Optimization of the efficiency and reliability of reverse-fabricated CMUT arrays

The electro-mechanical conversion efficiency and the long-term reliability of Capacitive Micromachined Ultrasonic Transducers (CMUT) are mainly limited by the parasitic capacitance and by charge injection phenomena. In this paper, we investigated the possibility of reducing both the parasitic capacitance, by patterning the CMUT electrodes in order to avoid any superposition of the two electrodes outside the cavity area, and the charge injection phenomena, by introducing high-quality Silicon Oxide (SiO2) buffer layers between the electrodes and the cavity Silicon Nitride (SixNy) passivation layers. Test capacitors were initially fabricated with the aim of measuring the dielectric characteristics of the SixNy, and to quantitatively evaluate the effects of the SiO2 buffer layers. Successively, 256-element CMUT arrays were designed and fabricated using both the classical and the modified electrode layouts and material stacks. The electrical impedance of single array elements was measured at different increasing and successively decreasing bias voltages in the 0-260V range in order to electrically stress the in-cavity dielectrics. A 33% reduction of the parasitic capacitance was achieved with the modified electrode layout, resulting in a 40% increase of the electro-mechanical coupling coefficient. A significant reduction of charge injection and charge trapping phenomena was demonstrated by a less hysteretic behavior of the capacitance-voltage characteristic, and by a negligible residual bias of the optimized device as compared to the conventional device.