Experimental Characterization of an Embossed Capacitive Micromachined Ultrasonic Transducer Cell (original) (raw)
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IEICE Electronics Express
An embossed capacitive micromachined ultrasonic transducer (CMUT) is a device with embossed membrane that works in the collapse mode to improve output pressure in transmission. In this paper, a six-mask sacrificial release process is proposed for fabricating embossed CMUT arrays. Based on this process, the embossed pattern CMUTs were firstly fabricated. By using of electroplating methods, annular embossed patterns made of nickel were grown on the full top electrodes of CMUTs. The dimension of the embossed pattern was about 3.0 µm in width and 1.4 µm in height. The resonant frequencies of the embossed CMUT array were 6.4 MHz and 8.7 MHz when the device worked in the conventional and the collapse mode, respectively.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2009
Capacitive micromachined ultrasonic transducers (CMUTs) featuring piston-shaped membranes (piston CMUTs) were developed to improve device performance in terms of transmission efficiency, reception sensitivity, and fractional bandwidth (FBW). A piston CMUT has a relatively flat active moving surface whose membrane motion is closer to ideal piston-type motion compared with a CMUT with uniformly thick membranes (classical CMUT). Piston CMUTs with a more uniform surface displacement profile can achieve high output pressure with a relatively small electrode separation. The improved device capacitance and gap uniformity also enhance detection sensitivity. By adding a center mass to the membrane, a large ratio of second-order resonant frequency to first-order resonant frequency was achieved. This improved the FBW. Piston CMUTs featuring membranes of different geometric shapes were designed and fabricated using wafer bonding. Fabricating piston CMUTs is a more complex process than fabricating CMUTs with uniformly thick membranes. However, no yield loss was observed. These devices achieved ~100% improvement in transduction performance (transmission and reception) over classical CMUTs. For CMUTs with square and rectangular membranes, the FBW increased from ~110% to ~150% and from ~140% to ~175%, respectively, compared with classical CMUTs. The new devices produced a maximum output pressure exceeding 1 MPa at the transducer surface. Performance optimization using geometric membrane shape configurations was the same in both piston CMUTs and classical CMUTs.
Capacitive micromachined ultrasonic transducers (cmuts) with piston-shaped membranes
IEEE Ultrasonics Symposium, 2005., 2005
Abstract— Compared to PZT transducers in medical applications, CMUTs reported on so far have broader fractional bandwidth (FBW) but lower transduction efficiency (TX and RX) [1]. Most fabricated CMUTs reported in the literature carried membranes of uniform thickness. Since there is a performance trade-off between transduction efficiency and FBW when designing CMUTs with uniform membrane thickness, there is limited room for performance improvement in these devices. However, wafer-bonding-based CMUT fabrication provides design flexibility by allowing fabrication of membranes with different thickness profiles. Herein, CMUTs featuring piston-shaped membranes are developed to improve device performance. According to our theoretical predictions, piston-shaped membranes should improve the CMUT performance in terms of output pressure, sensitivity, and broader fractional bandwidth. The large ratio of second resonant harmonic frequency to first resonant frequency improves FBW. Increased elect...
Capacitive micromachined ultrasonic transducer technology for medical ultrasound imaging
Proceedings of SPIE, 2005
Capacitive micromachined ultrasonic transducer (cMUT) technology has been recognized as an attractive alternative to the more traditional piezoelectric transducer technology in medical ultrasound imaging for several years now. There are mainly two reasons for the interest in this technology: Micromachining is derived from the integrated circuit technology and therefore shares the well-known advantages and experience of it. Also, capacitive transduction using thin membranes has fundamental superiorities over the piezoelectric transduction mechanism such as wide frequency bandwidth. Capacitive micromachined ultrasonic transducers are essentially capacitor cells where the two plates of the capacitor, the membrane and the substrate, are separated with a vacuum sealed cavity. Typically, a cMUT is made of many microscale capacitor cells operating in parallel. This paper describes a new fabrication technique for building cMUTs which is called the wafer-bonding method. In this method, the cavity and the membrane are defined on separate wafers and brought together by wafer-bonding in vacuum. The wafer-bonding method has several advantages over the traditional sacrificial release method of cMUT fabrication. It allows greater flexibility in the cMUT design which means better device performance. It reduces the number of process steps, device turnaround time, and increases the overall uniformity, reliability. and repeatability. Device examples of one-dimensional and two-dimensional arrays designed to work in the 1 to 50 MHz range with 100 % fractional bandwidth highlight the advantages of this method, and show that cMUT technology is indeed the better candidate for next generation ultrasonic imaging arrays.
Capacitive micromachined ultrasonic transducer with an open cells structure
The capacitive micromachined ultrasonic transducer (cMUT) has proved to be a viable alternative to the classical piezoelectric transducer in many practical applications like medical diagnostic and non-destructive testing. A cMUT consists of an array of closed electrostatic microcells obtained by surface micromachining a silicon substrate.
Characterization of capacitive micromachined ultrasonic transducers
2014 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2014
This communication describes numerical and experimental characterization of CMUTs for ultrasound transmission. Simulations based on finite elements method to model CMUTs electromechanical behaviour and to determine the dimensions of elementary cells are presented. In particular we analyze the collapse voltage variations for different parameters of a circular cell and the capacitance variations for different bias voltages. We report the deformations of non-metallized and metallized membranes and we determine eigenfrequencies, bandwidth and quality factors of cells. The fabrication of CMUTs is based on the anodic bonding of a SOI wafer on a borosilicate glass substrate and we compare experimental results with numerical results.
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000
We report experimental results from a comparative study on collapsed region and conventional region operation of capacitive micromachined ultrasonic transducers (CMUTs) fabricated with a wafer bonding technique. Using ultrasonic pulse-echo and pitch-catch measurements, we characterized single elements of 1-D CMUT arrays operating in oil. The experimental results from this study agreed with the simulation results: a CMUT operating in the collapsed region produced a higher maximum output pressure than a CMUT operated in the conventional region at 90% of its collapse voltage (3 kPa/V vs. 16.1 kPa/V at 2.3 MHz). While the pulse-echo fractional bandwidth (126%) was higher in the collapsed region operation than in the conventional operation (117%), the pulseecho amplitude in collapsed region operation was 11 dB higher than in conventional region operation. Furthermore, within the range of tested bias voltages, the output pressure monotonously increased with increased bias during collapsed region operation. It was also found that in the conventional mode, short AC pulses (larger than the collapse voltage) could be applied without collapsing the membranes. Finally, while no significant difference was observed in reflectivity of the CMUT face between the two regions of operation, hysteretic behavior of the devices was identified in the collapsed region operation.
Capacitive micromachined ultrasonic transducers for medical imaging and therapy
2011
Capacitive micromachined ultrasonic transducers (CMUTs) have been subject to extensive research for the last two decades. Although they were initially developed for air-coupled applications, today their main application space is medical imaging and therapy. This paper first presents a brief description of CMUTs, their basic structure, and operating principles. Our progression of developing several generations of fabrication processes is discussed with an emphasis on the advantages and disadvantages of each process. Monolithic and hybrid approaches for integrating CMUTs with supporting integrated circuits are surveyed. Several prototype transducer arrays with integrated frontend electronic circuits we developed and their use for 2-D and 3-D, anatomical and functional imaging, and ablative therapies are described. The presented results prove the CMUT as a MEMS technology for many medical diagnostic and therapeutic applications.
Advances in Capacitive Micromachined Ultrasonic Transducers
Micromachines, 2019
Capacitive micromachined ultrasonic transducer (CMUT) technology has enjoyed rapid development in the last decade. Advancements both in fabrication and integration, coupled with improved modelling, has enabled CMUTs to make their way into mainstream ultrasound imaging systems and find commercial success. In this review paper, we touch upon recent advancements in CMUT technology at all levels of abstraction; modeling, fabrication, integration, and applications. Regarding applications, we discuss future trends for CMUTs and their impact within the broad field of biomedical imaging.