Tip-sample Capacitance Research Articles

Accurate Electrostatic Force Measurements by Atomic Force Microscopy Using Proper Distance Control

Electrostatic force microscopy (EFM) enables us to examine the local electrostatic force between an AFM tip and sample surface, from which an electrical capacitance between them and surface potential can be evaluated, by applying direct current (DC) and alternating current (AC) voltages. From the dependences of the electrostatic force on the DC voltage and on the frequency of AC voltage in EFM, carrier density, carrier type, and deep level states in a semiconductor can also be investigated. Since EFM is basically operated as an atomic force microscopy (AFM), special care should be taken in an effect of the electrostatic force on a distance control between the tip and sample, and robust distance control is necessary even under the strong electrostatic force in order to realize proper measurements of both topography and electrostatic force. In this paper, we have examined the effectiveness of the usage of the oscillation amplitude of a cantilever as a feedback target for the distance control, which is referred to as Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> -mode operation, while the cantilever oscillation frequency is always kept at its primary resonance, like the conventional frequency modulation AFM (FM-AFM) which uses a resonant frequency shift as the feedback target. First, we have verified that a distance change due to the strong electrostatic force was significantly reduced compared with FM-AFM. Secondly, we have confirmed that the Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> -mode operation in EFM realized proper measurements of the dependance of a tip-sample capacitance including a surface depletion capacitance on the DC voltage and on the frequency of AC voltage, indicating good robustness of the method against the strong electrostatic force.

Local capacitance-voltage profiling and high voltage stress effect study of SiO2/SiC structures by time-resolved scanning nonlinear dielectric microscopy

We investigate SiO2/SiC structures by a nanoscale capacitance-voltage (CV) profiling method based on time-resolved scanning nonlinear dielectric microscopy (tr-SNDM). Owing to the high sensitivity to the variation in the tip-sample capacitance, wide bandwidth of the measurement, and high flexibility of offline processing, tr-SNDM permits the more accurate acquisition of the microscopic CV characteristics localized below the scanning tip. We demonstrate that tr-SNDM based local CV profiling is able to distinguish the different features of untreated and nitrided SiO2/SiC structures and image intrinsic spatial non-uniformity of the interfaces. We also investigate the nanoscale impacts of high voltage stress on the SiO2/SiC structures. By inspecting the local CV characteristics under high voltage stress, we suggest that the interface defect density locally increases during the stress application.

Signal Origin of Electrochemical Strain Microscopy and Link to Local Chemical Distribution in Solid State Electrolytes.

Electrochemical strain microscopy (ESM) is a distinguished method to characterize Li-ion mobility in energy materials with extremely high spatial resolution. The exact origin of the cantilever deflection when the technique is applied on solid state electrolytes (SSEs) is currently discussed in the literature. Understanding local properties and influences on ion mobility in SSEs is of utmost importance to improve such materials for next generation batteries. Here, the exact signal formation process of ESM when applied on sodium super ionic conductor (NASICON)-type SSE containing Na- and Li-ions is investigated. Changes in the dielectric properties, which are linked to the local chemical composition, are found to be responsible for the observed contrast in the deflection of the cantilever instead of a physical volume change as a result of Vegard´s Law. The cantilever response is strongly reduced in areas of high sodium content which is attributed to a reduction of the tip-sample capacitance in comparison to areas with high lithium content. This is the first time a direct link between electrostatic forces in contact mode and local chemical information is demonstrated on SSEs. The results open up new possibilities in information gain since dielectric properties are sensitive to subtle changes in local chemical composition.

Fast and reliable pre-approach for scanning probe microscopes based on tip-sample capacitance

Within the last three decades Scanning Probe Microscopy has been developed to a powerful tool for measuring surfaces and their properties on an atomic scale such that users can be found nowadays not only in academia but also in industry. This development is still pushed further by researchers, who continuously exploit new possibilities of this technique, as well as companies that focus mainly on the usability. However, although imaging has become significantly easier, the time required for a safe approach (without unwanted tip-sample contact) can be very time consuming, especially if the microscope is not equipped or suited for the observation of the tip-sample distance with an additional optical microscope. Here we show that the measurement of the absolute tip-sample capacitance provides an ideal solution for a fast and reliable pre-approach. The absolute tip-sample capacitance shows a generic behavior as a function of the distance, even though we measured it on several completely different setups. Insight into this behavior is gained via an analytical and computational analysis, from which two additional advantages arise: the capacitance measurement can be applied for observing, analyzing, and fine-tuning of the approach motor, as well as for the determination of the (effective) tip radius. The latter provides important information about the sharpness of the measured tip and can be used not only to characterize new (freshly etched) tips but also for the determination of the degradation after a tip-sample contact/crash.

