Beam steering in a Half-Frequency driven Airborne CMUT transmitter array (original) (raw)
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Electrically unbiased driven airborne capacitive micromachined ultrasonic transducer design
2012 IEEE International Ultrasonics Symposium, 2012
We present a design method for airborne capacitive micromachined ultrasonic transducers (CMVT). We use an equivalent lumped element circuit to model both electrical and mechanical properties of CMUT and analyze it in frequency domain using harmonic balance approach. We use this method to design CMUTs for large transmitted power generation at low drive voltage amplitude. We determine the dimensions of an airborne CMUT using the proposed method that works at 30 kHz with 5 rum radius, 240 11m membrane thickness and 11.8 11m effective gap height. The CMUT is designed such that an atmospheric depression of 70% of effective gap height is maintained.
Designing transmitting CMUT cells for airborne applications
IEEE transactions on ultrasonics, ferroelectrics, and frequency control, 2014
We report a new mode of airborne operation for capacitive micromachined ultrasonic transducers (CMUT), in which the plate motion spans the entire gap without collapsing and the transducer is driven by a sinusoidal voltage without a dc bias. We present equivalent-circuit-based design fundamentals for an airborne CMUT cell and verify the design targets using fabricated CMUTs. The performance limits for silicon plates are derived. We experimentally obtain 78.9 dB//20 μPa@1 m source level at 73.7 kHz, with a CMUT cell of radius 2.05 mm driven by 71 V sinusoidal drive voltage at half the frequency. The measured quality factor is 120. We also study and discuss the interaction of the nonlinear transduction force and the nonlinearity of the plate compliance.
2003
In this paper, a 400-pm x 40C-prn, 2-D eapacitive micromachined ultrasonic transducer array element is experimentally characterized, and the results are found to be in agreement with theoretical predictions. As a receiver the transducer has a 1.8 x lo-' n m / m displacement sensitivity, and, as a transmitter, it produces 16.4 kPa/V of output pressure at the transducer surface at 3 MHz. The transducer also has more than 100% fractional bandwidth around 3 MHz, which maker i t suitable for ultrasound imaging. The radiation pattern measurements indicate a 3-dB acceptance angle of * 35 degrees in agreement with the theoretical predictions.
Design and implementation of capacitive micromachined ultrasonic transducers for high power
2011 IEEE International Ultrasonics Symposium, 2011
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.
The characterization of capacitive micromachined ultrasonic transducers in air
Ultrasonics, 2002
Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. They exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz, when driven from a standard ultrasonic voltage source. Experiments are described using 1 mm square devices in air, operating in both pitch-catch and pulse-echo modes. The dependence on d.c. bias voltage is examined, together with calibration measurements using 1/8 in. microphones. The radiated beam profile, and the farfield directivity pattern, have been measured for both broad bandwidth and one-burst excitation, using a scanned miniature receiver. A 16 element square array is also presented, which has been used to measure the beam cross-sections from a focussed source.
Outperforming piezoelectric ultrasonics with high-reliability single-membrane CMUT array elements
Microsystems & Nanoengineering
It has long been hypothesized that capacitive micromachined ultrasound transducers (CMUTs) could potentially outperform piezoelectric technologies. However, challenges with dielectric charging, operational hysteresis, and transmit sensitivity have stood as obstacles to these performance outcomes. In this paper, we introduce key architectural features to enable high-reliability CMUTs with enhanced performance. Typically, a CMUT element in an array is designed with an ensemble of smaller membranes oscillating together to transmit or detect ultrasound waves. However, this approach can lead to unreliable behavior and suboptimal transmit performance if these smaller membranes oscillate out of phase or collapse at different voltages. In this work, we designed CMUT array elements composed of a single long rectangular membrane, with the aim of improving the output pressure and electromechanical efficiency. We compare the performance of three different modifications of this architecture: tra...
