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

P.I-14
Kelvin Force Microscopy Dynamic Behavior and Noise Propagation
Heinrich Diesinger, Dominique Deresmes, Jean-Philippe Nys, and Thierry Mélin
Institut d’Electronique, Microelectronique et Nanotechnologie, CNRS UMR 8520, Department
ISEN,Avenue Henri Poincaré, 59652 Villeneuve d’Ascq, France
The bandwidth of scanning probe control loops limits the sampling rate in data
acquisition. In this work, the dynamic behavior of amplitude detection (AM) and
frequency detection (FM) Kelvin force microscopy (KFM) setups is analyzed and
optimized. Since enhanced bandwidth alone would increase speed at the cost of tolerating
more noise, the origin of noise and its propagation within the control loops are studied in
parallel.
Laser Diode Photodetector
F N = 4γk B T
CPD
V PD
V PD, N = 10 μV/sqrt(Hz)
In
Lock-in
Osc
V
bias, AC
fres2 X
PI Amplifier
Kelvin Controller
+
Σ =
+
V Kelvin
open closed
Figure 1: Setup of the Kelvin control loop of an AM-KFM, consisting of a lock-in amplifier
exciting the probe electrostatically at its second resonance, and a PI error amplifier to close the
feedback loop and to apply the Kelvin voltage to the probe. The loop can be opened. Main noise
sources are the thermal probe excitation force and white noise at the output of the photodetector.
Fig. 1 shows the Kelvin control loop of an AM-KFM in ultrahigh vacuum, using the
second cantilever resonance for KFM while distance control is based on the first, similar
to the setup demonstrated by Kikukawa et al.[1]. The noise power spectral density of the
Kelvin output signal can be modeled after the transfer functions of the different stages
have been measured in open loop configuration. A benchmark criterium for comparing
hardware with respect to noise is issued. An FM KFM close to the configuration
suggested by Kitamura [2] is also analyzed. Advantages and drawbacks of both methods
in terms of bandwidth and signal to noise are discussed.
[1] A. Kikukawa, S. Hosaka, and R. Imura. Appl. Phys. Lett. 66, 3510 (1995).
[2] S. Kitamura and M. Iwatsuki. Appl. Phys. Lett. 72, 3154 (1998)
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