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 degrees 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 2.7x10(4)[1.9x10(4)-4.0x10(4)] s to 1.1x10(6)[0.9x10(6)-1.5x10(6)] s at 20 V-AC continuous unipolar actuation, by using a 40 nm thick Al2O3 humidity barrier. However, the initial maximum angular deflection, polarization, and dielectric permittivity decreased by about 6, 11, and 12%, respectively, for Al2O3 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.

Automated summary

The study evaluated the lifetime and reliability of PZT-based micromirrors operated under different humidity conditions, revealing the impact of humidity on the devices. The use of an Al2O3 humidity barrier increased the failure time of the devices, but also decreased their initial performance.