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50 Khz Capacitive Micromachined Ultrasonic Transducers for Generation of Highly Directional Sound with Parametric Arrays Ira O

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50 Khz Capacitive Micromachined Ultrasonic Transducers for Generation of Highly Directional Sound with Parametric Arrays Ira O IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, vol. 56, no. 1, JANUARY 2009 193 50 kHz Capacitive Micromachined Ultrasonic Transducers for Generation of Highly Directional Sound with Parametric Arrays Ira O. Wygant, Student Member, IEEE, Mario Kupnik, Member, IEEE, Jeffry C. Windsor, Wayne M. Wright, Mark S. Wochner, Goksen G. Yaralioglu, Member, IEEE, Mark F. Hamilton, and Butrus T. Khuri-Yakub, Fellow, IEEE Abstract—In this study, we examine the use of capacitive mode piezoelectric transducers, and PVDF (polyvinylidene micromachined ultrasonic transducers (CMUTs) with vac- fluoride) transducers. Conventional longitudinal-mode uum-sealed cavities for transmitting directional sound with piezoelectric transducers—for example, longitudinal-mode parametric arrays. We used finite element modeling to de- sign CMUTs with 40-μm- and 60-μm-thick membranes to have transducers made from PZT (lead zirconate titanate)— resonance frequencies of 46 kHz and 54 kHz, respectively. The are generally mechanically rugged and have good transmit wafer bonding approach used to fabricate the CMUTs pro- sensitivity. However, their large mechanical impedance vides good control over device properties and the capability mismatch with air results in a narrowband frequency re- to fabricate CMUTs with large diameter membranes and deep sponse; methods explored for increasing their bandwidth cavities. Each CMUT is 8 cm in diameter and consists of 284 circular membranes. Each membrane is 4 mm in diameter. include impedance matching [1]–[3] and using piezoelectric Characterization of the fabricated CMUTs shows they have composite materials [4]. Flextensional piezoelectric trans- center frequencies of 46 kHz and 55 kHz and 3 dB bandwidths ducers [5]–[11] also provide large displacement amplitudes of 1.9 kHz and 5.3 kHz for the 40-μm- and 60-μm-thick mem- per applied volt and typically have narrow bandwidths; for brane devices, respectively. With dc bias voltages of 380 V and large displacements, flextensional transducers can be high- 350 V and an ac excitation of 200 V peak-to-peak, the CMUTs generate average sound pressure levels, normalized to the de- ly nonlinear. Polyvinylidene fluoride (PVDF) transducers vice’s surface, of 135 dB and 129 dB (re 20 μPa), respectively. [12]–[14] in general have a low mechanical impedance and When used to generate 5 kHz sound with a parametric array, thus wide bandwidth and can be easily fabricated with we measured sound at 3 m with a 6 dB beamwidth of 8.7° and different shapes and curvatures; however, a low dielectric a sound pressure level of 58 dB. To understand how detec- constant, high dielectric loss, and low k limit their ability tor nonlinearity (e.g., the nonlinearity of the microphone used T to make the sound level measurements) affects the measured to produce high-intensity ultrasound [15]. sound pressure level, we made measurements with and without Capacitive air transducers can be categorized into an acoustic low-pass filter placed in front of the microphone; CUTs (capacitive ultrasonic transducers) and CMUTs the measured sound levels agree with numerical simulations (capacitive micromachined ultrasonic transducers). CUTs of the pressure field. The results presented in this paper dem- are manufactured by attaching a foil, commonly made of onstrate that large-area CMUTs, which produce high-intensi- ty ultrasound, can be fabricated for transmitting directional mylar or polyimide, to a grooved or pitted back plate (for sound with parametric arrays. example, see [16]). Conventional machining or microfabri- cation techniques create the grooves or pits. An insulating layer electrically isolates the foil from the back plate. I. Introduction The volume of air in the pits or grooves strongly influ- ences the CUTs’ frequency response. Early methods used or airborne ultrasound applications, capacitive trans- to fabricate CUTs could not precisely create cavities in Fducers provide good transmit and receive sensitivity the back plate. As a result, device performance was dif- and a wideband frequency response. Other transducer ficult to predict and control [17], [18]. Even with these technologies for generating airborne ultrasound include limitations, capacitive transducers compare favorably longitudinal-mode piezoelectric transducers, flextensional with piezoelectric ultrasound transducers for operation in air [16]. The advent of micromachining, particularly sili- Manuscript received April 19, 2008; accepted July 18, 2008. This work con micromachining for microelectromechanical systems was supported by DARPA SPAWAR Grant