Design of a micro-opto-mechanical ultrasound sensor for photoacoustic imaging

Optical ultrasound sensing is a promising technique for the emerging field of biomedical photoacoustic imaging. Previously at imec, micro-opto-mechanical sensors with integrated Mach-Zehnder interferometers were designed and demonstrated as highly sensitive for static pressure sensing. At quasi-static pressures, they are demonstrated to operate as a sensitive microphone. In this study, the application range is extended to include dynamic pressures targeting a proof of concept for a photoacoustic imaging application. To achieve this, the dynamic behavior of the pressure sensor is firstly characterized, showing its resonance frequency at 73.8 kHz. Since this is below the range desired in photoacoustic imaging, several approaches for resonance frequency increase are investigated, resulting positively for incomplete membrane release and new designs. These approaches are analyzed based on the evaluated sensor sensitivity, allowing the selection of optimal design. The mentioned evaluation of sensor sensitivity is possible due to a developed methodology based on measurement data, analytical and accurate acousto-mechanical finite element models. The developed methodology is demonstrated throughout a range of membrane sizes, frequencies and modes of vibration allowing the selection of maximum sensitivity design. In this study, a micro-opto-mechanical ultrasound sensor based on integrated Mach-Zehnder interferometer is designed for 1 MHz operation in a photoacoustic imaging environment and optimized for sensitivity.