Experimental characterization of collapse-mode CMUT operation

Abstract

Abstract-This paper reports on the experimental characterization of collapse-mode operation of capacitive micromachined ultrasonic transducers (CMUTs). CMUTs are conventionally operated by applying a direct current (DC) bias voltage less than the collapse voltage of the membrane, so that the membrane is deflected toward the bottom electrode. In the conventional regime, there is no contact between the membrane and the substrate; the maximum alternating current (AC) displacement occurs at the center of the membrane. In collapse-mode operation, the DC bias voltage is first increased beyond the collapse voltage, then reduced without releasing the collapsed membrane. In collapse-mode operation, the center of the membrane is always in contact with the substrate. In the case of a circular membrane, the maximum AC displacement occurs along the ring formed between the center and the edge of the membrane.The experimental characterization presented in this paper includes impedance measurements in air, pulse-echo experiments in immersion, and one-way optical displacement measurements in immersion for both conventional and collapse-mode operations. A 205-m 205-m 2-D CMUT array element composed of circular silicon nitride membranes is used in the experiments. In pulse-echo experiments, a custom integrated circuit (IC) comprising a pulse driver, a transmit/receive switch, a wideband lownoise preamplifier, and a line driver is used. By reducing the parasitic capacitance, the use of a custom IC enables pulseecho measurements at high frequencies with a very small transducer. By comparing frequency response and efficiency of the transducer in conventional and collapse regimes, experimental results show that a collapsed membrane can be used to generate and detect ultrasound more efficiently than a membrane operated in the conventional mode. Furthermore, the center frequency of the collapsed membrane can be changed by varying the applied DC voltage. In this study, the center frequency of a collapsed transducer in immersion is shown to vary from 20 MHz to 28 MHz with applied DC bias; the same transducer operates at 10 MHz in the conventional mode. In conventional mode, the maximum peakto-peak pressure is 370 kPa on the transducer surface for a 40-ns, 25-V unipolar pulse excitation. In collapse mode, a 25-ns, 25-V unipolar pulse generates 590 kPa pressure at the surface of the transducer.

An equivalent circuit for collapse operation mode of CMUTs

“…Applied electrostatic forces result in a transmission of a pressure signal. It has been demonstrated in previous studies [1] that the collapsed mode increases the pressure amplitude drastically. More recently, it has been shown that a full electrode coverage should be utilized with the collapsed mode [2] for highest pressure amplitudes at the output.…”

“…The collapse state of a suspended membrane is introduced for increasing the power output capability of capacitive micromachined ultrasonic transducers (CMUT) [1], [2]. However, only experiments and finite element method (FEM) simulations [3] have been utilized to understand the behavior of such a mode.…”

CMUT array element in deep-collapse mode

“…The center frequencies are 9MHz and 11MHz, respectively. The center frequency shift due to the increasing pulse amplitude was studied in the context of the collapse mode earlier in [5]. Both 25V and 40V electrical excitations force the CMUT plate with a collapse voltage of ∼3V to go into deep-collapse, resulting up to ∼50 kPa/V large signal transmission efficiency.…”

MEMS-Based Ultrasound Transducer: CMUT Modeling and Fabrication Process

“…For silicon and copper membranes higher than 800µm in diameter, the CMUT collapsed. The collapse of CMUT membrane happens when the displacement value is greater than the cavity thickness (8). As for the SU-8, the collapse of CMUT occurred even for membranes as small as around 400µm in diameter and 10µm in thickness.…”