Design and Fabrication of a Novel MEMS Capacitive ... - IEEE Xplore

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 61, NO. 3, MARCH 2014

Design and Fabrication of a Novel MEMS Capacitive Transducer With Multiple Moving Membrane, M3-CMUT Tahereh Arezoo Emadi, Member, IEEE, and Douglas A. Buchanan, Senior Member, IEEE

Abstract— A novel capacitive micromachined ultrasonic transducer is designed and fabricated. This transducer employs a stack of two deflectable membranes suspended over a fixed bottom electrode. In this configuration, the two moving membranes deflect simultaneously in response to a bias voltage, which results in a smaller effective cavity height compared with the conventional capacitive transducers. Electromechanical and acoustic analyses are conducted to investigate the transducer properties. A set of seven transducers with radii ranging from 30 to 55 µm were fabricated utilizing a sacrificial microelectromechanical system fabrication technology. Electrical measurements were performed and were compared with results from physical deflection measurements utilizing an optical vibrometer system. The results have been compared with analytical models as well as characterization of a set of five conventional, single membrane, transducers fabricated with the same technology. These experiments indicate a good agreement between the model and measured data. A larger membrane deflection and smaller cavity height are achieved from the double membrane devices. Therefore, this type of device may enhance the transducer acoustic power generation capability as well as increasing its sensitivity both of which result from the reduction in the transducer effective cavity height. Index Terms— Capacitive micromachined ultrasonic transducer (CMUT), deflectable membrane, microelectromechanical system (MEMS), multiple moving membrane CMUT (M3 -CMUT), sacrificial layer, static bottom electrode.

I. I NTRODUCTION

U

LTRASONIC technology is often used in imaging application and for the detection and location of various objects. This imaging technique employs transducers to generate an acoustic signal, which travels in the surrounding media and reflects back from the object. The returning signal is usually collected with the same transducer operating in a receiver mode. Traditionally, piezoelectric transducers have been employed for imaging purposes [1]. However, two decades ago, an alternative electrostatic, capacitive transducer was introduced to overcome some of the known piezoelectric transducers drawbacks [2]. Capacitive transducers can

Manuscript received March 21, 2013; revised December 16, 2013; accepted December 27, 2013. Date of publication January 14, 2014; date of current version February 20, 2014. This work was supported in part by the Natural Science and Engineering Research Council of Canada, in part by Canada Research Chairs, and in part by Manitoba Hydro. The review of this paper was arranged by Editor A. M. Ionescu. The authors are with the Department of Electrical and Computer Engineering, University of Manitoba, Winnipeg, MB R3T 5VR, Canada (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2013.2297677

be fabricated using microelectromechanical system (MEMS) technology. The advantages associated with employing MEMS ultrasonic transducers compared with the conventional piezo transducers include a much broader application area, a wider bandwidth with an improved resolution in imaging applications, better acoustic matching, and a highly miniaturized system. In addition, these transducers have been shown to considerably improve product uniformity when compared with the piezoelectric transducers [3], [4]. Capacitive micromachined ultrasonic transducers (CMUT) can employ a sacrificial MEMS fabrication technology. In this technique, an oxide is commonly used as the sacrificial layer, and the silicon substrate and films of polysilicon or silicon nitride are employed as the structural materials [5], [6]. Other techniques that may be used include a wafer bonding process, where a patterned wafer is bonded to a handle wafer with a predeposited layer, creating transducer cell cavity [7]. Atomic layer deposition can also be utilized when combined with diffusion bonding technique [8]. In the past, much has been published on improving the properties of capacitive transducers. These improvements include enhancing the detection properties [9], improving electrical safety [10], beam forming techniques, beam steering capability [11], [12], and improving the transducer effective capacitance using a concave bottom electrode [13]. In this paper, a novel capacitive micromachined transducer prototype is offered [14]. Unlike conventional CMUTs that consist of one moving membrane and a fixed bottom electrode, this new configuration employs two deflectable membranes and one static electrode. It is shown that this new design greatly enhances the transducer effective capacitance by reducing the effective cavity height that directly influences the generated acoustic power and sensitivity. II. M ULTIPLE M OVING M EMBRANE C APACITIVE M ICROMACHINED U LTRASONIC T RANSDUCER A. Principle of Operation Capacitive transducers typically include a metalized or conductive membrane suspended over an electrode with a defined cavity height. They can be modeled as a parallel plate capacitor where the membrane and electrode are attracted toward each other due to the electrostatic force, F, caused by an applied dc bias voltage, Vdc . This force is proportional to the amplitude of the bias voltage and the transducer effective

