Squeeze film damping and stiffening in circular CMUT with air-filled cavity: Influence of the lateral venting boundary conditions and the bias voltage

•The sealed air-filled cavity increases bandwidth of air-coupled CMUT.•For high squeeze number region, squeeze film effects do not depend on the lateral venting boundary conditions.•The analytical equation for the stiffness of non-uniform air-filled cavity is determined.•The stiffness ratio of 2 makes the resonant frequency independent on the bias voltage. The present paper deals with the analysis of the squeeze film effects in circular capacitive micromachined ultrasound transducers (CMUT) operating in air, with emphasis on improved bandwidth. Firstly, a 1D analytical approach based on parallel plate approximation is recalled. The opposing influences of the electrostatic softening and the squeeze film stiffening make the resonant frequency dependent on the bias voltage with respect to air spring constant to mechanical spring constant ratio. In a second part, FEM models using COMSOL Multiphysics® are built to analyze the influences of the plate flexibility and the lateral venting boundary conditions on the squeeze film effects. The associated numerical results show that viscous losses are involved in sealed air-filled cavities. Moreover, the dimensionless elastic and viscous damping forces do not depend on the lateral venting boundary conditions for high squeeze number range, usual for CMUT operation. Finally, 2D full coupled simulations of flexible CMUT are compared with experimental data. Thus, the squeeze film damping increases bandwidth of air-coupled CMUTs with both sealed and laterally vented cavities.