Keysight Technologies
Kelvin Force Microscopy
Using the 9500 AFM
Application Brief
Introduction
Scanning kelvin force microscopy (KFM) has been widely used in mapping surface potential distribution at
the nanoscale [1]. Due to direct and quantitative measurement of surface potential at high spatial resolution,
KFM has been used in various applications to reveal important information such as surface charging,
molecular dipole orientation in organic thin films, band bending and dopant concentration in semiconductor
materials, etc [2].
The principle of KFM is based on the measurement of electrostatic forces between the tip and the surface.
When a dc bias (Vdc) and a small ac modulation signal Vac sinωt are applied between the tip and the sample, the
induced capacitive force is
F(z)= Fdc + F ω + F 2 ω
= — 1 ∂c (Vdc — ϕ z)2 + 1 Vac2
2
2 ∂z
[
]
— ∂c (Vdc — ϕ)Vac sin ω t
∂z
+ 1 ∂c Vac2 cos 2 ω t,
4 ∂z
(1)
where ϕ is the contact potential difference (CPD) between the tip and the sample [3]. It is evident from the
Fω term in Equation (1) that Fω depends linearly on Vdc and becomes zero when Vdc = ϕ. Therefore, surface
potential can be measured directly by nullifying Fω. Since surface potential is measured here by nullifying the
amplitude of the Fω, it is named KFM-AM, meaning amplitude sensitive. Alternatively, surface potential can
be measured by nullifying the resonance frequency shift, Δfω, caused by the ac modulation (KFM-FM),
fo
fo 2
∂ c (V — ϕ) V s i n ω t,
∆fω = — —
F′
ω = —— —
ac
2k
2k ∂z2 dc
(2)
where f0 and k are the resonance frequency and spring constant of the cantilever, respectively. The force
component at 2ω is proportional to dC/dZ. Therefore, by mapping the F2ω response one can get spatial
variations of local dielectric behavior. In other words, KFM can provide an advanced characterization of local
electric and dielectric properties. Principally, KFM-AM and KFM-FM techniques measure surface potential
and local dielectric properties through the detection of electrostatic force and its gradient, respectively.
Instrumentation
The Keysight 9500 AFM/SPM microscope is a high-bandwidth digital controller instrument that delivers high
speed and high resolution imaging with integrated environmental control functions. The standard Keysight
9500 includes contact mode, acoustic AC mode, and phase imaging that comes with one universal scanner
operating in both Open-loop and Closed-loop mode. Switching imaging modes with the Keysight 9500 AFM/
SPM microscope is quick and convenient, a result from the scanner’s interchangeable, easy-to-load nose
cones. All 9500 AFMs come with low noise closed loop position sensors to provide the ultimate convenience
and performance in imaging, without sacrificing resolution and image quality.
03 | Keysight | M9037A PXIe Embedded Controller - Data Sheet
The Keysight 9500 AFM is equipped with a high-bandwidth,
FPGA-based digital controller with three dual phase lock-in
amplifiers (LIA). These FPGA based digital LIA have a broad
bandwidth up to 25 MHz. This allows the Keysight 9500 AFM
to perform single-pass KFM measurements by applying
dual-frequency excitation signals to the AFM tip simultaneously.
One excitation signal is used for modulating the mechanical
oscillation of the AFM tip for topography imaging. The second
excitation signal is applied for the modulation of the AFM-based
electrostatic tip-sample force, and is used for the measurement
of sample surface potential. The mechanical excitation is applied
at resonant frequency (ωmech) of cantilever whereas the small AC
voltage is applied at much lower frequency (ωelec << ωmech) between
tip and sample. The 3rd LIA can be set for monitoring various
signals, e.g., F2ω for dC/dZ imaging. Detailed instrumental setup/
operations are available in previous Keysight documents [4, 5].
A number of practical examples are shown below to demonstrate
the application of Keysight 9500 AFM in high resolution surface
potential measurement and spatial mapping of dielectric properties over the sample surface.
Surface Potential of Self-assembled
Fluoroalkanes
The fluoroalkane molecules FnHm [FnHm = CF3(CF2)n(CH2)mCH3]
form self-assembled structures, usually toroids or ribbons, on
silicon (Si) substrate. The F14H20 molecules have a dipole of 3.1D
oriented along the chain at the central -CH2-CF2- junction [6].
Macroscopic kelvin probe studies of Langmuir-Blodgett layers of
different FnHm revealed a strong surface potential of -0.8 V that is
assigned to vertically oriented molecular chains with fluorinated
parts facing air [7, 8]. Such a surface potential corresponding to
the self-assembled structures of fluoroalkanes can be studied by
KFM at the nanometer resolution.
Figure 1. Topography (A) and surface potential (B) images of fluoroalkane F14H20
self-assembly on silicon.
Scan size: 1.6μm x 1.6μm.
The topography and surface potential images of self-assembled
F14H20 structure on Si substrate are presented in Figure 1. The
surface potential of these self-assembled structures has a value
of around -0.75 V, which is consistent with predominantly vertical
alignment of the molecular chains.
Figures 2A, 2B and 2C show topography, phase and surface
potential images of zoomed in feature as compare to Figure 1
where an individual structure can be clearly seen. Similarly, Figure
2D shows the dC/dZ contrast which is associated with the local
dielectric properties of materials.
