An acousto-optic (AO)-based electric field sensor is presented for time domain measurement under magnetic resonance imaging (MRI). A fully MR-compatible sensor is designed and fabricated using a phase-shifted fiber Bragg grating mechanically coupled to a piezoelectric transducer. Mechanical resonance of the piezoelectric transducer is matched to the operating frequencies of commonly used MRI systems to increase the sensitivity of the sensor. Sensitivity of the sensor is measured as 1.27 mV/V/m, with a minimum detectable electric field of 4.4 mV/m/√Hz. Directivity of the sensor is measured with a 18 dB orthogonal component rejection. The dynamic range of the sensor is calculated as 117 dB/Hz, which allows the measurement of electric fields up to 3.2 kV/m. In MRI studies, the AO sensor was able detect local hot spots around a reference implant accurately with high signal-to-noise ratio. AO sensor exhibited similar or better performance when compared with commercially available MRI compatible electric field sensors. Furthermore, the small size of the sensor with the flexible fiber optic link could allow in situ measurements of electric fields during critical interventional procedures such as pacemaker lead or deep brain stimulator placement as an MRI dosimeter during diagnostic scans.
In this paper, a Luneburg lens is explored for omnidirectional structure-borne wave focusing both numerically and
experimentally. The proposed lens is formed by radially distributed blind holes with different diameters based on the
gradient index phononic crystal theory. The radial orientation and diameter of the holes are determined according to the
refractive index distribution which is guided by finite-element simulations of the lowest asymmetric mode Lamb wave
band diagrams. According to this design, the wave travels slower at the center of the lens and converges at the focal spot
which is on the circular lens boundary. Wave simulations are performed in COMSOL Multiphysics® under plane wave
excitation from a line source and wave focusing is observed at the opposite border of the lens with respect to the
incoming wave direction. Experimentally measured wave fields with a scanning laser vibrometer successfully validate
simulated wave focusing. Furthermore, omnidirectionality is verified by testing the lens under plane wave excitation
from different directions. With piezoelectric energy harvesters located at the boundary of the Luneburg lens substantially
larger power output can be obtained as compared to the baseline case of energy harvesting without the lens on the
uniform plate counterpart for the same incident plane wave excitation.
In this paper, we explore 3D-printed Gradient-Index Phononic Crystal Lens (GRIN-PCL) for structure-borne focusing
both numerically and experimentally. The proposed lens consists of an array of nylon stubs with different heights which
is fabricated by 3D printing the PA2200 nylon. The orientation and height of the stubs are determined according to the
hyperbolic secant gradient distribution of refractive index which is guided by finite-element simulations of the lowest
asymmetric mode Lamb wave band diagrams. The fabricated lens is then bonded to an aluminum plate to focus the wave
energy in the structure. The wave focusing performance is simulated in COMSOL Multiphysics® under plane wave
excitation from a line source indicating that the focal points are consistent with the analytical beam trajectory results.
Experiments are conducted with a scanning laser vibrometer and experimentally measured wave field successfully
validates the numerical simulation of wave focusing within the 3D-printed GRIN-PCL domain. With a piezoelectric
energy harvester disk located at the focal region of the GRIN-PCL larger power output is obtained as compared to the
baseline case of energy harvesting without the GRIN-PCL on the uniform plate counterpart for the same incident plane
wave excitation.
Vibration-based energy harvesting has been heavily researched over the last decade to enable self-powered small electronic components for wireless applications in various disciplines ranging from biomedical to civil engineering. The existing research efforts in this interdisciplinary field have mostly focused on the harvesting of deterministic or stochastic vibrational energy available at a fixed position in space. Such an approach is convenient to design and employ linear and nonlinear vibration-based energy harvesters, such as base-excited cantilevers with piezoelectric laminates. However, persistent vibrations at a fixed frequency and spatial point, or standing wave patterns, are rather simplified representations of ambient vibrational energy. As an alternative to energy harvesting from spatially localized vibrations and standing wave patterns, this work presents an investigation into the harvesting of one-dimensional bending waves in infinite beams. The focus is placed on the use of piezoelectric patches bonded to a thin and long beam and employed to transform the incoming wave energy into usable electricity while minimizing the traveling waves reflected and transmitted from the harvester domain. To this end, performance enhancement by wavelength matching, resistiveinductive circuits, and a localized obstacle are explored. Electroelastic model predictions and performance enhancement efforts are validated experimentally for various case studies.