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EMADI AND BUCHANAN: DESIGN AND FABRICATION OF A NOVEL MEMS CAPACITIVE TRANSDUCER

cavity height, deffective , and therefore, inversely, the device capacitance, C [15], [16]. A dc bias voltage, which is lower than the nominal pull down voltage (point of instability), is used to create an initial deflection of the membrane toward the bottom electrode. At the pull down bias, the effective displacement height is one third of the transducer cavity height (h) [2], [5], [17]. In transmitting mode, the top membrane is driven with an ac signal with an angular frequency ω that results in ultrasound generation [5]. The ac signal angular frequency is normally close to the natural resonant frequency of the transducer to give the maximum deflection. While still employing traditional MEMS fabrication technologies, capacitive transducers presented in this paper have been developed to include two deflectable membranes with a static bottom electrode deposited on a silicon substrate. In this new configuration, which is referred to as a multiple moving membrane capacitive micromachined ultrasonic transducer (M3 -CMUT), the lower deflectable membrane acts as a bottom electrode for the top membrane. Similar to conventional CMUTs, a series of individual M3 -CMUT cells can also be fabricated to form 1-D and 2-D arrays, with the desired number of cells and spacing. These M3 -CMUTs work on a principle of operation similar to that of conventional capacitive transducers. In transmitting mode, an ac signal is superimposed to the dc bias applied to the top membrane, which creates an electrostatic force related to the device geometry and dimensions similar to the conventional CMUTs [2]. The electrostatic force is given by F =

2 ε Amembrane + 2 Vdc Vac cos (ω t + ϕ) × Vdc 2 2h effective (t) (1) +Vac cos2 ( ω t + ϕ) ·

In the device operating bias voltage, the dc term is normally much larger than the ac term. Therefore, the force is dominated by the amplitude of the dc bias voltage [2], [5]. The generated electrostatic force, produced by the applied bias, is resisted by a mechanical restoring force known as the spring force, Fspring. For small membrane deflections, this force can be approximated linearly proportional to the membrane displacement, Fspring = −kU , where U is the membrane displacement and k is the membrane spring constant. The membrane spring constant is due to the stiffness of the membrane, and residual stress within the membrane material [15], [18]. Membrane stiffness is determined by membrane geometry, dimensions, and material and it is reduced by spring softening effect when a dc bias voltage is applied. The spring constant for a deflection point at the center of the circular membrane is given by (2) and this defines the membrane resonant frequency (ωr ) as follows: k=

3 16 π E membrane tmembrane

2 3 (1 − υ 2 ) rmembrane ε0 Amembrane V 2 − + 4 π σ tmembrane h 3effective ωr = 2π fr = k/m membrane

(2) (3)

where E membrane and ν are the Young’s modulus and Poisson’s ratio of the membrane material, respectively, tmembrane ,

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Fig. 1. Schematic view of the M3 -CMUT cell on silicon substrate, employing PolyMUMPs sacrificial technique and before the final release step.

