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

P.I-24
High-Speed Frequency Modulation Atomic Force Microscopy using
Wideband Digital Phase-Locked Loop Detector
Takeshi Fukuma 1,2 and Yuji Mitani 1
1 Frontier Science Organization, Kanazawa University, Kanazawa, Japan
2 PRESTO, Japan Science and Technology Agency, Kawaguchi, Japan
Recent advancement in the instrumentation of frequency modulation atomic force
microscopy (FM-AFM) has enabled to operate FM-AFM in liquid with true atomic
resolution. To date, liquid-environment FM-AFM has been used for several biological
applications, which has demonstrated its excellent force sensitivity and high spatial
resolution. However, unlike applications in vacuum, biological applications require highspeed
operation of FM-AFM due to the mobile nature, high complexity and large
dimension of biological systems. In this study, we develop a high-speed FM-AFM using
FPGA-based digital signal processing circuit in order to improve the applicability of this
method to practical biological studies.
Development of high-speed FM-AFM requires enhancing the bandwidth and resonance
frequencies of all the components involved in the tip-sample distance feedback loop. In
particular, a frequency detector has been one of the major speed limiting factors in the
feedback loop. A conventional frequency detector using a digital phase-locked loop
(PLL) typically utilizes multiplication-based phase comparator, which has limited its
bandwidth to less than 10 kHz. In this study, we have developed a wideband digital PLL
using a subtraction-based phase comparator (Fig. 1(a)). The developed PLL detector has
a bandwidth of wider than 1 MHz (Fig. 1(b)). A high-speed FPGA circuit has been used
for implementing the PLL as well as the cantilever excitation circuit, the distance and
amplitude feedback control circuits. Combined with our recently developed high-speed
scanner and wideband photo-thermal excitation system, we aim at high-speed operation
of FM-AFM in liquid.
Figure 1: (a) Block diagram and (b) frequency response of the developed PLL using a
subtraction-based phase comparator (fc : cutoff frequency of a low-pass filter at the output).
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