Japanese Journal of Applied Physics
Vol. 47, No. 1, 2008, pp. 718–720
#2008 The Japan Society of Applied Physics
Local Oxidation of Si Surfaces by Tapping-Mode Scanning Probe Microscopy:
Size Dependence of Oxide Wires on Dynamic Properties of Cantilever
Shinya N ISHIMURA, Takumi O GINO, Yasushi T AKEMURA1 , and Jun-ichi SHIRAKASHI
Department of Electrical and Electronic Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
1
Division of Electrical and Computer Engineering, Yokohama National University, Yokohama 240-8501, Japan
(Received July 12, 2007; accepted October 31, 2007; published online January 22, 2008)
Local-oxidation nanolithography by scanning probe microscopy (SPM) has enabled us to fabricate nanometer-scale oxide
wires on Si surfaces. Here, we investigate the size dependence of the wires on the dynamic properties (oscillation amplitude,
Q-factor) of the cantilever used in tapping-mode SPM local oxidation. With the enhancement of oscillation amplitude from 72
to 432 nm at a natural Q-factor of 505, the width and height of the Si oxide wires were controlled and ranged from 31.3 to
18.3 nm in width and from 1.5 to 0.4 nm in height, respectively. On the other hand, when the amplitude was fixed at a natural
value of 144 nm, local oxidation with a low Q-factor of 193 realized an oxide wire with a width of 33.7 nm and a height of
1.2 nm. At a high Q-factor of 1665, the width and height of the fabricated Si oxide wire were 24.9 and 1.0 nm, respectively.
These results imply that the size of the oxide wire is more strongly affected by the oscillation amplitude of the cantilever than
the Q-factor. [DOI: 10.1143/JJAP.47.718]
KEYWORDS: anodic oxidation, scanning probe microscopy (SPM), surface modification, Q-factor, amplitude, tapping mode
1.
of the dynamic properties of the cantilever such as the drive
amplitude and the quality factor (Q-factor), thus adjusting
the lateral size of the water meniscus. We have previously
revealed that the feature size of Si oxide wires can be well
controlled by varying the oscillation amplitude of the
cantilever and that the size uniformity of the wires does
not depend on the modulation amplitude of the cantilever.11)
In the present paper, in order to investigate the effect of
another important property, the Q-factor, we perform a
comparative study of the size dependence of the Si oxide
wires on the Q-factor and the oscillation amplitude of the
cantilever.
Introduction
Scanning probe microscopy (SPM) can achieve atomic
resolution on semiconductor and metallic surfaces. In
particular, local oxidation nanolithography by SPM is a
reliable method of producing ultrasmall patterns on material
surfaces.1) These modifications can be performed in contact
mode, tapping mode, or noncontact mode. An understanding
of the kinetics and mechanism of SPM local oxidation has
been achieved in the last decade. The key factors for
controlling the feature size of the oxide are the generation of
space charges within the oxide,2) the formation of a water
meniscus,3) the modulation of applied bias voltage,4) and the
operation mode5) of the SPM. In contact-mode SPM local
oxidation, it has been reported that voltage modulation
sweeps out space charges at the Si/SiO2 interface during the
reverse-bias cycle, allowing these charged ions to diffuse
into the bulk, which cannot occur under a static electric
field.2) The advantages of tapping-mode operation are the
elimination of the lateral force, which damages the surface
in the contact mode, and the improvement in resolution
compared with the noncontact mode. In noncontact-mode
operation, the tip is separated from the sample surface by a
gap of 3 –10 nm,3) and the minimum feature size of the Si
oxide has been reported to be from 13 nm6) to 30 nm.5) We
have reported that tapping-mode SPM local oxidation is a
suitable technique for producing oxide wires with good size
controllability and uniformity because of the enhanced
diffusion of space charges by electric-field modulation.7)
Furthermore, a recently developed tapping-mode technique
produced a Si oxide wire with sub-10 nm resolution.8)
Anodic oxidation occurs when the ionic current flows
between the SPM tip and the sample surface through a water
meniscus.9) The control of the water meniscus is of primary
importance for SPM local-oxidation nanolithography. The
lateral dimension of the water meniscus determines the
resolution of the oxide area, since the anodic oxidation
occurs within the meniscus.10) In tapping-mode operation,
the tip-sample distance is dependent on the oscillation
amplitude of the cantilever. Therefore, the active control of
the cantilever in tapping-mode operation allows the tuning
2.
