APPLIED PHYSICS LETTERS 92, 243119 共2008兲

High resonance frequency force microscope scanner usinginertia balance support
Takeshi Fukuma,1,a兲 Yasutaka Okazaki,2 Noriyuki Kodera,2 Takayuki Uchihashi,2 andToshio Ando2
1Frontier Science Organization, Kanazawa University, Kakuma-machi, 920-1192 Kanazawa, Japan andPRESTO, Japan Science and Technology Agency, 4-1-9 Honcho Kawaguchi, Saitama, Japan
2Physics Department, Kanazawa University, Kakuma-machi, 920-1192 Kanazawa, Japan

共Received 3 May 2008; accepted 5 June 2008; published online 19 June 2008兲
We have developed the atomic force microscope scanner with the high resonance frequency of540 kHz in the z axis using a piezosupport mechanism “inertia balance support.” In the method, acubic piezoactuator is supported at the four sides perpendicular to the extension axis, by which theresonance frequency of the scanner remains as high as that of the actuator in the free vibration. The
scanner allows driving at low voltage ⫾15 V for the practical z scan range of 330 nm. We
demonstrate the applicability of the scanner to the true-atomic-resolution imaging of mica inliquid. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2951594兴
The improvement of the imaging speed in atomic force
microscopy1 共AFM兲 has great advantages in the practicalapplications. The higher imaging speed gives the higher
throughput in defect testing of semiconductor devices, which
is one of the biggest industrial applications of AFM. For
biological applications, high-speed imaging enables visualizing dynamic behavior of proteins in physiologicalenvironments.2,3
The resonance frequency of the AFM scanner, especiallyin the z direction 共f z兲, is the major factor that limits theimaging speed in AFM. A piezotube scanner is the mostwidely used but has a relatively low f z 共typically ⬍1 kHz兲.
Efforts have been made for enhancing f z by employing shear
piezoactuators,4 counter balance,2 flexural support,5,6 etc.
Among the most sophisticated design was reported by Andoet al. in 2001.2 The scanner has a high f z of 180 kHz and has
been used for visualizing biological processes at video
rate.2,3
In this letter, we propose a design of the high-speed
AFM scanner using the “inertia balance support” mechanism. The distinctive features of the developed scanner include high f z of 540 kHz, simple structure and compact size
共7 ⫻ 7 ⫻ 3 mm3兲, applicability to the atomic-resolution imaging, and compatibility with low-voltage drive 共⫾15 V for z
scan range of 330 nm兲. The low-voltage drive eliminates therisk of electrocution in liquid environment applications. Inaddition, a low voltage buffer amplifier has a much higher
bandwidth and a better noise figure than a high-voltage amplifier, making it ideal for the high-speed and high-resolution
imaging in liquid. Here we show the detailed design andperformance of the developed scanner with demonstration of
the true atomic resolution by imaging mica in liquid.
In a typical design for the z scanner, one end of a piezoactuator with a resonance frequency of f 0 is fixed on a metal
plate and the other end is free to move. This fixation gives
rise to another vibration mode at the frequency f 0 / 2, leading
to a lower f z value. In addition, the whole actuator moves
toward the free end upon application of a driving voltage sothat the center of mass is also displaced in the same directiona兲Electronic mail: fukuma@staff.kanazawa-u.ac.jp.

to produce an impulsive force. The impulsive force is transmitted to the other mechanical components such as x and yscanners and excites the vibrations at low resonance frequencies. Such vibrations could produce a displacement in the z
direction or could be transmitted back to the z scanner. Consequently, f z is limited to values much lower than f 0.
In order to solve this problem, we have employed theinertia balance support mechanism for the fixation of a piezoactuator. Figures 1共a兲 and 1共b兲 show the schematic modeland the photograph of the developed AFM scanner. In thisdesign, the z scanner consists of 2 ⫻ 2 ⫻ 2 mm3 monolithic
piezoactuator placed inside a hole at the center of the support. The diameter of the hole is approximately 30 ␮mshorter than the diagonal of the cross section of the actuatorso that the actuator is supported by the lateral force applied
to the four sides perpendicular to the z axis. In this configuration, the actuator equally extends in both sides of the zaxis. Thus, f z remains as high as f 0. Moreover, the scannermotion hardly displaces the center of mass and hence theimpulsive force generated by the actuator is much smaller
than that in the typical fixation at one end.
Figures 2共a兲 and 2共b兲 respectively show the amplitude
and phase of the z vibration induced by driving the z scanner.
The z vibration was measured with a cantilever in contact
with the z scanner. The curve shows a clean single peak at540 kHz, which approximately agrees with the resonance

