Characterization of Ring Resonators for Ultrasound Sensing in a 45 nm Monolithic SPCLO Process

Monolithic integration of electronics and photonics enables low-power, high-density ultrasound receiver arrays based on silicon microring resonators (MRRs). This paper presents an array of MRRs fabricated in a high-volume 45 nm electronics-photonics CMOS-SOI process. We review a range of MRR-based sensors with different waveguide geometries, characterizing each sensor type electro-optically; we further evaluate sensitivity to ultrasound with and without the presence of undercut structures. The best performance is achieved using an undercut adiabatic microring resonator, which has an average intrinsic sensitivity to ultrasound of $23.63 \mathrm{fm} / \mathrm{kPa}$ at 15 MHz with a standard deviation of $11.97 \mathrm{fm} / \mathrm{kPa}$, while alternative ring geometries achieve comparable performance.