Experimental Procedure
Local-oxidation experiments were performed by SPM
operating in the tapping mode (SPA400/SPI4000, SII
NanoTechnology) in ambient air. The relative humidity
was kept around 30%. The sample was p-type silicon (100)
with a resistivity of 1 kcm. The spring constant and
resonance frequency of the Si cantilever were 40 N/m and
300 kHz, respectively. The oscillation amplitude of the
cantilever was changed from 72 to 432 nm by tuning the
excitation voltage of the cantilever. The Q-factor of the
resonance is defined by Q ¼ m!=, where ! is the
resonance frequency, m is the effective mass, and is the
damping factor of the cantilever. The control of the Q-factor
was achieved via the implementation of a positive feedback
loop. In general, it is difficult to set a stable desired Q-factor
in Q-factor-controlled SPM when executing measurements.
Therefore, even when a high Q-factor is set, the response is
sacrificed and in particular, the stability of the measurement
deteriorates. Further, when a low Q-factor is set, there is a
problem that high sensitivity in the measurement of the force
gradient is difficult.12–14) Hence, in this study, the Q-factor of
the cantilever, which had a natural value of around 500, was
varied from 193 to 1665 in order to avoid such difficulties.
3.
Results and Discussion
Figure 1 shows SPM images and cross sections of Si
oxide wires as a function of the oscillation amplitude of the
cantilever. The scanning speed and DC voltage, known to be
718
Jpn. J. Appl. Phys., Vol. 47, No. 1 (2008)
S. NISHIMURA et al.
Fig. 1. (Color online) SPM images and cross sections of Si oxide wires formed by amplitude modulation during tapping-mode SPM
local oxidation. The oscillation amplitude of the cantilever was varied from 72 to 432 nm. The quality factor was fixed at a natural
value of 505. The DC bias voltage applied to the tip and the scanning speed were set at 20 V and 20 nm/s, respectively. WA and HA
correspond to the average width and average height, taken from 10 cross sections and WSTD and HSTD represent the STD of the width
and height, respectively.
ments in ref. 11, which is a comparative study of contactmode and amplitude-modulated tapping-mode experiments.
Figure 3 shows the Si oxide wires formed with varying
the Q-factor. The cantilever initially had an oscillation
amplitude of 144 nm and a Q-factor of 537. As shown in
Fig. 4, the feature size of the oxide tends to decrease with
increasing the Q-factor. The width and height decreased
from 33.7 to 24.9 nm and from 1.2 to 1.0 nm, respectively,
with STDs of WSTD ¼ 2:0 { 3:6 nm and HSTD ¼ 0:21{ 0:26
nm. In particular, Figs. 2 and 4 show that the feature size of
the Si oxide wires is more strongly affected by the
oscillation amplitude of the cantilever than the Q-factor.
