuramochi et al. [4]. Figure 5(a)
shows normalized intensity of power spectrum as a function of recording density. It is observed that the intensity decreases with increase in recording density.
The intensity decreases with the increase of recording density. The attainable
resolution in the observation of recording media is obtained from intersecting
point of extrapolation of solid line and noise level line in Figure 5 [4]. The
noise level was obtained from DC-erased area in MFM image as shown in Figure
5(b). The intersecting point corresponding to attainable resolution is around
7.6 nm, which is comparable to the value reported in [4]. It should be noted
that in spite of the CNT-MFM probe by the present simple fabrication process
without coating of magnetic materials, the resolution has been improved compared
with conventional probe. The mass-scale fabrication of high-resolution CNT-MFM
probes is also expected by this direct growth method.Figure 5
Plot of normalized
intensity of power spectrum at the various recording bit length. The solid line
was obtained by linear fitting for experimental plot. The sloping dashed line
was obtained by extrapolating of solid line. The
noise level was obtained from DC-erased area in (b)
MFM image.


## 4. Conclusion

In conclusion, we have successfully
fabricated the CNT probes for MFM by direct growth using MPECVD. The CNTs of ~30 nm in diameter have been grown on the tip apex of Si
cantilever, and the magnetic particle (~20 nm) is located at the end. The MFM
observation using the fabricated CNT probe for magnetic recording media is
demonstrated. The CNT probe resolves a stripe
magnetic domain structure of ultrahigh-density recording media of 1000 and 1200 kfci much clearer than the conventional probe.



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*Source: 147204-2008-12-10.xml*