CMUT array element in deep-collapse mode

Abstract

Collapse and deep-collapse mode of operations have boosted the pressure outputs of capacitive micromachined ul- trasonic transducers (CMUTs) considerably. In this work, we demonstrate a CMUT element operating in the deep-collapse mode with 25 V pulse excitation and without the effects of charge trapping. The fabricated CMUT element consists of 4 by 4 circular cells with 20 m radius and 1 m thick plates suspended over a 50 nm cavity. The overall size of the element is 0.190 mm by 0.19 mm. The collapse voltage of the plates is measured to be approximately 3V. By driving the CMUTs with 25V pulses in the deep-collapse mode without any bias, we achieved 1.2 MPa peak-to-peak pressure output on the surface of the CMUT element with a center frequency of 9 MHz and 100% fractional bandwidth. We applied 1000 consecutive electrical pulses with alternating polarity to the element and witnessed no change in the transmitted acoustic pulse. Output power limitations of capacitive micromachined ultra- sonic transducers (CMUTs) have been one of the obstacles in front of the commercialization efforts, since ultrasound imag- ing applications require sufficiently high amplitude pressure waveforms transmitted into the target medium. A higher ampli- tude of the pressure waveform translates into more penetration depth and a better signal-to-noise ratio. It has recently been shown in (1) that it is possible to transmit high amplitude pressure pulses using the deep-collapse mode of CMUTs. The deep-collapse mode of operation requires voltages well above the collapse voltage of the transducer to be applied between the its electrodes. In this mode, CMUTs are designed to have very low collapse voltages and generating high amplitude pressure pulses does not require very high voltages. We show that it is possible to generate high amplitude pressure pulses using no bias and relatively low voltage levels that can be generated by a high-voltage CMOS process 1 . For most CMUT geometries, a dielectric insulation layer made from silicon oxide or silicon nitride is used between the transducer electrodes to provide electrical isolation. This layer sets the minimum electrode separation in the collapse- mode, which in turn determines the limit on the electrical attraction force applied on the plate. A thinner insulation layer is preferable to increase electrical force applied on the plate. However, a thin dielectric layer may pose a charge trapping problem when it is subject to high electric fields (2). The charge trapped in the insulation layer may degrade the device performance by creating a counter-force. In this paper, we 1 For example, H18 (0.18 m up to 50V) and H35 (0.35 m up to 120V) processes of Austria Microsystems AG, Unterpremst¨ atten, 8141, Austria. Fig. 1. An ideal pressure pulse in time generated by a CMUT, when a voltage pulse excitation is applied between CMUT electrodes. propose a simple way to eliminate the effects of the charge trapping without compromising the output pressure level. We first discuss the movement of the CMUT plate during the electrical pulse excitation. We use this discussion to determine the optimal electrical pulse shape to be applied to a CMUT during the deep-collapse mode. Finally, we demonstrate the deep-collapse mode of operation at relatively low voltage levels without the detrimental effects of the charge trapping.