SUBJECT: Ph.D. Dissertation Defense
   
BY: Eva Baumert
   
TIME: Friday, June 7, 2013, 2:00 p.m.
   
PLACE: Love Building, 210
   
TITLE: Influence of the Environment on the Fatigue Properties of Alumina Ultra-Thin Coatings and Silicon and Nickel Thin Films
   
COMMITTEE: Dr. Olivier Pierron, Chair (ME)
Dr. Steve Antolovich (ME)
Dr. Oliver Brand (ECE)
Dr. Samuel Graham (ME)
Dr. David McDowell (ME)
 

SUMMARY

This dissertation presents the investigation of three materials used in microelectromechanical systems (MEMS): alumina ultra-thin coatings and silicon and nickel thin films. For this purpose, novel experimental techniques were developed to study environmental eff ects and the underlying fatigue mechanisms in those materials. Knowledge of these mechanisms is necessary to improve the reliability of MEMS operating in harsh environments. MEMS resonators were used to investigate the fatigue and time-dependent behavior. This work is the first in using resonators to (1) investigate fatigue properties of coatings and metallic films and to (2) investigate the time-dependent fatigue behavior of silicon films. For fatigue testing, the resonators were subjected to fully-reversed loading at resonance (kHz range). Experiments were conducted in air at 30 C, 50% relative humidity (RH) or 80 C, 90% RH. The resonance frequency evolution proved to be a metric for the damage accumulation, which was further quantified using finite element analysis (FEM). Electron microscopy was used to examine the fatigue damage. For testing under static loads, the resonators were loaded using a micromanipulator and probe-tip. Experiments with atomic-layer-deposited alumina investigated the fatigue properties of coatings of 4 thicknesses ranging from 4.2 nm to 50.0 nm. Fatigue loading led to both cohesive and interfacial damage, while static loading did not result in any damage. The fatigue crack growth rates are about 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. Experiments with silicon micro-resonators investigated two aspects. (1) The influence of oxygen diff usion 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 at 80 C, 90% RH, which confirms reaction layer fatigue (RLF) as governing fatigue mechanism in silicon thin films and constitutes a possibility to significantly increase fatigue life. (2) Time-dependent failure in silicon was investigated and the observation of resonator failures under static loading suggest that time-dependent crack growth may be responsible for failures <1e7 cycles (<5 min). Experiments with metallic micro-resonators investigated the fatigue crack initiation in electro-deposited nickel under MEMS-relevant conditions, such as extreme stress gradients resulting in non-propagating cracks, fully-reversed loading, exposure to mild and harsh environments, and accumulation of billions of cycles. Under these circumstances, extrusions form locally at the notch root. Thick local oxides of up to 1100 nm were observed in the harsh environment, with thinner oxides in the mild environment. Non-propagating micro-cracks form in the oxide. Cyclic plasticity, oxidation, and micro-cracking change the resonance frequency.