Evanescent Microwave Microscope Research Articles

Spatially resolved electrical characterisation of graphene layers by an evanescent field microwave microscope

An evanescent field microwave microscope has been developed at the National Physical Laboratory. This instrument has multiple applications and has been developed to allow traceable measurements of local complex permittivity, unlike most other microwave scanning microscopes. In this paper we describe basic operation of the microscope and show measurements on graphene samples produced at Imperial College. The microscope obtains images by raster scanning of a wire probe in ‘contact mode’. Of particular interest to the graphene community is the possibility of being able to scan over large areas (up to 4×4mm2), and to be able to measure actual values of surface resistance without a requirement for metal contacts. As an ultrathin semimetal, a graphene layer being placed in the evanescent field of the probe is expected to behave like a lossy dielectric material, its microwave loss tangent is proportional to its conductivity. Employing a high Q dual mode re-entrant cavity as host resonator and a spherical metal probe of 180μm diameter, we found that spatial variations of the conductivity of graphene can be clearly resolved.

Structural investigations and magnetic properties of sol-gel Ni0.5Zn0.5Fe2O4 thin films for microwave heating

Nanocrystalline Ni0.5Zn0.5Fe2O4 thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673–1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation and remnant magnetization increased with increasing the grain size, while the coercivity demonstrated a maximum near a critical grain size of 21 nm due to the transition from monodomain to multidomain behavior. The complex permittivity of the Ni–Zn ferrite films was measured in the frequency range of 2–15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315–355 K was observed in the film close to the critical grain size.

Investigating the Dielectric Property of Abnormal Grain Growth Hexagonal BaTiO3

In this work, the structural and dielectric properties of hexagonal BaTiO 3 have been investigated. The samples are partially doped with Ga 3+ at Ti site to stabilize the hexagonal structure at room temperature. The abnormal grain growth (AGG) is observed on h-BaTi 1 − x Ga x O 3 samples at a higher sintering temperature. The Qxf values can be improved by Ga-doping in BaTiO 3 . By the evanescent microwave microscope scanning probe technology (EMP), it is found that the regions of the abnormal growth grains have the higher K-value than that of the normal growth grains. The K-value and Qxf value can be concurrently enhanced of the h-BaTi 1 − x Ga x O 3 samples with AGG have been reported.

Local complex permittivity measurements of porcine skin tissue in the frequency range from 1 GHz to 15 GHz by evanescent microscopy

The near-field evanescent microwave microscope is based on a coaxial transmission line resonator with a silver plated tungsten tip protruding through an end-wall aperture. The sensor is used to measure the local dielectric properties of porcine skin in the frequency range from 1 GHz to 15 GHz. The dielectric property of the skin within the near field of the tip frustrates the electric field and measurably changes the transmission line's resonant frequency and quality factor (Q). The shift of the resonator's frequency and Q is measured as a function of tip–sample separation, and a quantitative relationship between the real and imaginary parts of the local dielectric constant using the method of images is established. The associated changes in quality factor image scans of subsurface tissue structure and dielectric properties of skin surface lesions are presented.

Microwave Absorbing Ferrite Thin Films for Microwave Heating of Microstructured Reactors

AbstractNanocrystalline Ni0.5Zn0.5Fe2O4 thin films have been synthesized with various grain sizes by sol–gel method on polycrystalline silicon substrates. The morphology and microwave absorption properties of the films calcined in the 673–1073 K range were studied by using XRD, AFM, near–field evanescent microwave microscopy, coplanar waveguide and direct microwave heating measurements. All films were uniform without microcracks. The increase of the calcination temperature from 873 to 1073 K and time from 1 to 3h resulted in an increase of the grain size from 12 to 27 nm. The complex permittivity of the Ni-Zn ferrite films was measured in the frequency range of 2–15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315–355 K was observed in the film close to the critical grain size of 21 nm in diameter marked by the transition from single– to multi–domain structure of nanocrystals in Ni0.5Zn0.5Fe2O4 film and by a maximum in its coercivity.

