Noncontact Atomic Force Microscopy - Yale School of Engineering ...

P.II-19
Cantilever Holder Design for Spurious-Free Cantilever Excitation in
Hitoshi Asakawa 1 and Takeshi Fukuma 1,2
Liquid by Piezoactuator
1 Frontier Science Organization, Kanazawa University, Japan
2 PRESTO, JST, Japan
Recent advancement in frequency modulation atomic force microscopy (FM-AFM)
has enabled us to obtain atomic and molecular resolution images in liquid, which has
encouraged us to explore the possibilities of FM-AFM application in biological research.
In liquid-environment FM-AFM, a method for cantilever excitation is particularly
important for ensuring the stability and accuracy in imaging and quantitative force
measurements. This is because FM-AFM utilizes a cantilever deflection signal not only
for obtaining a distance feedback signal (i.e., frequency shift signal) but also for
producing a cantilever excitation signal. Although the cantilever excitation with a
piezoactuator is most commonly used, the method typically results in an excitation of
spurious resonances around the natural resonance frequency of a cantilever due to the
uncontrolled propagation of acoustic waves from a piezoactuator to the cantilever (Fig.
1(a)).
In order to suppress the influence of the spurious resonances, we propose a cantilever
holder design, where the propagation of an acoustic wave is restricted the surrounding
boundaries between two materials having significantly different specific acoustic
resistances (ρ·c [kg/m 2 ·s]). In our design, an acoustic wave is greatly reduced by the
reflection at the boundary between a piezoactuator (PZT, ρ·c=35×10 6 ) and its supporting
material (PEEK, ρ·c=3.3×10 6 ). Then the remaining acoustic wave is further reduced by
the reflection at the boundaries between the PEEK part and other components such as a
cantilever support (stainless steel 316, ρ·c=36×10 6 ) and an optical window (glass,
ρ·c=11×10 6 ). Figure 1 demonstrates the remarkable effect of the acoustic boundaries for
suppressing spurious vibrations induced by the acoustic waves.
a) b)
Figure 1: Frequency responses of cantilever oscillation amplitude induced by a piezoactuator.
(Excitation amplitude, : 0.5 V, : 1 V, : 1.5 V). Supporting materials for a piezoactuator: (a)
stainless steel 316 and (b) PEEK.
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