rmembrane , and Amembrane are the thickness, radius, and the area of the membrane, respectively, σ is the membrane residual stress, and m membrane is the mass of the membrane [15], [18]. The second term in (2) is the spring softening due to the applied dc voltage. In the receiver mode, the transducer is biased with a dc voltage, and the incoming signal causes a vibration in the transducer membrane. The membrane displacement results in a change in the transducer capacitance and can be detected through transducer current measurements [9]. The M3 -CMUT configuration is similar to that of CMUT, and the electrostatic force causes the top membrane to deflect toward the lower membrane and electrode. However, since the lower membrane is also free to move, it is attracted toward the top membrane due to the attractive force between them. Therefore, the effective gap height is even further decreased. As a result, the transducer effective capacitance increases. This enhances device properties that include the amplitude of vibration, generated power, and sensitivity. In this configuration, the generated electrostatic force leads to a piston-like vibration of the top membrane with the amplitude of Umembrane and acceleration of amembrane at a frequency equivalent to the frequency of the ac signal, ω [19]. In addition to the vibration of the top membrane, the lower membrane also follows the movement of the top membrane. The lower membrane movement arises from the change in the device electric field that directly influences the force on the lower membrane of M3 -CMUT. B. Fabrication A set of seven multiple moving membrane transducers were fabricated utilizing MEMSCAP sacrificial technique, Poly Multi-User MEMS Processes (PolyMUMPs) [20], with radii ranging from 30 to 55 μm. For comparison, five conventional CMUTs were also fabricated with the same technique on the same chip and with radii in the same range. In Fig. 1, a schematic view of the M3 -CMUT cell on a silicon substrate is shown before the final release. In this fabrication process, lowpressure chemical-vapor-deposition (LPCVD) grown polysilicon was used as the membrane and the static bottom electrode material. An LPCVD phosphor-silicate glass oxide was used as the sacrificial layer. A silicon nitride layer was used to isolate the device from the heavily doped silicon substrate. The sacrificial layers were removed using 49% hydrofluoric acid (HF) followed by a CO2 drying and an annealing process. In Table I, the physical properties of the materials are listed. Material layer thicknesses and sequence were restricted by the

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TABLE I P HYSICAL P ROPERTIES OF THE P ROPOSED M 3 -CMUT C ELL , U SING P OLY MUMP S FABRICATION T ECHNIQUE

Fig. 2. Membrane displacement (U P ) map of an individual, conventional CMUT cell with a 35-μm radius with a grounded bottom electrode and a dc voltage of 135 V applied to the membrane.

fabrication process. The deposited polysilicon in this technique has a residual stress of 10 ± 10 MPa, Young’s Modulus of 158 ± 10 GPa, Poisson’s ratio of 0.22 ± 0.01, and density of 2328 g/cm3 [20], [21]. In the M3 -CMUT cell structure, Poly0 was used as the static bottom electrode, Poly1 as the lower deflectable membrane, and Poly2 as the top deflectable membrane. The cavity heights between Poly2 and Poly1 and between Poly1 and Poly0 are defined by the (sacrificial layer) thicknesses of Oxide2 and Oxide1, respectively. The same technique was employed to fabricate conventional capacitive transducers. In the conventional CMUT structure, Poly0 and Poly1 are deposited directly onto each other and onto the substrate insulation layer, silicon nitride. This was done by employing an additional plasma etch step between the two polysilicon deposition steps to remove Oxide1. In this configuration, Poly0 and Poly1 act as the static bottom electrode and the deflectable membrane is created using a Poly2 layer, which is suspended over the bottom electrode with the cavity height defined by the Oxide2 layer thickness. C. Electromechanical Analysis The 3-D COMSOL electromecanics (emi) simulations [22] have been conducted to investigate the working properties of the M3 -CMUT cell. Both conventional, single deflectable membrane (CMUT), and multiple moving membrane (M3 -CMUT) have been investigated for devices with the same membrane radius and under the same bias condition. To simplify the simulated geometries, it was assumed that the membranes and electrode were initially flat. Moreover, the membrane edge curvatures (Fig. 1) that are associated with the fabrication process and the differences between layers thicknesses were ignored as well. The relatively small HF releasing holes, in the membrane, were also not included in the simulations. In the conventional CMUT devices, the bottom electrode is normally grounded. The dc bias applied to the membrane pulls it toward the bottom electrode, and the ac signal causes the membrane to vibrate with the same angular frequency, ω. The electromechanical, stationary simulation results for the membrane displacement map of the 35-μm radius conventional CMUT are presented in Fig. 2. The bottom electrode was grounded and the membrane was driven with a bias of Vdc = 135 V. Similar electromechanical stationary simulations were also performed for the M3 -CMUT structure, with two

Fig. 3. Membrane displacement map (U P0 , U P1 , and U P2 ) of an individual M3 -CMUT cell with a 35-μm radius. Both the bottom electrode and the middle membrane were grounded. A voltage of 135 V was applied to the top membrane.

