Pre-Charged Collapse-Mode Capacitive Micromachined Ultrasonic Transducers (CMUT) Receivers for Efficient Power Transfer

Capacitive Micromachined Ultrasonic Transducers (CMUTs) offer several advantages over standard PZT transducers, particularly for implantable devices. To eliminate their typical need for an external bias voltage, we embedded a charge storage layer in the dielectric. The objective of this study was to evaluate the performance of PECVD Si3N4 and ALD Al2O3 as materials for the charge storage layer, and two different dielectric layer thicknesses, focusing on their application as receivers in a wireless power transfer link. Capacitance-Voltage (C-V) measurements revealed that Si3N4 has a higher charge storage capacity compared to Al2O3. Additionally, a thicker dielectric layer between the bottom electrode and the charge storage layer (Bdiel) improved both charge trapping and retention, as assessed in dynamic accelerated lifetime transmit-mode tests. We then analysed the power conversion performance of the fabricated CMUTs through both simulations and experiments. We performed extensive modelling based on an equivalent circuit derived from electrical impedance measurements of the fabricated CMUTs. The model was used to predict the power conversion efficiency under various conditions, including the charging field strength, the operating frequency, and parasitic series resistance. Power transfer experiments at 1 and 2.4 MHz recorded efficiencies exceeding 80% with an optimally matched load and up to 54% with a purely resistive load. Results confirmed that, with optimal load matching, the efficiency of different CMUT variants is comparable, indicating that the optimal variant should be selected based on additional criteria, such as charge retention time.