Development of Silicon Photonic Opto-mechanical Ultrasound Sensor with Ultra-thin Membrane: Proof-of-concept study

The optical ultrasound sensing technique typically provides higher sensitivity and wider bandwidth, even though its application is limited due to the bulky optical system size. Silicon photonics has a great potential to fabricate large-scale optical systems on tiny silicon chips with sub-micron-sized components. In this study, we report on a silicon photonic-based new opto-mechanical ultrasonic sensor and its design scheme that allows easy fabrication while allowing to have a very thin membrane structure. Compared to previous work, which has a 2 μm-thick membrane defined by a wafer bonding technique, we have a much broader design space as the dimension and shape of the membrane are defined by lithography and etching. As a proof of concept, we implemented a sensor with only a 70 nm-thick membrane having acoustic resonance at 6.5 MHz. The sensitivity and fractional bandwidth of the device are 0.41μV/Pa and 62%, respectively.