Near-field Scanning Microwave Microscope Research Articles

Coplanar waveguide ground potential imbalance as a source of useful signal in near-field scanning microwave microscopy

Various techniques are available in order to obtain information on samples of a different nature in near-field scanning microwave microscopy (NSMM), with transmission-line resonator (TLR) techniques considered the most advanced in terms of sensitivity and resolution. In this paper, we focus on the development of TLR-based NSMM supplied by a new source of useful signal: coplanar waveguide ground potential imbalance. Electromagnetic modeling of the device and experimental scanning of two planar structures are conducted to examine the performance of the proposed technique. Both modeling and experimental results demonstrate the ability to distinguish symmetric inhomogeneity positions with respect to the central conductor of the coplanar waveguide by the phase of the imbalance signal. The thin-film structure scanning procedure displays the possibility of the proposed approach to resolve low-contrast structures with an accuracy of up to 98.7%.

An approach of flow velocity measurement on microfluidic chip by near-field scanning microwave microscopy

A novel approach for flow velocity measurements on a microfluidic chip using near-field scanning microwave microscopy (NSMM) is proposed. It does not require the tested sample to contain visible particles, as the intrinsic electromagnetic properties of the fluid can be detected. Compared with conventional flowmetry, NSMM can measure flow velocity of the tested specimen without any invasive detectors inside or in contact with the specimen, and it can preserve original characteristics of the sample. The flow velocity of different types of solutions is obtained in point scanning, and flow velocity distribution in a microfluidic channel is illustrated in line scanning of NSMM measurement. The flow velocity distribution in microfluidic channel demonstrates distinctive differences between flow velocities of adjacent points, and the microfluidic channel is recognized with a lateral resolution of 10 μm. This approach can help optimize localized flow velocity measurement in drug screening and chemical reaction, and it can be developed for 2D/3D flow velocity imaging in chemical industry and biomedical field.

Physics-Assisted Learning Scheme for Quantitative Imaging via Near-Field Scanning Microwave Microscopy

Quantitative determination of the dielectric property is important to the analysis and evaluation of the investigated samples’ performance. This article proposes a physics-assisted learning scheme (PALS) for the quantitative imaging via the near-field scanning microwave microscopy (NFSMM) in a nondestructive way. To avoid using the machine learning in a black-box way, an efficient input generation is realized by utilizing the Rayleigh approximation method. Numerical and experimental examples demonstrate that the PALS recovers the dielectric parameters of the subsurface perturbation more accurately and shows a better generalization ability. The proposed learning scheme provides a reasonable way to combine the machine learning with underlying physics in the NFSMM, rather than utilizing machine learning in a black-box way. The same principle is also potential to be applied in other types of scanning microscopies, such as optical and thermal ones.

Near-Field Microwave Microscopy for 3D Surface Assessment of Manufactured Structures

Using near-field scanning microwave microscopy as a contact and non-contacting investigative tool for 3D surface metrology with three differing measurement modes, it has been possible to analyse structures that may be difficult for existing metrology systems. The system utilizes the small change in capacitance between a coaxial resonant probe (at around 2 GHz) ending in an open circuit tip, and the sample surface. This is measured in the frequency domain by the shift in the resonance frequency of the voltage transmission coefficient |S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> |. It is also possible to investigate various materials (metallics, plastics etc.) owing to their differing dielectric properties. The probe has been tested on a computer-controlled 3D stage but is suitable for incorporation into a commercial co-ordinate measurement machine (CMM) to enhance its capability to inspect the inside surfaces of structures, e.g., threads in small bores.

Fabrication of Ultra-Sharp Tips by Dynamic Chemical Etching Process for Scanning Near-Field Microwave Microscopy.

This work details an effective dynamic chemical etching technique to fabricate ultra-sharp tips for Scanning Near-Field Microwave Microscopy (SNMM). The protruded cylindrical part of the inner conductor in a commercial SMA (Sub Miniature A) coaxial connector is tapered by a dynamic chemical etching process using ferric chloride. The technique is optimized to fabricate ultra-sharp probe tips with controllable shapes and tapered down to have a radius of tip apex around ∼1 μm. The detailed optimization facilitated the fabrication of reproducible high-quality probes suitable for non-contact SNMM operation. A simple analytical model is also presented to better describe the dynamics of the tip formation. The near-field characteristics of the tips are evaluated by finite element method (FEM) based electromagnetic simulations and the performance of the probes has been validated experimentally by means of imaging a metal-dielectric sample using the in-house scanning near-field microwave microscopy system.

Nanoscale Characterization of Graphene Oxide-Based Epoxy Nanocomposite Using Inverted Scanning Microwave Microscopy.

