Robert B. Givens

A microelectromechanical‐based magnetostrictive magnetometer

The principles of operation of a microelectromechanical (MEMS)‐based magnetometer designed on the magnetoelastic effect are described. The active transduction element is a commercial (001) silicon microcantilever coated with an amorphous thin film of the giant magnetostrictive alloy Terfenol‐D [(Dy0.7Te0.3)Fe2]. In addition to the magnetostrictive transducer, basic components of the magnetometer include: (a) mechanical resonance of the coated‐microcantilever through coupling to an ac magnetic field; and (b) detection by optical beam deflection of the microcantilever motion utilizing a laser diode source and a position‐sensitive detector. Currently, the sensitivity of this MEMS‐based magnetostrictive magnetometer is ∼1μT.

A high sensitivity, wide dynamic range magnetometer designed on a xylophone resonator

A novel magnetometer based on a classical xylophone resonator is described. The device consists of an aluminum bar supported by two wires placed at the nodal points of the fundamental resonance frequency. The wires also supply current of this frequency to the bar. In the presence of a magnetic field, the Lorentz force causes the resonator to vibrate. The amplitude of this vibration is proportional to a vector component of the magnetic field. The device is intrinsically linear, and by altering the drive current the sensitivity can range from nanoteslas to teslas.

MEMS-based resonating xylophone bar magnetometers

A novel magnetometer which utilizes the Lorentz force to measure vector magnetic fields has recently been described. The device, based on a classical resonating xylophone bar, has an extremely wide dynamic range and is ideally suited to miniaturization using a variety of technologies. The overall dimensions of the xylophone bar are limited by the width of the nodal supports which act as current electrodes and ultimately govern the resonance qualities. Minimum xylophone lengths of 10 and 5 mm mare attainable by electrostatic discharge machining and chemical milling of metal foils, respectively. Significantly smaller devices are achievable using polycrystalline silicon-based MEMS processing. However, the sheet resistivity of the silicon restricts the drive current through the xylophone bar and thus limits the sensitivity. This sensitivity can potentially by regained by replacing the silicon xylophone bar with a metal/piezoelectric/metal sandwich structure.

Polysilicon xylophone bar magnetometers

The recently developed JHU/APL magnetometer, which is based on a free-free (xylophone) resonating bar, is simple, small, light weight, has a low power consumption and utilizes the Lorentz force to measure vector magnetic fields. The device is intrinsically linear and has a wide dynamic range such that it can measure magnetic field strengths from nanoteslas to teslas. Furthermore, its sensitivity is independent of size for resonating bars of the same material and aspect ratio. This makes it ideally suited for miniaturization using MEMS techniques. Various polysilicon xylophone bars have been designed, processed, and characterized. The output response has verified the size-independent scaling law and sensitivities of the order of 100 nanoTesla have been achieved with drive currents as low as 20 microamps. This drive current is limited by the sheet resistance of the polysilicon support electrodes and directly affects the sensitivity. The electrodes also have a dramatic effect on the resonant frequency since they act as torsional stiffening members on the resonating bar. For example, for a 500 X 50 micron xylophone the resonant frequency varies from the designed 69 kHz to over 95 kHz for 10 micron wide support electrodes. The electrodes do not affect the mechanical Q-factors observed and values in excess of 20,000 at reduced pressures have been routinely obtained.

Microwave thermoreflectometry for detection of rebar corrosion

A microwave-based approach under development for detecting corrosion of rebar is described. The rebar inside the concrete is heated with an induction heater and then the surface temperature of the rebar inside the concrete is probed using a microwave reflectance method. This is in contrast to infrared thermographic approaches which monitor the surface temperature of the concrete and are dependent on waiting for considerable lengths of time for heat flow from the rebar to the concrete surface. Results will be presented for a series of test specimens produced by deliberately corroding rebar inside concrete in the laboratory. Microwave thermoreflectance measurements made in a 5 second measurement time are compared with conventional thermographic measurements of the temperature distribution at the concrete surface which require a 10 minute measurement time. Theoretical results are also presented of the predicted temperature versus time curves expected for rebar inside concrete with and without air defects at the rebar-concrete interface. These results predict that a rebar-concrete interface could be distinguished from a rebar-air interface with only 1 second of heating. The theoretical results further show that the presence of an air layer of finite thickness between rebar and concrete after about 2 seconds could be detected with a 2 second heating time.

