Massood Tabib-Azar

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 H2 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 H2 concentrations.

Non-destructive characterization of materials by evanescent microwaves

A microstrip quarter wavelength ( lambda g/4) resonator in conjunction with a small probe is used to resolve objects with characteristic dimensions as small as a thousandth of the wavelength ( lambda g/1000). The characteristic length for the decay of the evanescent waves at the tip of the probe was measured to be approximately 100-150 mu m at a microwave frequency of 1 GHz ( lambda free approximately=30 cm). The authors applied this technique to map microwave conductivity of metallic lines on glass and printed circuit boards and to investigate conductivity variations across a silicon wafer. It was possible to detect holes in printed circuit boards that were covered with solder and were not detectable otherwise.

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.

Mechanical properties of self-welded silicon nanobridges

Mechanical properties of self-welded [111] single-crystal silicon nanowire bridges grown between two silicon posts using metal-catalyzed chemical vapor deposition were determined using both dynamic and static measurements. The static tests were carried out using atomic force microscopy (AFM) to measure the nanowires’ Young’s modulus and the strength of the self-welded junctions. The AFM-measured Young’s modulus ranged from 93 to 250 GPa (compared to 185 GPa for bulk silicon in the [111] direction) depending on the nanowire diameter, which ranged from 140 to 200 nm. The self-welded wire could withstand a maximum bending stress in the range of 210–830 MPa (larger than bulk silicon), which also depended on the nanowire diameter and loading conditions. The beam broke close to the loading point, rather than at the self-welded junction, indicating the excellent bond strength of the self-welded junction. The vibration spectra measured with a network analyzer and a dc magnetic field indicated a dynamic Young’s mo...

Design and fabrication of scanning near-field microwave probes compatible with atomic force microscopy to image embedded nanostructures

Design, fabrication, and characterization of near-field microwave scanning probes compatible with an atomic force microscope (AFM) for imaging of embedded nanostructures are discussed. The microwave probe discussed here bridges the frequency gap between the existing local probe microscopy systems, and enables localized microwave spectroscopy and imaging of molecules and nanostructures. The probe consists of a coaxially shielded heavily doped silicon tip, and an aluminum (Al) coplanar waveguide. The coaxial tip structure was formed by a thick photoresist and plasma etching process, enabling the silicon apex to protrude through a well-defined aperture in the Al layer. Using this technique, probes with 10-/spl mu/m-high coaxial tips of 5-nm apex radius and 500-nm aperture radius were realized. The aperture confines the electromagnetic fields in the exposed tip region, allowing microwave measurements with high spatial resolution. The mechanical and electrical characterizations of the microwave probes were performed to ensure their compliance with the requirement of an AFM, as well as that of the microwave measurements. Finally, simultaneous AFM and microwave imaging of standard AFM samples with grid structures was performed for the first time. The lateral spatial resolution of the microwave scans was approximately 50 nm at 2.8 GHz, compared to 100 nm for the AFM scans. The ability of the microwave signal to penetrate inside the sample opens new possibilities in hyperspectral and multimodal imaging of nanostructures. Correlations between AFM images and the microwave images enable proper registration and referencing of the microwave properties to landmarks in the topographic AFM images.

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.

Gallium arsenide transistors: realization through a molecularly designed insulator.

A GaAs-based transistor, analogous to commercial silicon devices, has been fabricated with vapor-deposited cubic GaS as the insulator material. The n-channel, depletion mode, GaAs field-effect transistor shows, in addition to classical transistor characteristics, a channel mobility of 4665.6 square centimeters per volt per second, an interfacial trap density of 1011 per electron volt per square centimeter, and a transconductance of 7 millisiemens for a 5-micrometer gate length at a gate voltage of 8 volts. Furthermore, the GaAs transistor shows an on-to-off resistance ratio comparable to that of commercial devices.

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.

Electronic passivation of n‐ and p‐type GaAs using chemical vapor deposited GaS

We report on the electronic passivation of n‐ and p‐type GaAs using chemical vapor deposited cubic GaS. Au/GaS/GaAs fabricated metal‐insulator‐semiconductor (MIS) structures exhibit classical high‐frequency capacitor versus voltage (C‐V) behavior with well‐defined accumulation and inversion regions. Using high‐ and low‐frequency C‐V, the interface trap densities of ∼1011 eV−1 cm−2 on both n‐ and p‐type GaAs are determined. The electronic condition of GaS/GaAs interface did not show any deterioration after a six week time period.

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.