Modelling and characterization of optical micromachined ultrasound sensors with a silicon photonic ring resonator in a buckled acoustical membrane

Future applications of ultrasonography in (bio-)medical imaging require ultrasound sensor matrices with small sensitive elements. Promising are opto-mechanical ultrasound sensors (OMUS) based on a silicon photonic ring resonator embedded in a silicondioxide acoustical membrane. This work presents new OMUS modelling: acoustomechanical non-linear FEM and photonic circuit equations. We show that initial wafer stress needs to be considered in the design: the acoustical resonance frequency changes considerably and OMUS sensitivity differs for up- or downwards buckled membranes. Simulated acoustical resonance frequency agrees well with measurements, assuming realistic SOI wafer stress. Measured sensitivity showed large device-to-device variation and simulations agree within this order of magnitude. We conclude that careful modeling of stress is necessary for the design of robust and sensitive sensors.