deflectable membranes and a static bottom electrode. In Fig. 3, the membrane displacement map of an individual M3 -CMUT cell (35-μm radius) is illustrated. In this structure, both the bottom electrode and the lower deflectable membrane were grounded, whereas the top deflectable membrane was driven by a dc voltage of 135 V, similar to the conventional CMUT bias condition. Electromechanical eigen frequency simulations for the 35-μm membrane radius M3 -CMUT and CMUT showed a first mode resonant frequency of 4 MHz based on the geometry dimensions listed in Table I and the material properties of the layers from the COMSOL material library. In the M3 -CMUT cell, the electrostatic force between the two deflectable membranes attracts them toward each other. One immediate advantage of this configuration is that by allowing the lower membrane to be free to move, the effective gap between the two membranes is significantly reduced relative to a conventional CMUT. Therefore, the device sensitivity, which is proportional to the effective capacitance, is considerably enhanced. Moreover, comparisons between the simulation results for the M3 -CMUT (Fig. 3) and conventional CMUT (Fig. 2) indicate that for the same dc bias voltage of 135 V, the top membrane of M3 -CMUT deflects distinctly more than the CMUT membrane; U P2 = 90 nm compared with U P = 72 nm. Therefore, the device power generation capability in transmitting mode is also largely improved as the acoustic power is directly related to the amount of membrane deflection. Electromechanical voltage sweep analysis was also performed on the two transducers, for a dc bias voltage ranging from 10 to 150 V, applied to the top membrane for both devices. The static bottom electrodes were grounded in both cases, along with lower deflectable membrane in the M3 -CMUT device. In Fig. 4, the simulation results for the lower deflectable membrane, Udc,P1 , and the top deflectable membrane, Udc,P2 , of the M3 -CMUT and the membrane of the conventional CMUT, Udc,P are presented. The results indicate that with an increasing dc bias voltage, the M3 -CMUT


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TABLE II M EMBRANE D ISPLACEMENT A MPLITUDES AND C ORRESPONDING M EMBRANE A CCELERATIONS FOR THE M 3 -CMUT (F IG . 3) CMUT (F IG . 2), FOR AN AC S IGNAL OF 15 V S UPERIMPOSED ON A DC B IAS OF 135 V

AND THE

Fig. 4. Membrane displacements of the M3 -CMUT (Fig. 3) and the conventional CMUT (Fig. 2), for a dc bias voltage ranging from 10 to 150 V. Bottom electrodes are grounded in both cases.

Fig. 5. Transient response of actuated M3 -CMUT and CMUT membranes are shown at a frequency, dc bias, and ac voltage of 4 MHz, 135 V, and 15 V, respectively.

membrane displacement diverges significantly from that of the conventional CMUT. A greater displacement is achieved with the same driving voltage when compared with the conventional CMUT. This indicates a reduction of the transducer operating voltage for a given deflection amplitude. Electromechanical time-dependent analysis was also performed to show the behavior of the transducers under real, operational bias conditions, where an ac bias is superimposed on the dc bias voltage and for the same 35-μm radius M3 -CMUT and CMUT devices. The frequency of the applied ac signal was chosen to be 4 MHz, which is close to the first mode resonant frequency found from the electromechanical eigen frequency simulations of both devices. In Fig. 5, the transient simulation results are shown. It can be seen that for the same applied voltage, the M3 -CMUT top membrane vibrates with larger amplitude than the conventional CMUT. This is due to the greater membrane dc deflection, U0 , which enhances the membrane vibration in response to the applied ac signal. The results presented in Fig. 5 suggest that the M3 -CMUT lower membrane also vibrates with the same frequency and phase of the applied ac signal. This was the case for all the investigated transducers in this paper operating below or close to their first mode resonant frequencies, which are 20 dB higher than the CMUT, a direct result of the higher M3 -CMUT membrane deflection and acceleration. III. E XPERIMENTAL R ESULTS A set of seven single-cell M3 -CMUT and a set of five single-cell CMUT devices were fabricated employing PolyMUMPs [20] fabrication technique described in Section II-B. These transducers were designed based on the simulation results shown in Section II-C, and with radii of 30, 35, 40, 42, 45, 50, and 55 μm for the M3 -CMUTs, and 30, 35, 40, 45, and 55 μm for the traditional CMUTs. An image of the fabricated chip is shown on the left-hand side of Fig. 7. The chip dimensions are 4.75 mm × 4.75 mm. An optical image of the 55-μm radius M3 -CMUT is illustrated on the right-hand side of Fig. 7, along with the measured first mode deflection profile at 9 V dc bias superimposed by a 1 V ac signal and at a frequency of 1.48 MHz. The 3-D COMSOL simulations were conducted to investigate the deflection profile of the M3 -CMUTs at their higher natural resonant frequency modes, Section II-C. The simulation results for the first (top), second (middle), and fourth (bottom) natural frequencies (modes) of a 40-μm radius M3 -CMUT are illustrated on the left-hand side of Fig. 8. The simulated profiles are found similar to the conventional CMUTs profiles investigated in this paper. In order to validate the simulation results, the M3 -CMUT devices were tested using a laser vibrometer Polytec Micro System Analyzer, MSA-500 (Polytec Inc., CA, USA). The devices were characterized with 9 V dc bias and an ac signal of 1 V, which was limited by the vibrometer power supply. The 40-μm radius M3 -CMUT deflection profiles for the first (top), second (middle), and fourth (bottom) natural frequencies are presented on the right-hand side of Fig. 8. Comparing the images on the left-hand and right-hand sides of Fig. 8, the measured membrane vibration profiles are found to match the 3-D COMSOL electromecanics simulation. Similar modes and comparisons were observed for the CMUT devices. The measured and simulated natural frequencies for the 40-μm radius M3 -CMUT devices are 3.4 and 3.7 MHz (first mode), 6.3 and 5.0 MHz (second mode), and 11.1 and 10.3 MHz (fourth mode), respectively. The slight differences between the measured and simulated values can be attributed to the geometry simplification for the simulations (Section II-C).

Fig. 8. Simulated (left) and measured (right) deflection profiles of a 40-μm radius M3 -CMUT at the first, second, and fourth natural resonant frequencies from top to bottom, respectively. Deflection profiles were captured employing a Polytec Micro System Analyzer, MSA-500.

Fig. 9. Natural resonant frequencies of the transducers biased with 30 V dc; analytical model (black), M3 -CMUTs (blue), and conventional CMUTs (red). Error bars in analytical model indicate uncertainties in the fabrication process.

The Polytec Micro System Analyzer was used to measure the frequency response of the 40-μm radius M3 -CMUT and CMUT over a frequency range from 0–15 MHz (step = 3.1 KHz). The transducers were biased at 9 V dc superimposed with 1 V ac. Both of the transducers frequency response profiles show five resonant modes within the investigated frequency range. The transducers first natural resonant frequencies were observed to be close to each other, 3.4 MHz for the M3 -CMUT device and 2.8 MHz for the CMUT device. An Agilent Precision Impedance Analyzer 4294A was used to measure the transducers resonant frequencies at higher dc voltages, 40 V maximum, which was limited by the impedance analyzer power supply. Impedance measurements were performed and resonant frequencies of each device were extracted from the peaks in the measured impedance. Employing (2) and (3), the results were compared with the analytical model, using the material properties and dimensions described in Section II-B. The measured resonant frequencies at 30 V dc, superimposed with a 50 mV ac signal are presented in Fig. 9 for the fabricated M3 -CMUTs (blue) and CMUTs (red), along with the analytical model. Error bars in the analytical model represent the uncertainty in the material properties used in the PolyMUMPs fabrication process, described before. The model assumes an initially flat membrane and does not include the curvature of the layers associated with the fabrication process


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Fig. 11. Shift in the first natural resonant frequency peaks due to the spring softening effect for a dc bias increasing from 20 to 40 V. Blue points represent the measured data for the M3 -CMUT devices and red points indicate the result for the conventional CMUTs devices. The error bars are associated with the limitation of the accuracy of the Impedance Analyzer. Inset: variation in the membrane spring constant versus device dimensions and for both M3 -CMUT and CMUT devices.

