High Power CMUTs: Design and experimental verification

Cataloged from PDF version of article.Capacitive micromachined ultrasonic transducers (CMUTs) have great potential to compete with piezoelectric transducers in high-power applications. As the output pressures increase, nonlinearity of CMUT must be reconsidered and optimization is required to reduce harmonic distortions. In this paper, we describe a design approach in which uncollapsed CMUT array elements are sized so as to operate at the maximum radiation impedance and have gap heights such that the generated electrostatic force can sustain a plate displacement with full swing at the given drive amplitude. The proposed design enables high output pressures and low harmonic distortions at the output. An equivalent circuit model of the array is used that accurately simulates the uncollapsed mode of operation. The model facilities the design of CMUT parameters for high-pressure output, without the intensive need for computationally involved FEM tools. The optimized design requires a relatively thick plate compared with a conventional CMUT plate. Thus, we used a silicon wafer as the CMUT plate. The fabrication process involves an anodic bonding process for bonding the silicon plate with the glass substrate. To eliminate the bias voltage, which may cause charging problems, the CMUT array is driven with large continuous wave signals at half of the resonant frequency. The fabricated arrays are tested in an oil tank by applying a 125-V peak 5-cycle burst sinusoidal signal at 1.44 MHz. The applied voltage is increased until the plate is about to touch the bottom electrode to get the maximum peak displacement. The observed pressure is about 1.8 MPa with −28 dBc second harmonic at the surface of the array

Design and implementation of capacitive micromachined ultrasonic transducers for high power

Capacitive micromachined ultrasonic transducers (CMUTs) have a strong potential to compete piezoelectric transducers in high power applications. In a CMUT, obtaining high port pressure competes with high particle velocity: a small gap is required for high electrostatic force while particle displacement is limited by the gap height. On the other hand, it is shown in [1] that CMUT array exhibits radiation impedance maxima over a relatively narrow frequency band. In this paper, we describe a design approach in which CMUT array elements resonate at the frequency of maximum impedance and have gap heights such that the generated electrostatic force in uncollapsed mode, can sustain particle displacement peak amplitude up to the gap height. The CMUT parameters are optimized for around 3 MHz of operation, using both a SPICE model and FEM. The optimized parameters require a thick membrane and low gap heights to get maximum displacement without collapsing membrane during the operation. We used anodic bonding process to fabricate CMUT arrays. A conductive 100 μm silicon wafer is bonded to a glass wafer. Before the bonding process, the silicon wafer is thermally oxidized to create an insulating layer which prevents break down in the operation. Then, the cavities are formed on the insulating layer by a wet etch. The gap height is set to 100 nm. Meanwhile, the glass wafer is dry etched by 120 nm and the etched area is filled by gold evaporation to create the bottom electrodes. The wafers are dipped into piranha solution and bonding process is done afterwards. The fabricated CMUTs are tested in an oil tank. To eliminate the DC voltage which may cause charging problem in the operation, we tried to drive the CMUT array with large continuous wave signals at half of the operating frequency. We observed 1MPa peak to peak pressure with -23 dB second harmonic at the surface of the array (Fig. 1). The proposed design further extends the operation of CMUTs. Observing low harmonic distortions at high output pressure levels, without any charging problem, make CMUT a big candidate for high power applications. © 2011 IEEE

CMUT array element in deep-collapse mode

Collapse and deep-collapse mode of operations have boosted the pressure outputs of capacitive micromachined ultrasonic transducers (CMUTs) considerably. In this work, we demonstrate a CMUT element operating in the deep-collapse mode with 25 V pulse excitation and without the effects of charge trapping. The fabricated CMUT element consists of 4 by 4 circular cells with 20 μm radius and 1 μm thick plates suspended over a 50 nm cavity. The overall size of the element is 0.190 mm by 0.19 mm. The collapse voltage of the plates is measured to be approximately 3V. By driving the CMUTs with 25V pulses in the deep-collapse mode without any bias, we achieved 1.2 MPa peak-to-peak pressure output on the surface of the CMUT element with a center frequency of 9 MHz and 100% fractional bandwidth. We applied 1000 consecutive electrical pulses with alternating polarity to the element and witnessed no change in the transmitted acoustic pulse. © 2011 IEEE

An equivalent circuit for collapse operation mode of CMUTs

Collapse mode of operation of the capacitive mi-cromachined ultrasonic transducers (CMUTs) was shown to be a very effective way for achieving high output pressures. However, no accurate model exists for understanding the mechanics and limits of the collapse mode. In this work, we extend the analyses made for CMUTs working in uncollapsed mode to collapsed mode. We have developed an equivalent nonlinear electrical circuit that can accurately simulate the mechanical behavior of a CMUT under any large signal electrical excitation. The static and dynamic deflections of a membrane predicted by the model are compared with the finite element simulations. The equivalent circuit model can estimate the static deflection within 1% and the transient behavior of a CMUT membrane within 3% accuracy. The circuit model is also compared to experimental results of pulse excitation applied to fabricated collapse mode CMUTs. The model is suitable as a powerful design and optimization tool for the collapsed as well as the uncollapsed case of CMUTs. © 2010 IEEE

Wafer bonded capacitive micromachined underwater transducers

In this work we have designed, fabricated and tested CMUTs as underwater transducers. Single CMUT membranes with three different radii and 380 microns of thickness are fabricated for the demonstration of an underwater CMUT element. The active area of the transducer is fabricated on top of a 3″ silicon wafer. The silicon wafer is bonded to a gold electrode coated glass substrate wafer 10 cm in diameter. Thermally grown silicon oxide layer is used as the insulation layer between membrane and substrate electrodes. Electrical contacts and insulation are made by epoxy layers. Single CMUT elements are tested in air and in water. Approximately 40% bandwidth is achieved around 25 KHz with a single underwater CMUT cell. Radiated pressure field due to second harmonic generation when the CMUTs are driven with high sinusoidal voltages is measured. ©2009 IEEE

Massive lower gastro-intestinal bleeding due to small bowel diverticula. A report of two cases

Gastro-intestinal bleeding from the small bowel is a rare entity. It is difficult to determine the source of bleeding because of the unavailability of routine small bowel endoscopy. The most common reasons for bleeding from the small bowel are tumours, arteriovenous malformations and inflammatory bowel diseases. Diverticula of the small bowel are very uncommon. We present two cases of gastro-intestinal bleeding due to small bowel diverticula. Both of them were diagnosed on laparotomy. One had a short segment of small bowel, with six diverticula, which was resected. The second case had a long segment of small bowel with multiple diverticula. This patient was treated by isolating and excising the bleeding diverticulum. Haemodynamically unstable lower gastro-intestinal bleeding mandates exploratory laparotomy. Mesenteric angiography and Tc 99 labelled erythrocyte scintigraphy can detect the bleeding site. Intra-operative endoscopy can be performed safely via an enterotomy and can localize the bleeding site

THE ROLE OF PERITONEAL-LAVAGE IN THE MANAGEMENT OF PENETRATING ABDOMINAL INJURY

Preventing negative laparotomies is one of the most challenging problems in the management of penetrating abdominal injuries. The term "selective laparotomy" has been therefore introduced and has found an ever increasing acceptance. The peritoneal lavage is a useful tool in patient selection for laparotomy but the main problem is where to set the boundary between a positive and a negative peritoneal lavage. The manipulation of this boundary leads to significant changes in the sensitivity and specificity of the peritoneal lavage. Here we are presenting 162 consecutive cases of penetrating abdominal trauma and discussing our methods of evaluation and management