Scanning microwave microscopy (SMM) is a novel metrological tool that advances the quantitative, nanometric, high-frequency, electrical characterization of a broad range of materials of technological importance. In this work, we report an inverted near-field scanning microwave microscopy (iSMM) investigation of a graphene oxide-based epoxy nanocomposite material at a nanoscopic level. The high-resolution spatial mapping of local conductance provides a quantitative analysis of the sample's electrical properties. In particular, the electrical conductivity in the order of ∼10-1 S/m as well as the mapping of the dielectric constant with a value of ∼4.7 ± 0.2 are reported and validated by the full-wave electromagnetic modeling of the tip-sample interaction.

Learning-Based Subsurface Quantitative Imaging via Near-Field Scanning Microwave Microscopy

Accurate characterization of sample dielectric property is essential for scientists and engineers to analyze and evaluate the performance of the invested sample. This article proposes a learning-based (LB) method for quantitative subsurface imaging via coaxial-resonator-based near-field scanning microwave microscopy (CR-NFSMM) in a nondestructive way. To efficiently generate the database, a well-designed forward solver is critical since many samples will be included in the database, each sample needs a scanning procedure, and each scanning point involves solving the forward problem once. A fast forward problem solver is used to evaluate the tip–sample interaction in CR-NFSMM that avoids repeated meshing during the scanning process and it can apply to an arbitrary tip shape. Once the neural network is trained, it generates the reconstructed image within 1 s. Numerical and experimental results show that the proposed LB method could improve the resolution of the image and recover the dielectric properties of the subsurface perturbation pixel-by-pixel. Moreover, the proposed method outperforms the traditional objective function approach in terms of image resolution and time cost. The proposed method is promising to realize a nondestructive and real-time local dielectric evaluation of the subsurface object.

A Systematic Material Characterization Method via Near-Field Scanning Microwave Microscopy

Accurate measurement of material dielectric property is essential for engineers to synthesize microwave devices. This article proposes a systematic material characterization method via the coaxial resonator (CR)-based near-field scanning microwave microscopy (NFSMM) with an arbitrary tip shape, from the modeling method to the inversion algorithm. The effective interaction region in the CR-based NFSMM is investigated and determined, which contributes most to the contrast capacitance caused by the tip-sample interaction. Then the Huygens’ principle is applied in the forward solver of NFSMM when the samples under test are homogeneous. The relative error of our model is less than 2.1% in numerical validations. Based on the proposed forward solver, a dielectric characterization method via the conjugate gradient (CG) algorithm is proposed and verified with experimental results. The retrieved permittivity of unknown samples matches well with that measured by the transmission line method, and the discrepancy between the two results is less than 3%. The proposed systematic method is promising to provide a new pathway to quantitatively determine the dielectric property of homogeneous samples.

High-Resolution Detection of Rock-Forming Minerals by Permittivity Measurements with a Near-Field Scanning Microwave Microscope.

The identification of the minerals composing rocks and their dielectric characterization is essential for the utilization of microwave energy in the rock industry. This paper describes the use of a near-field scanning microwave microscope with enhanced sensitivity for non-invasive measurements of permittivity maps of rock specimens at the micrometer scale in non-contact mode. The microwave system comprises a near-field probe, an in-house single-port vectorial reflectometer, and all circuitry and software needed to make a stand-alone, portable instrument. The relationship between the resonance parameters of the near-field probe and the dielectric properties of materials was determined by a combination of classical cavity perturbation theory and an image charge model. The accuracy of this approach was validated by a comparison study with reference materials. The device was employed to determine the permittivity maps of a couple of igneous rock specimens with low-loss and high-loss minerals. The dielectric results were correlated with the minerals comprising the samples and compared with the dielectric results reported in the literature, with excellent agreements.

Application of Near-Field Scanning Microwave Microscopy in Liquid Environment

In this article, a method of illustrating the electromagnetic properties of liquid specimens is proposed by our homemade near-field scanning microwave microscopy (NSMM). By introducing the fundamental theorem of NSMM and conducting a simulation on the configuration of the whole system, parameters including quality factor and resonant frequency in electrolyte liquid specimen vary with types of the solution as well as their concentration. By conducting the line scanning testing method, the relation of concentration of a specified solution concerning its corresponding electromagnetic properties is clarified. Eventually, yeast cells are tested at living or dead status with point scanning and line scanning separately. The NSMM experiment results exhibit a substantial capacity for biologic samples in a liquid environment.

Detection of Anti-Counterfeiting Markers through Permittivity Maps Using a Micrometer Scale near Field Scanning Microwave Microscope.

