REVIEW OF SCIENTIFIC INSTRUMENTS
VOLUME 73, NUMBER 9
SEPTEMBER 2002
Tapping mode atomic force microscopy in liquid with an insulated
piezoelectric microactuator
B. Rogers,a) D. York, N. Whisman, M. Jones, K. Murray, and J. D. Adamsb)
Department of Mechanical Engineering and the Nevada Ventures Nanoscience Program,
University of Nevada, Reno, Reno, Nevada 89557
T. Sulchek
E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305
S. C. Minne
NanoDevices Inc., 5571 Ekwill Street, Santa Barbara, California 93111
~Received
11 March 2002; accepted for publication 10 June 2002!
Tapping mode atomic force microscopy in liquids is enhanced using an insulated cantilever with an
integrated piezoelectric microactuator. When vibrating the cantilever via direct force modulation by
the actuator, a single resonance peak appears in the plot of rms cantilever amplitude versus excitation
frequency, eliminating the spurious resonances typical of acoustic excitation in a liquid medium. This
simplifies selection of the cantilever’s natural resonance frequency for improved tuning accuracy and
speed. Acoustic excitation can excite cantilever modes that do not displace the tip of the cantilever but
vibrate the microscope’s detection system and create unwanted liquid-coupled acoustic waves
between the liquid-cell and the sample. These modes are eliminated by directly forcing the cantilever.
Insulated microactuated probes offer a simple and more direct alternative solution to recently
presented magnetic tuning methods. © 2002 American
Institute
of
Physics.
@DOI:10.1063/1.1499532#
modes of the fluid cell.7 A frequency sweep yields numerous
Tapping mode atomic force microscopy ~AFM!1 is a very peaks in the rms cantilever amplitude in the general range of
important tool capable of nanometer scale resolution on the desired cantilever resonance. This makes selection of the
biological samples. Because there is intermittent contact with cantilever’s natural frequency difficult .8 9
Han, Lindsay, and Tianwei and others have magnetized
the sample surface, the frictional forces that plague contact
mode are negligible in tapping mode, allowing fragile or cantilevers and applied external magnetic fields using a soloosely attached samples to be imaged without significant lenoid to vibrate the cantilever directly. This type of direct
force modulation requires modification of the AFM cantilever
damage.
Paramount in enabling investigation of delicate biological and additional hardware, such as the Magnetic Actuated Drive
specimens in their natural environments was the development TappingMode for the MultiMode scanning probe microscope,
of tapping mode atomic force microscopy in liquids, initially available from Digital Instruments ~DI!. An unmagnetized
used to image deoxyribonucleic acid plasmids on mica in cantilever has also been oscillated by applying an ac current to
on the cantilever in the presence of a permanent
water.2 Liquid operating advantages include the elimination of custom traces
8
capillary forces,3 and the ability to generate high-resolution magnet. Cantilevers having various geometries have also
been oscillated using localized application of acoustic
images of biological samples in physiological conditions.4,5
10
In the case of tapping mode the probe must first be forced radiation pressure. This technique was used to characterize
cantilevers,
not
to
image.
into vibration, or ‘‘tuned,’’ at its natural frequency. For tunes
Revendo and Proksch11 constructed a fluid cell capable of
in liquid this is typically accomplished using a large piezotube
housed inside the atomic force microscope ~AFM! head. When imaging the same sample area using both acoustical excitation
tuned in this fashion, a liquid cell holding the cantilever with a piezotube and magnetic excitation with a built in
undergoes forced vibration in order to excite the cantilever magnetic drive. While the piezotube-driven tuning curve
resonance. This form of acoustically excited tapping mode has exhibited a complicated acoustic spectrum, the magnetic
been shown to generate unwanted mechanical excitation of the method produced a cleaner resonance response. The two
detection system, rendering it less sensitive to the approach of techniques were shown to produce images with similar resothe sample.5,6 Furthermore, indirect forcing causes spurious lution. However, acoustic modulation resulted in sonication of
molecular samples, causing sample instability and motion.
resonances due to acoustic
The drawbacks of magnetic imaging were higher complexity
and cost, contamination of samples by magnetic metal ions,
a!
Author to whom correspondence should be addressed; electronic mail:
and heating of the fluid cell by the electromagnet.
rogers@unr.edu
In another attempt to simplify the liquid tuning response, a
b!
Electronic mail: jdadams@unr.edu, Web: http://www.nano.unr.edu/adams/
conventional AFM liquid cell with acoustic tuning and acI. INTRODUCTION
0034-6748/2002/73(9)/3242/3/$19.00
3242
© 2002 American Institute of Physics
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Rev. Sci. Instrum., Vol. 73, No. 9, September 2002
Tapping AFM in liquid
3243
FIG. 1. Active probe with an integrated ZnO piezoelectric microactuator
~photo courtesy of NanoDevices!.
tive quality factor control was used to show that complicated
cantilever rms amplitude profiles having irregular resonance
peaks can be simplified to a clearly recognizable, single
peak—that of the cantilever alone. 12 However, this technique
works by raising the effective Q as high as 250, which can
negatively affect image quality and limit scan speed. The
system bandwidth is inversely proportional to the quality
factor, and, as shown by Sulchek et al., 13 Q can be reduced
as a means of increasing tapping mode scan speed.
