Evanescent Microwave Probe Research Articles

Self-oscillating evanescent microwave probes for nondestructive evaluations of materials

The design and operation of a self-oscillating evanescent microwave probe (SO-EMP) for very high spatial resolution imaging of material nonuniformities are discussed. Composed of a microstripline resonator in a feedback loop across a 10-dB amplifier with 2.5-GHz bandwidth centered at 1.75 GHz, these oscillator probes are very compact and suitable for high-resolution imaging of materials. A wire tip connected to one end of the resonator enables the microwave probe to interact with a sample located nearby which affects the resonant frequency (f/sub 0/) and the quality factor (Q) of the oscillator. Variations in the material properties can be detected by scanning the wire tip over the sample while monitoring f/sub 0/ and Q that automatically track the permittivity, permeability, and dissipation in the sample. The SO-EMP outputs versus position of its tip over different high- and low-contrast samples were monitored and pseudo-color maps were generated to image material nonuniformities. Due to the amplifier nonlinearity that comes into play in the oscillation characteristics of this probe and the resonant contribution of the probe-sample interaction, the SO-EMP sensitivity and spatial resolution are improved compared to the other modes of operations we have reported in the past.

Dielectric mapping of a Pb(Ni1/3Nb2/3)O3–PbZrO3–PbTiO3 ternary phase spread

Lead perovskite compounds possess very rich and complex phase diagrams. They also have great potential in a variety of commercial applications. However, due in part to the very complicated nature of these materials, very few systematic studies of their ternary phase diagrams with respect to electrical properties have been performed. In this letter, we report the microwave dielectric property mapping of a ternary Pb(Ni1/3Nb2/3)O3–PbZrO3–PbTiO3 composition spread using a scanning evanescent microwave probe. We identified composition regions with dielectric constants higher than 600 at 1 GHz.

Evanescent microwave probe measurement of low-k dielectric films

An evanescent microwave probe (EMP) has been used to characterize low-k dielectric films grown on Si wafers at 2 GHz. Several families of low-k dielectric films with varying film thicknesses have been examined. The EMP signal (shift in resonant frequency) was found to scale with the “reduced electrical length,” defined as the film thickness/dielectric constant. The universal functional dependence derived from the experimental results is quantitatively consistent with the results of both analytical modeling and finite element simulations. These results demonstrate the unique capability of EMP to accurately characterize the application relevant parameter of low-k thin films.

Strong correlation between high-temperature electronic and low-temperature magnetic ordering inLa1−xCaxMnO3continuous phase diagram

Optical and microwave impedance and magnetic mapping of continuous phase diagram of thin film ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ was achieved on a single chip. The optical and microwave impedance images obtained by charge-coupled device camera and scanned evanescent microwave probe showed abrupt variations in visible wavelength reflectance and microwave impedance as functions of composition at room temperature. The low-temperature magnetic order probed by scanning superconducting quantum interference device microscope is directly correlated with the room-temperature electrical impedance variation. Most of these variations may arise from remnant electronic orbital ordering persisting to room temperature in these compounds. We also observed a narrow singular stripe in composition with dramatically increased conductivity.

Nondestructive imaging of grain boundaries in polycrystalline materials using evanescent microwave probes

Nondestructively imaging electrical properties of materials with very high spatial resolution is of great importance in analyzing electronic, semiconductor, superconductor, biological, and other materials. In this work, we introduce and discuss a new family of probes that use evanescent microwave fields to image microwave properties of materials. We discuss and explain new equipment and programming methods that have been incorporated into our setup to improve its resolution and imaging capabilities. In addition, new scanning techniques are discussed that allow for the affluence region of microwave probe to be characterized. Finally, images that were obtained from ceramics, diamond, and other types of materials are discussed to demonstrate the improved imaging capability of the evanescent microwave probe in detecting grain boundaries and film uniformities among other parameters.

Tip–sample distance feedback control in a scanning evanescent microwave probe for nonlinear dielectric imaging

We implemented tip–sample distance control in a scanning evanescent microwave probe for nonlinear dielectric microscopy. With the analytic expression of the tip–sample capacitance as a function of tip–sample distance, we can quantitatively regulate the tip–sample separation and independently measure the dielectric nonlinearity by application of an ac bias voltage. Simultaneous imaging of topography and ferroelectric domains has been demonstrated on periodically poled LiNbO3 single crystals.

Real-time imaging of semiconductor space-charge regions using high-spatial resolution evanescent microwave microscope

A high-resolution evanescent microwave probe (EMP) was used to detect and image depletion regions in solar cell p-n junctions in real time. The EMP uses a microwave resonator operating around 10 GHz that is coupled to a thin wire probe. Unable to travel beyond the waveguide discontinuity, the microwave fields set up evanescent fields in the tip’s vicinity. When coupled to an object nearby, these evanescent fields are modified and change the resonant characteristics of the resonator. The microwave conductivity of the nearby object affects the extent of the modification of the probe’s output which is monitored as the probe is scanned over the object. Using these EMP scans, steady-state and transient expansions/contractions of the p-n junction’s depletion region under dc and pulsed reverse/forward biases are mapped. These experimental data along with the conductivity calibration of the EMP were then used to quantitatively calculate doping concentrations, diffusion lengths, and carrier recombination lifetimes in the junction. Junctions are one of the most crucial building blocks of semiconductor devices and these studies clearly show the ability of the EMP in quantitative and nondestructive evaluations of electronic devices and circuits.