Novel solid state sensor platform

A unique solid-state optical sensor configuration has been invented that can serve as a development platform for a host of chemical and biochemical sensors in either gaseous or liquid environments. We present results from measurements from the first adaptation of the device to oxygen sensing via fluorescence quenching and note the distinct advantages over existing electrochemical and more recent fiber-optic methods. The platform technology itself features greatly enhanced energy efficiency, high sensitivity, low-power consumption, ease of miniaturization, low cost, high-volume manufacturability using standard methods, very fast response/recovery profiles, and high reliability. The oxygen sensor embodiment has been demonstrated to operate well over the temperature range from -20 to 50 degrees C, not to be interfered with by other common gases including water vapor at high levels, and capable of response times less than 100 milliseconds.

Design and Properties of a Thin-Film, Mems-Based Magnetostrictive Magnetometer

The principles of operation of a MEMS-based magnetometer designed on the magnetoelastic effect are described. The active transduction element is a commercial (001) silicon microcantilever sputter coated with an amorphous thin film of the giant magnetostrictive alloy Terfenol-D [(Dy 0.7 Tb 0.3 )Fe 2 ]. The easy axis of magnetization of the Terfenol-D film lies in the plane of the microcantilever and along the acicular direction. In addition to the magnetostrictive transducer, basic components include: (a) mechanical resonance of the coated-microcantilever through coupling to an ac magnetic field; and, (b) detection by optical beam deflection of the microcantilever motion utilizing a laser diode source and a position-sensitive detector. The sensitivity of this magnetostrictive magnetometer is currently 1 μT, which is three orders of magnitude better than a magnetometer based on a similar-sized piezoresistive cantilever.

Electron-Induced Ionization and Thermalization in CdS Using Optical Beam Deflection Imaging

The spatial dependence of electron energy loss in semiconducting cadmium sulphide has been investigated as a function of incident electron beam energy using time resolved internal optical beam deflection techniques. Spatial resolution of approximately 0.51.im has been achieved. Ionization and direct electron injection have been shown to dominate the response for times less than 200 nsec, with local thermalization due to electron relaxation effects dominating for times less than several microseconds. Thermal diffusion dominates the beam deflection response at longer times. From these measurements thermal diffusion coefficients have been measured and primary electron ranges estimated. Time resolved images of the primary electron interaction volume have been obtained.

Time Resolved Optical Detection of Electron Loss and Migration in CdS

A beam of energetic electrons incident on a semiconductor produces a variety of effects depending on the primary beam energy and on the properties of the semiconductor. These effects include electron penetration, internai ionization and thermal deposition as well as external effects such as secondary and back scattered electrons, electron beam induced current (EBIC) and lattice strain. Modulated electron beams have been used for thermal wave imaging by the use of piezoelectric detectors in contact with the sample to monitor the modulated strain produced by the electron beam. This technique is termed Scanning Electron Acoustic Microscopy (SEAM). SEAM studies of integrated circuits have shown that subsurface features are imaged at depths controlled by the energy of the electron beam [1]. However, there is no adequate theory which describes this effect or the more generai question of image contrast in SEAM. This arises in part because SEAM images represent a convolution of thermal, acoustic and electron transport effects as well as the initial electron loss profile in the semiconductor. There is a need for improved understanding of electron injection, scattering, trapping and thermalization especially as they apply to the use of electron excitation beams for thermal wave imaging.

Optical Beam Deflection in Semiconductors with Electron Beam Excitation

Optical beam deflection experiments performed in a scanning electron microscope are used to investigate electron-beam-specimen interactions and thermal and electronic diffusion in semiconductors.