Text preview for : 5991-3183EN Surface Potential Measurements Using the Keysight 7500 AFM - Application Note c20141020 part of
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Keysight 5600LS AFM
Surface Potential Measurements
Using Keysight 7500 AFM
Application Brief
Introduction cantilever, respectively. lows the Keysight 7500 AFM to perform
single-pass KFM measurements by ap-
Scanning kelvin force microscopy (KFM) The force component at 2v is proportion- plying dual-frequency excitation signals
has been widely used in mapping surface al to dC/dZ. Therefore, by mapping the to the AFM tip simultaneously. One
potential (SP) distribution at the na- F2v response one can get spatial varia- excitation signal is used for modulating
noscale. The principle of KFM is based on tions of local dielectric behavior. In other the mechanical oscillation of the AFM tip
the measurement of electrostatic forces words, KFM can provide an advanced and for topography imaging. The second
between the tip and the surface. When a characterization of local electric and excitation signal is applied for the modu-
dc bias (Vdc) and a small ac modulation dielectric properties. lation of the AFM-based electrostatic
signal Vac sin vt are applied between the tip-sample force, and is used for the
tip and the sample, the induced capaci- Instrumentation measurement of sample surface poten-
tive force is tial. The 3rd LIA can be set for monitor-
The Keysight Technologies, Inc. 7500 ing various signals, e.g., F2v for dC/dZ
AFM/SPM microscope is a high- imaging. Detailed instrumental setup/
performance instrument that delivers operation are available in other Key-
high resolution imaging with integrated sight documents. A number of practical
environmental control functions. The examples are shown below to demonstrate
standard Keysight 7500 includes contact the application of Keysight 7500 AFM in high
(1) mode, acoustic AC mode, and phase resolution surface potential measurement
where f is the contact potential dif- imaging that comes with one universal and spatial mapping of dielectric properties
ference (CPD) between the tip and the scanner operating in both Open-loop and over the sample surface.
sample. It is evident from the Fv term Closed-loop mode. Switching imaging
modes with the Keysight 7500 AFM/SPM
in Equation (1) that Fv depends linearly Surface Potential of
on Vdc and becomes zero when Vdc = f. microscope is quick and convenient, a re-
sult from the scanner's interchangeable,
Self-assembled Fluoroalkanes
Therefore, SP can be measured directly
by nullifying Fv. Since SP is measured easy-to-load nose cones. All 7500 AFM's Fluoroalkanes FnHm [FnHm = CF3(CF2)
here by nullifying the amplitude of the come with the lowest noise closed loop n(CH2)mCH3] form self-assembled struc-
Fv, it is named AM-KFM, meaning am- position detectors to provide the ultimate tures, usually toroids or ribbons, on Si
plitude sensitive. Alternatively, SP can convenience and performance in imaging, substrate. Those structures exhibit strong
be measured by nullifying the resonance without sacriicing resolution and image surface potential due to the vertical
frequency shift, fv, caused by the ac quality. orientation of the chains carrying the mo-
modulation (FM-KFM), lecular dipole at the -CH2-CF2- bond [4].
The Keysight 7500 AFM is equipped with A set of F14H20 self-assembled structures
an AC Mode controller that has three (most are toroids) on Si substrate was
(2) dual phase lock-in ampliiers (LIA). These examined in KFM (AM-AM) mode, the
where f0 and k are the resonance digitally-controlled analog LIA have a corresponding topography and SP images
frequency and spring constant of the broad bandwidth up to 6MHz. This al- are shown in Figure 1. As it revealed that
02 | Keysight | Surface Potential Measurements Using Keysight 7500 AFM - Application Brief
there are only a number of patches of self-
assembled F14H20 molecules show strong
dark contrast in the SP image. Other areas
of the surface are covered with randomly
adsorbed F14H20 molecules and show a
relatively homogeneous potential back-
ground. The surface potential of these
self-assembled structures has a value of
around -0.7V, which is consistent with
predominantly vertical alignment of the
molecular chains.
KFM measurement can be done in either
AM-AM mode or AM-FM mode with the
Keysight 7500 AFM, depending on the
input signal of the surface potential servo.
In general the AM-FM mode has better Figure 1. KFM topography (left) and surface potential (right) images of luoroalkane F14H20 self-assembly on Si.
Scan size: 4