Microwave characterization of nanostructured ferroelectricBa0.6Sr0.4TiO3 thin films fabricated by pulsed laser deposition

A series of nanostructured ferroelectric thin films of barium strontium titanate werefabricated using a pulsed laser deposition system with real-time in situ processcontrol. Pulsed laser deposition parameters were controlled during the growth ofnanostructured thin films for use in the development of high frequency tunable microwavedevices. The thin films were all grown at the same substrate temperature and laserbeam energy density, but the chamber oxygen partial pressure (COPP) was variedsystematically from 19 mTorr through 1000 Torr. Structural and electromagneticcharacterization was performed using atomic force microscopy and evanescent microwavemicroscopy, respectively. Atomic force microscopy showed a linear increase in grain sizewith increases in the ambient oxygen pressure from 38 to 150 mTorr and from300 mTorr to 1000 Torr. The correlation of the microwave properties with the epitaxialfilm microstructure can be attributed to stresses and polarizability in the film.Microwave characterization showed that a COPP of 75 mTorr yielded the mostdesirable film in terms of tunability and loss tangent over a wide frequency range.

The effects of dielectric loss and tip resistance on resonator Q of the scanning evanescent microwave microscopy (SEMM) probe

Scanning evanescent microwave microscopy (SEMM) has significant potential for measuring dielectric properties. The loss tangent appears in a linear term in the relation between frequency shift and resonator quality factor (Q) shift, but the coefficient of this term was not generally known. The derivation of an analytic expression for the coefficient is presented here; good agreement is seen with simulations. In addition, it is shown that the resistance of the evanescent microwave probe (EMP) tip gives rise to a quadratic term in the expression for Q shift.

Quantitative measurement of piezoelectric coefficient of thin film using a scanning evanescent microwave microscope

This article describes a new approach to quantitatively measure the piezoelectric coefficients of thin films at the microscopic level using a scanning evanescent microwave microscope. This technique can resolve 10 pm deformation caused by the piezoelectric effect and has the advantages of high scanning speed, large scanning area, submicron spatial resolution, and a simultaneous accessibility to many other related properties. Results from the test measurements on the longitudinal piezoelectric coefficient of PZT thin film agree well with those from other techniques listed in literatures.

Electromagnetic characterization ofYBa2Cu3O7−δ thin films with calcium doping for bi-crystal grain boundary conductivity enhancement

The objective of this study was to examine the transport properties of twoYBa2Cu3O7−δ thin filmswith (Y0.9Ca0.1)2BaCuO5 additionsdeposited on vicinal SrTiO3 6° bi-crystal substrates and to investigate the possible correlations between spatialcalcium distribution and local electromagnetic properties across bi-crystal grainboundaries using evanescent microwave microscopy (EMM) and atomic forcemicroscopy (AFM). The samples under consideration differed in transport criticalcurrent measurements by a factor of two although they were deposited on thesame type of bi-crystal substrate. A near-field evanescent microwave microscopebased on a coaxial transmission line resonator with an end-wall aperture wasused to measure changes in conductivity local to the bi-crystal boundary ofYBa2Cu3O7−δ thin films below (79.2 K) and above (room temperature) the superconducting transitiontemperature. Atomic concentration measurements by electron microprobe analysiswere performed in the same regions, and a clear correlation between calciumdistribution and conductivity at 79.2 K (as represented by the change in quality factor)was found. Surface potential imaging (SPI) and quality factor scans in the areaof the bi-crystal grain boundaries were performed at room temperature usingAFM and EMM, respectively, to evaluate local electromagnetic properties in thenormal state and investigate their correlation with superconducting properties.

