Nanoporous polymer layers are being considered for a range of emerging nanoscale applications, from low permittivity materials for interlayer dielectrics in microelectronics and anti-reflective coatings in optical technologies, to biosensors and size-selective membranes for biological applications. Polymer thin films have inherently low elastic modulus, strength and hardness, but exhibit fracture properties that are higher than those reported for glass, ceramic, and even some metal layers. However, constraint of a ductile polymer between two elastic layers is expected to affect the local plasticity ahead of a crack tip and its contribution to the film adhesion with films below a micron in thickness. Additionally, nanoporosity would be expected to have a deleterious effect on mechanical properties, producing materials and layers that are structurally weaker than fully dense versions they replace. Therefore, the integration of these nanoporous polymer layer at nanometer thicknesses would present significantly processing and mechanical reliability challenges.
In this dissertation, surprising evidence is presented that nanoporous polymer films exhibit increasing fracture energy with increasing porosity. Such behavior is in stark contrast to a wide range of reported behavior for porous solids. A ductile nano-void growth and coalescence fracture mechanics-based model is presented to rationalize the increase in fracture toughness of the voided polymer film. The model is shown to explain the behavior in terms of a specific scaling of the size of the pores with pore volume fraction. It is demonstrated that the pore size must increase with close to a linear dependence on the volume fraction in order to increase rather than decrease the fracture energy. Independent characterization of the pore size as a function of volume fraction is shown to confirm predictions made by the model.
The fracture behavior of these constrained polymer films are also examined with film thickness as low as 7.5 nm. Unlike a continuous decrease in adhesion observed in a dense polymer with decreasing film thickness, the adhesion of nanoporous layers is also shown to be insensitive to film thickness down to thickness of ∼50 nm. A sharp transition in fracture behavior is observed below this critical film thickness. Extension of the nano-void growth model can be used to predict this transition point for a nanoporous layer.
Finally, subcritical crack growth mechanism like environmentally-assisted crack and fatigue are also considered for these ultrathin polymers films. These crack growth behaviors also have key parameters that are influenced by the level of porosity and thickness of the film.
|Advisor:||Dauskardt, Reinhold H.|
|School Location:||United States -- California|
|Source:||DAI-B 69/05, Dissertation Abstracts International|
|Subjects:||Mechanics, Materials science|
|Keywords:||Adhesion, Fatigue, Fracture, Nanoporous, Polymers, Thin films|
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