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.

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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.

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Capacitive Micromachined Ultrasound Transducers (CMUTs) have many advantages compared to other ultrasonic transducer technologies, especially for implantable devices. However, they require a high bias voltage for efficient operation. To eliminate the need for an external bias voltage, a charge storage layer can be embedded in the dielectric. This study aims to compare the performance of Si 3 N 4 and Al 2 O 3 when used as a charge storage layer. By measuring the shift in the C-V curve, Si 3 N 4 exhibits a larger shift than Al 2 O 3 , indicating a better charge-trapping capability. When using the pre-charged CMUTs as power receivers, the Si 3 N 4 version harvested up to 80 mW -only a few mW more than the Al 2 O 3 - with an efficiency of about 50 %. Accelerated Lifetime Tests predict a lifetime of about 7.8 and 1.2 years for Si 3 N 4 and Al 2 O 3 respectively.@en

Recently, the applications of ultrasound transducers expanded from high-end diagnostic tools to point of care diagnostic devices and wireless power receivers for implantable devices. These new applications additionally require that the transducer technology must comply to biocompatibility and manufacturing scalability. In this respect, Capacitive Micromachined Ultrasound Transducers (CMUTs) have a strong advantage compared to the conventional PZT based transducers. However, current CMUTs require a large DC bias voltage for their operation, which limits the miniaturizability of these devices. In this study, we propose a pre-charged collapse-mode CMUT for immersive applications that can operate without an external bias by means of a charge trapping Al2O3 layer embedded in the dielectrics between the top and bottom electrodes. The built-in charge layer was analytically modeled and four layer stack combinations were investigated and characterized. The measurement results of the CMUTs were then used to fit the model and to quantify the amount and type of trapped charge. It was found that these devices polarize due to the ferroelectric-like behavior of the Al2O3, and the amount of charge stored in the charge-trapping layer was estimated to be approximately 0.02 C/m2. Their acoustic performance shows a transmit and receive sensitivity of 8.8 kPa/V and 13.1 V/MPa respectively. In addition, we show that increasing the charging temperature, the charging duration, and the charging voltage results in a higher amount of stored charge. Finally, results of ALT tests showed that these devices have a lifetime of more than 2.5 years at body temperature.@en