2002 IEEE Ultrasonics Symposium, 2002. Proceedings.
Capacitive micromachined ultrasonic transducers (CMUTs) have long been studied. Past research has shown that CMUTs indeed have remarkable features such as wide bandwidth and high efficiency. This paper introduces an inclusion to the CMUT technology that uses the wafer-bonding technique to fabricate membranes on silicon. This new technology enables the fabrication of large membranes with large gaps, and expands the frequency span of CMUTs to 10 kHz in the low end. CMUT devices with different frequency spans are fabricated using both technologies, and tested. Electromechanical coupling efficiency, k T 2 , value as high as 0.85 and fractional immersion bandwidth as wide as 175 % are measured.
Medical Imaging 2005: Ultrasonic Imaging and Signal Processing, 2005
We have designed, fabricated, and characterized two-dimensional 16x16-element capacitive micromachined ultrasonic transducer (CMUT) arrays. The CMUT array elements have a 250-µm pitch, and when tested in immersion, have a 5-MHz center frequency and 99% fractional bandwidth. The fabrication process is based on standard silicon micromachining techniques and therefore has the advantages of high yield, low cost, and ease of integration. The transducers have a Si 3 N 4 membrane and are fabricated on a 400-µm thick silicon substrate. A low parasitic capacitance through-wafer via connects each CMUT element to a flip-chip bond pad on the back side of the wafer. Each through-wafer via is 20 µ m in diameter and 400 µ m deep. The interconnects form metal-insulator-semiconductor (MIS) junctions with the surrounding high-resistivity silicon substrate to establish isolation and to reduce parasitic capacitance. Each through-wafer via has less than 0.06 pF of parasitic capacitance. We have investigated a Au-In flip-chip bonding process to connect the 2D CMUT array to a custom integrated circuit (IC) with transmit and receive electronics. To develop this process, we fabricated fanout structures on silicon, and flip-chip bonded these test dies to a flat surface coated with gold. The average series resistance per bump is about 3 Ohms, and 100% yield is obtained for a total of 30 bumps.
2005
We have designed, fabricated, and characterized two-dimensional 16x16-element capacitive micromachined ultrasonic transducer (CMUT) arrays. The CMUT array elements have a 250-µm pitch, and when tested in immersion, have a 5-MHz center frequency and 99% fractional bandwidth. The fabrication process is based on standard silicon micromachining techniques and therefore has the advantages of high yield, low cost, and ease of integration. The transducers have a Si 3 N 4 membrane and are fabricated on a 400-µm thick silicon substrate. A low parasitic capacitance through-wafer via connects each CMUT element to a flip-chip bond pad on the back side of the wafer. Each through-wafer via is 20 µ m in diameter and 400 µ m deep. The interconnects form metal-insulator-semiconductor (MIS) junctions with the surrounding high-resistivity silicon substrate to establish isolation and to reduce parasitic capacitance. Each through-wafer via has less than 0.06 pF of parasitic capacitance. We have investigated a Au-In flip-chip bonding process to connect the 2D CMUT array to a custom integrated circuit (IC) with transmit and receive electronics. To develop this process, we fabricated fanout structures on silicon, and flip-chip bonded these test dies to a flat surface coated with gold. The average series resistance per bump is about 3 Ohms, and 100% yield is obtained for a total of 30 bumps.
Capacitive micromachined ultrasonic transducer (CMUT) arrays for medical imaging
Microelectronics Journal, 2006
Capacitive micromachined ultrasonic transducers (CMUTs) bring the fabrication technology of standard integrated circuits into the field of ultrasound medical imaging. This unique property, combined with the inherent advantages of CMUTs in terms of increased bandwidth and suitability for new imaging modalities and high frequency applications, have indicated these devices as new generation arrays for acoustic imaging. The advances in microfabrication have made possible to fabricate, in few years, silicon-based electrostatic transducers competing in performance with the piezoelectric transducers. This paper summarizes the fabrication, design, modeling, and characterization of 1D CMUT linear arrays for medical imaging, established in our laboratories during the past 3 years. Although the viability of our CMUT technology for applications in diagnostic echographic imaging is demonstrated, the whole process from silicon die to final probe is not fully mature yet for successful practical applications. q