Surface Potential Measurements via Single-Pass Scanning Kelvin Force Microscopy

Table of Contents

Example 1: Surface Potential of Self-Assembled Fluoroalkanes
Example 2: Differentiation of Heterogeneous Polymer Materials with dC/dZ
Examples 3 & 4: KFM Measurements on Metals & Semiconducting Materials
Example 5: KFM Measurement on Materials for Renewable Energy Research
AFM Instrumentation from Keysight Technologies


Scanning Kelvin force microscopy (KFM) has been widely utilized in mapping surface potential (SP) distribution at the nanoscale. 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 Vacsin ωt are applied between the tip and the sample, the induced capacitive force is


whereΦ is the contact potential difference (CPD) between the tip and the sample. It is evident from the Fω term in Equation (1) that Fω depends linearly on Vdc and becomes zero when Vdc= Φ. Therefore, SP can be measured directly by nullifying Fω. Since SP is measured here by nullifying the amplitude of the Fω, it is named AM-KFM, meaning amplitude sensitive. Alternatively, SP can be measured by nullifying the resonance frequency shift, Δ fω, caused by the ac modulation (FM-KFM),


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 F response one can get spatial variations of local dielectric behavior. In other words, KFM can provide advanced characterization of local electric and dielectric properties.


The Keysight 7500 AFM/SPM microscope is a high-performance scientific instrument that delivers high-resolution imaging with integrated environmental control functions. The standard Keysight 7500 includes contact mode, acoustic AC mode, and phase imaging with one universal scanner that operates in both open-loop mode and closed-loop mode.

Switching imaging modes with the 7500 AFM/SPM microscope is quick and convenient, owing to the scanner’s interchangeable, easy-to-load nose cones. All 7500 AFM/SPM microscopes come with the lowest-noise closed-loop position detectors available so as to provide the ultimate convenience and performance in imaging — without sacrificing resolution or image quality.

The Keysight 7500 is equipped with an AC mode controller that has three dual-phase lock-in amplifiers (LIAs). These digitally controlled analog LIAs have a broad bandwidth, up to 6MHz. This allows the Keysight 7500 to perform single-pass KFM measurementsby applying dual-frequency excitation signals to the AFM tip simultaneously.

One excitation signal is used for modulating the mechanical oscillation of the AFM tip and 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 third LIA can be set for monitoring various signals (e.g., F for dC/dZ imaging).

Details of instrumental setup/operation are available in other Keysight documents. A number of practical examples are shown below to demonstrate the use of the Keysight 7500 for high-resolution surface potential measurement and spatial mapping of dielectric properties over the sample surface.

Example 1: Surface Potential of Self-Assembled Fluoroalkanes

Fluoroalkanes FnHm [FnHm = CF3(CF2)n(CH2)mCH3] form self-assembled structures, usually toroids or ribbons, on Si substrate. The structures exhibit strong surface potential due to the vertical orientation of the chains carrying the molecular dipole at the -CH2-CF2- bond [4].

A set of F14H20 self-assembled structures (most are toroids) on Si substrate was examined in KFM (AM-AM) mode; the corresponding topography and SP images are shown in Figure 1.

Figure 1. KFM topography (left) and surface potential (right) images of fluoroalkane F14H20 self-assembly on Si. Scan size: 4 µm x 4 µm.

As revealed in Figure 1, 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 background.

The surface potential of these self-assembled structures has a value of around –0.7V, which is consistent with the 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, depending on the input signal of the surface potential servo. In general, the AM-FM mode has better sensitivity for surface potential values.

The surface potential values obtained in the AM-FM mode are higher than those obtained in the AM-AM mode. The better sensitivity of theAM-FM detection also leads to higher spatial resolution achieved in mapping of surface potential.

Example 2: 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 in local mechanical properties and variations of energy dissipated in the tip-sample interactions.

Surface potential images can play a similar role in the identification of surface locations having different electric properties. In addition, dC/dZ imaging can be particularly useful for differentiating components having different dielectric properties. The range of polymer materials that might be sensed by KFM and dC/dZ can be expanded by including acrylic polymers.

The KFM images of a 20nm-thick film of 3PS/7PMMA (weight ratio of the components is 3:7), which was deposited on Si substrate by spin-casting, are shown in Figure 2.

Figure 2. Topography (A), phase (B), SP (C), and dC/dZ (D) images of PS(3)-PMMA(7). Scan size: 8µm x 8µm.

