Highly reliable CMUT cell structure with reduced dielectric charging effect

Abstract

A highly reliable cell structure of capacitive micromachined ultrasound transducer (CMUT) was proposed. This structure equips with spacers (bumps) beneath the membrane and partial openings just above the respective spacers in the top electrode. The spacers prevent the membrane from direct contacting with the bottom surface of the cavity even when the total driving voltage exceeds collapse voltage of the CMUT. The direct current flow in the dielectric between the top and bottom electrodes in the event of the contacting was reduced by limiting the contact area of the membrane by spacers. Moreover, partial openings in the top electrode reduced the electric field concentration in each spacer where the distance between two electrodes is closest at the maximum displacement of vibrating membrane. This significantly improved the dielectric reliability of this structure compared with the conventional one. The proposed CMUT cell structure reduced the receiving sensitivity fluctuation below 1 dB after membrane vibration over 6×1011 cycles at the driving voltage over the collapse voltage of the CMUT. At this driving condition, transmitted ultrasound pressure reached three times higher than that of the conventional one. The proposed structure thus enables CMUT device to be widely used in medical imaging applications.