Browsing by Subject "Capacitive Micromachined Ultrasonic Transducers"

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Open Access
Deep collapse mode capacitive micromachined ultrasonic transducers
(2010) Olçum, Selim
Capacitive micromachined ultrasonic transducers (CMUTs) are suspended microelectromechanical membrane structures with a moving top electrode and a rigid substrate electrode. The membrane is actuated by electrical signals applied between the electrodes, resulting in radiated pressure waves. CMUTs have several advantages over traditional piezoelectric transducers such as their wider bandwidth and microfabrication methodology. CMUTs as microelectromechanical systems (MEMS), are fabricated using CMOS compatible processes and suitable for batch fabrication. Low cost production of large amount of CMUTs can be fabricated using already established integrated circuit (IC) technology infrastructure. Contrary to piezoelectrics, fabricating large 2-D arrays populated with transducer elements using CMUTs is low-cost. The technological challenges of CMUTs regarding the fabrication and integration issues were solved during the past 15 years, and their successful operation has been demonstrated in many applications. However, commercialization of CMUTs is still an overdue passion for CMUT community. The bandwidth of the CMUTs are inherently superior to their piezoelectric rivals due to the nature of the suspended membrane structure, however, their power output capability must be improved to achieve superior signal-to-noise ratio and penetration depth. In this thesis, we gave a comprehensive discussion about the physics and functionality of CMUTs and showed both theoretically and experimentally that their power outputs can be increased substantially. Using the conventional uncollapsed mode of CMUTs, where the suspended membrane vibrates freely, the lumped displacement of the membrane is limited. Limited displacement, unfortunately, limits the power output of the CMUT. However, a larger lumped displacement is possible in the collapsed state, where the membrane gets in contact with the substrate. By controlling the movement of the membrane in this state, the power output of the CMUTs can be increased. We derived the analytical expressions for the profile of a circular CMUT membrane in both uncollapsed and collapsed states. Using the profiles, we calculated the forces acting on the membrane and the energy radiated to the medium during an applied electrical pulse. We showed that the radiated energy can be increased drastically by utilizing the nonlinear forces on the membrane, well beyond the collapse voltage. Using the analytical expressions, we developed a nonlinear electrical equivalent circuit model that can be used to simulate the mechanical behavior of a transmitting CMUT under any electrical excitation. Furthermore, the model can handle different membrane dimensions and material properties. It can predict the membrane movement in the collapsed state as well as in the uncollapsed state. In addition, it predicts the hysteretic snap-back behavior of CMUTs, when the electric potential across a collapsed membrane is decreased. The nonlinear equivalent circuit was simulated using SPICE circuit simulator, and the accuracy of the model was tested using finite element method (FEM) simulations. Better than 3% accuracy is achieved for the static deflection of a membrane as a function of applied DC voltage. On the other hand, the pressure output of a CMUT under large signal excitation is predicted within 5% accuracy. Using the developed model, we explained the dynamics of a CMUT membrane. Based on our physical understanding, we proposed a new mode of operation, the deep collapse mode, in order to generate high power acoustic pulses with large bandwidth (>100% fractional) at a desired center frequency. We showed both by simulation (FEM and equivalent circuit) and by experiments that the deep collapse mode increases the output pressure of a CMUT, substantially. The experiments were performed on CMUTs fabricated at Bilkent University by a sacrificial release process. Larger than 3.5 MPa peak-to-peak acoustic pulses were measured on CMUT surface with more than 100% fractional bandwidth around 7 MHz using an electrical pulse amplitude of 160 Volts. Furthermore, we optimized the deep collapse mode in terms of CMUT dimensions and parameters of the applied electrical pulse, i.e., amplitude, rise and fall times, pulse width and polarity. The experimental results were compared to dynamic FEM and equivalent circuit simulations. We concluded that the experimental results are in good agreement with the simulations. We believe that CMUTs, with their high transmit power capability in the deep collapse mode can become a strong competitor to piezoelectrics.
Open Access
Lumped element modeling of circular CMUT in collopsed mode
(2014) Aydoğdu, Elif
Capacitive micromachined ultrasonic transducer is a microelectromechanical device, which serves as an acoustic signal source or sensor, in a variety of applications including medical ultrasound imaging, ultrasound therapy, airborne applications. It has a suspended membrane with an electrode inside, and at the underlying substrate there is another electrode, so that the membrane can be deflected by the electrical field formed between the electrodes. Similarly, any mechanical disturbance on the membrane can be sensed as a change in the capacitance of the two electrodes. CMUT is a nonlinear device which has a distributed mechanical operation. Although, it is a mass-spring system basically, the nonlinear electrical force and the radiation force makes it impossible to solve CMUT through analytical equations. In order to predict its behavior, and design a CMUT towards the needs of a specific application, either finite element analysis or equivalent electrical circuit modeling should be utilized. Finite element analysis (FEA) can predict the distributed CMUT operation with high accuracy, but its usage is limited to designs employing low number of CMUTs because of the computation cost. Recently, advances in equivalent circuit modeling, made it possible to simulate arrays employing very high number of CMUTs, with high accuracy. These models assume uncollapsed mode operation and except collapsed mode operation as it is highly nonlinear. This thesis focuses on obtaining an accurate equivalent circuit model for a circular CMUT in collapsed mode. The outcome is a parametric circuit model, that can simulate a CMUT of any physical and material parameters, under an arbitrary electrical or mechanical excitation. In collapsed mode, the compliance of the membrane is no longer constant as in uncollapsed mode, and it increases with increasing contact radius. Similarly, the capacitance, the electrical force and the radiation impedance all have new behavior regarding the contact radius. As there is no analytical solution for those parameters, we perform numerical calculations and extract expressions for each of them. First, we calculate the collapsed membrane deflection, utilizing the exact electrical force distribution in the analytical formulation of membrane deflection. Then we use the deflection profile to calculate the capacitance, electrical force, and compliance. Performing a set of calculations for different CMUT dimensions, different pressure and voltage levels, we obtain dependencies of those parameters on rms deflection. Then we develop a lumped element model of collapsed membrane operation, expressing the parameters as functions of rms deflection. The radiation impedance for the collapsed mode is also included in the model. The model is then merged with the uncollapsed mode model to obtain a simulation tool that handles all CMUT behavior, in transmit or receive. Large- and smallsignal operation of a single CMUT can be fully simulated for any excitation regime. The results are in good agreement with FEM simulations.