Electrochemically driven degradation of chemical solution deposited ferroelectric thin-films in humid ambient

Abstract

The ambient humidity significantly accelerates the degradation of lead zirconate titanate (PZT) films in microelectromechanical systems; the cause of such degradation is under debate. Here, it is shown that the degradation of chemical solution derived PZT thin-films in humid conditions is driven by the system's electrochemical activity toward water electrolysis. The layer stacks with Pt-based electrodes exhibited a faster degradation rate owing to their higher electrocatalytic activity compared to Au. A degradation model is proposed based on the electrolysis of liquid or gaseous H2O, involving the evolution of oxygen and hydrogen gas at the top and bottom electrodes. Degradation proceeds above the threshold voltage for a given electrode system and is driven by the evolution and pressure build-up of gaseous species at the PZT/electrode interfaces. The pressure build-up causes film cracking, delamination of the film and electrodes, electrothermal breakdown events, and eventually time-dependent dielectric breakdown. Significantly larger post-breakdown crater sizes in humid than in dry conditions suggests that larger cracks through which dielectric breakdown through humidified air can occur. Overall, these effects are shown to cause sample failure up to six orders of magnitude of time earlier than for operation in dry conditions. Thus, in order to improve the resilience of thin-film systems in humid conditions, it is imperative to protect the electrochemically active electrode components of the device.

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

“…Post-failure, the ETB-craters of the bare devices covered an average 150 ± 40μm 2 of the electrode surface area, with most of the craters appearing along the electrode edges. From elemental mapping of post-failed devices (not shown here), residuals of Si, PZT, Pt, and Au were found both inside the post-failure craters and scattered across the electrode surface, consistent with degradation observed in previous work [24], [27] and with other literature reports [24]. For the present device design the critical flaw was located at the 20 μm wide electrode routings, which make up about 1/3 of the total electrode circumference (approx.…”

“…g) and electrochemical compression of gas trapped at the piezoelectric thin film/electrode interfaces, as the driving mechanism [24]. Degradation occurs through cracking, delamination, and failure by ETB, caused by dielectric breakdown through humidified air separating the top and bottom electrodes in cracks and voids.…”

“…Ensuring a reliable long-term device operation in realistic and harsh operating conditions is technologically and scientifically important for commercialization of thin-film piezoelectric MEMS (piezoMEMS) [17]- [20]. As such, humidity is a severe ambient stressor [21]- [24]. Operation in humid conditions causes cracking, resistance degradation, excessive leakage, and time-dependent dielectric breakdown (TDDB) considerably faster than for operation under dry conditions [25], [26].…”

Tunable Molecular Electrodes for Bistable Polarization Screening

“…Atmospheric adsorbates cannot be considered the most optimal screening species because: i) Their composition and adsorption is difficult to control and predict, sometimes leading to undesirable effects such as sample degradation, [ 7 ] dependence on sample's history [ 8 ] or polarization switching [ 4,5,9–11 ] and ii) The charge sign or preferred dipole orientation of screening species tends to stabilize one ferroelectric polarity over the opposite, i.e., screening species can be considered as unipolar, requiring the dipole rotation and change of the interfacial chemical bond upon switching. [ 12 ]…”

“…External screening by adsorbates coming from the atmosphere (such as H 2 O and O 2 ) [1][2][3][4][5] has been shown to be more effective in stabilizing the ferroelectric phase compared to metallic electrodes with finite screening length. [6] Atmospheric adsorbates cannot be considered the most optimal screening species because: i) Their composition and adsorption is difficult to control and predict, sometimes leading to undesirable effects such as sample degradation, [7] dependence on sample's history [8] or polarization switching [4,5,[9][10][11] and ii) The charge sign or preferred dipole orientation of screening species tends to stabilize one ferroelectric polarity over the opposite, i.e., screening species can be considered as unipolar, requiring the dipole rotation and change of the interfacial chemical bond upon switching. [12] In view of this, it is desirable to find a screening mechanism that combines the screening efficiency of chemical adsorbates with the predictability and bipolar screening ability of metallic electrodes.…”