KEYWORDS: Signal to noise ratio, Point spread functions, Ultrasonography, Intravascular ultrasound, Real time imaging, Imaging arrays, Transducers, Image restoration, Image resolution, Phased arrays
Designing a mechanically flexible catheter based volumetric ultrasonic imaging device for intravascular and intracardiac imaging is challenging due to small transducer area and limited number of cables. With a few parallel channels, synthetic phased array processing is necessary to acquire data from a large number of transducer elements. This increases the data collection time and hence reduces frame rate and causes artifacts due to tissue-transducer motion. Some of these drawbacks can be resolved by different array designs offered by CMUT-on-CMOS approach. We recently implemented a 2.1-mm diameter single chip 10 MHz dual ring CMUT-on-CMOS array for forward looking ICE with 64-transmit and 56-receive elements along with associated electronics. These volumetric arrays have the small element size required by high operating frequencies and achieve sub mm resolution, but the system would be susceptible to motion artifacts. To enable real time imaging with high SNR, we designed novel arrays consisting of multiple defocused annular rings for transmit aperture and a single ring receive array. The annular transmit rings are utilized to act as a high power element by focusing to a virtual ring shaped line behind the aperture. In this case, image reconstruction is performed by only receive beamforming, reducing total required firing steps from 896 to 14 with a trade-off in image resolution. The SNR of system is improved more than 5 dB for the same frequency and frame rate as compared to the dual ring array, which can be utilized to achieve the same resolution by increasing the operating frequency.
A micrograting interferometer has been fabricated to use in measuring the static and dynamic performance of MEMS devices. These measurements aid in qualifying the functionality of fabricated MEMS devices, as well as improving fabrication techniques. The metrology system uses a phase sensitive diffraction grating for interferometric axial resolution and a microfabricated lens for improved lateral resolution. In addition, active control is applied to the system to reduce the impact of mechanical vibrations and insure a high degree of measurement sensitivity. The control scheme is demonstrated successfully in the scanning of MEMS devices in the experiment. A deformable grating, which controls measurement sensitivity, has been fabricated and integrated with optoelectronics in small volume. Experiments with the integrated package demonstrate that the measurement sensitivity can be adjusted by actuating the deformable grating. This integrated single device illustrates that the deformable grating sensor can be expanded to form arrays for parallel measurement of MEMS device.
Intravascular ultrasound (IVUS) imaging has become an essential imaging modality for the effective diagnosis and treatment of cardiovascular diseases during the past decade enabled by innovative applications of piezoelectric transducer technology. The limitations in the manufacture and performance of the same piezoelectric transducers have also impeded the improvement of IVUS for emerging clinically important applications such as forward viewing arrays for guiding interventions and high resolution imaging of arterial structure such as vulnerable plaque and fibrous cap, and also implementation of techniques such as harmonic imaging of the tissue and of the contrast agents. Capacitive micromachined ultrasonic transducer (CMUT) technology shows great potential for transforming IVUS not only to satisfy these clinical needs but also to open up possibilities for low-cost imaging devices integrated to therapeutic tools. We have developed manufacturing processes with a maximum process temperature of 250°C to build CMUTs on the same silicon chip with integrated electronics. Using these processes we fabricated CMUT arrays suitable for forward viewing IVUS in the 10-20MHz range. We characterized these array elements in terms of pulse-echo response, radiation pattern measurements and demonstrated its volumetric imaging capabilities on various imaging targets.
A micro grating interferometer has been fabricated to use in measuring the dynamic performance of MEMS devices. The system uses a phase sensitive diffraction grating for interferometric axial resolution and a microfabricated lens for improved lateral response. Early experimental results using a non-deformable grating interferometer show that both the transient and steady state vibration of MEMS devices can be measured and mapped using the micro interferometer. These initial results also reveal vibrational noise and sample alignment problems. To avoid these obstacles and to maximize the sensitivity of the interferometer, a PID control unit is introduced. Analysis has been performed on the interferometer system to improve the controller design. A deformable grating interferometer has also been fabricated using microfabrication techniques and tested to show proper range of actuation under DC bias. This grating also demonstrates the ability to maintain a high sensitivity during operation.
