Surface micromachined capacitive ultrasonic immersion transducers

Capacitive micromachined ultrasonic transducers ~CMUTs!, introduced about a decade ago, have been shown to be a good alternative to conventional piezoelectric transducers in various aspects, such as sensitivity, transduction efficiency, and bandwidth. In this paper, we discuss the principles of capacitive transducer operation that underlie these aspects. Many of the key features of capacitive ultrasonic transducers are enabled with micromachining technology. Micromachining allows us to miniaturize device dimensions and produce capacitive transducers that perform comparably to their piezoelectric counterparts. The fabrication process is described briefly, and the performance of the CMUT transducers is evaluated by demonstrating characterization results. It is shown that the transduction efficiency as defined by the electromechanical coupling coefficient can be close to unity with proper device design and operating voltage. It is also shown that CMUTs provide large bandwidth ~123% fractional bandwidth! in immersion applications which translate into high temporal and axial resolution. Finally, the feasibility of using CMUTs is demonstrated by showing imaging examples in air and in immersion.

Electrostatic or capacitive ultrasonic transducers (EUTs or CUTs) were originally developed from the condenser transmitters and microphones with grooved and roughened backplates in the 1930s. Significant progress in theoretical and experimental research has been made since the introduction of micromachining techniques at the end of the 1980s. We have seen the advancement not only from the roughened ultrasonic transducers to the 2‐D array micromachined capacitive ultrasonic transducers (mCUTs), but also from the grooved ultrasonic transducers to the micromachined capacitive Lamb wave transducers (mCLWTs). Thus, the theoretical TDK model originally developed for grooved ultrasonic transducers [L.‐F. Ge, Proceedings 16th Intl. Conf. Acoustics/135th ASA Meeting, l08l–1082, 1998; and Sciences in China (A), (in English), 42, 1308–1315 (1999)] can be used for mCLWTs. It is most important for a reliable model to be able to predict the transducer’s natural frequencies, at which the lowest‐order antisymmetric Lamb wave (A0 wave) is excited. As an example using a typical transducer [M. H. Badi et al., IEEE Trans. Ultrason. Ferroelectr. Freq. Control 50(9), 1191–1203 (2003)], the fundamental frequency predicted using the TDK model is 2.18 MHz, which agrees well with the measured result of 2.1 MHz. [Work supported by NSFC (60374044).]

Since the proposing of capacitive micromachined ultrasonic transducer by Khuri Yakub group in 1994 that this kind of transducer occupying the advantages of wide bandwidth, impedance matching well with the propagation medium especially in fluid and air and high sensitivity, has shown a great potential for wide ranges of applications. This Letter reports kind of micro‐electro‐mechanical systems (MEMS) capacitive ultrasonic transducer with the novel cavities embedded in the device layer of silicon on insulator wafer bonded with a glass substrate. The optimum geometric dimensions are confirmed by both mechanical vibrating of the membrane and the electrical characteristics analysis. Finite‐element analysis is adopted to determine the operation mode. The safety and reliability of the proposed device is ensured by the obtained deflections and equivalent stress under operation/collapse voltage. The bottom electrodes of the proposed transducer are fabricated on the top surface of the glass substrate. The parallel parasitic capacitance is reduced, thus improving the electromechanical coupling coefficient. The test results show that the electromechanical coupling coefficient is 69.65%, which demonstrates that this proposed MEMS capacitive ultrasonic transducer structure can enhance the performance significantly.

Capacitive micro-fabricated ultrasonic transducers for biometric applications

Investigations into the characteristics of water-coupled ultrasonic capacitance transducers have been undertaken for a range of transducer configurations. The radiated fields have been scanned in water using a miniature hydrophone detector, and the results compared to theory based on a plane piston approach. Micromachined backplates in conjunction with thin Mylar and mica membranes have been investigated, together with aperture modifications such as an annulus and Fresnel zone plate. The measured results agree well with theory, thus demonstrating that wideband predictable performance (>8 MHz) is obtainable with such transducers. Additionally, pulse-echo C-scans of a Plexiglas plate containing an artificial defect have been undertaken with the capacitance transducer, in order to demonstrate that the capacitance transducer has sufficient sensitivity to allow routine nondestructive testing within immersion applications.

