Capacitive micromachined ultrasonic transducers with piston-shaped membranes: fabrication and experimental characterization (original) (raw)
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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...
Ultrasonics …, 2005
In this work we define a performance measure for capacitive micromachined ultrasonic transducers (cMUT) in the form of a gain-bandwidth product to investigate the conditions that optimize the gain and bandwidth with respect to device dimensions, electrode size and electrical termination resistance. For the transmit mode, we define the figure of merit as the pressurebandwidth product. Fully-metallized membranes achieve a higher pressure-bandwidth product compared to partially metallized ones. It is shown that the bandwidth is not affected by the electrode size in the transmit mode. In the receive mode, we define the figure of merit as the gain-bandwidth product. We show in this case that the figure of merit can be maximized by optimizing the electrode radius. We present normalized charts for designing an optimum cMUT cell at the desired frequency with a given bandwidth for transmit or receive modes. The effect of spurious capacitance and liquid loading effect are considered. Design examples are given to clarify the use of these charts.
Experimental Characterization of an Embossed Capacitive Micromachined Ultrasonic Transducer Cell
Micromachines
Capacitive Micromachined Ultrasonic Transducer (CMUT) is a promising ultrasonic transducer in medical diagnosis and therapeutic applications that demand a high output pressure. The concept of a CMUT with an annular embossed pattern on a membrane working in collapse mode is proposed to further improve the output pressure. To evaluate the performance of an embossed CMUT cell, both the embossed and uniform membrane CMUT cells were fabricated in the same die with a customized six-mask sacrificial release process. An annular nickel pattern with the dimension of 3 μ m × 2 μ m (width × height) was formed on a full top electrode CMUT to realize an embossed CMUT cell. Experimental characterization was carried out with optical, electrical, and acoustic instruments on the embossed and uniform CMUT cells. The embossed CMUT cell achieved 27.1% improvement of output pressure in comparison to the uniform CMUT cell biased at 170 V voltage. The fractional bandwidths of the embossed and uniform CMUT ...
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.
New fabrication process for capacitive micromachined ultrasonic transducers
2003
In this paper, we introduce a new method to fabricate Capacitive Micromachined Ultrasonic Transducers (CMUT) that uses a wafer-bonding technique. The transducer membrane and cavity are defined separately on a Silicon-On-Insulator (Sol) wafer and on a prime quality silicon wafer, respectively. Using silicon direct bonding in a vacuum environment, the two wafers are bonded forming the transducer. This new process offers many advantages over surface micromachining on the fabrication of the transducers with different cavity and membrane configurations. ChlUTs with different dimensions have been successfully fabricated and characterized. For the first time, sub-MHz operation is achieved with CMUTs. The test results show that the new process is a promising method to fabricate CMUTs for operation in air and water at different frequency ranges.
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.
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.
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.
Precision Engineering, 2002
The fabrication of capacitive ultrasonic transducers by means of surface micromachining techniques and a low-temperature process is presented. Investigation of main process steps is reported. The use of polyimide as sacrificial layer, possible as the process is at low temperature, guarantees precise control of active transducer cells, thanks to its etching selectivity against the structural materials employed, and to the lithographic definition of the sacrificial layer into islands before the deposition of the membrane layer (pre-patterning). Control of the mechanical properties of free-standing membranes has been gained with the optimization of silicon nitride deposition and following thermal annealing steps.