KFM measurement can be done in either AM-AM mode or AM-FM
mode with the Keysight 9500 AFM, depending on the input signal
of the surface potential servo. In general, the AM-FM mode has
better sensitivity on the surface potential value, leading to higher
spatial resolution in mapping of surface potential [9].
Figure 2. Topography (A), phase (B), surface potential (C), and dC/dZ
amplitude (D) images of F14H20 self-assembled mainly toroid on Si-substrate.
Scan size: 200nm x 200nm.
04 | Keysight | M9037A PXIe Embedded Controller - Data Sheet
Differentiation of Heterogeneous Polymer
Materials with dC/dZ
Phase imaging has often been used in mapping of heterogeneous
polymer materials. The phase contrast is assigned to differences
of local mechanical properties and variations of energy dissipated
in the tip-sample interactions. Surface potential images can play
a similar role in identifying the surface locations with different
electric properties. In addition, dC/dZ imaging can be particularly
useful in differentiating components of different dielectric
properties.
Figure 3 shows an example of KFM imaging of a binary blend
of polystyrene (darker region in Figure 3A) and polyethylene
(bright circular region in Figure 3A). The phase image (Figure
3B) indicates the islands on the topography (Figure 3A), which
correspond to polyethylene that has softer mechanical property.
Similarly, surface potential and dC/dZ images also clearly
revealed the difference in electrical and dielectric behavior of two
components. The measured surface potential difference between
two polymers is about 90 mV.
KFM Measurements of Semiconducting Materials
Another important KFM application is the characterization of
semiconductor devices, in both fabrication and failure analysis.
The surface potential measured using KFM is correlated to the
local work function of the semiconductor sample, which in turn
depends on the material and dopant level near the surface.
Through careful experiment and tip calibration, evaluation of
localized Fermi level on semiconductor surface is possible using
KFM. It is also interesting to look at the electric field distribution
around certain elements in an IC device, particularly in the case
of hot circuit with current flowing. As an example, a KFM image of
a piece of SRAM de-processed on the bare silicon level, exposing
the PMOS and NMOS structures is presented in Figure 4. The
surface potential image in Figure 4C clearly reveals the different
potential levels correlating to the different doped regions on the
surface.
Figure 3. Topography (A), phase (B), surface potential (C) and dC/dZ amplitude
(D) images of polymer blend of Polystyrene (PS) and Polyethylene (PE) on silicon
substrate. Scan size: 10μm x 10μm
Summary
Single-pass KFM mode offers high sensitivity and spatial resolution for surface potential measurement. KFM are applicable to a
wide range of materials, for both surface potential measurement
and dielectric characterization. It also complements phase
imaging for compositional imaging by advancing to earlier
not accessible areas such as semiconductors. KFM studies
in different environments are also bringing new and valuable
information about morphology of heterogeneous polymers. The
area of environmental AFM will further benefit from local electric
and mechanical measurements with multiple frequency detection.
Figure 4. Topography (A), phase (B), surface potential (C), and dC/dZ amplitude
(D) images of SRAM. Scan size: 15μm x 15μm.
05 | Keysight | M9037A PXIe Embedded Controller - Data Sheet
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Reference
[1]. W. Melitz, J. Shena, A. C. Kummela, S. Lee, Kelvin probe force microscopy and its application Surf. Sci. Rep., 66
(2011), pp. 1–27.
[2]. Th. Glatzel, M.Ch. Lux-Steiner, E. Strassburg et al., Scanning Probe Microscopy (2007), pp. 113-131.
[3]. S. Magonov and J. Alexander, S. Belikov, Exploring the Surfaces of Materials using Atomic Force Microscopy,
(2013), pp. 236-253.
[4]. S. Magonov, J. Alexander and , Compositional Mapping of Materials with Single-Pass Kelvin Force Microscopy,
Application Note, Keysight Technologies Inc., 5990-5480EN, (2014).
[5]. S. Magonov and J. Alexander, Advanced Atomic Force Microscopy: Exploring Measurement of Local Electric
Properties. Keysight Technologies Inc., 5989-9740EN., (2014).
[6]. J. Alexander, S. Magonov, and M. Moeller, Topography and surface potential in kelvin force microscopy of
perfluoroalkyl alkanes self-assemblies, J. Vac. Sci. Techn. B, 2009, 27,903-911.
[7]. A. El Abed, M.-C Faure, E. Pouzet, O. Abillon, Experimental evidence for an original two-dimensional phase
structure: An antiparallel semifluorinated monolayer at the air-water interface. Phys. Rev. E 2002, 5, 051603.
doi:10.1103/PhysRevE.65.051603.
[8]. Broniatowski, M.; Minores, J., Jr.; Dynarowicz-Latka, P., Semifluorinated chains in 2D- perfluorododecyl)-alkanes
at the air/water interface ,J. Colloid Interface Sci. 2004, 279, 552– 558. doi:10.1016/j.jcis.2004.06.080.
[9]. S. Magonov, J. Alexander, Single-pass Kelvin force microscopy and dC/dZ measurements in the intermittent
contact: applications to polymer materials, Beilstein J. Nanotechnol., 2 (2011), pp. 15–27.
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© Keysight Technologies, 2007 - 2016
Published in USA, June 7, 2016
5992-1619EN
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