Influence of the environment on the fatigue properties of alumina ultra-thin coatings and silicon and nickel thin films
Baumert, Eva K.
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This dissertation presents the investigation of three thin film materials used in microelectromechanical systems (MEMS): alumina, silicon, and nickel. For this purpose, novel experimental techniques to test thin films under MEMS-relevant loading conditions were developed in order to study environmental effects and the underlying fatigue mechanisms of amorphous alumina ultra-thin coatings, mono-crystalline brittle silicon thin films, and poly-crystalline ductile nickel thin films. Knowledge of these mechanisms is necessary to improve the reliability of MEMS, especially of those devices operating in harsh environments. MEMS resonators were used to investigate both the fatigue and time-dependent behavior of alumina, silicon, and nickel. While micro-resonators were used in prior studies to research the fatigue properties of mono- and polycrystalline silicon, this work is the first in (1) using them to investigate fatigue properties of ultra-thin coatings and metallic films and in (2) using micro-resonators to investigate the time-dependent fatigue behavior of silicon films. For fatigue testing, the micro-resonators were subjected to fully-reversed loading at resonance (≈40 kHz for alumina-coated silicon, ≈8 kHz for nickel). Experiments were conducted in air at 30 °C, 50% relative humidity (RH) or 80 °C, 90% RH and testing was carried out over a broad range of applied stresses. The resonance frequency evolution proved to be a metric for the accumulated damage, which could be further quantified using finite element analysis. In addition, scanning and transmission electron microscopy were used to examine the extent of fatigue damage. For testing under static loads, the resonators were subjected to external loading using a micromanipulator and probe-tip. Experiments with atomic-layer-deposited alumina investigated the cohesive and interfacial fatigue properties of alumina coatings of four different thicknesses, ranging from nominally 4.2 nm to 50.0 nm on silicon micro-resonators. Fatigue loading led to both cohesive and interfacial damage, while static loading did not result in any damage. Both the cohesive and interfacial fatigue crack growth rates are approximately one order of magnitude higher at 80 °C, 90% RH than at 30 °C, 50% RH and seem to increase with increasing strain energy release rate. A combination of compressive loading and the silicon sidewall's surface roughness is believed to cause the observed fatigue behavior. Experiments with 10-micrometer-thick deep reactive ion etched silicon micro-resonators investigated two aspects: whether surface oxidation is the critical parameter in silicon thin film fatigue and time-dependent failure in silicon as a potential underlying cause of resonator failures in the low cycle fatigue (LCF, <17 cycles, corresponding to ≈5 min) regime. To confirm whether surface oxidation is the critical parameter in silicon thin film fatigue, the influence of oxygen diffusion barrier alumina coatings on the fatigue behavior was investigated. The coatings led to an increase in fatigue life by at least two orders of magnitude compared to uncoated devices in the harsh environment, which not only confirms reaction layer fatigue (RLF) as governing fatigue mechanism in silicon thin films, but also constitutes a practical solution to significantly increase fatigue lifetimes. Previous LCF data were inconsistent with the RLF model, given that thick surface oxidation is unrealistic for tests lasting only few minutes. Accordingly, time-dependent failure in silicon was investigated as underlying cause and the observation of resonator failures under static loading indeed suggest that time-dependent crack growth may be responsible for LCF failures. Experiments with metallic micro-resonators investigated the fatigue crack initiation in 20-micrometer-thick electro-deposited nickel under MEMS-relevant conditions, such as extreme stress gradients resulting in non-propagating cracks, fully-reversed loading (over a large range of stress amplitudes), exposure to mild and harsh environments, and accumulation of billions of cycles. Under these circumstances, extrusions form locally at the notch root (within few million cycles at high stress amplitudes). Very thick local oxides (only at the location of the extrusions) of up to 1100 nm were observed in the harsh environment, with thinner oxides (up to 700 nm) in the mild environment. Micro-cracks form in the oxide but do not propagate given the extreme stress gradients. Finite element analysis confirmed that oxidation and micro-cracking lead to changes in the resonance frequency, which are consistent with the experimental results. Accumulation of cyclic plasticity appears to also lead to a decrease in resonance frequency which scales with applied strain.