A Comparative Study of Si3N4 and Al2O3 as Dielectric Materials for Pre-Charged Collapse-Mode CMUTs

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.