Fabrication and Characterization of CMUTs realized by Wafer Bonding

This paper presents the fabrication process and electrical characterization of Capacitive Micromachined Ultrasonic Transducers (CMUTs) realized by fusion bonding. The transducer array with electrical connections was realized by four photolithography steps. The electrical characterization of two different geometries is presented. The smallest structure have 120 nm deep circular cavities with radius of 5.7 mum separated by a pitch of 12.5 mum and are vacuum-sealed by a 100 nm silicon nitride membrane. The dimensions in the second geometry are twice as large except of the cavity gap which is the same. The electrical measurements of the input impedance were carried out by a vector network analyzer on one row of 52 elements, each with four CMUTs. In non-collapsed mode in air the small CMUTs have a resonance frequency of 30 MHz with a Q value of 70 when DC biased by 30 V. When immersed in oil, the resonance frequency was reduced to 8 MHz and the peak was broadened as expected. The large CMUT structures show somewhat distorted resonances at about 13 MHz in air, with Q-values up to 50 to 60 for the better devices. Simulations indicate resonance frequencies that are about 20% lower. Hence some process is taking place in these devices that are not included in the theory. No hysteresis was observed in any of the measurements