Surface Electron Spin Resonance Study on Ruby Crystal Using Evanescent Microwave Microscopy Techniques

As the silicon technology approaches to its physical limit, the future electronic devices will depend on behaviors of a few electrons. This study is to explore the possibility of detecting a single electron spin transition by using nondestructive evanescent microwave microscopy (EMM) techniques. To enhance the RF magnetic field and minimize the dielectric losses, the sample is placed at the center of the conventional electron spin resonance (ESR) microwave cavity that does not have scanning image capabilities. In this paper, a magnetic dipole probe (MDP) is presented that not only has the advantages of the microwave cavity, but is also capable of surface scanning at high speeds. At present, the minimum detectable electron spin transitions are 20 000 on the ruby crystal (Cr3+ doped in Al2O3) surface, whereas the commercially available ESR microwave cavity has a resolution of 106 minimum detectable spins limit. Three ESR energy absorption spikes were detected at 3.77 and 3.73 GHz with the ruby crystal placed inside and outside of the MDP conductor loop, respectively. The measured ESR energy absorption spectra are consistent with theoretical analysis and the conventional ESR experimental results. The current MDP sensor has a 500-mum spatial resolution with a 1-mm radius conductor loop made by 150-mum copper wires. The nondestructive and noninvasive natures of the EMM microscopy are suitable for many biomedical applications, such as DNA sequencing, Alzheimer, and other biological tissue studies. Future efforts will be focused on integration of the MDP on the atomic force microscopy with carbon-nanotube bridges

Quantitative microscopy of nonlinear dielectric constant using a scanning evanescent microwave microscopy

Quantitative characterization of dielectric nonlinearity in ferroelectric materials has been successfully performed using a scanning evanescent microwave microscope, and a key calibration coefficient for quantitative microscopy of nonlinear dielectric constant is derived. This unique technique has advantages of high spatial resolution and simultaneously accessing of other related properties such as dielectric constant and magnetoelectric coefficient. Samples of LiNbO3 single crystal and PbTiO3 thin film are measured, which demonstrates that this technique can access the nonlinear dielectric constant of a microregion with a sensitivity of 1.0×10−21F∕V.

Near-field microwave microscope and electron-spin-resonance detection: ruby crystal surface

Microwave photons can image a surface by using near-field geometry with spatial resolution close to the nanometer-length scale. We detected electron-spin resonance (ESR) on ruby surfaces by using microwave photons at the S-band frequency (3.73 GHz). The spatial locations of the electron-spin centers were pinpointed with localized incident microwave photons generated by using evanescent microwave microscopy (EMM). We show that the EMM probe is capable of resolving 20,000 spin transitions, compared with the approximately 10(10) minimum detectable spins of the conventional continuous-wave ESR spectrometer. This represents roughly a 6-order-of-magnitude enhancement in sensitivity. Our ultimate goal is to achieve the minimum detectable spin transition of a single electron and nanometer-level spatial resolution by using microfabricated atomic force microscopy-EMM probes.

Sensitivity and resolution of evanescent microwave microscope

A near-field evanescent microwave microscope is based on a /spl lambda//4 coaxial TEM transmission-line resonator with a silver-plated tungsten tip extending through an end-wall aperture. This microwave microscope is used to characterize local electromagnetic properties of dielectrics, conductors, and superconductors. The resolution of the probe is verified experimentally by scanning etched gold lines on a sapphire substrate. A first-order sensitivity estimation generated from a unified equivalent circuit model of the probe and sample is investigated. The sensitivity inherent to the resonant probe and system noise is discussed. Experimental validation of sensitivity is given.

Measurement of the magnetoelectric coefficient using a scanning evanescent microwave microscope

A quantitative magnetoelectric coefficient measurement method was developed based on scanning evanescent microwave microscopy. This unique technique does not require electrodes and has advantages that it has a high spatial resolution and can simultaneously measure other related properties such as the nonlinear dielectric constant. We have demonstrated that this technique can detect the magnetoelectric coefficient of thin film samples as low as 10mV∕cmOe.

Local dielectric and strain measurements inYBa2Cu3O7−δ thin films by evanescent microscopy and Raman spectroscopy

