This work describes a method to estimate the mean collapse and snapback voltages of CMUT elements and their statistical distribution by extracting the voltage-dependent capacitance from fast dynamic excitation of the device, enabling fast analysis of fabrication-related intra-element and inter-element variability. While both voltage excitation and current excitation of the CMUT element provide comparable array-level results in terms of mean and deviation of positive and negative collapse and snapback voltages, it is demonstrated that improved detection of isolated collapse and snapback events is achieved through current-driven excitation, implementing charge-controlled actuation of the membranes.

Ultrasound (US) is a promising modality for wirelessly powering implantable devices, requiring encapsulated receivers to ensure long-term stability. Traditional hermetic packaging often limits acoustic transmission, making polymer-based encapsulation a more suitable alternative. This study investigates how implant-grade polymers, thermoplastic polyurethane (TPU), parylene-C, and medical-grade silicones (MED-1000 and MED2-4213), affect the receive performance of piezoelectric micromachined ultrasonic transducers (PMUTs). Simulations and measurements between 1 and 7 MHz show that all tested materials exhibit transmission coefficients above 94% at nanometer- and micrometer-scale thicknesses, confirming their acoustic transparency. The results show that although coated PMUTs are acoustically well matched with the surrounding water medium, the added mechanical load of the coating can hinder membrane motion and reduce the energy transferred to the PMUTs. Modeling and experimental data demonstrate that stiffer coatings, such as parylene-C, lead to a reduced sensitivity when similar thicknesses are used. Likewise, residual stress in materials like MED-1000 can also degrade the performance. These effects are not evident from acoustic transmission measurements alone, underscoring the need to assess both acoustic and mechanical properties when selecting encapsulation materials. In general, softer materials offer excellent acoustic performance for PMUT encapsulation, while stiffer materials must be applied in thinner layers to avoid impairing PMUT function.

Capacitive micromachined ultrasonic transducers (CMUTs) offer several advantages over standard lead zirconate titanate (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 plasma-enhanced chemical vapor deposition (PECVD) Si3N4 and atomic layer deposition (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 (CV) 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 (TX)-mode tests. We then analyzed the power conversion performance of the fabricated CMUTs through both simulations and experiments. We performed extensive modeling 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.

Estimation of the collapse (V_col) and snapback (V_sb) voltages of Capacitive Micromachined Ultrasonic Transducers (CMUTs) is usually performed by extracting C-V curves from low frequency impedance measurements at different bias points. However, impedance analysis in several bias conditions is time-consuming, making this technique unsuitable for wafer-level testing. Additionally, prolonged exposure to high electric fields may lead to charge injection and trapping phenomena in the CMUT in-cavity insulation layers. This paper proposes an adjustment to the conventional impedance analysis technique aimed at enhancing estimation accuracy and introduces a novel technique for fast C-V assessment enabling rapid wafer-level characterization. Results from both techniques are compared, demonstrating the validity of the proposed approaches.