A numerically stable and systematic implementation of the rigorous coupled-wave analysis (RCWA) for the general multilayered grating structures is presented for both TE and TM modes. Numerical results of the approach are shown for the diffraction-based optical device as an example and are compared with the scalar diffraction method to illustrate the limited applicability of the scalar analysis.
A diffraction-based interferometric optical detection method for micromachined acoustic sensors can provide better sensitivity as compared to conventional capacitance detection schemes especially at low frequency range. The optical detection method, complete with optoelectronics readout, can be integrated with a capacitive micromachined acoustic transducer. The method is utilized on a 19×19 capacitive micromachined ultrasonic transducer (cMUT) array to demonstrate ultrasonic imaging of wire targets at 750kHz in air. A silicon photodiode (PD) array is also designed and fabricated in a standard 1.3μm CMOS technology, and through-wafer etching of holes for optical interconnect is performed on the same silicon platform. Further improvement of displacement sensitivity in a resonant-cavity-enhanced (RCE) acoustic sensor is theoretically analyzed including the loss effect in the mirror, and the theoretical results are experimentally verified by measurements on devices with a thin metallic bottom mirror made of silver.
We present a technique in which atomic force microscopy (AFM) at ultrasonic frequencies is used to measure the contact stiffness between an AFM tip and thin films on silicon substrates. In this method, the resonance frequencies of the cantilever flexural modes are used to determine the tip-sample contact stiffness. We present experimental results, showing that the contact stiffness is highly sensitive to the thickness of thin metal and polymer films. These results are compared with those from out theoretical model, which we call the Contact Stiffness Algorithm (CSA), that may be used to calculate the contact stiffness between an AFM tip and an arbitrarily layered sample. Unlike transmission electron microscopy (TEM) or scanning electron microscopy (SEM) on a cross-section of the sample, this film thickness measurement technique is non- destructive. It is also capable of high lateral spatial resolution, provided that a sharp AFM tip is used. We present images of a photoresist film on silicon with contrast resulting from the elastic properties of the sample.
A high frequency ultrasonic technique has been developed to monitor photoresist processing in situ during semiconductor manufacturing. Photoresist pre-exposure bake and development have been monitored using the sensor, and the post-exposure bake has been studies as well. The in-situ glass transition temperature (Tg) was determined during the prebake for I-line films down to 0.6micrometers as well as for chemically- amplified DUV resist of similar thicknesses. Using classical reflection theory, photoresist properties such as the density, thickness and acoustic velocity were determined during processing. This in situ parameter inversion method can be used to determine process endpoint if the optimal density, velocity, and thickness are predetermined. The Tg for post-exposure bake of I-line resists is expected to be the Tg of the novolac resin alone, without solvent present. Measurements using the described sensor have confirmed that the resin Tg during postbake is 118 degrees C, the value of Tg provided by Shipley. This provides a measurement of postbake as well as a confirmation that the sensor is measuring Tg accurately. The development process was also monitored using this sensor. Results prove the usefulness of this sensor for in situ measurements of resist thickness changes during development. This was verified for different exposure doses and for resist coated on a wafer with circuit topography.
Conventional methods of ultrasonic non-destructive evaluation (NDE) use liquids to couple sound waves into the test samples. This either requires immersion of the parts to be examined or the use of complex and bulky water squirting systems that must be scanned over the structure. Air-coupled ultrasonic systems eliminate these requirements if the losses at air-solid interfaces are tolerable. Micromachined capacitive ultrasonic transducers (cMUTs) have been shown to have more than 100 dB dynamic range when used in the bistatic transmission mode. In this paper, we present results of a pitch-catch transmission system using cMUTs that achieves a 103 dB dynamic range. Each transducer consists of 10,000 silicon nitride membranes of 100 micrometers diameter connected in parallel. This geometry result in transducers with a resonant frequency around 2.3 MHz. These transducers can be used in transmission experiments at normal incident to the sample or to excite and detect guided waves in aluminum and composite plates. In this paper we present ultrasonic defect detection results from both through transmission and guided Lamb wave experiments in aluminum and composite plates, such as those used in aircraft.