FIG. 1. 共Color online兲 共a兲 Schematic model and 共a兲 the photograph of thedeveloped scanner. The scanner has the dimension of 7 ⫻ 7 ⫻ 3 mm3.

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Fukuma et al.

FIG. 2. 共Color online兲 共a兲 Amplitude and 共b兲 phase of the z vibration induced by driving the z scanner. 共c兲 Amplitude and 共d兲 phase of the z vibration induced by driving the x scanner. The characteristics for the y scanner isapproximately the same as that for the x scanner. The z vibration was measured from the deflection of a cantilever 共NCH: Nanosensors兲 in contact
with the z scanner. The amplitude is normalized at the low frequency values.
The dotted lines in 共a兲 and 共c兲 corresponds to the values of ⫾1 dB.

frequency of the piezoactuator in the free vibration
共510– 550 kHz兲. This demonstrates that the inertia balance
support can prevent the reduction of f z due to the fixation.
The amplitude and phase responses show typical profiles fora second order vibration system so that they can be easily
reproduced with a simple LCR circuit. Thus, it is not difficult
to suppress the resonance peak at 540 kHz using the active Q
control method7 for further extending the effective f z value.
The piezoactuators for the x and y axes are fixed on theside of the z scanner as shown in Fig. 1. This configuration isessentially the same as that in the tripot scanner.8 Namely,
the displacement of the x and y actuators induce the displacement of the z scanner as well as the distortion in the wholescanner. Due to the small displacement 共nanometer range兲
compared to the scale of the scanner 共millimeter range兲, thedisplacement of the z scanner caused by the distortion isnegligible.
This structure allows designing the scanner to be simple
and compact, as shown in Fig. 1共a兲. Since the x and y scanners are not placed either over or under the z scanner, themechanical coupling between the x, y scanners and the z
scanner remains relatively small. In addition, the free space
under the z scanner makes it possible to add a counterbalance
for compensating the possible weight increase on the upperside of the z scanner.
The free space in the scanner is filled with elastomer
having a high loss factor to absorb the energy of the spurious
mechanical vibrations 关Fig. 1共b兲兴. This also helps to mechanically isolate the z scanner from the other mechanicalcomponents.
In Figs. 2共a兲 and 2共b兲, there are small peaks at
20– 30 kHz. These minor peaks are caused by the coupling
between the z scanner and other mechanical components.
However, the magnitude of the amplitude peak is less than
1 dB as indicated by the dotted lines in Fig. 2共a兲. The magnitude of the phase peak is as small as the slight phase delayof the z actuator itself. Therefore, these minor peaks are unlikely to cause problems in a practical application.

