Unusual fracture behavior of nanoporous polymeric thin-films
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BB8.9.1
Unusual fracture behavior of nanoporous polymeric thin-films
Andrew V. Kearney and Reinhold H. Dauskardt Department of Materials Science, Stanford University, Stanford, CA 94305-2205 Carol E. Mohler and Michael E. Mills Advanced Electronics Materials, The Dow Chemical Company, Midland, MI 48674
ABSTRACT We present surprising evidence that the fracture resistance of porous forms of poly(arylene) ether (PAE) films exhibit increasing fracture resistance with increasing porosity. Such behavior is in stark contrast to the fracture toughness of porous solids, which typically decrease markedly with increasing porosity. A fracture mechanics based model is presented to rationalize the increase in fracture toughness of the voided polymer film and explain the behavior in terms of the pore size and volume fraction. It is shown that a certain dependence of pore size and volume fraction is required to increase rather than decrease the fracture resistance. The research has implications for the optimum void size and volume fraction needed to enhance the fracture resistance of porous ductile polymer films.
INTRODUCTION Nanoporous polymer layers are being considered for a range of applications in thin-film devices and their packages, sensor and actuator technologies. However, the introduction of nanometer-sized pores typically has a deleterious effect on mechanical properties. For example, adding porosity to a dense solid has been shown to significantly reduce the modulus of the material [1,2]. The fracture toughness of voided solids is also expected to decrease sharply, especially if the voids are clustered or closely spaced [3]. Such nanoporous materials and layers are therefore structurally weaker than the fully dense materials they replace, creating challenges for integration in existing process technologies. The present study will explore how nanoscale porosity affects the adhesion of porous poly(arylene) ether (PAE) thin films. Surprising evidence is presented showing that the interfacial fracture resistance of porous forms of the PAE film to an adjacent elastic layer increases with increasing porosity. Such behavior is in stark contrast to the fracture resistance or toughness of porous solids, which typically decreases with increasing porosity. A model is developed to explore the relation between the PAE film pore size and volume fraction and the resulting adhesion to the adjacent elastic layer.
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EXPERIMENTAL The nanoporous polymer selected for this study was a modified polyarylene ether, PAE (porous SiLK* Y dielectric resin, The Dow Chemical, Midland, MI) thermosetting matrix material with a polystyrene sacrificial porogen [4]. The porogen loading was adjusted to achieve final pore volume fractions of 5.4, 10.8, 12.8 and 17.4 vol. %. Fully dense forms of the polymer films were also prepared for comparison. Polymers were spun onto 200 mm diameter Si wafers previously coated with 0.2 µm thick SiNx barrier layer. Wafers were prepared with polymer film thicknesses of 120 nm and 250 nm. The films were then ca
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