APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 23
10 JUNE 2002
Electrostatic force microscopy using a quartz tuning fork
Yongho Seo and Wonho Jhea)
Center for Near-field Atom-Photon Technology and School of Physics, Seoul National University,
Seoul 151-742, Korea
Cheol Seong Hwang
School of Materials Science and Engineering, Seoul National University, Kwanak-ku, Seoul 151-742, Korea
共Received 11 February 2002; accepted for publication 20 April 2002兲
We demonstrate an electrostatic force microscopy based on a quartz tuning fork with 50 nm spatial
resolution and 1 pN force sensitivity. We use a tuning fork with a spring constant of 1300 N/m and
a Q factor of 3000. A sharpened nickel tip is attached to a prong of the tuning fork as well as
electrically connected to the electrode of the prong. By applying a dc bias to the tip, ferroelectric
domain patterns are recorded and read out on piezoelectric thin film. © 2002 American Institute of
Physics. 关DOI: 10.1063/1.1485312兴
Since local electrostatic surface charge or potential distributions were imaged by scanning force microscope,1–3
there have been several experimental efforts to obtain nanoscale electrostatic images of the ferroelectric polarization,4,5
charges in metallic nanoparticles,6 and bound surface charges
in semicondutors7 using electrostatic force microscopy
共EFM兲. In conventional EFM, the Si cantilever is employed
as a force sensor and optical deflection detection is employed
to measure its motion. A microfabricated cantilever has such
a small spring constant 共0.1–10 N/m兲 that it has a high force
sensitivity (⬃ pN). Besides delicate optical alignment, however, the cantilever has some weak points: 共i兲 high resolution
EFM is difficult because the dithering amplitude is large
(ⲏ10 nm) and 共ii兲 laser light should be employed, which is
undesirable, particularly for EFM of semiconductors and metallic nanoparticles because the laser light modifies the
charge distribution due to the photoionization effect.8
Recently, to overcome these problems, EFM using a piezoresistive cantilever has been developed.9,10 The
piezoresistive-cantilever-based EFM uses piezoresistivity of
boron-doped Si: when the cantilever is deflected, the resistance of the boron doped layer changes. Because no light is
needed for deflection measurement, it is possible to operate
in dark conditions, avoiding unwanted photoionization effect. However, there are also some disadvantages: 共i兲 the
change in resistance ⌬R/R is very small (⬃1 ppm/pnm),
which indicates that to measure its frequency shift in EFM,
the dithering amplitude of the cantilever should be on the
order of 100 nm and 共ii兲 the dissipative power due to Joule
heating is serious, particularly in low temperature experiments. For the resistance and applying voltage of ⬃2 k⍀
and ⬃5 V, respectively, the resultant dissipated power
amounts to 10 mW.
In this letter, we demonstrate nonoptical EFM that employs a quartz tuning fork using the piezoelectric effect.
Since Guethner et al.11 introduced the quartz tuning fork as a
force sensor in atomic force microscopy, there have been
several attempts to employ the tuning fork in near-field scana兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
ning optical microscopy,12 atomic force microscopy,13–15
magnetic force microscopy,16,17 and acoustic near-field
microscopy.18 There are important advantages in tuning-forkbased force microscopy over using the piezoresistive cantilever: a high Q-value (103 – 104 ), small oscillation amplitude
(⬃1 nm), and low dissipative power (⬃1 nW).
A prong of the tuning fork is of 2.2 mm in length, 190
␮m in thickness, and 100 ␮m in width. Note that this geometry of the tuning fork corresponds to a spring constant of
k⯝1300 N/m. Ni wire with a 150 ␮m diameter is first etched
electrochemically in H3 PO4 acid and the wire diameter is
reduced to 10 ␮m. Then the thin wire is inserted into a platinum wire loop containing H3 PO4 , so that the wire section
goes through the electrolyte. The tuning fork is then moved
so that the end of Ni wire is attached to the prong of the
tuning fork with silver paste. As a result, the Ni wire is
connected mechanically as well as electrically to the electrode of the prong. After curing, a dc voltage between the
electrode (⫹) and the platinum wire loop (⫺) is applied,
which makes the etched Ni wire sharp and separated into two
parts. All the processes are done with a home-made micromanipulator.
The length of the protruded tip is ⬃100 ␮ m and the tip
diameter is ⬃100 nm. Due to additional mass, the resonance
frequency of 33 kHz was reduced slightly by 100 Hz. At
resonance, the full width at half maximum is about 12 Hz
and the corresponding Q value is about 3000, which is 30
times larger than that of a conventional microfabricated cantilever. The minimum detectable capacitance limited by the
thermal noise is given by1
C min⫽
冉
16k B TB 共 ⑀ A 兲 2 k
V 4Q ␻
冊
1/4
,
共1兲
where k B is the Boltzmann constant, T the temperature, ⑀ the
permittivity, and A the area of the capacitor. For example, for
the tuning-fork-based EFM with a bandwidth of B⫽10 Hz at
room temperature, the minimum detectable capacitance is 2
⫻10⫺20 F.
A dc voltage V dc and an ac voltage V 0 cos ␻t are applied
to the electrode that is connected to the tip. The dc voltage is
for writing as well as reading out the ferroelectric domains,
0003-6951/2002/80(23)/4324/3/$19.00
4324
© 2002 American Institute of Physics
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Appl. Phys. Lett., Vol. 80, No. 23, 10 June 2002
Seo, Jhe, and Hwang
4325
FIG. 2. EFM images of a PZT film after 共a兲 poling and 共b兲 line drawing. The
scanning areas are 15⫻15 and 0.9⫻0.9 ␮ m2 , respectively.
