On the Effect of Water-Induced Degradation of Thin-Film Piezoelectric Microelectromechanical Systems

Lifetime and reliability in realistic operating conditions are important parameters for the application of thin-film piezoelectric microelectromechanical systems (piezoMEMS) based on lead zirconate titanate (PZT). Humidity can induce time-dependent dielectric breakdown at a higher rate compared to dry conditions, and significantly alter the dynamic behavior of piezoMEMS-devices. Here we assess the lifetime and reliability of PZT-based micromirrors with and without humidity barriers operated at 23°C in an ambient of 0 and 95 % relative humidity. The correlation of the dynamic response, as well as the ferroelectric, dielectric, and leakage properties, with degradation time was investigated. In humid conditions, the median timeto-failure was increased from 2.7×10 ⁴ [1.9×10 ⁴ -4.0×10 ⁴ ] s to 1.1×10 ⁶ [0.9×10 ⁶ -1.5×10 ⁶ ] s at 20 VAC continuous unipolar actuation, by using a 40 nm thick Al ₂ O ₃ humidity barrier. However, the initial maximum angular deflection, polarization, and dielectric permittivity decreased by about 6, 11, and 12 %, respectively, for Al ₂ O ₃ capped devices. For both bare and encapsulated devices, the onset of electrothermal breakdown-events was the dominant cause of degradation. Severe distortions in the device's dynamic behavior, together with failure from loss of angular deflection, preceded time-dependent dielectric breakdown in 95% relative humidity. Moreover, due to the film-substrate stress transfer sensitivity of thin-film devices, water-induced degradation affects the reliability of thin-film piezoMEMS differently than bulk piezoMEMS.