Finite Element Analysis of CMUTs with Pressurized Cavities Nikhil Apte, Kwan Kyu Park, and Butrus T. Khuri-Yakub Edward L. Ginzton Laboratory Center for Nanoscale Science and Engineering, Stanford University, Stanford, CA 94305 U.S.A npapte@stanford.edu Abstract – We propose using CMUTs with pressurized cavities in environments with extreme pressure variations. By controlling the pressure inside the cavity, the pressure differential across the CMUT plate can be kept low, ensuring a stable operating point and preventing mechanical failure. In such CMUTs, a squeeze film is formed between the plate and the substrate, which provides additional damping as well as stiffening. The damping from the squeeze film helps increase the bandwidth of the CMUT. We present a new method for performing a finite element analysis for such structures using ANSYS. We fabricated a variety of vented CMUTs in the frequency range of ~100-200 kHz, which exhibited a quality factor of 25-30 in air at 1 atm pressure. Our finite element model successfully predicts the center frequency and quality factor for these devices. Keywords: CMUT, FEA, varying pressure, squeeze-film damping I. INTRODUCTION Capacitive Micromachined Ultrasound Transducers (CMUTs) are versatile devices which have applications in varied fields such as medical imaging, ultrasonic flow metering, ranging, chemical sensing, etc. Some of these applications, e.g. flow metering often involve operation in a varying ambient pressure. Conventional CMUTs are inherently unsuitable for operating in such environments. A conventional CMUT consists of a fixed substrate (bottom electrode) and a moving plate (top electrode). The cavity between the plate and the bottom substrate is hermetically sealed during fabrication of the CMUT, typically under vacuum [1]. The vacuum cavity offers the advantage of reduced loading and increased transduction efficiency, however having a fixed pressure inside the cavity limits the operating pressure range for the CMUT. The static deflection of the CMUT plate depends on the pressure differential across it and the transmit and receive sensitivity of the CMUT depend on this static deflection. In a varying ambient pressure, the static deflection would vary considerably, and the CMUT would have an unstable operating point. To overcome this problem, M.-C. Ho, et. al. proposed operating CMUTs in a permanent contact mode even under 1 atm pressure [2]. This would enable a more stable operating point over a wider operating pressure range. However, even such a CMUT would still be limited by the mechanical strength of the structure. Beyond a certain pressure, such a CMUT would fail mechanically. We propose making CMUTs with cavities which are vented to an external fluid source. The cavity could simply be vented to the ambient environment thus ensuring a zero differential pressure across the plate, or it could be pressurized using a variety of gases / liquids, with the pressure being controlled independently. The pressure differential can be kept constant irrespective of the absolute ambient pressure. This will ensure a stable operating point for the CMUT. Also, with this approach the pressure differential across the plate will be limited and hence such a CMUT should be able to operate in any pressure condition. In such vented CMUTs, a fluid squeeze film will be formed inside the cavity. This squeeze film will provide additional stiffening as well as damping for the plate. The additional damping will help increase the fractional bandwidth at the cost of some loss of transmit and receive sensitivity. An improved bandwidth would be especially useful for applications like airborne ultrasound imaging. Also in flow metering applications, a broad bandwidth would help relax the frequency matching requirement for the transmit and receive transducers. Squeeze films are often seen in MEMS devices like accelerometers [3], micromirrors, etc. In such cases, the entire moving structure is usually vented to ambient pressure at the edges. However in a CMUT, most of the cavity is sealed at the edges by the post supporting the plate and the cavity needs to be vented through channels made through the side (post) or through the substrate [Fig. 1]. The location, number and size of these channels will affect the stiffening and damping, and can be optimized for the design of such CMUTs. II. SQUEEZE FILM THEORY The squeeze film theory comes into picture whenever there is squeezing of a thin fluid film between two surfaces moving normal to each other. Reynolds equation from lubrication theory is used to analyze the fluid structure interaction when squeeze films are involved [4,5].   (    ) Where, d = local gap separation ρ = density t = time η = dynamic viscosity