Appl. Phys. Lett. 92, 243119 共2008兲

Figures 2共c兲 and 2共d兲 show the amplitude and phase of
the z vibration induced by driving the x scanner. The z vibration was measured from the deflection of the cantilever incontact with the z scanner. Although these curves do notshow the pure characteristics of the x scanner itself, they
practically determine the maximum driving frequency for thex scanner. The y scanner shows similar characteristics to
those of the x scanner.
Figure 2共c兲 shows that the gain variation is kept less than
⫾1 dB up to 8 kHz. For obtaining a 256⫻ 256 pixel imageat 30 frame/ s, the required driving frequency of the x scanner is 7.7 kHz. Thus, the maximum x driving frequency of
8 kHz is high enough even for the video-frame imaging.
The piezoactuators used for the x, y, and z scanners havethe driving voltage range from −20 to 100 V, which corresponds to the displacement range from −440 nm to 2.2 ␮m
共22 nm/ V兲. Since the z actuator equally extends to the bothsides of the z axis, the displacement becomes half of thewhole extension 共11 nm/ V兲. For the low drive voltage⫾15 V, the scan range is 660 nm in xy and 330 nm in z. This
scan range is large enough for most of the high-resolution
imaging applications.
We have integrated the developed scanner into a homebuilt frequency modulation AFM 共FM-AFM兲 with a low
noise cantilever deflection sensor.9,10 The deflection noise
density of the sensor is less than 10 fm/ 冑Hz,10 which is essential for oscillating a stiff cantilever with small amplitude
共⬍0.5 nm兲 and thereby obtaining the true atomic
resolution.11
The scanner was driven with a home-built buffer amplifier with an output range of ⫾15 V. The root-mean-square
共rms兲 output noise of the driver is about 35 nV/ 冑Hz, whichcorresponds to the z displacement noise of 0.27 pm for a
bandwidth of 500 kHz and the piezosensitivity of 11 nm/ V.
In order to obtain clear true atomic-resolution images, the z
displacement noise level in the feedback condition should beless than 10 pm. Thus, the instrumentation noise is required
to be less than 1 pm. The combination of the developed
scanner and the low voltage buffer amplifier can satisfy thesestringent requirements even for a wide bandwidth of
500 kHz.
For the purpose of comparison, we also measured theoutput noise of a commercially available high-voltage amplifier 共ENP-4014B: Echo Electronics兲. We confirmed that theamplifier is applicable to the true atomic resolution imaging
at a relatively low imaging speed 共⬍1 frame/ min兲 with apiezotube scanner. The output noise density was 2 ␮V / 冑Hz
for the gain from 0 to 26 dB. The corresponding z noiselevel for 500 kHz bandwidth is 15 pm. This could preventthe atomic-resolution imaging at a high imaging rate.
Figure 3共a兲 shows an FM-AFM image taken in phosphate buffer solution using the developed scanner driven
with the home-built low voltage buffer amplifier. The image
shows the honeycomblike structure which is characteristic of
the cleaved mica surface 关Fig. 3共b兲兴. The individual atomsconstituting the honeycomb lattice are resolved in the image,which demonstrates that the developed scanner is applicableto the atomic-resolution imaging.
The tip was scanned at the velocity of 732 nm/ s, whichis relatively fast for the atomic-resolution imaging. For example, obtaining a 128⫻ 64 pixel image on 5 ⫻ 2.5 nm2 areawith this tip velocity takes 0.87 s. The present imaging speed

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Appl. Phys. Lett. 92, 243119 共2008兲

Fukuma et al.

by enhancing the bandwidth and reducing the noise level. We
presented the way to enhance the scanner bandwidth using
the inertia balance support and to reduce the z displacement
noise using the low-voltage buffer amplifier.
This research was supported by PREST, Japan Scienceand Technology Agency.
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1

FIG. 3. 共Color online兲 共a兲 FM-AFM image of mica in phosphate buffer
solution. 2 ⫻ 1.2 nm2, ⌬f = −50 Hz, tip velocity: 732 nm/ s. A Si cantilever
共NCH: Nanosensors兲 with a resonance frequency of about 130 kHz and Q
factor of 8 in liquid was used. The scanner was driven with a low voltage
buffer amplifier 共⫾15 V兲. 共b兲 Schematic model of the cleaved mica surface.

of our AFM is no longer limited by the resonance frequencyof the z scanner but by the bandwidth of the FM detector
共⬃1.2 kHz兲. The high signal-to-noise ratio of the image
shown in Fig. 3共a兲 suggests that the imaging speed is not
limited by the force sensitivity at the feedback bandwidth.
Therefore, further improvement of the imaging speed should
be possible by enhancing the bandwidth of the FM detector.
In this study, we developed the high-speed AFM scannerwith the high resonance frequency of 540 kHz. We also applied the developed scanner to the atomic-resolution imaging
by FM-AFM in liquid. For developing a high-speed andhigh-resolution AFM, all the components involved in themain tip-sample distance feedback loop must be optimized

2Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp