In this article, a method for contactless amplitude and phase measurements of local voltages within microwave devices is developed. A conductive cantilever probe with a sharp tip is used as capacitively coupled sensor to record radio frequency (RF) voltages at well-defined distances between the probe tip and the device surface. The recorded voltage is compared to a nonlinear capacitance versus distance model to separate the local voltage on the device from long-range crosstalk contributions. To determine both amplitude and phase of the local voltage, a complex parameter identification procedure is proposed and implemented. Thus, amplitude and phase measurements of local voltages at mu m spatial resolution are enabled. Additionally, the cantilever probe is integrated on a PCB to enable the transmission of RF signals from the tip to the detector over a wide bandwidth. The proposed method, together with the developed probe, are verified by performing voltage measurements on an open-ended transmission line at frequencies from 1 to 26.5 GHz. The measurements closely match the analytically calculated standing wave patterns on the transmission line.

In this paper, a method for contactless amplitude and phase measurements of local voltages within microwave devices is developed. A conductive cantilever probe with a sharp tip is used as capacitively coupled sensor to record radio frequency (RF) voltages at well-defined distances between the probe tip and the device surface. The recorded voltage is compared to a nonlinear capacitance vs. distance model to separate the local voltage on the device from long-range cross-talk contributions. To determine both amplitude and phase of the local voltage, a complex parameter identification procedure is proposed and implemented. Thus, amplitude and phase measurements of local voltages at μm spatial resolution are enabled. Additionally, the cantilever probe is integrated on a PCB to enable the transmission of RF signals from the tip to the detector over a wide bandwidth. The proposed method, together with the developed probe, are verified by performing voltage measurements on an open-ended transmission line at frequencies from 1 GHz to 26.5 GHz. The measurements closely match the analytically calculated standing wave patterns on the transmission line.

The measurement of electrical surface charges and their associated potentials at the nanoscale plays a crucial role in understanding important molecular processes, such as corrosion or biological tissue interactions. Measurement of these potential distributions, especially in aqueous environments, is not always possible with standard atomic force microscopy (AFM)-based techniques. The herein proposed single-harmonic response open-loop Kelvin-probe force microscopy (SH-KPFM) mode circumvents issues of common methods and enables such investigations in water via a suitable choice of the electrical excitation signal. The mode is validated using parameter sweeps on calibration samples and compared to conventional KPFM in air. Furthermore, SH-KPFM is applied to investigate the potential distribution and time-dependent depolarization of a charged PMMA surface immersed in deionized water, demonstrating its ability to analyze complex electrostatic interactions on the nanoscale.

The detailed understanding of parasitic circuit elements is crucial for the development of integrated radio frequency (RF) devices. In this paper, a contactless voltage sensing method is used to identify parasitic capacitances within an integrated RF circuit. The voltage distribution within a resistive voltage divider is measured with mu m spatial resolution at frequencies of 1 GHz to 26.5 GHz. Parasitic capacitances between circuit test points within the voltage divider are identified by comparing the measured frequency-dependent voltage distribution to an equivalent circuit model. The results are verified by performing measurements on a second voltage divider with the same dimensions, but different resistor values.

Electric charge distributions and the associated surface potentials at the nanoscale play a key role in many areas from material sciences to biology. The increasing field of sample analysis in liquid environments of biologically relevant ionic concentration demands for invasive-free measurements, hardly achievable with current techniques. This paper presents the development of a novel Atomic force Microscopy measurement mode, termed AC-EFM, enabling quantitative surface potential measurements at the nanoscale. It circumvents the use of a dcbias, which leads to parasitic electrochemical effects in liquids in conventional methods, by the application of an amplitude modulated high frequency voltage. The surface potential is measured through the shift in cantilever resonance frequency, which itself is detected by a phase-locked loop. Measurements with externally applied sample potentials validate the derived model and experiments on a standard KPFM sample show improved spatial resolution, when compared to conventional methods. The capability of AC-EFM to quantitatively measure surface potentials at the nanoscale without the use of a dc-bias is demonstrated.

Contactless sensing methods using capacitively cou-pled probes can enable local radio frequency (RF) voltage measurements without the need for large contact pads. This enables a measurement of internal voltage distributions and can significantly facilitate the development of integrated microwave devices. The achievable spatial resolution of these methods is typically limited by parasitic capacitive cross-talk between the probe and adjacent circuit parts. When RF voltage measurements are performed at multiple tip-surface distances, cross-talk can be reduced by employing a suitable model of the distance dependent tip-circuit capacitance. In this paper, the achievable spatial reso-lution and its limitation by cross-talk induced errors is analysed. Electrostatic simulations of the capacitance between a probe tip and different test structures on a passivated circuit are performed and the results are verified by RF voltage measurements on $\mu \mathrm{m}$ -sized test structures at a frequency of 13 GHz. The analysis shows, that the achievable spatial resolution is mainly limited by the passivation layer and that cross-talk induced measurement errors limit the minimum structure size to two times the layer thickness.

Knowledge of electric charge and potential distributions at the nanoscale is of great interest in the fields of material science and biology. The required high measurement accuracy, spatial resolution, and applicability to aqueous environments are not always provided by conventional techniques such as Kelvin probe force microscopy (KPFM) due to averaging artifacts and the use of a dc bias. This article presents the development of an atomic force microscopy measurement mode, enabling quantitative surface potential measurements of nanoscale structures with high measurement accuracy in air and liquid (aqueous) environments. Averaging artifacts caused by the influence of the cantilever cone, cantilever beam and tip-sample distance in dc-bias-free KPFM (AC-KPFM) are eliminated by the implemented heterodyne detection and single-pass operation. The accuracy of the potential measurement as compared to amplitude-modulated KPFM (AM-KPFM) modes is greatly improved while keeping the advantages of closed-loop and dc-bias-free operation. Experiments on a gold-aluminum test sample and collagen fibrils show quantitative surface potential measurements on nanoscale structures and operability in an aqueous environment.