Quantitative Theory for Probe-Sample Interaction With Inhomogeneous Perturbation in Near-Field Scanning Microwave Microscopy

A general approach for calculating tip–sample capacitance variation in near-field scanning microwave microscopy is presented. It can be applied to arbitrary tip shapes, thick and thin films, and variation due to inhomogeneous perturbation. The computation domain for the tip–sample interaction problem is reduced to a block perturbation area by applying Green’s theorem, and thus it can save substantial time and memory during calculating either electric field or contrast capacitance for three-dimensional models of near-field microwave microscopy. We show that this method can accurately calculate capacitance variation due to inhomogeneous perturbation in insulating or conductive samples, as verified by finite-element analysis results of commercial software and experimental data from microwave impedance microscopy. More importantly, the method in this paper also provides a rigorous framework to solve the inverse problem, which has great potential to improve resolution by deconvolution.

(Invited) Characterization of Buried Interfaces with Scanning Probe Microscopes

Scanning probe microscopes (SPMs) have some capability to image sub-surface structure, including the details of buried interfaces. This paper describes the theoretical and practical basis for obtaining information about shallow buried interfaces, subsurface compositional variations, and electrical potential variations with SPMs. Three techniques are discussed: scanning microwave microscopy (SMM) to image the capacitance of buried metal lines, scanning Kelvin force microscopy (SKFM) to image the potential of buried metal lines, and electric force microscopy (EFM) phase imaging to see buried interface surface roughness. COMSOL simulations of the SMM resonator response to small variations in tip-sample capacitance, explaining the contrast reversal phenomena, are described. SKFM and EFM images of NIST designed potential variation test structures show the potential of each technique for buried interface characterization.

Scanning nonlinear dielectric potentiometry.

Measuring spontaneous polarization and permanent dipoles on surfaces and interfaces on the nanoscale is difficult because the induced electrostatic fields and potentials are often influenced by other phenomena such as the existence of monopole fixed charges, screening charges, and contact potential differences. A method based on tip-sample capacitance detection and bias feedback is proposed which is only sensitive to polarization- or dipole-induced potentials, unlike Kelvin probe force microscopy. The feasibility of this method was demonstrated by simultaneously measuring topography and polarization-induced potentials on a reconstructed Si(111)-(7 × 7) surface with atomic resolution.

Low-Temperature Scanning Capacitance Probe for Imaging Electron Motion

Novel techniques to probe electronic properties at the nanoscale can shed light on the physics of nanoscale devices. In particular, studying the scattering of electrons from edges and apertures at the nanoscale and imaging the electron profile in a quantum dot, have been of interest [1]. In this paper, we present the design and implementation of a cooled scanning capacitance probe that operates at liquid He temperatures to image electron waves in nanodevices. The conducting tip of a scanned probe microscope is held above the nanoscale structure, and an applied sample-to-tip voltage creates an image charge that is measured by a cooled charge amplifier [2] adjacent to the tip. The circuit is based on a low-capacitance, high- electron-mobility transistor (Fujitsu FHX35X). The input is a capacitance bridge formed by a low capacitance pinched-off HEMT transistor and tip-sample capacitance. We have achieved low noise level (0.13 e/VHz) and high spatial resolution (100 nm) for this technique, which promises to be a useful tool to study electronic behavior in nanoscale devices.

Quantifying electrostatic force contributions in electrically biased nanoscale interactions

A study of the validity of analytical methods for calculating the electrostatic force interaction in alternating current electrostatic force microscopy is presented. Using a simple harmonic oscillator model, two analytical frameworks aimed at relating the electrostatic force between the cantilever tip and the sample with measurable parameters (amplitude and phase of the cantilever) are derived. The validity of the frameworks is examined based on two parameters that define the oscillation amplitude of the cantilever (tip voltage and tip-sample distance). Results are compared with an analytical model of the electrostatic interaction between tip and sample (tip-sample capacitance) and the range of validity of these two frameworks is provided. Our analysis confirms that the commonly used interpretation of the amplitude and the phase as a measure for the electrostatic force and for the derivative of the electrostatic force is only valid for very small oscillation amplitudes and depends on the tip geometry. Furthermore, this study demonstrates that these two techniques suffer from sensitivity limitations at large tip-sample distances. Finally, we compare the two frameworks with an alternative technique for the quantification of the tip-sample electrostatic interaction we have recently proposed and we discuss and experimentally demonstrate its advantages in terms of reliability and sensitivity, providing an example of dielectric constant measurement of a thin insulating film.