Parametric equalization is often used to achieve a desired response from an audio transmitter, but is rarely applied to ultrasonic transducer systems. The ability of a broadband ultrasonic transmission and reception system to adapt its frequency and time domain response to changing acoustic conditions would be a distinct advantage in certain applications. Ultrasonic remote monitoring systems would benefit significantly from this ability, as signal levels could be minimized and consequentially the transmitter power consumption decreased. This work presents a real-time adaptive ultrasonic parametric equalizer using optimization driven Matlab code to control the coefficients of a switched capacitor filter network implemented in a Cypress PSOC (Programmable System On a Chip). In this work, adaptive parametric magnitude equalization of a through-transmission ultrasonic system using CUTs (Capacitive Ultrasonic Transducers) has been achieved in real time by tracking a desired SNR (signal to noise ratio) across the operational frequency spectrum. A Matlab general radial basis function (GRBF) artificial neural network (ANN) was developed to control the equalization filter coefficients based on the received frequency response data. The adaptive parametric equaliser adjusts the magnitude of the driving signal to maintain the desired SNR as closely as possible. The neural network was trained using PSO (Particle Swarm Optimization) back-propagation, based on a state space model of the system developed from frequency response data. The developed equalization circuitry, which is switched capacitor based and was fully implemented on the PSOC, is also described.

Fabrication of capacitive ultrasonic transducers by a low temperature and fully surface-micromachined process

A study is presented of the function of an air-coupled capacitiveultrasonic transducer with uniform net backplate. Net backplates are an easy,fast-to-assemble alternative to grooved backplates. The frequency responses ofthe transducers were measured and the resonance frequencies were estimated. Aprocedure to study the electrostatics of the transducer with net backplategeometry is described with calculations of surface charge density distribution,capacitance and sensitivity.

Since the proposing of capacitive micromachined ultrasonic transducer by Khuri Yakub group in 1994 that this kind of transducer occupying the advantages of wide bandwidth, impedance matching well with the propagation medium especially in fluid and air and high sensitivity, has shown a great potential for a wide range of applications such as medical imaging, medical ultrasound diagnosis, therapy and underwater ultrasonic imaging. This paper presents a kind of MEMS capacitive ultrasonic transducer with the novel cavities embedded in the device layer of Silicon On Insulator. Firstly, the geometric dimensions are derived from the theoretical analysis of the vibrating membrane and the electrical analysis of the transducer. Secondly, Finite Element Analysis is adopted to confirm the operation mode and the deflection and equivalent stress under operation/collapse voltage are obtained. Then, the fabrication process is depicted and the fabricated transducer is shown. The test results show that the electromechanical coupling coefficient is 69.65%.

A new method to manufacture capacitive micromachined ultrasonic transducers (cMUT) combining a wafer bonding and a sacrificial layer processes is introduced. Devices with monocrystalline silicon membranes over a polysilicon electrode have been manufactured. From impedance measurements in air of a monocell test device biased under 140 V, a resonance at 8.0 MHz and an electromechanical coupling coefficient of 45% have been found. Underwater pulse-echo experiments of a cell array are presented.

The mechanical actuation of suspended structures in microelectromechanical system (MEMS) switches plays an important role in obtaining an open or short circuit in radio-frequency (RF) transmission lines. These micromachined switches have moving parts which are realized by a sacrificial layer. Several sacrificial materials have been utilized in the fabrication of MEMS structures. The traditional method of using metal such as copper or nickel as a sacrificial layer is difficult due to metal etchant compatibility issues. A photoresist is the best choice for this process. Generally, the HiPR photoresist has been found to be suitable␣for the role of sacrificial layer. However, the HiPR is no longer available in the commercial market. As an alternative to the HiPR, the AZ4620 photoresist can be used. In this paper, a unit process optimization technique is described featuring an AZ4620 photoresist as a sacrificial layer for the microfabrication of a suspended structure. The AZ4620 meets all the requirements for the development of RF MEMS switches. Hence, the recipe and fabrication technique are optimized according to the required thickness. Preliminary measurements of the fabricated switch beam indicate that the required gap has been achieved. This optimized process is compatible with standard MEMS technology.

