Wafer Bonded Capacitive Micromachined Transducers for Underwater Applications (original) (raw)
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Wafer bonded capacitive micromachined underwater transducers
2009 IEEE International Ultrasonics Symposium, 2009
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.
2005
This paper introduces a new method for fabricating 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 vacnum environment, the two wafers are bonded forming the transducer. Among the many advantages this wafer-bonding technique, and probdbly Ihe most important for low frequeucy transducer application% is the ability to define relatively large membranes and large gaps easily. The particular device reported in this paper is designed to operate in the 10 kHz-150 liHz range as a transminer only for a sonar application. In this paper, we describe the new fabrication process to build CMUTs, and present the first experimental results obtained from this particular device that demonstrate wide-band operation in the above mentioned frequency range.
Acoustic coupling in capacitive microfabricated ultrasonic transducers: Modeling and experiments
In the design of low-frequency transducer arrays for active sonar systems, the acoustic interactions that occur between the transducer elements have received much attention. Because of these interactions, the acoustic loading on each transducer depends on its position in the array, and the radiated acoustic power may vary considerably from one element to another. Capacitive microfabricated ultrasonic transducers (CMUT) are made of a twodimensional array of metallized micromembranes, all electrically connected in parallel, and driven into flexural motion by the electrostatic force produced by an applied voltage. The mechanical impedance of these membranes is typically much lower than the acoustic impedance of water. In our investigations of acoustic coupling in CMUTs, interaction effects between the membranes in immersion were observed, similar to those reported in sonar arrays. Because CMUTs have many promising applications in the field of medical ultrasound imaging, understanding of crosscoupling mechanisms and acoustic interaction effects is especially important for reducing cross-talk between array elements, which can produce artifacts and degrade image quality. In this paper, we report a finite-element study of acoustic interactions in CMUTs and experimental results obtained by laser interferometry measurements. The good agreement found between finite element modeling (FEM) results and optical displacement measurements demonstrates that acoustic interactions through the liquid represent a major source of cross coupling in CMUTs.
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.
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.
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.
2003
Equivalent circuit model has been widely used to predict the bandwidth of capacitive micromachined ultrasonic transducers (CMUTs). According to this model, the lower cutoff of the bandwidth is determined by the time constant of the parallel RC where R is dictated by the radiation and C is determined by the electrical capacitance of the transducer. The higher cutoff, on the other hand, is determined by the membrane's anti-resonance. In the mechanical part of the model, the radiation impedance is simply added to the membrane impedance assuming that the membrane impedance does not change when it operates in the immersion medium. Therefore, the mass loading effect of the medium is neglected. Our finite element method calculations showed that the mass loading on the membrane impedance drastically lowers the membrane anti-resonance frequency degrading the bandwidth. In this paper, we present results of equivalent circuit modeling combined with finite element analysis. We constructed a 3D finite element model for one element of a 1D array. The element has 7 hexagonal membranes in the width dimension and it is assumed that the membranes are replicated in the length dimension infinitely by using symmetry boundary conditions. By combining membrane impedance with equivalent circuit model, we found that the center frequency of operation is 11 MHz and the bandwidth is 12.5 MHz close to the collapse voltage. We also investigated the effect of the DC bias on the center frequency. Decreasing the bias voltage increased the center frequency without affecting the bandwidth assuming the source impedance is zero.
2021
Capacitive Micromachined Ultrasonic Transducer (CMUT) provides an alternative to commercial piezoelectric-based ultrasonic transducers due to its wide bandwidth, improved e ciency, sensitivity, and design exibility [1, 2]. In this paper, Finite Element Method-based design and simulations of circular capacitive micromachined ultrasonic transducer (CMUT) is presented. The FEM simulation of air-coupled CMUT was accomplished by using MEMCAD tools CoventorWare® and COMSOL™. The resonance frequency of 3.9 MHz was achieved for the designed circular CMUT device. A favourable agreement was found for the resonance frequency and pull-in voltage of the device using MEMSCAD tools and analytical calculations. For the proposed CMUT design, a circular cavity will be formed inside the glass substrate. Then, a free-standing membrane will be released using active layer of silicon-on-insulator (SOI) wafer. The bulk silicon of SOI wafer will be removed after bonding it on the glass substrate using anodic bonding technique as described in fabrication process ow for CMUT.
Fabricating capacitive micromachined ultrasonic transducers with wafer-bonding technology
… Systems, Journal of, 2003
This paper introduces a new method for fabricating capacitive micromachined ultrasonic transducers (CMUTs) that uses a wafer bonding technique. The transducer membrane and cavity are defined on an SOI (silicon-on-insulator) wafer and on a prime wafer, respectively. Then, using silicon direct bonding in a vacuum environment, the two wafers are bonded together to form a transducer. This new technique, capable of fabricating large CMUTs, offers advantages over the traditionally micromachined CMUTs. First, forming a vacuum-sealed cavity is relatively easy since the wafer bonding is performed in a vacuum chamber. Second, this process enables better control over the gap height, making it possible to fabricate very small gaps (less than 0.1 m). Third, since the membrane is made of single crystal silicon, it is possible to predict and control the mechanical properties of the membrane to within 5%. Finally, the number of process steps involved in making a CMUT has been reduced from 22 to 15, shortening the device turnaround time. All of these advantages provide repeatable fabrication of CMUTs featuring predictable center frequency, bandwidth, and collapse voltage. Using this new technique, we have fabricated CMUTs that have membrane sizes between 12 m and 750 m, and thicknesses between 0.34 m and 4.5 m. This paper presents the fabrication process and some experimental results obtained from the wafer-bonded devices. [929] Index Terms-Capacitive micromachined ultrasonic transducers (CMUT), silicon-on-insulator (SOI) wafer, ultrasonic transducer, wafer bonding.
Journal of Microelectromechanical Systems, 2000
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 vacnum environment, the two wafers are bonded forming the transducer. Among the many advantages this wafer-bonding technique, and probdbly Ihe most important for low frequeucy transducer application% is the ability to define relatively large membranes and large gaps easily. The particular device reported in this paper is designed to operate in the 10 kHz -150 liHz range as a transminer only for a sonar application. In this paper, we describe the new fabrication process to build CMUTs, and present the first experimental results obtained from this particular device that demonstrate wide-band operation in the above mentioned frequency range.