Analysis of the charging problem in capacitive micro-machined ultrasonic transducers

The charging property of dielectrics between the upper and lower electrodes in capacitive micro-machined ultrasonic transducers (CMUTs) was investigated. In particular, the dependence of the capacitance of a CMUT cell on DC voltage (i.e., C-V curve) was measured before and after DC or DC+AC voltage stress was applied between the two electrodes. The charging was evaluated from the C-V curve shift. Silicon dioxide and silicon nitride were investigated as dielectric materials. The investigation found that the charging starts when the stress voltage reaches the collapse voltage of the CMUT cell, and it becomes larger with increasing stress voltage. Silicon dioxide causes less charging than silicon nitride. The charging by DC+AC voltage stress is higher than that by DC voltage stress only. The charge quantity accumulated in the dielectrics saturates at a constant value depending on the stress voltage. Moreover, the CV-curve shift, which degrades the sensitivity of the CMUT, can be estimated from DC bias voltage, AC driving voltage, and collapse voltage of CMUT. To avoid this sensitivity degradation, it is thus necessary that a system using CMUT devices must optimally control the operation voltage in accordance with the estimated shift.