Biocompatible Hermetic Encapsulation of PMUTs for Usage in Implantable Medical Devices

This study reports on the encapsulation of piezoelectric micromachined ultrasonic transducer (PMUT) devices by hermetic Parylene F-VT4 (PVT4) and multilayer stacks of PVT4 combined with atomic layer deposition films. These encapsulations are evaluated for their potential to enable implantable PMUT devices for biomedical applications, providing necessary protection for both the device and the patient while preserving device performance. The mechanical and acoustic properties of coated and uncoated single PMUTs and 64x64 PMUT arrays were characterized in water. Results indicate that the resonance frequencies of both the single elements and the arrays change non-monotonically with increasing PVT4 thickness due to alterations in the equivalent mechanical properties of the composite membrane. While single PMUTs exhibited a monotonic decrease in displacement, encapsulation significantly improved the array's pressure output, attributed to changes in acoustic wavelength and inter-element coupling. This enhancement underscores the effectiveness of encapsulation in optimizing overall performance.