Pd-coated elastooptic fiber optic Bragg grating sensors for multiplexed hydrogen sensing

We report a new type of optical hydrogen sensor with a fiber optic Bragg grating (FBG) coated with palladium thin film. The sensing mechanism in this device is based on mechanical stress that is induced in the palladium coating when it absorbs hydrogen. The stress in the palladium coating stretches and shifts the Bragg wavelength of the FBG. Using FBGs with different wavelengths many such hydrogen sensors can be multiplexed on a single optical fiber. Here multiplexing two sensors is demonstrated. Moreover, hydrogen and thermal sensitivities of the sensors were calculated using a simple elastic model. Additionally, to quantify the amount of stress in the palladium film as a function of hydrogen concentration, a novel and very sensitive method was devised and used to detect deflections in a Pd-coated cantilever using an evanescent microwave probe. This stress was in the range of 5.26–8.59×10 −7 Pa for H 2 concentrations of 0.5–1.4% at room temperature, which is about three times larger than that found in the bulk palladium for the same range of H 2 concentrations.

Novel physical sensors using evanescent microwave probes

Local probes, such as electron and photon tunneling, atomic force, and capacitance probes, are excellent sensing means for displacement and other related sensors. Here we introduce applications of a new local probe using evanescent microwave probe (EMP) in displacement sensing with a very high vertical spatial resolution (0.01 μm at 1 GHz), very high bandwidth (100 MHz), and stability. The EMP has been used in the characterization and mapping of the microwave properties of a variety of materials in the past and its application in gas sensing and thermography was recently explored and reported. The interesting feature of the EMP is that its characteristics can be easily altered for a specific sensing application by changing its geometry and frequency of operation.

Evanescent microwave probes on high-resistivity silicon and its application in characterization of semiconductors

In this article we report the design, fabrication, and characterization of very high quality factor 10 GHz microstrip resonators on high-resistivity (high-ρ) silicon substrates. Our experiments show that an external quality factor of over 13 000 can be achieved on microstripline resonators on high-ρ silicon substrates. Such a high Q factor enables integration of arrays of previously reported evanescent microwave probe (EMP) on silicon cantilever beams. We also demonstrate that electron–hole pair recombination and generation lifetimes of silicon can be conveniently measured by illuminating the resonator using a pulsed light. Alternatively, the EMP was also used to nondestructively monitor excess carrier generation and recombination process in a semiconductor placed near the two-dimensional resonator.

Nondestructive superresolution imaging of defects and nonuniformities in metals, semiconductors, dielectrics, composites, and plants using evanescent microwaves

We have imaged and mapped material nonuniformities and defects using microwaves generated at the end of a microstripline resonator with 0.4 μm lateral spatial resolution at 1 GHz. Here we experimentally examine the effect of microstripline substrate permittivity, the feedline-to-resonator coupling strength, and probe tip geometry on the spatial resolution of the probe. Carbon composites, dielectrics, semiconductors, metals, and botanical samples were scanned for defects, residual stresses, subsurface features, areas of different film thickness, and moisture content. The resulting evanescent microwave probe (EMP) images are discussed. The main objective of this work is to demonstrate the overall capabilities of the EMP imaging technique as well as to discuss various probe parameters that can be used to design EMPs for different applications.

0.4 μm spatial resolution with 1 GHz (λ=30 cm) evanescent microwave probe

In this article we describe evanescent field imaging of material nonuniformities with a record resolution of 0.4 μm at 1 GHz (λg/750 000), using a resonant stripline scanning microwave probe. A chemically etched tip is used as a point-like evanescent field emitter and a probe–sample distance modulation is employed to improve the signal-to-noise ratio. Images obtained by evanescent microwave probe, by optical microscope, and by scanning tunneling microscope are presented for comparison. Probe was calibrated to perform quantitative conductivity measurements. The principal factors affecting the ultimate resolution of evanescent microwave probe are also discussed.

Evanescent Electromagnetics: A Novel, Super-Resolution, and Non-Intrusive, Imaging Technique for Biological Applications

Scanning tunneling and atomic force microscopes (STM and AFM) are used to study biological materials. These methods, often capable of achieving atomic resolutions, reveal fascinating information regarding the inner workings of these materials. However, both STM and AFM require physical contact to the specimen. In the case of STM, the specimen needs to be conducting as well. Here we introduce a new method for imaging biological materials through air or a suitable liquid using decaying or evanescent fields at the tip of a properly designed microwave resonator. This novel method involves the use of an evanescent microwave probe (EMP) and is capable of imaging a variety of non-uniformities in biological materials including conductivity, permittivity, and density variations. EMP is a non-contact and non-destructive sensor and it does not require conducting specimens. Its spatial resolution is currently around 0.4 μm at 1 GHz. We have used this probe to map non-uniformities in a variety of materials including metals, semiconductors, insulators, and biological and botanical samples. Here we discuss applications of EMP imaging in bone, teeth, botanical, and agricultural specimens.

Evanescent microwaves: a novel super-resolution noncontact nondestructive imaging technique for biological applications

Scanning tunneling microscopes (STM) and atomic force microscopes (AFM) are used to study biological materials. These methods, often capable of achieving atomic resolutions, reveal fascinating information regarding the inner workings of these materials. However, both STM and AFM require physical contact to the specimen. In the case of STM the specimen needs to be conducting as well. Here we introduce a new method for imaging biological materials through air or a suitable liquid using decaying or evanescent fields at the tip of a properly designed microwave resonator. This novel method involves the use of an evanescent microwave probe (EMP) and it is capable of imaging a variety of nonuniformities in biological materials including conductivity, permittivity, and density variations. EMP is a noncontact and nondestructive sensor and it does not require conducting specimens. Its spatial resolution is currently around 0.4 /spl mu/m at 1 GHz. We have used this probe to map nonuniformities in a variety of materials including metals, semiconductors, insulators, and biological and botanical samples. Here we discuss applications of EMP imaging in bone, teeth, botanical, and agricultural specimens.