The near-field evanescent microwave microscope is based on a coaxial transmissionline resonator with a silver-plated tungsten tip protruding through an end-wallaperture. The sensor is used to measure local dielectric properties of thin-filmYBa2Cu3O7−δ deposited onthree different SrTiO3 bi-crystal substrates having mismatch grain boundary angles of3°,6°,and 12°. The measurements in the superconducting state are below critical temperature atT = 79.4 K. The dielectric property of the superconductor within the near field of thetip frustrates the electric field and measurably changes the transmission line’sresonant frequency. The shift of the resonator’s frequency is measured as afunction of tip–sample separation and associated changes in quality factor(ΔQ) image scans of the thin film are presented. A quantitative relationship between the real andimaginary parts of the local dielectric constant and the frequency shift using the method ofimages is established. The comparison between experimental data and theory based on thismethod is given and discussed. Raman measurements of the intergranular strain within theYBa2Cu3O7−δ thin filmdeposited on each SrTiO3 substrate in the region of the bi-crystal junction showed excellent correlation between grainboundary mismatch and induced grain boundary strain. Compressive strains normal to thea axis (i.e. tensilestrains normal to the b axis) were detected across the grain boundary. The magnitude of induced strain as well asits spread away from the grain boundary increased as the mismatch angle increased.

Characterization of Local Dielectric Properties of Superconductor<tex>$rm YBa_2rm Cu_3rm O_7-delta$</tex>Using Evanescent Microwave Microscopy

A near-field evanescent microwave microscope based on a coaxial transmission line resonator with a tungsten tip protruding through an end-wall aperture is used to measure local dielectric properties of thin film YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// in superconducting state below critical temperature T/sub c/=91 K at T=79.4 K and in normal state at room temperature (T=298 K). The dielectric property of the superconductor within the near field of the tip frustrates the electric field and measurably changes the transmission line's resonant frequency. The shift of the resonator's frequency is measured as a function of tip-sample separation and associated change in quality factor (Q) image scans of the thin film is obtained. A quantitative relationship between the real and imaginary parts of the local dielectric constant and the frequency shift using the method of images is established. The comparison between experimental data and theory based on this method is given and discussed for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film deposited on LaAlO/sub 3/ substrate.

Quantitative scanning evanescent microwave microscopy and its applications in characterization of functional materials libraries

This paper gives a comprehensive review on the advances in the field of scanning evanescent microwave microscopy, as a high-throughput characterization tool for electrical properties. Theoretical model analyses used for performing quantitative non-destructive characterization of various materials are presented. Examples of applications of the microwave microscopy to the rapid measurements of dielectric/ferroelectric libraries are given.

Lossy AlN–SiC composites fabricated by spark plasma sintering

Dense lossy AlN–SiC composites doped with Y2O3 were fabricated by spark plasma sintering (SPS). Both the microstructure and the dielectric properties are different from those of hot-pressed samples. Microstructure analysis reveals little solid solution (AlN)x(SiC)1-x is formed. Scanning evanescent microwave microscopy images show that the materials by SPS exhibit large contrast in dielectric permittivity, whereas the hot-pressed materials show very mild fluctuation in dielectric permittivity over the samples. The results indicate that AlN–SiC composites fabricated by SPS can be treated approximately as a mechanical mixture of AlN and SiC when estimating complex permittivity of the composite in the microwave range. The complex permittivity of the composites with different SiC contents can be phenomenologically predicted by effective medium approximation.

Quantitative microwave evanescent microscopy of dielectric thin films using a recursive image charge approach

A recursive image charge approach has been successfully developed for quantitative microwave microscopy of dielectric thin films using the scanning evanescent microwave microscope. With this approach and the recursion-to-circulation algorithm, frequency shift of the microscope as functions of the thickness of the film, dielectric constants of the film and the substrate can be efficiently computed in a circulation way.

Measurement of Dielectric Property by Evanescent Microwave Microscope

Microwave dielectric properties of (Bi1.5Zn1.0Nb1.5)O7/MgO (BiZN/MgO) thin films were measured at 2.85 GHz by an evanescent microwave probe (EMP) technique. This is the first time that the dielectric properties of a thin film have been characterized in the microwave frequency region. The dielectric constant of the BiZN/MgO films increases with the deposition temperature, which is apparently due to the increase in crystallinity and the change in the preferred orientation of the films. High-resolution EMP measurement reveals that the films consist of ultrasmall grains about 0.4 µm in diameter. The dielectric constant (K) of BiZN/MgO films is comparable to that of the BiZN bulk materials, whereas the loss factor (tan δ) of the films is markedly larger than that of the ceramics.