N66001-06-1-2032. Work was (MEMs), makes it possible to control the dimensions of performed in part at the Stanford Nanofabrication Facility (a member the pits or grooves in the back plate precisely. This better of the National Nanotechnology Infrastructure Network), which is sup- ported by the National Science Foundation under Grant ECS-9731293, control results in easier-to-design and better-performing its lab members, and the industrial members of the Stanford Center for devices [19]–[22]. Integrated Systems. CMUTs are capacitive transducers made entirely us- I. O. Wygant, M. Kupnik, G. Yaralioglu, and B. T. Khuri-Yakub are with the Edward L. Ginzton Laboratory, Stanford University, Stanford, ing microfabrication techniques [23], [24]. As a result, the CA (e-mail: [email protected]). CMUT membrane is a permanent part of the device and J. C. Windsor, W. M. Wright, M. S. Wochner, and M. F. Hamilton are can be designed to tight geometric specifications.C MUTs with the Applied Research Laboratories, University of Texas at Austin, Austin, TX. can have air-filled cavities, like typical CUTs, or they can Digital Object Identifier 10.1109/TUFFC.2009.1019 have have vacuum-sealed cavities. Because of the micro- 0885–3010/$25.00 © 2009 IEEE 194 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, vol. 56, no. 1, JANUARY 2009 machining methods used to seal the CMUT cavities, the radiated directly by the transducer. For example, consider cavity vacuum is expected to last indefinitely. Vacuum- a transducer that transmits an amplitude-modulated ul- sealed cavities avoid the damping effects of air inside the trasound signal with carrier frequency f0 and modulation cavity and thus can improve transmit and receive sensi- frequency fdiff/2. The modulated signal consists of ultra- tivity. However, for some low-frequency designs, vacuum- sonic frequencies f1 = f0 – fdiff/2 and f2 = f0 + fdiff/2, which sealed cavities result in excessive static membrane dis- are referred to as the primary frequencies. As this signal placement and stress; additionally, the damping effect of propagates, self-demodulation results in audible sound at air inside the cavity is sometimes desired to increase the the difference frequency fdiff = |f2 – f1|. The challenge of transducer’s bandwidth. transmitting sound with parametric arrays in air is to gen- There are two basic approaches to CMUT fabrica- erate ultrasound waves with sufficient intensity to produce tion. First-generation CMUTs were made using sacrificial desirable sound pressure levels in the audio band. fabrication methods [25]. With these methods, built-up In this paper, we examine the use of CMUTs for gen- sacrificial material such as polysilicon defines the cavity eration of high-intensity ultrasound for parametric arrays. volume. Material such as silicon nitride covers the sacrifi- We present the finite element model (FEM) simulations cial material. Removal of the sacrificial material leaves the used to make two CMUT designs. We describe a fabri- membrane and cavity, which are the basic components of cation process for making CMUTs with large diameter a CMUT. Recently, CMUT manufacturing methods based membranes and deep cavities. Finally, we discuss the char- on wafer bonding [25] have gained popularity. Wafer-bond- acterization of the fabricated CMUTs and present mea- ing methods provide tight control over device dimensions surements and simulations of the transmission of highly and result in a single-crystal silicon membrane, which has directional 5 kHz sound. predictable mechanical properties. Additionally, wafer- bonded CMUTs are typically easier to fabricate. For the CMUTs designed for this study, wafer-bonding methods II. Design enable the large-diameter membranes and deep cavities that are required for low frequencies and high output pres- We designed the CMUTs for an ultrasound carrier sures. frequency of 50 kHz and a dc bias voltage of less than We designed the CMUTs in this study for transmit- 1000 V. We chose a 50 kHz carrier frequency as a tradeoff ting highly directional sound. A directional sound source between being sufficiently high to produce a reasonably could be used, for example, in a quiet office space where narrow sound beam and being sufficiently low to avoid the source transmits a narrow beam of sound that is au- excessive absorption due to viscosity, heat conduction, dible only to listeners directly on-axis with the source. and molecular relaxation in the air [35]. Limiting the bias We generate the directional sound using the parametric voltage to less than 1000 V allows us to use an insulating array effect. The parametric array was first discovered by layer thickness of several micrometers, which is a thickness Westervelt in 1960 [26], [27] as a means of creating a beam easily grown with thermal oxidation and which is compa- of sound that is narrower than conventionally allowed by rable to the insulating
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