Optimization Of collapsed mode CMUT receiver for maximum off-resonance sensitivity

Abstract

Capacitive Micromachined Ultrasonic Transducer (CMUT) has a flexural conductive plate suspended over a fixed substrate with metal electrode deposited over it. This circular suspended plate is fixed at the rim but is free to move under ambient medium loading or applied bias between CMUT plate and bottom electrode. In presence of an acoustic medium, when the bias is applied the center of the plate de ects more towards the substrate owing to the electric field which is established in the cavity structure. This electric field and ambient force are balanced by plate restoring force in stable region of un-collapsed mode of plate operation. As bias is increased further, the electrostatic forces overwhelms the plate restoring force and the center of the plate contacts with the substrate with a thin insulation layer in between. We call this state, collapse mode of CMUT operation as long as the center of the plate stays in contact with the substrate. In this work, we etch small cavities and employ thin CMUT plate which is easily depressed by the atmospheric force over an evacuated cavity to produce a stable contact with the bottom electrode without any bias. CMUTs have widely been used as sensors in a wide range of applications such as underwater ow metering sensing, airborne applications, medical ultrasound imaging where CMUTs have been characterized for wide bandwidth and high sensitivity than piezoelectric ceramics. Commercial scanners using 1-D CMUT arrays are also reported to have produced clinical-quality images. Despite the success of CMUTs in medical ultrasound over the past three decades, to this day no effort has been made to optimize the receive sensitivity of a collapsed-mode CMUT. Traditional practice is to bias the CMUT plate close to collapse voltage to acheive higher coupling coe cent and sense the incoming ultrasound as an open circuit receive voltage (OCRV) signal of the transducer using a voltage pre-amplifier or short circuit receive current (SCRC) employing a transimpedance amplifier. Maintaining plate in the vicinity of collapse threshold is rather di cult as any mechanical disturbance may collapse the plate and compromise the sensitivity of the transducer. In this work we propose ambient pressure collapse CMUT transducer capable of providing a stable performance at off-resonance frequencies. When the plate of a collapsed CMUT is subjected to incoming ultrasound its receive signal becomes a strong function of bias. The electric field sustained between the biased collapsed plate and the substrate is larger than an un-collapsed plate owing to small insulation layer gap at the contact center. This results in an increased input capacitance of collapsed CMUT which together with higher electromechanical turns ratio makes collapsed CMUT a viable choice for a higher acoustic output than its coventional counterpart. First, we derive and use a linear-equivalent circuit model under small signal conditions to assess the performance of collapsed CMUT as a function of bias. We derive both OCRV and SCRC normalized to incident pressure for a collapsed CMUT in terms of lumped circuit elements. The performance curves as a function of DC bias for varying CMUT operating conditions are obtained. We compare SCRC performance with OCRV and show that simulated SCRC performance with a transimpedance amplifier is not impaired by electrical losses. We then optimize the SCRC performance with bias for any given CMUT operating conditions or geometry. To characterize the model sensitivity we design and fabricate CMUT cells based on anodic wafer technology. We employ a transimpedance amplifier to measure and verify experimentally through fabricated CMUT cells, -60 dB V/Pa sensitivity at 100 kHz when the CMUT is biased between 50 to 65 Volts.