This paper describes the use of microwave technology to identify anti-counterfeiting markers on banknotes. The proposed method is based on a robust near-field scanning microwave microscope specially developed to measure permittivity maps of heterogeneous paper specimens at the micrometer scale. The equipment has a built-in vector network analyzer to measure the reflection response of a near-field coaxial probe, which makes it a standalone and portable device. A new approach employing the information of a displacement laser and the cavity perturbation technique was used to determine the relationship between the dielectric properties of the specimens and the resonance response of the probe, avoiding the use of distance-following techniques. The accuracy of the dielectric measurements was evaluated through a comparative study with other well-established cavity methods, revealing uncertainties lower than 5%, very similar to the accuracy reported by other more sophisticated setups. The device was employed to determine the dielectric map of a watermark on a 20 EUR banknote. In addition, the penetration capabilities of microwave energy allowed for the detection of the watermark when concealed behind dielectric or metallic layers. This work demonstrates the benefits of this microwave technique as a novel method for identifying anti-counterfeiting features, which opens new perspectives with which to develop optically opaque markers only traceable through this microwave technique.

Quantitative Characterization of Platinum Diselenide Electrical Conductivity With an Inverted Scanning Microwave Microscope

Near-field scanning microwave microscopy is a technique with increasing popularity for the study of nanometer-scale electrical properties of samples. Here, we present an approach to quantify sample properties in images obtained with an inverted scanning microwave microscope (iSMM), recently introduced by our group. In particular, this study reports the analysis of the local electrical conductivity of a platinum diselenide sample and proves its semimetal behavior. The approach is validated by a full-wave numerical model, reproducing the complete iSMM operation as well as all steps of the calibration algorithms. To extract local sample properties, this article provides two calibration procedures, respectively, for transmission and reflection mode measurements, based on a two-port equivalent circuit of the iSMM. This enables the high-frequency quantitative characterization of a wide variety of samples and surfaces.

A Microwave Near-field 3D Super-resolution Imaging Method

Microwave imaging is widely used in many areas involving non-destructive testing, biomedical imaging, radar detection and imaging, etc. However, there is a lack of microwave 3D imaging methods with lateral and depth super-resolution. We propose a microwave near-field 3D super-resolution imaging method based on the near-field scanning microwave microscopy (NSMM) technique, enabling lateral and depth super-resolution simultaneously. A resonant-cavity probe is designed to scan the first group of step samples to establish a linear equation between the phase of the normalized reflection coefficient (NS11) and the tip-sample distance ( $$\Delta H$$ ). The second group of samples is scanned at the resonance frequencies corresponding to $$\Delta H$$ , realizing the layer-by-layer imaging of the letter patterns. Finally, we have calculated the height information through the fitted linear equation and reconstructed the second group of step samples with letter patterns. The 3D reconstruction of the second group of step samples with letter patterns is completed. The depth variation of the step and the height of the letter patterns are 200 $$\mu m$$ (~ λ/570), which can be identified approximatively through experiments. And the letter patterns with a lateral width of 720 $$\mu m$$ (~ λ/160) are resolved. The experimental results show that our method has good feasibility in realizing microwave 3D imaging with depth and lateral super-resolution.

Real-Time Removal of Topographic Artifacts in Scanning Microwave Microscopy

Near-field scanning microwave microscopy (SMM) employs microwave radiation to image and characterize samples down to the atomic scale, including soft biological structures or inorganic materials, such as ferroelectric films. However, SMM generally also senses sample topography; hypersensitivity to topography becomes problematic with additional parasitic contributions and may partially or completely mask nontopographic sample features in the data, such as contrast in electrical conductivity or permittivity. This work proposes a simple and effective procedure to remove unwanted parasitic effects from SMM images. Differently from existing procedures, the method is applicable either in postprocessing or in real time, i.e., during the scanning operation. This allows the immediate visualization of nontopographic sample features in the instrument screen. As a proof of concept, hafnium zirconium oxide (HfZrO) ferroelectric film with high surface roughness is studied; unwanted contributions were removed from SMM data, providing a clear map of the sample ferroelectric structure that was originally hidden.

Improvement of spatial resolution by tilt correction in near-field scanning microwave microscopy

The limitation of mechanical manufacturing will result in a small tilt angle of the sample stage in the horizontal direction, which decreases the spatial resolution of imaging in near-field scanning microwave microscopy (NSMM). In this paper, we focus on the tilt correction and improve the spatial resolution of the NSMM image. The results of electromagnetic simulation and line scan measurement demonstrate the critical parameters affecting the sensitivity of NSMM, such as the length of the probe extending out of the cavity, the tip–sample distance, and the tip apex size. The tilt images can rotate successfully to the horizontal plane with the tilt correction methods, and the local average and re-interpolation are applied to denoise the images. Experimental NSMM images of copper thin film grid, coin texture, lithography mask, and leaf vein are obtained. The images before and after correction verify the improvement of the spatial resolution with all the above methods.