EFM data mapped into 2D images of tip-sample contact potential difference and capacitance second derivative

We report a simple technique for mapping Electrostatic Force Microscopy (EFM) bias sweep data into 2D images. The method allows simultaneous probing, in the same scanning area, of the contact potential difference and the second derivative of the capacitance between tip and sample, along with the height information. The only required equipment consists of a microscope with lift-mode EFM capable of phase shift detection. We designate this approach as Scanning Probe Potential Electrostatic Force Microscopy (SPP-EFM). An open-source MATLAB Graphical User Interface (GUI) for images acquisition, processing and analysis has been developed. The technique is tested with Indium Tin Oxide (ITO) and with poly(3-hexylthiophene) (P3HT) nanowires for organic transistor applications.

Dynamic electrostatic force microscopy technique for the study of electrical properties with improved spatial resolution

The need to resolve the electrical properties of confined structures (CNTs, quantum dots, nanorods, etc) is becoming increasingly important in the field of electronic and optoelectronic devices. Here we propose an approach based on amplitude modulated electrostatic force microscopy to obtain measurements at small tip–sample distances, where highly nonlinear forces are present. We discuss how this improves the lateral resolution of the technique and allows probing of the electrical and surface properties. The complete force field at different tip biases is employed to derive the local work function difference. Then, by appropriately biasing the tip–sample system, short-range forces are reconstructed. The short-range component is then separated from the generic tip–sample force in order to recover the pure electrostatic contribution. This data can be employed to derive the tip–sample capacitance curve and the sample dielectric constant. After presenting a theoretical model that justifies the need for probing the electrical properties of the sample in the vicinity of the surface, the methodology is presented in detail and verified experimentally.

Nanoparticles charge response from electrostatic force microscopy

Electrostatic force microscopy (EFM) allows measurement of tiny changes in tip-sample capacitance. When nanoobjects are studied by EFM, they only contribute a very small fraction of the total capacitance between the tip and the sample. We show that the analysis of 3D maps of the EFM signal allows extracting the contribution of the nanomaterial to the total capacitance. This opens the way to applications of EFM as a measure of the dielectric coefficient of electrically insulating nanomaterials or the quantum capacitance of conducting nanomaterials. We apply this method to study the charge response of magnetite, Fe3O4, nanoparticles.

Simultaneous measurement of tunneling current and atomic dipole moment on Si(111)-(7 × 7) surface by noncontact scanning nonlinear dielectric microscopy

Non-contact scanning nonlinear dielectric microscopy (NC-SNDM) can resolve the topography and dipole moment distribution of a Si(111)-(7 × 7) surface on an atomic level. We discuss the origin of the atomic contrast in dipole moment images based on simultaneously acquired time-averaged tunneling current images. The dipole moment images are found to have the same characteristics as the simultaneously acquired current images. This similarity between these two images does not originate from circuit crosstalk since it is suppressed by our circuit design that decouples the tip-sample capacitance and the tunneling current. Constant-height images also indicate that the atomic contrast is not artificially caused by feedback crosstalk. These results suggest that the atomic contrast is due to the variation in the tip-sample capacitance caused by modulation of atomic dipole moments by the local density of states of the surface.

Water assisted gate induced temporal surface charge distribution probed by electrostatic force microscopy

In this paper, we present a quantitative method to measure charge density on dielectric layers using electrostatic force microscopy. As opposed to previous reports, our method, which is based on force curve measurements, does not require preliminary knowledge of the tip-sample capacitance and its derivatives. Using this approach, we have been able to quantify lateral and temporal SiO2 surface charge distribution and have unveiled a gate-induced charge redistribution mechanism which takes place in the vicinity of grounded electrodes. We argue that this mechanism constitutes a dominant factor in the hysteresis phenomenon, which is frequently observed in the transfer characteristics of nano-scale devices.

Force gradient sensitive detection in lift-mode Kelvin probe force microscopy

We demonstrate frequency modulation Kelvin probe force microscopy operated inlift-mode under ambient conditions. Frequency modulation detection is sensitive toforce gradients rather than forces as in the commonly used amplitude modulationtechnique. As a result there is less influence from electric fields originating from thetip’s cone and cantilever, and the recorded surface potential does not suffer fromthe large lateral averaging observed in amplitude modulated Kelvin probe forcemicroscopy. The frequency modulation technique further shows a reduced dependenceon the lift-height and the frequency shift can be used to map the second orderderivative of the tip–sample capacitance which gives high resolution materialcontrast of dielectric sample properties. The sequential nature of the lift-modetechnique overcomes various problems of single-scan techniques, where crosstalkbetween the Kelvin probe and topography feedbacks often impair the correctinterpretation of the recorded data in terms of quantitative electric surface potentials.