In this study, a subminiature implantable capacitive pressure sensor is proposed for biomedical applications. The proposed pressure sensor comprises an array of elastic silicon nitride (SiN) diaphragms formed by the application of a polysilicon (p-Si) sacrificial layer. In addition, using the p-Si layer, a resistive temperature sensor is also integrated into one device without additional fabrication steps or extra cost, thus enabling the device to measure pressure and temperature simultaneously. The sensor with a size of 0.5 × 1.2 mm was fabricated using microelectromechanical systems (MEMS) technology and was packaged in needle-shaped metal housing that is both insertable and biocompatible. The packaged pressure sensor immersed in a physiological saline solution exhibited excellent performance without leakage. The sensor achieved a sensitivity of approximately 1.73 pF/bar and a hysteresis of about 1.7%, respectively. Furthermore, it was confirmed that the pressure sensor operated normally for 48 h without experiencing insulation breakdown or degradation of the capacitance. The integrated resistive temperature sensor also worked properly. The response of the temperature sensor varied linearly with temperature variation. It had an acceptable temperature coefficient of resistance (TCR) of approximately 0.25%/°C.

Capacitive Micromachined Ultrasonic Transducers (CMUTs), introduced about few decades ago, have been shown to be a good alternative to conventional piezoelectric transducers in various aspects, such as sensitivity, transduction efficiency, and bandwidth. It is used in a wide range of applications including Non destructive testing applications, medical imaging, flow metering, micro/nanoelectronics and industrial cleaning etc. The advent of MEMS technology has lead to intensive study of micro machined ultrasonic transducers. They are similar to other capacitance transducers as they employ a vibrating membrane to send and receive ultrasound in air and in water. The advantage of hexagonal Capacitance micro machined ultrasonic transducers (CMUTs) is that they can be operated in the high temperature range and the transducer array can be fabricated at a lower cost. Presented in this paper is a study on some aspects of the design of a modified CMUT, which the authors believe would lead to a CMUT with many advantages, including less loading effect and higher packing density. The focus of discussion in the paper is FEM simulation of a single cell hexagonal CMUT using polysilicon as membrane material. Also discussed is the simulation study of the critical parameters which influence the operation of the CMUTs. The simulations were carried out using MATLAB and IntelliSuite 8.0. The study indicates that in the range of study, the best performance is achieved at the natural resonance frequency, which is very desirable.

Flip chip technology is one of the fastest growing segments of microelectronics packaging because of its ability to satisfy the increasing demands of high input/output density, package miniaturization, and reduced cost. A critical element in the successful application of flip chip technology is the reliability of solder bumps. In this paper, a nondestructive inspection method combining ultrasonic excitation with modal analysis is proposed for flip-chip solder bump defect detection. The signal generator and power amplifier are utilized to drive the capacitive air-coupled ultrasonic transducer to produce continuous ultrasonic waves for exciting the test chips. The vibration velocities of the chips are measured by the laser scanning vibrometer to extract the modal shapes and resonance frequencies. The results prove that the defective chips can be distinguished from the good chip by the modal shapes, and the resonance frequencies of the chips decrease with the increase of the open solder bumps. Therefore, this detection method may provide a new path for the improvement and innovation of flip chip on-line inspection systems.

The current state of novel technology, surface microfabricated ultrasonic transducers, is reported. Experiments demonstrating both air and water transmission are presented. Air-coupled longitudinal wave transmission through aluminum is demonstrated, implying a 110 dB dynamic range for transducers at 2.3 MHz in air. Water transmission experiments from 1 to 20 MHz are performed, with a measured 60 dB SNR at 3 MHz. A theoretical model is proposed that agrees well with observed transducer behavior. Most significantly, the model is used to demonstrate that microfabricated ultrasonic transducers constitute an attractive alternative to piezoelectric transducers in many applications.