Graduate Studies
Embargoed Master's Theses
First Advisor
Fadi Alsaleem
Committee Members
Moe Alahmad, Joseph Turner, Robert Macdonald (GE Aerospace)
Date of this Version
5-2025
Document Type
Thesis
Citation
A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science
Major: Mechanical Engineering and Applied Mechanics
Under the supervision of Professor Fadi Alsaleem
Lincoln, Nebraska, May 2025
Abstract
Stiction—unintended adhesion between microsurfaces—remains a critical reliability challenge for microelectromechanical systems (MEMS), particularly in high-g inertial sensors where movable elements repeatedly contact travel stops. Existing surface treatments, like self-assembled monolayers, suffer from limited thermal stability and integration complexities, while wafer-level quantitative adhesion characterization methods remain scarce. This work presents a production-oriented framework to quantitatively measure and understand MEMS stiction under varying contact conditions and surface states.
Custom paddle test structures were fabricated across a 4-inch wafer, providing uniform displacement and well-defined contact areas. By varying beam thickness and stop counts (1–5 discrete stops and continuous-contact designs), devices spanned stiffnesses of ~1–4 N/m without altering geometry. Electrostatic pull-in and release voltages were recorded automatically using a Keysight LCR meter and automatic probe station, with triple-sweep trials demonstrating repeatability within ~0.2 V. A lumped-mass electromechanical model, incorporating bending rigidity, parallel-plate electrostatics, and tensile residual stress (~10–15 N/m), was calibrated to experimental pull-in data. Deviations from ideal (no-adhesion) release behavior yielded a closed-form expression for stiction forces, enabling purely electrical wafer-level extraction of adhesion forces.
To investigate surface modification effects, a conformal 50 nm HfO2 film was deposited by Atomic Layer Deposition (ALD) at 350 °C. Analytical modeling showed only a negligible stiffness increase (~0.3 N/m), confirmed by post-ALD testing. Devices were then systematically evaluated using the developed methodology across surface conditions and contact geometries. A subsequent 500 °C anneal showed no significant changes in stiction behavior relative to the as-deposited state. Importantly, while a measurable stiction increase was observed for the ALD-coated devices relative to uncoated silicon, this reflects the specific experimental conditions and should not be interpreted as a general conclusion regarding HfO2 coatings.
Overall, the methodology integrates automated design, predictive modeling, and high-throughput electrical testing, providing a quantitative framework to distinguish stiction contributions from mechanical and surface effects. The findings highlight that while the ALD coating introduced negligible mechanical changes, the developed methodology successfully captured and quantified stiction behavior variations, emphasizing the importance of comprehensive characterization for MEMS design and surface treatment evaluation.
Advisor: Fadi Alsaleem
Comments
Copyright 2025, Mohammad S. Megdadi. Used by permission