Schöberle, Bernd Michael:
ISBN 9783899597608

Evaluation of Viscoelastic Materials for MEMS by Creep Compliance Analysis. Scientific Reports on Micro and Nanosystems Vol. 5 # Pb., Großformat (24x17), 190 S., 76 Abb., 26 Tab.

SCHLAGWORTE:
MEMS
Polymer
Creep
Material Test
Bulge Test
Resonant Vibrometry

This thesis advances the current state-of-the-art in microscale material testing in two aspects. First, a new methodology is established to determine the long term creep compliance of non-linear viscoelastic thin films using fully clamped diaphragms under tensile stress. Second, the mechanical properties creep compliance, fracture strength and intrinsic in-plane stress of several photosensitive polymers are investigated and quantified with the introduced approach. The measurement methodology employs the bulge test method, the resonant vibrometry test method and the uniaxial tensile test method. Theoretical models for non-linear viscoelastic behavior of membranes are established and experimentally verified. They include the influence of the specimens' geometry as well as the complex loading conditions. The intrinsic tensile in-plane stress and strain of the polymeric thin films are integrated. Simulations support the model assumption of plain-strain conditions in the chosen geometry of the diaphragms. A statistically firm experimental analysis of time-dependent mechanical behavior of MEMS-relevant polymers is performed under consideration of material constraints and the measurement technique. The materials are chosen on the basis of a physical property evaluation which reviews their suitability to serve in mechanically active microelectromechanical systems (MEMS). Three different negative photoresist formulations are thoroughly characterized. Experimental results are presented for the highly cross-linked epoxy resin SU-8-2002, the polyimide PI2737 and for GLM2060, a composite version of SU-8, filled with silica nanoparticles. Based on a series of creep experiments, the isothermal long-term creep behavior of the polymeric thin films in their non-linear viscoelastic regime is extrapolated according to the time-stress superposition principle. Master creep compliance curves, predicting the creep compliance up to t > 1e10 are constructed. The shift factors of the individual creep compliance curves can be described by the Eyring theory of stress activated plastic flow for all experiments. The fracture strengths are quantified and the von-Mises model is verified for the description of the latter under complex loading conditions for SU-8. Reference measurements of the fracture strength and the glassy modulus are taken on bone shaped SU-8 samples with an uniaxial tensile test setup. Furthermore, the time and temperature dependent characteristics of the intrinsic in-plane stress of SU-8-2002 membranes are investigated. Additionally, a fourth material is subject to experimental studies: GCM3060, a conductive SU-8 composite filled with silver microparticles. It is characterized in its mechanical short term behavior.

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