A system of in situ ultrasonic sensors has been developed that can be used to monitor the photoresist prebake process. A high frequency phase measurement monitors the resist film properties while a lower frequency time of flight measurement monitors the corresponding wafer temperature. The high frequency measurement involves calculating the phase of an ultrasound signal as it is reflected from the silicon/photoresist interface. As the photoresist film changes in thickness and viscoelastic properties, the phase of the reflected signal will change. In this way, it is possible to follow how the photoresist film changes as it bakes; the solvent evaporates from the resist, decreasing the thickness and increasing the density. Results indicate that there is a phase minimum at a repeatable temperature, believed to be the softening or glass transition temperature (Tg). The lower frequency (200 kHz) time of flight measurement employs PZT-5H piezoelectric transducers bonded to a quartz buffer rod. The transducer generates a Lamb wave in the wafer which is then detected at another location by an identical transducer. The time of flight of the Lamb wave through the wafer depends linearly on temperature. Using these two sensors, we can measure the wafer temperature and the photoresist properties during prebake; providing us with the information necessary for in situ process control.
In the last five years we have been actively developing capacitive micromachined ultrasonic transducers (cMUT) since they have potential advantages over piezoelectric transducers, such as ease of fabrication in single elements and arrays, broad bandwidth and high efficiency. We report on research efforts in the theoretical understanding of cMUT with an improved electrical equivalent circuit model as well as its actual implementation through microfabrication. First, we present a process sequence that has allowed us to make reproducible devices with sealed membranes. The impact of electrode metalization on the impedance, bandwidth and efficiency of the devices will be discussed, and experimental results will be compared to theoretical models. Our study in the paper indicates that the best cMUT performance can be achieved with the appropriate fabrication process under certain constraints. Our most recent devices have been designed to have an input impedance with areal part of 50 Ohms at a frequency around 5 Mhz. A through transmission measurement gave a dynamic range of better than 100 dB while operating in the frequency range of 1-10 MHz. Operated without tuning, the devices are capable of operation from dc to 10s of MHz which is achieved because the devices are not resonant. In summary, we will present a novel technology capable of delivering surface micromachined ultrasonic transducers that are efficient, and easy to make single element and multiple elements array transducers. These devices can also be integrated on chip with transmitter and receiver electronics.
We describe a novel technique to measure in situ, simultaneously, temperature and thin film thickness during semiconductor processing. The measurement is based on the principle that the velocity of an ultrasonic Lamb wave propagating in a silicon wafer is a function of both the wafer temperature and the thin film coating on the wafer surface. Because sensitivities of Lamb wave velocity to temperature and film thickness change differently with frequency, with a simple linear inversion method, we are able to obtain both the processing temperature an film thickness simultaneously with two sets of sensors operating at two distinct frequencies, 0.5MHz and 1.5MHz. This technique is demonstrated in an aluminum sputtering system. We have achieved a temperature measurement accuracy of +/- 0.15 degree C and an aluminum film thickness resolution of +/- 170 angstrom. The measurement does not depend on the optical or the electrical properties of either the wafer or the film materials, and is insensitive to the processing environment. With its high measurement accuracy and setup simplicity, this sensor system carries great potential in semiconductor process monitoring and control.
We propose a novel technique that utilizes point source excitation and detection of Lamb waves through dry, elastic contacts to monitor thickness changes of plate-like structures. A pair of pin transducers are used to excite and detect the A0 mode Lamb wave in a test plate or a pipe wall and the wave velocity is obtained by time of flight measurement. Any change in plate thickness can be detected by the change in the Lamb wave velocity due to the dispersive nature of the A0 mode. We demonstrate the power of this approach in ultrasonic pipe erosion/corrosion monitoring and its potential application in aircraft skin defect imaging. We present results of thickness measurements of a test plate with 1 percent accuracy, and erosion/corrosion monitoring in a section of pipe that was removed from service, as well as imaging of defects in an aluminum thin plate.
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