Fig. 10. Normalized real part of the impedance for a M3 -CMUT (blue) and a conventional CMUT (red) both with a membrane radius of (a) 40 μm and (b) 55 μm. Solid lines represent the real impedance at 40 V and dashed line at 20 V. The shift in the resonant frequency peaks is due to the spring softening effect.

and layer anchoring to the substrate. From Fig. 9, it can be seen that the results for both M3 -CMUTs and CMUTs are in a good agreement with the analytical model for the same membrane radius. The Polytec Micro System Analyzer was employed to measure the resonant frequencies of all the fabricated transducers at 10 V dc and were compared with Impedance Analyzer results. All the values were found to be in a good agreement and within ±1% of each other. The normalized real part of the impedance for the 40- and 55-μm radius M3 -CMUT (blue lines) and CMUT (red lines) are illustrated in Fig. 10(a) and (b), respectively. The results are presented for a bias of 40 V dc (solid lines) and 20 V (dashed lines), both with a superimposed, 50 mV ac signal. From Fig. 10, it can be observed that the resonant frequencies decrease when the dc bias voltage increases. This is due to the spring softening effect. By employing (2) and (3), this change in the resonant frequency can be directly translated as the change in the membrane spring constant. Assuming the same material and fabrication process, this frequency shift, fr , can therefore be represented by the second term in (2) for a given device, dimension, and voltage change, V . Therefore, a larger reduction in the device resonant frequency at a given V can be directly attributed to a smaller effective cavity height. The measured reduction in the resonant frequencies of the M3 -CMUTs (blue line) and CMUTs (red line) are illustrated in Fig. 11, for a dc bias change from 20 to 40 V. Error bars represent the Impedance Analyzer measurement accuracy of ±1.3 kHz. The equivalent change of the spring constant is shown in the inset of Fig. 11 where it can be seen that the M3 -CMUT devices experience a significantly larger resonant frequency shift for devices with radius above 35 μm. The shift also diverges from the CMUT frequency shift for larger radii.

Therefore, the effective cavity heights for these M3 -CMUT devices are much smaller than the CMUT cavity height at a given voltage. This confirms the findings in previous sections, and that the effective cavity height of M3 -CMUT is further reduced by the summation of both top and lower membranes deflections. The opposite phenomena for very small devices (30-μm radius) is due to the fact the devices are operating well below their nominal operating voltage (>100 V calculated from the analytical model). IV. C ONCLUSION The CMUTs provide several advantages over traditional transducers. They offer a wider bandwidth, better acoustic matching, higher sensitivity, and improved electrical safety. In addition, they have the ability to produce large and uniform arrays with different number of cells, and in a highly miniaturized system. Moreover, they show effective beam steering capability, with the potential for mass fabrication. However, despite all the improvements made in MEMS ultrasonic transducers in the past decade, some limitations are still noted, such as a high driving voltage required for operation. In addition, the current demands for high resolution imaging require generating even higher acoustic power and pressure, especially for imaging complex geometries such as in multilayer underground power cables. The higher sensitivity is often needed when the transducers operate in the receiving mode especially since the reflected wave can be weakened as it passes through several layers of different materials. In this paper, a novel capacitive MEMS ultrasonic transducer is introduced. Unlike conventional CMUT, where the device includes one moving membrane and a fixed bottom electrode, the developed capacitive MEMS transducer (M3 -CMUT) employs two deflectable polysilicon membranes as well as a bottom electrode, fabricated on silicon substrate using a MEMS sacrificial fabrication technique. Two sets of M3 -CMUT and CMUT devices were fabricated (on the same chip) and with similar membrane radii. Electrical measurements were conducted at different bias conditions and are verified optically using a Polytec Micro System Analyzer. Device deflection profiles were measured at different resonant frequency modes and the results have been compared and found to be in a good agreement with the simulation results.

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It is shown in this paper that in the M3 -CMUT configuration, the generated electrostatic force due to the applied bias causes a downward deflection of the top membrane and an upward motion of the lower membrane. Therefore, the transducer effective gap is significantly less than that for conventional CMUTs. The higher measured resonant frequency shifts found for a given device dimension and voltage indicates a larger spring softening effect confirming a smaller M3 -CMUT effective cavity height. The significant reduction in the M3 -CMUT cavity height can largely enhance the transducer operational properties, such as sensitivity and power generation capability. For the same dc bias, the deflection of the top membrane has been shown to be greater than that of the CMUT with the same dimensions. It has been shown that in this novel design, the required driving voltage has also been reduced and creates an increased membrane deflection for a given voltage thus enhancing the device acoustic output properties. The lower membrane is also deflectable and therefore the top membrane, where the contact is made, may be grounded while still preserving vibrational properties. This reverse biasing is especially beneficial in health related applications, where the top membrane, which might come in contact with a patient’s body, can be grounded instead to being biased at high voltages. The sensitivity of this new design is also largely enhanced, as the effective gap between the two near membranes is reduced, which may be crucial for imaging complex geometries where the reflected acoustic wave is often weak.

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ACKNOWLEDGMENT The authors would like to thank Dr. E. Cretu for providing access and assistance with the Polytec Microsystem Analyzer system and CMC Microsystems Canada for their COMSOL simulation support and for their support with the MEMSCAP device fabrication and packaging. R EFERENCES [1] T. L. Szabo, Diagnostic Ultrasound Imaging—Inside Out. San Diego, CA, USA: Academic Press, 2004. [2] M. I. Haller and B. T. Khuri-Yakub, “A surface micromachined electrostatic ultrasonic air transducer,” in Proc. IEEE Ultrason. Symp., Feb. 1994, pp. 1241–44. [3] B. T. Khuri-Yakub, O. Oralkon, and M. Kupnik, “Next-gen ultrasound,” IEEE Spectrum, vol. 46, no. 5, pp. 44–54, May 2009. [4] O. Oralkon, A. S. Ergun, J. A. Jhonson, M. Karaman, U. Demirci, K. Kaviani, et al., “Capacitive micromachined ultrasonic transducers: Next-generation arrays for acoustic imaging?” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 49, no. 11, pp. 1596–1610, Nov. 2002. [5] I. Ladabaum, X. Jin, H. T. Soh, A. Atalar, and B. T. Khuri-Yakub, “Surface micromachined capacitive ultrasonic transducers,” IEEE Trans. Ultrason., Ferroelectron., Freq. Control, vol. 45, no. 3, pp. 678–690, May 1998. [6] X. Jin, I. Ladabaum, F. L. Degertekin, S. Calmes, and B. T. Khuri-Yakub, “Fabrication and characterization of surface micromachined capacitive ultrasonic immersion transducers,” IEEE J. Microelectromech. Syst., vol. 8, no. 1, pp. 100–114, Mar. 1999. [7] A. S. Logan and J. T. W. Yeow, “Fabricating capacitive micromachined ultrasonic transducers with a novel silicon-nitride-based wafer bonding process,” IEEE Trans. Ultrason., Freq. Control, vol. 56, no. 5, pp. 1074–1084, May 2009. [8] L. L. Liu, O. M. Mukdadi, C. F. Herrmann, R. A. Saravanan, J. R. Hertzberg, S. M. George, et al., “A novel method for fabrication capacitive micromachined ultrasonic transducers with ultra-thin membranes,” in Proc. IEEE Ultrason. Symp., vol. 1. Aug. 2004, pp. 497–500.

Tahereh Arezoo Emadi (M’06) received the Ph.D. degree from the University of Manitoba, Winnipeg, MB, Canada, in 2011. She is a Post-Doctoral Fellow with the Department of Electrical and Computer Engineering, University of Manitoba. Her current research interests include microelectromechanical systems (MEMS), sensors, and MEMS transducers.

Douglas A. Buchanan (SM’81) received the B.Sc. and M.Sc. degrees from the University of Manitoba, Winnipeg, MB, Canada, in 1981 and 1982, respectively, and the Ph.D. degree from the University of Durham, Durham, NC, USA, in 1986. He was with the T. J. Watson Research Center, Yorktown Heights, NY, USA, and is currently a Professor of electrical and computer engineering with the University of Manitoba.