The development of highly integrated microwave devices greatly benefits from precise knowledge of internal device voltages. Contact-based probing techniques can only provide external measurements and are limited by the size of the necessary contact pads. The contactless voltage sensing method presented in this article enables measurements of radio frequency (RF) voltages within microwave devices. By using a conductive cantilever probe with a sharp tip as capacitively coupled sensor, measurements can be performed at precisely known tip–surface distances. In combination with the proposed model-based crosstalk compensation method, this enables measurements at high spatial resolution. The RF sensing system is implemented and experimentally verified. Voltages at frequencies up to 13 GHz on 2- $\mu \text{m}$ -sized structures are measured while improving the measurement sensitivity by a factor of 4.9 with respect to conventional contactless sensing techniques.

This paper presents the application of mechatronic demodulation for performing dynamic mode AFM measurements with self-sensing cantilevers in liquid. In the proposed approach the bridge circuits with the piezoresistive sensing elements are supplied by an ac voltage that corresponds to the cantilever oscillation frequency. In contrast to conventional read-out circuits, no dc voltages are applied to the piezoresistive elements. Therefore, electrochemical effects leading to a deterioration of the self-sensing cantilever are avoided. This is demonstrated by immersing a self-sensing cantilever in water and comparing microscope images for operation with dc and ac voltages. To verify the feasibility of using mechatronic demodulation for dynamic mode AFM imaging in water the topography of a calibration grating is measured.

This paper presents surface potential measurements by closed-loop AC-KPFM in aqueous solutions. In contrast to conventional KPFM, the proposed method omits the use of a dc-bias, therefore preventing electrochemical & electrokinetic effects, enabling operation in water. The ability for reproducible surface potential measurements in aqueous solutions is demonstrated and the influence of key imaging parameters on its performance are investigated, in particular the excitation frequency of the drive signal. It is found that proper operation of AC-KPFM in water is only possible in a regime, where movement of the ions in the solution is suppressed. In the case of highly deionized water, an excitation frequency of >30 kHz is necessary to achieve a comparable performance as conventional KPFM operation in air.

Performing electrical Atomic Force Microscopy measurements in aqueous solutions is of paramount importance in a vast range of scientific fields. As with operation at ambient conditions, it is important to ensure that the force on the cantilever is purely of electrostatic origin. However, with the insertion of water and therefore mobile ions and polar molecules into the tip-sample system come several unwanted effects. Here, an experimental study is carried out, analyzing the influence of parameters such as drive-frequency, -amplitude and ionic concentration on the feasibility to perform electrical AFM measurements in aqueous solutions. To this end the system is theoretically modelled and parameter sweeps are preformed, leading to transition frequencies above which the electrostatic force has the predominant impact on the cantilever.

This paper presents the development of a novel Atomic Force Microscopy mode, enabling dc-bias-free surface potential measurements with high spatial resolution. The influence of the cantilever cone, cantilever beam and lift height in dc-biasfree Kelvin Probe Force Microscopy (AC-KPFM) is reduced due to the proposed heterodyne detection principle, which is sensitive to the electrostatic force gradient. The accuracy of the potential measurement as compared to amplitude modulated KPFM modes is improved, while keeping the advantages of closed-loop and dc-bias-free operation. Measurements on a gold-aluminum test-sample show an increase in spatial resolution of 37% and lift height independent, quantitative surface potential measurements.

This paper presents the application of a mechatronic lock-in amplifier for demodulation of cantilever oscillations in dynamic Atomic Force Microscopy (AFM) measurement modes. The method is implemented using self-sensing AFM cantilevers with integrated piezoresistive deflection sensors, which are configured in AC bridge circuits for direct demodulation at the bridge circuit output. Dynamic AFM topography and phase measurements are carried out and the imaging performance is analysed. Comparison to demodulation by a conventional digital lock-in amplifier shows that the mechatronic demodulation method enables AFM imaging with significantly reduced sampling frequency without loss of imaging performance.

Mapping charged chemical groups at the solidliquid interface is important in many areas, ranging from colloidal systems to biomolecular interactions. However, classical methods to measure surface charges either lack spatial resolution or-like Kelvin-probe force microscopy (KPFM)-cannot be applied in aqueous solutions because a DC bias voltage is used. Here, we show that using AC Kelvin probe force microscopy (AC-KPFM), in which the DC bias is replaced with an AC voltage of sufficiently high frequency, the surface potential of spatially fixated, charged surface groups can be mapped in aqueous solution. We demonstrate this with micropatterned, functionalized alkanethiol layers which expose ionized amino- and carboxy-groups. These groups are representative of the charged groups of most biomolecules such as proteins. By adjusting the pH of the solution, the charge of the groups was reversibly altered, demonstrating the electrostatic nature of the measured signal. The influence of the electric double layer (EDL) on the measurement is discussed, and we, furthermore, show how charged, micropatterned layers can be used to spatially direct the deposition of nanoparticles of opposite charge.

The concept of on-chip double-resonant-tunneling-diode (RTD) patch-antenna oscillator has the virtues of compactness, simplicity, high isolation from the external circuitry, and increased output power. Relying on this concept, we demonstrate an order of magnitude increase in the output power compared to previous reports on patch-antenna RTD oscillators: 10 μW at the fundamental frequency of 525 GHz and 70 μW at 330 GHz. Estimates show that significantly higher frequencies and powers are achievable with this type of oscillator. Only optical lithography has been used in the fabrication process of the oscillators.