Microscopic origin of inhomogeneous transport in four-terminal tellurene devices

Tellurene—the 2D form of elemental tellurium—provides an attractive alternative to conventional 2D semiconductors due to its high bipolar mobilities, facile solution processing, and the possibility of dopant intercalation into its 1D van der Waals lattice. Here, we study the microscopic origin of transport anisotropy in lithographically defined four-terminal tellurene devices using spatially resolved near-field scanning microwave microscopy (SMM). Our conductivity- and carrier type-sensitive SMM imaging reveals that the overall p-type transport measured between adjacent and opposite terminals originates from strong p-type character at the device edges. Despite using an atomic layer deposition-grown conformal overcoat that n-dopes the device interior, we observe only weak n-type transport along the main device channel at positive backgate voltages. This weak n-type transport along the device channel is shown to arise from local p-doping within a few micrometers of the electrodes, which produces a transport barrier from the n-type interior to the electrodes. These results reveal how the backgate-dependent conduction anisotropy could be leveraged to weigh different inputs for non-von Neumann architectures.

Converse magneto-electric effects in a core\u2013shell multiferroic nanofiber by electric field tuning of ferromagnetic resonance

This report is on studies directed at the nature of magneto-electric (ME) coupling by ferromagnetic resonance (FMR) under an electric field in a coaxial nanofiber of nickel ferrite (NFO) and lead zirconate titanate (PZT). Fibers with ferrite cores and PZT shells were prepared by electrospinning. The core–shell structure of annealed fibers was confirmed by electron- and scanning probe microscopy. For studies on converse ME effects, i.e., the magnetic response of the fibers to an applied electric field, FMR measurements were done on a single fiber with a near-field scanning microwave microscope (NSMM) at 5–10 GHz by obtaining profiles of both amplitude and phase of the complex scattering parameter S11 as a function of bias magnetic field. The strength of the voltage-ME coupling Av was determined from the shift in the resonance field Hr for bias voltage of V = 0–7 V applied to the fiber. The coefficient Av for the NFO core/PZT shell structure was estimated to be − 1.92 kA/Vm (− 24 Oe/V). A model was developed for the converse ME effects in the fibers and the theoretical estimates are in good agreement with the data.

Spatially Resolved Persistent Photoconductivity in MoS2-WS2 Lateral Heterostructures.

The optical and electronic properties of 2D semiconductors are intrinsically linked via the strong interactions between optically excited bound species and free carriers. Here we use near-field scanning microwave microscopy (SMM) to image spatial variations in photoconductivity in MoS2-WS2 lateral multijunction heterostructures using photon energy-resolved narrowband illumination. We find that the onset of photoconductivity in individual domains corresponds to the optical absorption onset, confirming that the tightly bound excitons in transition metal dichalcogenides can nonetheless dissociate into free carriers. These photogenerated carriers are most likely n-type and are seen to persist for up to days. Informed by finite element modeling we reveal that they can increase the carrier density by up to 200 times. This persistent photoconductivity appears to be dominated by contributions from the multilayer MoS2 domains, and we attribute the flake-wide response in part to charge transfer across the heterointerface. Spatial correlation of our SMM imaging with photoluminescence (PL) mapping confirms the strong link between PL peak emission photon energy, PL intensity, and the local accumulated charge. This work reveals the spatially and temporally complex optoelectronic response of these systems and cautions that properties measured during or after illumination may not reflect the true dark state of these materials but rather a metastable charged state.

Defect Detection in Graphene Preparation Based on Near-Field Scanning Microwave Microscopy

In this letter, a nondestructive local characterization of graphene fabricated by the reduction method on graphene oxide aqueous was demonstrated by using a near-field scanning microwave microscopy (NSMM). The dielectric properties of graphene films have been extracted to analyze the quality of graphene. Clear images of defects on graphene film were obtained by mapping the resonant frequency, $f_{r}$ , and quality factor, $Q$ , of a given area. The results and images obtained demonstrate that NSMM can be employed in thin-film analysis for characterization of local electrical properties of materials in a nondestructive manner and for obtaining a map of conductivity distribution.

Ultra-subwavelength focusing and giant magnetic-field enhancement in a low-loss one-way waveguide based on remanence

The subwavelength focusing based on surface plasmon polaritons has been widely explored in tapered metallic structures. However, the efficiency of energy localization is relatively weak, largely due to high propagation loss and strong back reflection. Here, we propose a three-dimensional (3D) straight-tapered one-way surface magnetoplasmon (SMP) waveguide with the end surface of ∼10−4λ0 × 10−4λ0 to achieve energy focusing in the microwave regime. Due to low propagation loss of SMP, we achieve huge magnetic field enhancement in such an ultra-subwavelength area, by five orders of magnitude. Instead of using an external static magnetic field, our proposed SMP waveguide relies on remanence, which is very convenient for operating practical 3D applications. These results show promising applications in magnetic-field enhancing or quenching fluorescence, luminescence or nonlinearity of two-dimensional materials, novel scanning near-field microwave microscopy and energy storage.