For amplitude modulation with a fixed Q-factor, a larger
amplitude of the cantilever may reduce the spatial dimension
of the water meniscus formed between the SPM tip and the
sample surface, so that the width of the fabricated oxide can
be controlled over a wide range by varying the oscillation
amplitude. Additionally, the amplitude enhancement causes
a decrease in the average intensity of the electric-field
strength between the tip and the sample. This also contributes to the improved size controllability of the fabricated
oxide with a smaller resolution. On the other hand, for SPM
local oxidation with a controlled Q-factor, since the increase
of the Q-factor effectively promotes greater force sensitivity
of the cantilever, it is possible to control the motion of the
cantilever precisely. Therefore, the fluctuation of the tipsample distance is considerably suppressed with the enhancement of the Q-factor. Thus, it seems that the water
meniscus between the tip and the sample is more stably
formed with a higher Q-factor value, resulting in the precise
control of the oxide size. In fact, as shown in Figs. 1 and 3,
the STDs of the size of the fabricated oxide wires are
approximately 2 – 3 nm in width and 0.1– 0.2 nm in height
and are less dependent on the dynamic properties of the
cantilever. It is suggested that the size uniformity of the
fabricated oxide wires is unaffected by both the enhancement of the oscillation amplitude and that of the Q-factor.
3
50
Width
Height
2
30
20
1
10
0
Height (nm)
Width (nm)
40
Natural Q = 505
DC voltage = 20 V
Scanning speed = 20 nm/s
0
100
200
300
400
0
500
Amplitude (nm)
Fig. 2. Size dependence of Si oxide wires on the amplitude of the
cantilever.
the main control parameters during Si oxide formation, were
fixed at 20 nm/s and 20 V, respectively, throughout the
experiments. The Q-factor of the cantilever was set at a
natural value of 505. All SPM images in this study were of
areas of 1 1 mm2 . The size dependence of the Si oxide
wires on the oscillation amplitude is also plotted in Fig. 2.
With enhancing the oscillation amplitude of the cantilever,
the size of the fabricated oxide wires was clearly decreased.
By changing the oscillation amplitude of the cantilever from
72 to 432 nm, the feature size of the fabricated oxide was
well controlled and ranged from 31.3 to 18.3 nm in width
and from 1.5 to 0.4 nm in height, respectively. The size
uniformity of the oxide was evaluated by the standard
deviation (STD) of measurements taken from 10 cross
sections along the X-axis. The STDs of the Si oxide wires
were typically 2.1– 3.2 nm in width (WSTD ) and 0.1– 0.2 nm
in height (HSTD ), which were smaller than those (WSTD ¼
9:2 nm, HSTD ¼ 0:3 nm) obtained by contact-mode experi719
Jpn. J. Appl. Phys., Vol. 47, No. 1 (2008)
S. NISHIMURA et al.
Fig. 3. (Color online) SPM images and cross sections of Si oxide wires formed by Q-factor control during SPM local oxidation.
The Q-factor of the cantilever was varied from 193 to 1665. The cantilever initially had an oscillation amplitude of 144 nm and
a Q-factor of 537. DC voltage and scanning speed are the same as those in Fig. 1.
Width (nm)
2
30
20
1
Height (nm)
Width
Height
40
Natural Q = 537
Amplitude = 144 nm
DC voltage = 20 V
Scanning speed = 20 nm/s
10
0
oscillation amplitude of the cantilever than the Q-factor.
Furthermore, the STD of the fabricated oxide was measured
to be around 2 – 3 nm in width and 0.1– 0.2 nm in height in
spite of the active control of the cantilever dynamics.
Therefore, amplitude modulation in combination with Qfactor control during SPM local oxidation is effective for the
fabrication of Si oxide wires with high controllability and
good size uniformity.
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0
400
800
1200
1600
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Q-factor
Fig. 4. Size dependence of Si oxide wires on the Q-factor of the
cantilever.
Therefore, tapping-mode SPM local oxidation combining
both amplitude modulation and Q-factor control is a useful
technique for realizing nanometer-scale Si oxide wires with
good size uniformity.
4.
Conclusions
The nanometer-scale modification of Si surfaces is
demonstrated by active control of the cantilever dynamics
in tapping-mode SPM local oxidation. The feature size of Si
oxide wires was well controlled by varying the oscillation
amplitude and Q-factor of the cantilever. The size controllability of the Si oxide was more strongly affected by the
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