Residual stress in capacitive micromachined ultrasonic transducers fabricated with Anodic Bonding using SOI wafer

This paper focuses on a fabrication process for CMUTs using anodic bonding of a silicon on insulator wafer on a glass wafer. This technology makes possible the fabrication of large membranes and can extend the frequency range of CMUTs to lower frequencies of operation. Silicon membranes having radii of 50, 70, 100 and 150 mu m and a 1.5 mu m thickness are fabricated and electromechanically characterized. Resonant frequencies from 0.6 to 2.3 MHz and an electromechanical coupling coefficient around 58% are reported. The discrepancies between experimental and theoretical values of the first resonance frequency can be explained by a combined effect of a tensile residual stress (between 90 and 110 MPa) in the membranes and experimental boundary conditions that can be modeled by a torsional stiffness of 2.10-7 N.m.rad(-1) instead of a perfectly clamped ring. (C) 2014 The Authors. Published by Elsevier Ltd.