|Ph.D Student||Ben-David Eran|
|Subject||Investigation of the Mechanical Response and Characteristics|
of Thin Free-Standing Films at Various Strain
|Department||Department of Mechanical Engineering||Supervisors||Professor Doron Shilo|
|Professor Daniel Rittel|
|Full Thesis text|
Thin metallic films are commonly employed as structural features and electrode components in Micro and Nano Electro-Mechanical-Systems (MEMS/NEMS), e.g., RF-MEMS switches, transducers, etc. Thin films in MEMS are frequently subjected to various mechanical conditions which may result in plasticity, friction and wear, creep or fatigue. The investigation of the mechanical response and characteristics of thin free-standing films is crucial for the design and fabrication of more reliable MEMS/NEMS devices. It is also of high interest from the scientific and materials engineering views.
This thesis includes three main parts, each serve a different research objective, and the combination of all of them is essential for the advancement of the study of the mechanical characteristics and behavior of thin films. The first part is focused on the development of a novel apparatus and a procedure for tensile testing of thin free-standing films under a wide range of strain rates, from quasistatic to high, almost comparable with those obtained in Hopkinson bar tests. To provide this capability, a unique displacement measurement method was applied and a micro device, which meets several strict requirements, was implemented. The apparatus capabilities were demonstrated both in the quasistatic regime and at the maximal strain rate possible, which was about 500 sec-1.
In the second part of the study we present tensile tests of thin free-standing aluminum films under quasi-static, medium and high strain rates. A large strain rate effect was revealed as the ultimate tensile strength increased by more than 400% compared to quasi-static tests. An analysis of the kinetic relation for plastic flow showed that all commonly used kinetic laws could not explain our results. Instead, we suggest an elaborated kinetic law that is in good agreement with the results over the entire range of strain rates.
The third part of the study presents a comprehensive characterization of the relationships between deposition condition, microstructure and mechanical behavior in thin aluminum films. A particular focus is placed on the effect of porosity, which is present in all thin films deposited by evaporation or sputtering techniques. The influences of the deposition temperature on the grain size, pore size and crystallographic texture were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical behavior of the films was investigated using four different methods. Each method is suitable for characterizing different properties and for testing the material at different length scales. Nanoindentation was used to study hardness at the level of individual grains; resonance ultrasound spectroscopy (RUS) was used to measure the elastic moduli and porosity; and bulge and tensile tests were used to study released films under biaxial and uniaxial tensions. Our results demonstrated that even low porosities may have tremendous effects on the mechanical properties and that different methods allow capturing different aspects of these effects. Therefore a combination of several methods is required to obtain a comprehensive understanding of the mechanical behavior of a film. Our results show that porosity with different pore size leads to very different effects on the mechanical behavior.