The phase image (Figure 2B) indicates that the islands on the topography image (Figure 2A) correspond to the 70% of the film composed of PMMA, which has a stiffer mechanical property. The SP image (Figure 2C) shows little difference between the PMMA and PS domains (about 60mV, determined from a separate measurement). The dC/dZ image (Figure 2D), on the other hand, clearly reveals the difference in dielectric behavior between the two components.

Examples 3 & 4: KFM Measurements on Metals & Semiconducting Materials

Surface potentials of metals are directly related to their work functions. KFM can be used to measure the difference in work functions between different metals and semiconducting materials. In addition, dC/dZ imaging is also important in the characterization of semiconductor materials. One example, shown in Figure3, is the KFM and dC/dZ imaging of Ti-coated carbon nanotubes deposited on ITO.

Figure 3. Topography (A), SP (B), dC/dZ amplitude (C), and dC/dZ phase (D) images of Ti-coated CNT deposited on ITO. Scan size: 2µm x 2µm.

Carbon nanotubes coated with Ti clusters are clearly seen in the topography image (Figure 3A). They generally show a higher surface potential than the underlying ITO substrate (Figure 3B). The dC/dZ amplitude shows the difference between the ITO and the Ti-coated nanotubes as well (Figure 3C).

A detailed examination of the dC/dZ phase image (Figure 3D) further reveals the difference between the carbon nanotubes and the Ti clusters on top of them. This also indicates that not all of the particles seen in the topography image are Ti clusters; some are carbon residues from the CNT preparation.

Another important KFM application is the characterization of semiconductor devices for 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 the dopant level near the surface.

Through careful experiment and tip calibration, evaluation of localized Fermi level on the semiconductor surface is possible using KFM. There is also interest in looking at the electric field distribution around certain elements of an IC device, particularly in the case of a hot circuit with currents flowing.

As an example, a KFM image of a piece of SRAM de-processed to the bare silicon level, exposing the PMOS and NMOS structures, is presented in Figure 4.

Figure 4. Topography (top) and surface potential (bottom) images of SRAM. Scan size: 25µm x 25µm.

The potential image (Figure 4, bottom) clearly reveals the different potential levels correlating to the different doped regions on the surface.

Example 5: KFM Measurement on Materials for Renewable Energy Research

KFM has been widely used for energy-related research (e.g., batteries, fuel cells, photovoltaic cells) to investigate electronic properties of candidate materials.

It has been employed to study the work function differences between different facets of crystal orientation on single grains of CuGaSe2, to characterize the grain boundary structures within polycrystalline absorber materials, and to study the cross-section of the junction region through a complete solar cell device. KFM has also been utilized to study organic solar cells (OPV).

The investigation of a classical organic solar cell system consisting of MDMO-PPV/PCBM is well suited to analysis using KFM, together with high-resolution SEM data, resulting in identification of a barrier for electron transmission from the electron-rich PCBM nanoclusters toward the extracting cathode, and correlation of the power conversion efficiency to the nanoscale morphology in the bulk heterojunction.

KFM was used to differentiate the work function of the dopants from the polymer matrix, which further helped in identifying the type of charge carrier in the system.

Figure 5 shows an example of KFM imaging of dual-patterned conducting polymer structure on Au substrate.

Figure 5. Topography (A), SP (B), and dC/dZ (C) images of conducting polymer patterned on Au substrate. Scan size: 3µm x 3µm.

The honeycomb frame of the structure is formed by conductive polymer, while the hole is backfilled with SAM layers of insulating organic molecules. The surface potential image (Figure 5B) clearly reveals the difference between the conducting polymer frame and the SAM layer.

It also shows the imperfection existing in the structure that gives rise to a slightly higher potential. The dC/dZ image (Figure 5C) shows the different dielectric properties of the two materials.


Single-pass Kelvin force microscopy mode offers high sensitivity and spatial resolution for surface potential measurements on a broad range of materials. Single-pass KFM is also utilized for dielectric characterization. Furthermore, in compositional imaging, it complements phase imaging by providing information for previously inaccessible areas (e.g., metal alloys and semiconductors).

KFM studies in different environments are also yielding new and valuable information about morphology of heterogeneous polymers. Environmental AFM will further benefit from local electric and mechanical measurements via multiple-frequency detection. Finally, KFM is widely used in energy-related research to investigate electronic properties of candidate materials for batteries, fuel cells, and photovoltaic cells.

AFM Instrumentation from Keysight Technologies

Keysight Technologies offers high-precision, modular AFM solutions for research, industry, and education. Exceptional worldwide support is provided by experienced application scientists and technical service personnel. Keysight’s leading-edge R&D laboratories are dedicated to the timely introduction and optimization of innovative, easy-to-use AFM technologies.

Source: Keysight Technologies

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