Ultra-Low-Voltage Capacitive Micromachined Ultrasonic Transducers with Increased Output Pressure Due to Piston-Structured Plates

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) represent an accepted technology for ultrasonic transducers, while high bias voltage requirements and limited output pressure still need to be addressed. In this paper, we present a design for ultra-low-voltage operation with enhanced output pressure. Low voltages allow for good integrability and mobile applications, whereas higher output pressures improve the penetration depth and signal-to-noise ratio. The CMUT introduced has an ultra-thin gap (120 nm), small plate thickness (800 nm), and is supported by a non-flexural piston, stiffening the topside for improved average displacement, and thus higher output pressure. Three designs for low MHz operation are simulated and fabricated for comparison: bare plate, plate with small piston (34% plate coverage), and big piston (57%). The impact of the piston on the plate mechanics in terms of resonance and pull-in voltage are simulated with finite element method (FEM). Simulations are in good agreement with laser Doppler vibrometer and LCR-meter measurements. Further, the sound pressure output is characterized in immersion with a hydrophone. Pull-in voltages range from only 7.4 V to 25.0 V. Measurements in immersion with a pulse at 80% of the pull-in voltage present surface output pressures from 44.7 kPa to 502.1 kPa at 3.3 MHz to 4.2 MHz with a fractional bandwidth of up to 135%. This leads to an improvement in transmit sensitivity in pulsed (non-harmonic) driving from 7.8 kPa/V up to 24.8 kPa/V.

Fully Printed Flexible Ultrasound Transducer for Medical Applications

“…[16,17] Despite their potential advantages, the need for constant DC bias voltage, as well as limited sensitivity and power delivery can pose design and application challenges. [18] Another potential lead-free alternative to conventional piezoelectric ceramics is the use of softer piezoelectric polymers. [19] Polyvinylidene fluoride (PVDF) and its copolymers have several advantages compared to piezoceramics: their processing requires lower temperatures (typically below 150 °C, compared to above 1000 °C for piezoceramics) [20] and it is compatible with solutionbased deposition methods, such as several printing and additive manufacturing techniques.…”

“…[ 16,17 ] Despite their potential advantages, the need for constant DC bias voltage, as well as limited sensitivity and power delivery can pose design and application challenges. [ 18 ]…”

Power Efficiency Characterization with Various Gate Oxide Thicknesses in Class DE Amplifiers for HIFU Applications

“…In order to have high-power driving and high-power efficiency, research is being conducted on GaN devices in addition to CMOS devices [18]. In the driving topology, there were efforts to lower the high DC voltage bias [19,20] and to simultaneously control a large array [21,22]. There were also attempts to increase the range of operating frequencies [23] and to try to implement them digitally [24,25].…”