The magnetic methods for vibrating a microcantilever in
liquids avoid the acoustic tuning drawbacks, yet require the
generation of external magnetic fields which can also add
unwanted heat and contaminating materials to the imaging
environment. Moreover, magnetically driven cantilevers can
exhibit spurious amplitude signals in the 35–50 kHz range
due to normal excitation of modes of the cantilever substrates
caused by the magnetic field.6 The many scattered spikes in
the signal make tuning such cantilevers difficult.
In this article, we present a simpler method of tuning and
imaging in liquid that does not require additional hardware or
magnetic field generation. Piezoelectric microactuated
probes,14,15 now commercially available16 and compatible
with the DI Nanoscope IV System Controller for use in air,
have been insulated for use in liquid with much improved
cantilever tuning. By vibrating only the cantilever and not the
entire cell, the microactuator provides the direct drive
benefits similar to the earlier referenced magnetic methods,
but without the magnetic hardware and sample-holder modifications. The system’s response does not exhibit the spurious
peaks associated with an external drive. This makes the use
of smaller oscillation amplitudes possible, which can diminish the energy transferred to the sample from the tip and
help minimize sample damage.6
FIG. 2. ~a! Piezoelectric microactuated cantilever frequency response ~0–50
kHz! during tune in water using large z-piezo tube as the tuning actuator
instead of the piezoelectric microactuator. Amplitude is the thick trace and
phase is the thin trace; resonance frequency is shown by the arrow. Note the
numerous resonance peaks corresponding to components of the system other
than the cantilever itself. ~b! Frequency response ~0–50 kHz! of cantilever
used in ~a! during tune in water using integrated microactuator instead of piezo
tube. The single resonance peak and corresponding 2180° phase shift is easily
identified.
that described by Sulchek et al., 17 was used for this experiment. The cell attaches to four pins at the bottom of the
Dimension AFM just like conventional cantilever holders. The
bulk silicon die from which the cantilever extends was
attached with epoxy to a ceramic substrate and the dual
II. EXPERIMENT
A Digital Instruments ~DI! Dimension 3000 microscope
with a removable self-made Plexiglas liquid cell, similar to
FIG. 3. A 5123512 pixel tapping mode image of a calibration standard ~200
nm deep, 10 mm pitch! taken in normal saline solution ~0.9% NaCl!, using a
NanoDevices microactuated cantilever. The scan rate was 1 Hz; scan size was
50 mm.
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3244
Rev. Sci. Instrum., Vol. 73, No. 9, September 2002
traces on the die were wire bonded to the substrate’s bond
pads. The ac tapping signal was accessed using a DI signal
access module and routed to wires soldered to the bond pads.
Figure 1 shows the type of NanoDevices cantilever, with a
zinc-oxide ~ZnO! microactuator used in the experiments.
During imaging the cantilever was entirely immersed in the
liquid, so the exposed electrical traces on top of the cantilever
die, including the entire cantilever and actuator portions, were
first insulated from the conductive liquid medium with a 1%
fluoropolymer solution in fluorosolvent18 sprayed from an
airbrush. This insulation scheme has been described previously
for limited use in contact mode AFM.17
Insulated cantilevers were successfully used in deionized
water, tap water, and normal saline solution ~0.9% NaCl!, to
tune and take many images. The insulation scheme proved
sufficient for continuous imaging over periods of hours with
drive amplitudes exceeding 2 V, including one test case in
which more than 100 consecutive images ~30 mm scan size,
512 3512 pixels! were taken continuously in water with the
same cantilever.
I I I . R E S U L T S AN D D I S C U S S I O N
Figures 2~a! and 2~b! show the rms cantilever amplitude
as a function of frequency ranging from 0 to 50 kHz using the
large piezotube and the integrated microactuator as vibration
actuators, respectively. The spurious resonances visible in the
former are removed by forcing the cantilever directly, enabling
automatic tunes with the microscope controller. The manual
tune using the piezotube required a drive amplitude of 5 V.
The microactuated cantilever used a drive amplitude of 0.380
mV and was tuned automatically. Both oscillated with the
same free air rms amplitude once tuned.
Figure 3 shows a tapping mode image taken in normal
saline solution ~0.9% NaCl! using a microactuated cantilever
to create the tapping oscillation and the microscope’s piezotube as the topography actuator.
Many such images were taken in a variety of liquids, the
tuning process proving simpler and quicker than conventional
tapping mode in liquid. Without spurious resonance peaks,
tuning to the natural frequency of the cantilever probe
Rogers e t a l .
can be performed quickly and more accurately. Direct-force
modulation reduces acoustic effects during imaging and helps
minimize tip-sample interaction forces.
ACKNOW LEDGM ENTS
The authors are grateful for support and funding from the
Nevada Ventures Nanoscience Program and wish to thank
Mike Lemich, Wade Cline, Nelson Publicover, Carl Marsh,
Nicholaus Halecky, and Steven Malekos for their help on this
project. S. C. M. was supported by the National Science
Foundation under Grant No. 0091549. The authors also thank
C. F. Quate for his support and advice.
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