Low-Frequency Acoustic Microscopy

Since acoustic microscopy was first invented by Quate and Lemons,1 many workers in the field have built acoustic microscopes ranging in frequency from tens of megahertz to hundreds of gigahertz, nd for a wide variety of applications in materials characterization, integrated circuits evaluation, and medical applications. In this work, we use the acoustic microscope as a quantitative nondestructive evaluation tool, our main purpose being the detection and characterization of defects present within 1 mm of the surface of a sample

Analytic modeling of loss and cross-coupling in capacitive micromachined ultrasonic transducers

The structural loss mechanism of capacitive micromachined ultrasonic transducer (cMUT) is investigated using finite element analysis and the normal mode theory. A single micromachined transducer membrane on an infinite silicon substrate is simulated by incorporating absorbing boundary conditions in the finite element method. This enables direct evaluation of the mechanical impedance of the membrane. Furthermore, the field distribution along the thickness of the silicon substrate due to outward radiating wave modes is obtained. The normal mode theory is applied to extract the contributions of different wave modes to the complicated field distributions. It is found that, the lowest order Lamb wave modes are responsible for the loss. Evaluation of absolute and relative power losses due to individual modes indicate that the lowest order anti-symmetric (A0) mode is the dominant radial mode in agreement with experimental measurements. The results of the analysis are used to derive a detailed equivalent circuit model of a cMUT with structural loss

Low-Frequency Acoustic Microscopy

Since acoustic microscopy was first invented by Quate and Lemons,1 many workers in the field have built acoustic microscopes ranging in frequency from tens of megahertz to hundreds of gigahertz, nd for a wide variety of applications in materials characterization, integrated circuits evaluation, and medical applications. In this work, we use the acoustic microscope as a quantitative nondestructive evaluation tool, our main purpose being the detection and characterization of defects present within 1 mm of the surface of a sample.</p

A novel parametric-effect MEMS amplifier

This paper presents the theory and measurements of a mechanical parametric-effect amplifier with a 200-kHz input signal and a 1.84-MHz output signal. The device used is a MEMS time-varying capacitor which is composed of an array of low-stress metallized silicon-nitride diaphragms, and is pumped by a large-signal voltage at 1.64 MHz. This induces a large change in the capacitance, and results in parametric amplification of an input signal at 200 kHz. The parametric amplifier capacitance is 500 pF, resulting in an output impedance of 140 Ω. A higher impedance can also be achieved with a lower capacitance. To our knowledge, this device is the first-ever MEMS mechanical up-converter parametric-effect amplifier developed with an up-conversion ratio of 9:1. The measurements agree very well with theory, including the effect the series resistance and the and of the MEMS time-varying capacitor. The application areas are in amplifiers which operate at very high temperatures (200°C-600°C), under high particle bombardment (nuclear applications), in non-semiconductor-based amplification, and in low-noise systems, since parametric amplifiers do not suffer from thermal, shot, or 1/f noise problem

Silicon micromachined ultrasonic immersion transducers

Broadband transmission of ultrasound in water using capacitive, micromachined transducers is reported. Transmission experiments using the same pair of devices at 4, 6, and 8 MHz with a signal-to-noise ratio greater than 48 dB are presented. Transmission is observed from 1 to 20 MHz. Better receiving electronics are necessary to demonstrate operation beyond this range. Furthermore, the same pair of transducers is operated at resonance to demonstrate ultrasound transmission in air at 6 MHz. The versatile transducers are made using silicon surface micromachining techniques. Computer simulations confirm the experimental results and are used to show that this technology promises to yield immersion transducers that are competitive with piezoelectric devices in terms of performance, enabling systems with 130 dB dynamic range. The advantage of the micromachined transducers is that they can be operated in high-temperature environments and that arrays can be fabricated at lower cost. © 1996 American Institute of Physics

Novel parametric-effect MEMS amplifiers/transducers

A parametric effect amplifier has been built at 200 kHz (input) and 1.84 MHz (output) using a MEMS time-varying capacitor. The capacitor is composed of a thin low stress metallized silicon-nitride diaphragm and is pumped by a large signal voltage at 1.64 MHz. This results in a large change in the capacitance, and parametric amplification of an input signal at 200 kHz. To our knowledge, this device is the first-ever mechanical up-converter parametric-effect amplifier, with an up-conversion ratio of 9:1.Anglai