FIG. 1. Approach curves obtained at bias voltages of 0, 1, and 2 V between
the tip and the sample.
whereas the ac voltage is oscillating the tuning fork at its
resonance frequency. Consequently, on the other electrode of
the tuning fork, current is induced due to piezoelectricity of
the tuning fork, which is measured by an current–voltage
converter. To measure the shift in frequency associated with
the tip approach, phase detection mode is employed. A
home-made phase detector consisting of a voltage comparator and an exclusive OR gate is used with a low pass filter
(RC⯝10 ms). The signal to noise ratio is ⯝104 and the
frequency resolution is about 2 mHz.
With the dc bias voltage V dc applied to the tip, the resonance frequency shifts are measured as a function of the
distance, as shown in Fig. 1. The sample surface is a clean,
grounded Al layer. Note that when the Al layer is biased with
the tip grounded, we also obtain the same results. The dithering amplitude was 17 nm the drive voltage V 0 of 50 mV.
When no bias voltage is applied, there is a shallow dip that
indicates van der Waals attractive interaction.15,19 As the tip
becomes closer to the sample, the frequency increases rapidly. This suggests that repulsive interaction due to contact
occurs. When V dc was applied, deeper dips appeared which
indicate attractive electrostatic forces given by,1
F e⫽
1 ⳵C 2
V ,
2 ⳵ z dc
共2兲
where V dc⫽1 V, C⫽ ⑀ 0 A/z, A⯝10⫺14 m2 , and z⯝5 nm.
The force gradient expected, ⳵ F e / ⳵ z, is 2⫻10⫺2 N/m.
On the other hand, when a small force gradient is applied to the tip, the shift in frequency ⌬ f is obtained as
⌬f⯝
1 f ⳵Fe
,
2 k ⳵z
共3兲
where f is the resonance frequency(⫽33 kHz). With frequency resolution of 2 mHz, the measurable force gradient is
10⫺4 N/m. Taking into consideration that the dithering amplitude of the tuning fork can be reduced to 10 nm, the minimum detectable force difference becomes 1 pN. Note that the
electrostatic force between the tip and the sample is typically
⬃1 nN. The shift in frequency at the dips in Fig. 1, ⌬ f
⯝0.1 Hz, corresponds to the force gradient F e⬘ ⯝1
⫻10⫺2 N/m calculated from Eq. 共3兲, which is of the same
order as that expected from Eq. 共2兲.
In addition to capacitive force, there can be Coulombic
force when the sample has spontaneous polarization or is
charged by an external potential. Assuming that the surface
polarization field of the sample E s is uniform and that the
charge at the tip q t does not modify E s , the Coulombic force
can be expressed as E s q t , where E s ⫽ ␴ /2⑀ 0 and q t
⫽CV dc . 20 Therefore the shift in frequency due the ferroelectric polarization is
⌬f⯝
1 f ␴ V dc ⳵ C
.
4 k ⑀0 ⳵z
共4兲
Figure 2共a兲 shows an EFM image of a poled
PbZr0.5Ti0.5O3 共PZT兲 thin film of 250 nm thickness deposited
on a Pt layer by the sol–gel process 共the Pt layer is grounded
during measurements兲. For a poling process, the tip biased at
V dc⫽⫺10 V is scanned on a 7⫻7 ␮ m2 area of the PZT
sample in contact mode. Then a larger area of 15⫻15 ␮ m2 is
scanned at a constant gap separation of 50 nm between the
tip and the sample and at V dc⫽2 V. Its maximum contrast
corresponds to a frequency shift of ⌬ f ⯝30 mHz. From Eq.
共4兲, where z⫽300 nm because z is the distance from the tip
end to the grounded Pt layer, the polarization is estimated to
be ␴ ⯝2 ␮ C/cm2 , which is a reasonable when one considers
that the saturated polarization of the sample measured with a
large parallel capacitor was ⬃10 ␮ C/cm2 .
Narrow lines were written and read out as shown in Fig.
2共b兲. The image is 0.9⫻0.9 ␮ m2 , the width of the left line is
about 150 nm, and its expected spatial resolution is better
than 50 nm. This spatical resolution, which is limited by the
tip diameter, can be improved further by replacing the tip
with a sharper one. In the writing process, the scanning speed
does not influence the remanent polarization and thus the
patterns written remained for longer than 5 h.
Figure 3 shows EFM images showing various patterns.
The scanning areas are 4⫻4 and 7⫻7 ␮ m2 , respectively.
FIG. 3. Some patterns recorded on the PZT sample with scanning areas of
4⫻4 and 7⫻7 ␮ m2 , respectively.
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4326
Appl. Phys. Lett., Vol. 80, No. 23, 10 June 2002
The check pattern in Fig. 3共a兲 was written by applying V dc
⫽⫺10 V 共dark兲 and ⫹10 V 共bright兲 alternately, although
the bright dots are not very clear. We attribute the imperfectness to the initial charge distribution or the imprinted polarization of the PZT film.21,22 Some characters were also recorded with V dc⫽⫺10 V and read out with V dc⫽2 V 关Fig.
3共b兲兴. Note that with 300 nm width, all letters are fully resolved.
In summary, we have demonstrated tuning-fork-based
EFM. With 50 nm spatial resolution and 1 pN force sensitivity achieved, such simple EFM has important advantages: 共i兲
it is convenient and inexpensive to manufacture, 共ii兲 it is
possible to operate the EFM in a dark environment so that
optical deflection detection is not needed, 共iii兲 its low power
dissipation allows operation at low stable temperatures.
This work was supported by the Creative Research Initiatives of the Korean Ministry of Science and Technology.
The authors acknowledge Crystal Bank 共KOSEF–Pusan National University兲 for providing the PZT sample.
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