Calibrated nanoscale capacitance measurements using a scanning microwave microscope

A scanning microwave microscope (SMM) for spatially resolved capacitance measurements in the attofarad-to-femtofarad regime is presented. The system is based on the combination of an atomic force microscope (AFM) and a performance network analyzer (PNA). For the determination of absolute capacitance values from PNA reflection amplitudes, a calibration sample of conductive gold pads of various sizes on a SiO(2) staircase structure was used. The thickness of the dielectric SiO(2) staircase ranged from 10 to 200 nm. The quantitative capacitance values determined from the PNA reflection amplitude were compared to control measurements using an external capacitance bridge. Depending on the area of the gold top electrode and the SiO(2) step height, the corresponding capacitance values, as measured with the SMM, ranged from 0.1 to 22 fF at a noise level of ~2 aF and a relative accuracy of 20%. The sample capacitance could be modeled to a good degree as idealized parallel plates with the SiO(2) dielectric sandwiched in between. The cantilever/sample stray capacitance was measured by lifting the tip away from the surface. By bringing the AFM tip into direct contact with the SiO(2) staircase structure, the electrical footprint of the tip was determined, resulting in an effective tip radius of ~60 nm and a tip-sample capacitance of ~20 aF at the smallest dielectric thickness.

Nanoscale electrical properties of cluster-assembled palladium oxide thin films

The electrical properties of cluster-assembled nanostructured palladium oxide $({\text{ns-PdO}}_{x})$ thin films grown by supersonic cluster beam deposition have been characterized by means of a customized ac current-sensing atomic force microscope. Scanning impedance microscopy is shown to provide a deep picture of the electrical properties of thin nanostructured interfaces even in the case of very soft and poorly adherent films. In particular, the dielectric constant of ${\text{ns-PdO}}_{x}$ can be quantitatively determined as well as its $I\text{\ensuremath{-}}V$ characteristics. Moreover, the measurement of the tip-sample parasitic capacitance can be exploited to probe the overall mesoscale conductive character of thin films and to give a complementary and more precise view of the oxidation of ${\text{ns-PdO}}_{x}$ obtained by x-ray photoemission spectroscopy.

An analytical model for the capacitance between probe tip and dielectric film deduced by high-frequency electromagnetic-field simulations

An analytical model for the capacitance between a probe tip and a dielectric film is presented based on the high-frequency electromagnetic-field simulation of the three-dimensional tip and dielectric film system. We examined the effects of the probe-tip geometry and film properties on the tip-sample capacitance and deduced empirically the analytical model expressed by a simple logarithmic dependence on the tip-film distance, Z. Furthermore, the parameters in the analytical model could describe the probe-tip shape, dielectric permittivity, and film thickness. Based on these results, we discuss the capabilities to probe quantitatively the nanoscale dielectric properties of a thin film.

Low-temperature and high magnetic field dynamic scanning capacitance microscope

We demonstrate a dynamic scanning capacitance microscope (DSCM) that operates at large bandwidths, cryogenic temperatures, and high magnetic fields. The setup is based on a noncontact atomic force microscope (AFM) with a quartz tuning fork sensor for the nonoptical excitation and readout in topography, force, and dissipation measurements. The metallic AFM tip forms part of a rf resonator with a transmission characteristics modulated by the sample properties and the tip-sample capacitance. The tip motion gives rise to a modulation of the capacitance at the frequency of the AFM sensor and its harmonics, which can be recorded simultaneously with the AFM data. We use an intuitive model to describe and analyze the resonator transmission and show that for most experimental conditions it is proportional to the complex tip-sample conductance, which depends on both the tip-sample capacitance and the sample resistivity. We demonstrate the performance of the DSCM on metal disks buried under a polymer layer and we discuss images recorded on a two-dimensional electron gas in the quantum Hall effect regime, i.e. at cryogenic temperatures and in high magnetic fields, where we directly image the formation of compressible stripes at the physical edge of the sample.

Tip-to-Sample Distance Dependence of dC/dZ Imaging in Thin Dielectric Film Measurement

We have developed scanning capacitance microscopy (SCM) with a self-sensing conductive probe that can be used to obtain static capacitance (dC/dZ) images by virtue of the vertical vibration of the probe tip. This technique for dC/dZ imaging can delineate features, such as thickness variations or fixed charge distributions, within a dielectric film and provide a lateral resolution comparable to that of simultaneously obtained topography images. In this work, we have experimentally revealed that the lateral resolution of a dC/dZ image is insensitive to the probe tip amplitude, and the sensitivity of dC/dZ images strongly depends on the distance of the gap between the probe tip and the sample surface. These results and the force–distance characteristics of the self-sensing conductive probe indicate that the dC/dZ signal is mostly determined by the probe tip–sample capacitance and also that the spatial resolution of dC/dZ imaging is not affected by the surface-adsorbed meniscus layer under a vacuum environment measurement. Finally, we have demonstrated sub-10-nm spatial resoluton in dC/dZ imaging for thin dielectric film measurement.